**Biomass as an Alternative for Gas Production**

Liliana Pampillón-González and

José Ramón Laines Canepa

Additional information is available at the end of the chapter

http://dx.doi.org/10.5772/67952

#### **Abstract**

Natural gas comes from the decomposition of organic material under anaerobic conditions in a process that occurred around 150 million years ago, which allows the gas trapping between rock pore spaces (porous system). Even though natural gas has become one of the most used fuels around the world, there are other spontaneous, continuous, ongoing, or inducing processes that can produce a similar gas in a short time (considering human scale); we refer to biogas. The aim of this chapter is to describe the biomass potential from organic residues for biogas production. The first part explains the biomass as an energy source, a comparison between natural gas reserves and sources of biogas with a global perspective of their energy contribution. The main biomass conversion technologies followed by case studies are shown in the second part. Finally, the biomethanization process is covered as a promising way to valorize some biomass residues into natural gas. Information about where and how the biogas can be contained, controlled, and distributed is provided. This chapter focuses in considering biogas as an alternative in the fuel demand with the advantage of coming from a renewable source, providing electricity, heat, or transport, and the generation of by-products.

**Keywords:** organic residues, biomass conversion, biomethane, biogas, renewable energy

### **1. Introduction**

Nowadays, the impact of the climate change around the world is undeniable. Most of the environmental, social, and economic problems that all societies face are associated to the

© 2017 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

energy consumption and water demand, as well as other services. Crude oil and natural gas have been used for decades, the main energy source in the major economies. Nevertheless, it has been proved that the majority of anthropogenic greenhouse gas (GHG) emissions account to the consumptions of these fossil fuels [1], increasing the global warming.

The concern is not only about the negative impacts on environment; it is also the dwindling of the fossil fuel reserves. This situation is disquieting and has focused the world's attention on the search and adoption of alternative energy sources. One of them, in this case study, is biogas production. The latter is one of the biofuels in gas form that are made from biological sources and brings an option for sharing the energy demand through the treatment of some biomass residues.

In this perspective, this chapter focuses on the description of biogas production through the use of biomass with the adoption of biological technologies as a promising way for contributing the safe and sustainable energy supply, providing heat, electricity, and biomethane (similar to natural gas).

### **2. Biomass as an energy source**

Energy is manifested by heat or electricity that is derived from fossil fuels. In some countries, not only fossil fuels can be used for this goal; there are other elements like some plants, agricultural residues, and municipal organic wastes that can also provide it.

As the law of conversation of energy states, "energy can neither be created nor destroyed; it can only be transformed from one form to another." For instance, the chemical energy stored in some organic residues can be converted to other forms of energy.

This is exactly what the bioenergy look for: the use of the stored energy from organic materials. Here is where the concept of *biomass* is introduced as a raw organic material that can be treated to generate heat and electricity from liquid, solid, or gaseous biofuels. In this respect, biomass resources represent a biogas production source. It is also one of the most abundant resources and comprises all biological materials including living or recently living organism and is considered a renewable organic resource [2].

The biomass resources take their energy from the sun, as most of the other renewable energies sources. For example, photovoltaic energy captures the solar radiation in a direct way by specialized equipment providing energy. Also, the solar energy that is transferred through the space causes the moving of air masses by heating results in wind, which can be used through turbines and generates electricity. Energy is also transferred to the water flows. The precipitation of water vapor due to the combination of wind and heat from solar energy causes the rain, which turns rivers on. The force of the water flow also can be exploiting to produce energy (hydroelectricity) and so on.

Energy from biomass is not the exception. The so-called bioenergy can harness solar energy stored in various biomass resources. Plants, for example, use solar energy to convert inorganic compounds assimilated into the organic compounds (Eq. (1)).

Photosynthesis process:

energy consumption and water demand, as well as other services. Crude oil and natural gas have been used for decades, the main energy source in the major economies. Nevertheless, it has been proved that the majority of anthropogenic greenhouse gas (GHG) emissions account

The concern is not only about the negative impacts on environment; it is also the dwindling of the fossil fuel reserves. This situation is disquieting and has focused the world's attention on the search and adoption of alternative energy sources. One of them, in this case study, is biogas production. The latter is one of the biofuels in gas form that are made from biological sources and brings an option for sharing the energy demand through the treatment of some

In this perspective, this chapter focuses on the description of biogas production through the use of biomass with the adoption of biological technologies as a promising way for contributing the safe and sustainable energy supply, providing heat, electricity, and biomethane

Energy is manifested by heat or electricity that is derived from fossil fuels. In some countries, not only fossil fuels can be used for this goal; there are other elements like some plants, agri-

As the law of conversation of energy states, "energy can neither be created nor destroyed; it can only be transformed from one form to another." For instance, the chemical energy stored

This is exactly what the bioenergy look for: the use of the stored energy from organic materials. Here is where the concept of *biomass* is introduced as a raw organic material that can be treated to generate heat and electricity from liquid, solid, or gaseous biofuels. In this respect, biomass resources represent a biogas production source. It is also one of the most abundant resources and comprises all biological materials including living or recently living organism

The biomass resources take their energy from the sun, as most of the other renewable energies sources. For example, photovoltaic energy captures the solar radiation in a direct way by specialized equipment providing energy. Also, the solar energy that is transferred through the space causes the moving of air masses by heating results in wind, which can be used through turbines and generates electricity. Energy is also transferred to the water flows. The precipitation of water vapor due to the combination of wind and heat from solar energy causes the rain, which turns rivers on. The force of the water flow also can be exploiting to produce

Energy from biomass is not the exception. The so-called bioenergy can harness solar energy stored in various biomass resources. Plants, for example, use solar energy to convert inorganic

cultural residues, and municipal organic wastes that can also provide it.

in some organic residues can be converted to other forms of energy.

and is considered a renewable organic resource [2].

compounds assimilated into the organic compounds (Eq. (1)).

energy (hydroelectricity) and so on.

to the consumptions of these fossil fuels [1], increasing the global warming.

biomass residues.

(similar to natural gas).

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**2. Biomass as an energy source**

$$6\text{CO}\_2 + 12\text{H}\_2\text{O} \rightarrow \text{C}\_6\text{H}\_{12}\text{O}\_6 + 6\text{H}\_2\text{O} + 6\text{O}\_2\tag{1}$$

An animal that eats plants takes advantage of the stored energy from these and generates biomass. Biomass works as a type of storage (battery) of solar energy transferred from one trophic level to another. The transfer of energy is evident in all processes of living beings (**Figure 1**).

Around the world, there are different sources of biomass which can be used for its conversion into energy, which includes material of biological origin, like living plants and animals and resulting residues, crops and forestry residues, sea weeds, agro-industrial residues, sewage, and municipal solid waste. Biomass can be almost all the organic material, excluding fossilized organic material embedded in geological formation [3].

Most of these biomass resources represent an environmental problem if they are not managed, transported, or disposed properly. Consequently, if energy is generated by the use of them, we can contribute for reducing the environmental pollution [4]. Furthermore, this source of energy has the advantage of not releasing CO2 into the atmosphere due to the carbon capture and storage, serving as an effective carbon sink [2].

Moreover, biomass can be multiplied in different forms of energy, that is, heat from wood and forestry residues, chemical energy from hydrogen and some biofuels, and electrical energy from the use of biogas in certain motor engines. In this chapter, we will focus in biogas, which represents a biofuel generated by biomass conversion technologies (anaerobic digestion) and an alternative for gas production.

**Figure 1.** Energy from different biomass sources.

#### **2.1. Is biogas the same as natural gas?**

The answer is no. Natural gas comes from the decomposition of organic material under anaerobic conditions but was exposed to intense heat and pressure, in a process that occurred around 150 million year ago, which allows the gas trapping between rock pore spaces (porous systems). The gas produced during this period of time is located various meters below the surface of the earth. It is not considered a renewable resource. The process for natural gas production considers mainly extraction from the subsurface, collection, treatment, transportation, and distribution services.

On the other hand, biogas is the term employed to refer to the gas obtained in a short time (considering human scale) by the anaerobic digestion of biomass resources. The process occurs sometimes as a spontaneous, continuous, ongoing, or inducing way but always is very sensible to biological process. Indeed, specific microorganisms, in a four-step process (hydrolysis, acidification, acetogenesis, and methanogenesis), achieve the anaerobic digestion of organic material (**Figure 2**). To do so, certain physico-chemical parameters such as temperature, pH, daily organic load, available nutrients, retention time, agitation, and other inhibitory factors must be adequate or adjusted for generating biogas [5].

The main difference between natural gas and biogas is related to the carbon dioxide content. The latter is contained in 25–45% of the total composition of biogas, while natural gas contains less than 1% (**Table 1**). Moreover, natural gas contains other hydrocarbons rather than methane. The methane content strongly influences the calorific value of these gases. Energy content of biogas similar to natural gas can be obtained if carbon dioxide from biogas is removed

**Figure 2.** Stages of anaerobic digestion process. Source: modified from Ref. [6].


**Table 1.** Composition of biogas and natural gas.

**2.1. Is biogas the same as natural gas?**

176 Advances in Natural Gas Emerging Technologies

tation, and distribution services.

The answer is no. Natural gas comes from the decomposition of organic material under anaerobic conditions but was exposed to intense heat and pressure, in a process that occurred around 150 million year ago, which allows the gas trapping between rock pore spaces (porous systems). The gas produced during this period of time is located various meters below the surface of the earth. It is not considered a renewable resource. The process for natural gas production considers mainly extraction from the subsurface, collection, treatment, transpor-

On the other hand, biogas is the term employed to refer to the gas obtained in a short time (considering human scale) by the anaerobic digestion of biomass resources. The process occurs sometimes as a spontaneous, continuous, ongoing, or inducing way but always is very sensible to biological process. Indeed, specific microorganisms, in a four-step process (hydrolysis, acidification, acetogenesis, and methanogenesis), achieve the anaerobic digestion of organic material (**Figure 2**). To do so, certain physico-chemical parameters such as temperature, pH, daily organic load, available nutrients, retention time, agitation, and other

The main difference between natural gas and biogas is related to the carbon dioxide content. The latter is contained in 25–45% of the total composition of biogas, while natural gas contains less than 1% (**Table 1**). Moreover, natural gas contains other hydrocarbons rather than methane. The methane content strongly influences the calorific value of these gases. Energy content of biogas similar to natural gas can be obtained if carbon dioxide from biogas is removed

inhibitory factors must be adequate or adjusted for generating biogas [5].

**Figure 2.** Stages of anaerobic digestion process. Source: modified from Ref. [6].

in an upgrading process [7]. The presence of hydrogen sulfide (H<sup>2</sup> S) in biogas must be cleaning or upgrading to methane in order to diversify the end use of biogas in several ways.

#### **2.2. Natural gas reserves and sources of biogas**

Natural gas is a fossil fuel often found under the oceans, near oil deposits, trapped between the rock pores spaces (porous systems), and beneath the earth's surface. Similarly to the oil exploration, there are natural gas reservoirs around the planet classified as proved and undiscovered technically recoverable resources. A reservoir is a location where large volumes of methane can be trapped in the subsurface of the earth. In this respect, proved reserves of natural gas are estimated quantities that analyses of geological and engineering data have demonstrated to be economically recoverable from known reservoir in the future [10]. According to the International Energy Statistics, in 2014 there were 6973 proved reserves worldwide [10], in which the countries of Middle East and Eurasia represent the vast majority of it (**Figure 3**).

Even though natural gas has become one of the most used fuels around the world and the trends point to increase in number of proved reserves due to the application of new technologies, the world population will continue to grow and still demand more energy, so the amount of fossil fuels is not an enough resource for all the countries. As well as, the ongoing price increase of fossil resources and the visible impacts on the global warming.

Under this scenario, a versatile fuel that comes from a wide variety of biomass is biogas. It can provide a renewable source of energy and can lead to reduce impacts of pollution by inadequate waste disposal. Whereas undiscovered technically recoverable resources of natural gas are still growing, a large quantity of solid waste is also generating. Most of the countries

**Figure 3.** Proved reserves of natural gas worldwide in 2014 (with data from Ref. [10]).

around the world deal with their residues; they represent a social-environmental problem due to the lack of management. This biomass can be a harnessing nature's potential to produce energy. It is continously produced, free in many countries and widely available.

In this respect, the future role for biogas in the world is related with the availability of different types or organic feedstock which depends on a number of economic, social, technological, environmental, and regulatory factors. Examples of various biomass feedstocks for biogas production by sector are shown in **Table 2**.

It is predicted that by 2020, renewables will represent the 14% from the total EU energy mix, in which biomass accounts with the 54% of the 251 million tons of oil equivalents (Mtoe) (**Figure 4**). Unfortunately, most of this biomass is used in a direct way as wood, so biogas potential studies can be evaluated considering certain type of biomass.

For 2010, primary production of biogas in Europe was 10.9 Mtoe, in which 27% of the biogas was produced from landfill, 10% from sewage sludge, and 63% from decentralized agricultural plants, municipal solid waste, methanization plants, co-digestion, and multiproduct plants [13]. This biogas production increases to 31% compared to 2009. Germany is one of the countries that have doubled biogas production in the last years, and it is also one of the main biogas-producing countries for the 2020 in the EU (**Figure 5**). The acceptance and the rapidly growth of the technology show how biogas can make an important contribution to the energy supply in a short term.

Similarly to biomass demand, the biogas demand has a number of end user sectors, which have different characteristics in terms of application, economic value added, customers, social benefits, and environmental impact [14]. If biogas is conditioned or cleaned, it will be an


Source: modified from Ref. [11].

around the world deal with their residues; they represent a social-environmental problem due to the lack of management. This biomass can be a harnessing nature's potential to produce

In this respect, the future role for biogas in the world is related with the availability of different types or organic feedstock which depends on a number of economic, social, technological, environmental, and regulatory factors. Examples of various biomass feedstocks for biogas

It is predicted that by 2020, renewables will represent the 14% from the total EU energy mix, in which biomass accounts with the 54% of the 251 million tons of oil equivalents (Mtoe) (**Figure 4**). Unfortunately, most of this biomass is used in a direct way as wood, so biogas potential studies

For 2010, primary production of biogas in Europe was 10.9 Mtoe, in which 27% of the biogas was produced from landfill, 10% from sewage sludge, and 63% from decentralized agricultural plants, municipal solid waste, methanization plants, co-digestion, and multiproduct plants [13]. This biogas production increases to 31% compared to 2009. Germany is one of the countries that have doubled biogas production in the last years, and it is also one of the main biogas-producing countries for the 2020 in the EU (**Figure 5**). The acceptance and the rapidly growth of the technology show how biogas can make an important contribution to the energy

Similarly to biomass demand, the biogas demand has a number of end user sectors, which have different characteristics in terms of application, economic value added, customers, social benefits, and environmental impact [14]. If biogas is conditioned or cleaned, it will be an

energy. It is continously produced, free in many countries and widely available.

**Figure 3.** Proved reserves of natural gas worldwide in 2014 (with data from Ref. [10]).

production by sector are shown in **Table 2**.

178 Advances in Natural Gas Emerging Technologies

supply in a short term.

can be evaluated considering certain type of biomass.

**Table 2.** Sources and type of biomass by sector.

**Figure 4.** EU energy mix 2020 [12].

**Figure 5.** Biogas potential for 2020 in the EU.

outstanding solution for a variety of applications commonly known for natural gas with the addition of the versatility of its end uses. Some examples include: motor fuel, electricity, heat, combined electricity and heat, and recently replace carbon compound into plastic products [11] and also the generation of by-products that can be used as an organic fertilizer.

#### **2.3. Advantages of biomass energy**

There is an important environmental advantage of biomass utilization in terms of reduction of natural resource depletion [15], carbon neutral resource in its life cycle (Asian Biomass Handbook), and sustainable energy systems [16]. It has been estimated that by the year 2020, 50% of the present gas consumption in the Europe Union could be covered by biomethane from digested feedstock [17] contributing to the greenhouse gas capture, like methane. Also the fermentation process is an alternative for wet-bases raw residues treatment, and particularly anaerobic digestion because of the cost-effective [18, 19]. Biogas can be burned directly in boiler for heat or/and engine for cogeneration, while upgrade biogas can be injected in the natural gas grid and used directly at the consumer in boilers and small combined heat and power (CHP) [20].

### **3. Biomass conversion technologies**

Since the last century (1897), some Asian countries, like China and India, started their first trials in using biogas [21], through a stabilization process that allows the use in household and farm-scale applications. Similarly, England reported using it in the 1930s for lighting streets [11]. In both cases, the main biomass source to produce biogas was taken from sewage in order provide a fuel for cooking and lighting. In a brief context, the use of biomass to provide energy has been fundamental to the development of societies.

Nowadays, the demand on energy and the impact on climate change have led to calls for an increase in the use of biogas in different ways. In this section, the main process or conversion technologies employed for the biomass are presented with specific regard to biogas production.

#### **3.1. Biomass conversion process**

The biomass conversion technologies are closely related to the type of biomass, quantity, the availability, the cost-effective, and the end user requirement of the biofuel. The selection of the technology depends on the main interest of the "producer." For all the cases, the main biomass treatments that can be applied are encompassed in four conversion technologies: direct combustion, thermochemical, biochemical and biotechnology, and nanotechnology (**Figure 6**).

It is important to note that a pretreatment of the biomass is necessary before applying a conversion technology. In some cases, biomass has to be harvested, collected, transported, or stored [22]. Further, resource availability varies from region to region, according to weather conditions, soil type, geography, population density, and productive activities, which makes the choice of technology for processing more complex.

#### **3.2. Direct combustion**

outstanding solution for a variety of applications commonly known for natural gas with the addition of the versatility of its end uses. Some examples include: motor fuel, electricity, heat, combined electricity and heat, and recently replace carbon compound into plastic products

There is an important environmental advantage of biomass utilization in terms of reduction of natural resource depletion [15], carbon neutral resource in its life cycle (Asian Biomass Handbook), and sustainable energy systems [16]. It has been estimated that by the year 2020, 50% of the present gas consumption in the Europe Union could be covered by biomethane from digested feedstock [17] contributing to the greenhouse gas capture, like methane. Also the fermentation process is an alternative for wet-bases raw residues treatment, and particularly anaerobic digestion because of the cost-effective [18, 19]. Biogas can be burned directly in boiler for heat or/and engine for cogeneration, while upgrade biogas can be injected in the natural gas grid and used directly at the consumer in boilers and small combined heat and

Since the last century (1897), some Asian countries, like China and India, started their first trials in using biogas [21], through a stabilization process that allows the use in household and farm-scale applications. Similarly, England reported using it in the 1930s for lighting streets

[11] and also the generation of by-products that can be used as an organic fertilizer.

**2.3. Advantages of biomass energy**

**Figure 5.** Biogas potential for 2020 in the EU.

180 Advances in Natural Gas Emerging Technologies

**3. Biomass conversion technologies**

power (CHP) [20].

One of the oldest uses in which biomass has been utilized for energy in the world is through the burning wood (combustion). This action represents a traditional use of biomass, particularly in rural zones. It is considered an essential resource to the economic development of societies [23]. Nevertheless, when the wood is burnt in an open fire stove, around 80% energy is lost [24]. Recently, technologies suggest the use of energy efficiency stoves, which not only has a better thermal efficiency but also avoids indoor air pollutions. Other specialized equipment involves furnaces, boilers, steam turbines, and turbogenerator. The combustion of biomass allows the recovery of the chemical energy stored. In general, combustion processes

**Figure 6.** Conversion technologies of biomass into energy. Source: modified from Ref. [2].

involve direct oxidation of matter in air, that is, ignition or burning of organic matter in an air atmosphere sufficient to react with oxygen fuel.

#### **3.3. Thermochemical process**

Thermochemical process, as the direct combustion, has a core axis, the temperature. One of the main differences is an induced atmosphere in which conversion of biomass took place. This oxidation process can occur in the presence or absence of a gasifying medium. The conversion of biomass depends on temperature and pressure variables. For example, if the substrate to transform is in the presence of a gas such as oxygen, water vapor, or hydrogen, producing fuel is performed through gasification. If, however, material degradation occurs in the absence of oxygen, that is, nitrogen, under controlled pressure and temperature, then the process is called pyrolysis.

There are some good experiences in the pyrolysis of certain materials, in which a charcoal, bio-oil, and a fuel gas can be recovered [25].

### **3.4. Biochemical process**

Biochemical treatment unlike thermochemical process achieves power generation through biological transformation of organic compounds, employing anaerobic digestion, or fermentation of biomass. Fermentation is usually used to produce biofuels, as ethanol, from sugar crops, and starch crops [22]. Nevertheless, there is another route, in which biomass conversion is done, the anaerobic digestion.

Among the general background information about conversion technologies, anaerobic digestion is the main focus in this section due to the direct biogas production. The anaerobic process is analog to ruminant digestion process. The biomass is degraded by a consortium of bacteria within an anaerobic environment, producing a principal product, gas. This gas, called biogas, represents a proven technology and its use is widely spreading through Europe.

For biogas production, there are some types of biomass that are more accurate, like the ones with high moisture content in organic wastes (80–90%) or wet biomass residues as manures, municipal organic solid waste, and sewage sludge [22]. The anaerobic digestion process generally occurs in reactors or tanks in a single, multistage process or dry digestion.

Anaerobic digester can be categorized, designed, and operated by different configurations: batch or continuous, temperature (mesophilic or thermophilic), solid content (high or low solid content), and complexity (single stage or multistage) [26]. Another specific configuration considering the organic rate load, digester, is divided into passive systems (covered lagoons), low rate systems (complete mix reactor, plug flow, and mixed plug flow), and high rate systems (contact stabilization, fixed film, suspended media, and sequencing batch) [27]. All these types of reactors perform the anaerobic digestion, but each one operates for salient features with a variety of applications of the end products.

An experience in the livestock sector in Mexico using covered lagoon anaerobic digestion reactor shows benefits in the use of biogas not only on environmental aspects as improving


the quality of wastewater but also economically due to the avoid of penalties for the water discharges and the social acceptance of the livestock activity in the region (**Table 3**).

**Table 3.** Biogas production experience in livestock sector in Mexico.

involve direct oxidation of matter in air, that is, ignition or burning of organic matter in an air

Thermochemical process, as the direct combustion, has a core axis, the temperature. One of the main differences is an induced atmosphere in which conversion of biomass took place. This oxidation process can occur in the presence or absence of a gasifying medium. The conversion of biomass depends on temperature and pressure variables. For example, if the substrate to transform is in the presence of a gas such as oxygen, water vapor, or hydrogen, producing fuel is performed through gasification. If, however, material degradation occurs in the absence of oxygen, that is, nitrogen, under controlled pressure and temperature, then the process is called pyrolysis. There are some good experiences in the pyrolysis of certain materials, in which a charcoal,

Biochemical treatment unlike thermochemical process achieves power generation through biological transformation of organic compounds, employing anaerobic digestion, or fermentation of biomass. Fermentation is usually used to produce biofuels, as ethanol, from sugar crops, and starch crops [22]. Nevertheless, there is another route, in which biomass conver-

Among the general background information about conversion technologies, anaerobic digestion is the main focus in this section due to the direct biogas production. The anaerobic process is analog to ruminant digestion process. The biomass is degraded by a consortium of bacteria within an anaerobic environment, producing a principal product, gas. This gas, called biogas,

For biogas production, there are some types of biomass that are more accurate, like the ones with high moisture content in organic wastes (80–90%) or wet biomass residues as manures, municipal organic solid waste, and sewage sludge [22]. The anaerobic digestion process gen-

Anaerobic digester can be categorized, designed, and operated by different configurations: batch or continuous, temperature (mesophilic or thermophilic), solid content (high or low solid content), and complexity (single stage or multistage) [26]. Another specific configuration considering the organic rate load, digester, is divided into passive systems (covered lagoons), low rate systems (complete mix reactor, plug flow, and mixed plug flow), and high rate systems (contact stabilization, fixed film, suspended media, and sequencing batch) [27]. All these types of reactors perform the anaerobic digestion, but each one operates for salient features

An experience in the livestock sector in Mexico using covered lagoon anaerobic digestion reactor shows benefits in the use of biogas not only on environmental aspects as improving

represents a proven technology and its use is widely spreading through Europe.

erally occurs in reactors or tanks in a single, multistage process or dry digestion.

atmosphere sufficient to react with oxygen fuel.

bio-oil, and a fuel gas can be recovered [25].

sion is done, the anaerobic digestion.

with a variety of applications of the end products.

**3.3. Thermochemical process**

182 Advances in Natural Gas Emerging Technologies

**3.4. Biochemical process**

In this example, the different benefits of biogas production in livestock sector highlighted the use of biogas in energy generation. Against other energy sources, in this case, the biogas produced is used in the farm for their own consumption by a gas combustion engine. The heat generated by the motors can be used for heating the reactor or drying waste. Biogas has the quality that does not have to be consumed at the moment of production. The production of this biofuel also impacts in macro- and microeconomic aspects, due to the generation of new sources of employs and access to energy in a remote place. Moreover, the livestock producer is selling an organic fertilizer obtained by high-quality digestate obtained in the biogas production.

Furthermore, odor reduction and the removal of pathogenic organism in livestock residues are achieved. The methane emission of the manures is captured, reducing the release of methane to the atmosphere. Methane (CH4 ) is considered one of the largest contributors to the GHG emissions by livestock sector, with a global warming potential 25 times more than carbon dioxide (CO<sup>2</sup> ) [29, 30].

In general, the biomass conversion technologies mentioned above can be integrated into the concept of biorefinery. Analog to oil process, the different biomass feedstocks offer a wide range of products that can be used as fuel, including gas, oil, or chemical, offering greater possibility of using cogeneration systems and supply facilities in the transport sector.

### **4. Biomethanization process**

When the major end product in a biogas plant is methane, similar to natural gas, this upgraded gas is called biomethane. The methane content determines the energetic value in the biogas [11]. In this respect, one of the main reasons for upgrading biogas to a degree equivalent to natural gas is to inject to the gas distribution network and thus diversify some natural gas sources.

Biomethanization process opens new paths to achieve this goal: first, because the gas storage in an extended way allows the injection into a distribution system and second due to the variety use of fuel in transport stations, mainly.

As we see in the sections above, the main biogas uses in development countries are lighting, cooking, and further in gas turbines. In industrial countries biogas is produced in large-scale digester (biogas plants) with an interest in the concentration of methane from biogas to fulfill natural gas standards. Depending on the end use, different biogas treatments (cleaning or upgrading) are necessary. For example, vehicle gas fuel requires a biogas similar to natural gas quality so a biogas upgrading process is needed. In other words, biomethanization allows biogas to be contained, controlled, and distributable.

### **4.1. Biogas cleaning**

There are some undesirable components in biogas that promote corrosion in many materials and engines: H<sup>2</sup> S, oxygen, nitrogen, water, siloxanes, and particle traces (see **Table 1**). These impurities can induce or promote corrosion in many parts of the biogas system or equipment in which biogas is used. Overall, these components must be removed in order to allow the concentration of methane in biogas.

Water content in biogas can cause corrosion in pipelines due to the formation of carbonic acid in a reaction derived from water and carbon dioxide [31]. Fortunately, it can be removed by cooling, compression, absorption, or adsorption (activated carbon, sieves, or SiO2 ). Hydrogen sulfide (H<sup>2</sup> S), another unwanted component in biogas, is of corrosive nature, leading the damage of motor engine, pipes, etc. It is a highly toxic gas that attempts to destroy the human health. The removal of hydrogen sulfide can be done by precipitation, adsorption on active carbon for H2 S removal (US 8669095 B2 patent) [32]. Siloxanes also constitute an impurity in biogas. It can affect combustion equipment, as gas engine, through the formation of silicon oxide. The most common methods for removing siloxane components are adsorption on activated carbon, activated aluminum, or silica gel, mainly [31].

After desulfurization and drying process of biogas, it can generate electricity and heat in cogeneration systems, combined heat and power (CHP), or can be transformed to energy products with higher value, density, and calorific value.

#### **4.2. Biogas upgrading**

Around the world, the number of upgrading biogas plants has increased, reaching 100 during 2009 [7]. This facility has gained the world's attention due to the rising oil and natural gas prices.

The biogas obtained during anaerobic digestion of biomass contains important amounts of carbon dioxide that result in lower energy content. In order to improve this characteristic, the separation of carbon dioxide through an upgrading process is requested. Cleaning the gas before upgrading is recommended.

Compared with the common uses of biogas, the upgrading of biogas brings several advantages related to transportation of the gas and offering the chance to increase the overall efficiency of gas utilization. In this part, it is important to clear up that cleaning biogas refers to the separation of impurities, while upgrading refers to the separation of CO2 .

Biomethanization process opens new paths to achieve this goal: first, because the gas storage in an extended way allows the injection into a distribution system and second due to the vari-

As we see in the sections above, the main biogas uses in development countries are lighting, cooking, and further in gas turbines. In industrial countries biogas is produced in large-scale digester (biogas plants) with an interest in the concentration of methane from biogas to fulfill natural gas standards. Depending on the end use, different biogas treatments (cleaning or upgrading) are necessary. For example, vehicle gas fuel requires a biogas similar to natural gas quality so a biogas upgrading process is needed. In other words, biomethanization allows

There are some undesirable components in biogas that promote corrosion in many materials

impurities can induce or promote corrosion in many parts of the biogas system or equipment in which biogas is used. Overall, these components must be removed in order to allow the

Water content in biogas can cause corrosion in pipelines due to the formation of carbonic acid in a reaction derived from water and carbon dioxide [31]. Fortunately, it can be removed by

age of motor engine, pipes, etc. It is a highly toxic gas that attempts to destroy the human health. The removal of hydrogen sulfide can be done by precipitation, adsorption on active

biogas. It can affect combustion equipment, as gas engine, through the formation of silicon oxide. The most common methods for removing siloxane components are adsorption on acti-

After desulfurization and drying process of biogas, it can generate electricity and heat in cogeneration systems, combined heat and power (CHP), or can be transformed to energy products

Around the world, the number of upgrading biogas plants has increased, reaching 100 during 2009 [7]. This facility has gained the world's attention due to the rising oil and natural gas prices. The biogas obtained during anaerobic digestion of biomass contains important amounts of carbon dioxide that result in lower energy content. In order to improve this characteristic, the separation of carbon dioxide through an upgrading process is requested. Cleaning the gas

Compared with the common uses of biogas, the upgrading of biogas brings several advantages related to transportation of the gas and offering the chance to increase the overall efficiency of

S), another unwanted component in biogas, is of corrosive nature, leading the dam-

S removal (US 8669095 B2 patent) [32]. Siloxanes also constitute an impurity in

cooling, compression, absorption, or adsorption (activated carbon, sieves, or SiO2

vated carbon, activated aluminum, or silica gel, mainly [31].

with higher value, density, and calorific value.

before upgrading is recommended.

S, oxygen, nitrogen, water, siloxanes, and particle traces (see **Table 1**). These

). Hydrogen

ety use of fuel in transport stations, mainly.

184 Advances in Natural Gas Emerging Technologies

biogas to be contained, controlled, and distributable.

**4.1. Biogas cleaning**

concentration of methane in biogas.

and engines: H<sup>2</sup>

sulfide (H<sup>2</sup>

carbon for H2

**4.2. Biogas upgrading**

Currently, there are several technologies for biogas cleaning and upgrading, commercially available, like pressure swing adsorption (PSA) (US 6340382 B1 patent) [33], water scrubbing, organic physical scrubbing, and chemical scrubbing. Most of them are a combination or one or two processes for biogas cleaning or upgrading (**Figure 7**).

**Figure 7.** Different biogas cleaning and upgrading of biogas. Source: adapted from Ref. 34.

If biogas is upgraded to biomethane with approximately 98% of methane content in biogas, it can have the same properties as natural gas [35]. By these standards, biomethane can be fed into the available gas network or be used for any purpose for which natural gas is used. The overall environmental benefits of the use of biogas are, however, highest when the biogas is used as a vehicle fuel replacing oil or diesel [4].

In fact, the selection of the optimal technology for biogas upgrading depends on the quality and quantity of the raw biogas to be upgraded, the desired biomethane quality and the final use of the biogas, the anaerobic digestion system, the continuity of the biomass, as well as the local circumstances [36].

### **5. Opportunities for bio-based economy (green natural gas)**

The current leader in the deployment of biogas technology is Germany. In the last decade, the number of digester plant increased ten times compared to 1996 (Poeschl et al., 2010). The German scheme is a clear example for biogas technology promotion; it highlights the employment of key instruments for helping to spread out the technology, that is, economic incentives.

Broadly, biogas production in different countries is still dependent on subsidies for attracting investors, producers, and I&D groups and promoting its scalability. Certification systems, feed-in tariffs, and investment support are examples of measures that are widely applied (**Table 4**). Some of the policy documents and directives that are related to bioenergy are included in three EU regulatory frameworks: the Renewable Energy Directive (2009/28/EC), the Directive on Waste Recycling and Recovery (2008/98/EC), and the Directive on Landfill (1999/31/EC) [37].


**Table 4.** Examples of incentives schemes for biogas production.

### **6. Conclusion**

Most of the countries around the world are still dependent on energy supplies, mainly by fossil fuels. Societies need to secure the energy demand, through social equality and mitigating the environmental impact. In this respect, biogas production is not only a promising way but is currently one of the most renewable technologies capable of offer energy, as such fossil fuel does.

Biogas can play the pivot role in the renewable sustainable energy systems in the near future due to its versatility, availability, storability, and energetic value. In this context, adequate public policy (regulation) for promoting economic, social, and cultural conditions for biogas production is still necessary.

Even though the technology has been adopted by many countries in Europe, there is still a necessity for developing and applying more adequate technology for cleaning and upgrading biogas to biomethane in places in which the use is limited (grid injection), which is becoming a present challenge.

Biogas and biomethane benefits promoting is required to overcome the reliability of the anaerobic process and the use of the by-products, increase the ability of the enterprises to satisfy the market necessities, and involve the government, public, private, and actor in this important task for reaching to a sustainable energy system.

### **Acknowledgments**

**5. Opportunities for bio-based economy (green natural gas)**

**Country Incentive Scope of support**

The UK Feed-in tariff Electricity from biogas

**Table 4.** Examples of incentives schemes for biogas production.

(1999/31/EC) [37].

186 Advances in Natural Gas Emerging Technologies

**6. Conclusion**

Source: modified from Ref. [37].

does.

The current leader in the deployment of biogas technology is Germany. In the last decade, the number of digester plant increased ten times compared to 1996 (Poeschl et al., 2010). The German scheme is a clear example for biogas technology promotion; it highlights the employment of key instruments for helping to spread out the technology, that is, economic incentives. Broadly, biogas production in different countries is still dependent on subsidies for attracting investors, producers, and I&D groups and promoting its scalability. Certification systems, feed-in tariffs, and investment support are examples of measures that are widely applied (**Table 4**). Some of the policy documents and directives that are related to bioenergy are included in three EU regulatory frameworks: the Renewable Energy Directive (2009/28/EC), the Directive on Waste Recycling and Recovery (2008/98/EC), and the Directive on Landfill

Germany Feed-in tariff Electricity and heat from biogas. Tariff according to system size and fuel

Gas processing bonus Upgraded biogas for grid injection and transport

Renewable obligation order % RES from electricity production (>5 MW) Climate change levy Favors any type of renewable energy generation

landfill gas

Renewable heat incentive Biomethane injection and biogas combustion, except from

Energy taxation Tax benefits for electricity, heat, and transport from biogas

Market premium Biogas and biomethane

Flexibility premium Electricity from biogas

Sweden Certification system Certificates for electricity from biogas

Investment support Farm-based biogas production

Most of the countries around the world are still dependent on energy supplies, mainly by fossil fuels. Societies need to secure the energy demand, through social equality and mitigating the environmental impact. In this respect, biogas production is not only a promising way but is currently one of the most renewable technologies capable of offer energy, as such fossil fuel The support and valuable comments from a Master of Science (Latin: Magister Scientiae). Oscar Silván-Hernández and Dr Alejandro Ordáz-Flores are greatly appreciated.

### **Author details**

Liliana Pampill�n-Gon��le�\* and �os� Ram�n Laines Canepa

\*Address all correspondence to: lilianapg@hotmail.com; liliana.pampillon@ujat.mx

División Académica de Ciencias Biológicas, Universidad Juárez Autónoma de Tabasco, Villahermosa, Tabasco, Mexico

### **References**


[19] Fehrenbach H., Giergrich J., Reinhardt G., Sayer U., Gretz M., Lanje K., Schmitz J., Ktirerien einer nachhaltigen Bioenergienutzungim globalel Mabstab, UBA.Forschungsbericht, 2008, 6, 41-112.

[4] Lantz M., Svensson M., Björnsson L., Börjesson, The prospect for an expansion of biogas system in Sweden-Incentives, barriers and potentials, Energy Policy, 2007, 35, 1830-1843.

[5] Deepanraj B., Sivasubramanian V., and Jayaraj S., Biogas generation through anaerobic digestion process: An overview, Research Journal of Chemistry and Environment, 2014,

[6] Nasir I.M., Ghazi T.I.M., Omar R., Anaerobic digestion technology in livestock manure treatment for biogas production: A review, Engineering in Life Science, 2012, 12, 258-269.

[7] Petersson A., Wellinger A., editors. Biogas Upgrading Technologies developments and Innovations, 2009, Switzerland, IEA Bioenergy Task 37, 20 p, DOI: http://www.iea-biogas.net/files/daten-redaktion/download/publi-task37/upgrading\_rz\_low\_final.pdf. [8] Deublein D., Steinhauser A., editors, Biogas from Waste and Renewable Resources, and

[9] Rasi S., Veijanen A., Rintala J., Trace compounds of biogas from different biogas produc-

[10] EIA, U.S. Energy Information Administration. Independent Statistic & Analysis [Internet], 2016, Available from http://www.iea-biogas.net/files/daten-redaktion/down-

[11] Wellinger A., Murphy J., Baxter D., editors, The Biogas Handbook: Science, Production and Applications, 2013, IEA Bioenergy, Woodhead Publishing series in Energy,

[12] IEA World Energy Outlook, 2011, Paris, Available from https://www.iea.org/publications/freepublications/publication/WEO2011\_WEB.pdf [Accessed: 2016-06-02].

[13] Tricase C., Lombardi M., State of art and prospects of Italian biogas production from animal sewage: Technical-economic considerations, Renewable Energy, 2009, 34, 477-485.

[14] Slade R., Saunders R., Gross R., Bauen A., Energy from Biomass: The Size of the Global Resource 2011,, London, Imperial College Centre for Energy Policy and Technology and

[15] Carpentieri M., Corti A., Lombardi L., Life cycle assessment (LCA) of an integrated biomass gasification combined cycle (IBGCC) with CO2 removal, Energy Conservation and

[16] Islas J., Manzini F., Masera O., A prospective study of bioenergy use in Mexico, Energy,

[17] Thrän D., Seiffert M., Müller-Langer F., Plätter A., Vogel A., Möglinckleiten einer europäischen Biogaseinspeisungsstrategie, Studie im Auftrag der Fraktion der Stadwerke Aachen (STAWAG), 2007, Leipzig, Germany, Institut für Energetik und Umwelt gGmbH(IE) ed.

[18] Hansen L.C., Cheong D.Y., Agricultural waste management in food processing, Handbook of Farm, Dairy and Food Machinery Engineering, Myer Kutz Associates, Inc., Delmar,

Introduction, 2008, London, UK, Wiley-VCH Publishing, 578 p.

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2007, 32, 2306-2320.

Cambridge, UK. 512 p.

18(5), 80-94.

188 Advances in Natural Gas Emerging Technologies


**Chapter 9**

## **Shale Gas in Poland**

[32] Grill J., Method for the Treatment of Process Gas for Biological H2

Renewable and Sustainable Energy Reviews, 2010, 14, 1728-1797.

http://infohouse.p2ric.org/ref/22/21262.pdf [Accessed: 2016-03-18].

[33] Alli Baksh M.S., Ackley M.W., Pressure Swing Adsorption Process for the Production of

[34] Schmuderer M., Overview, operational experience and perspectives of biogas upgrading technologies, On the Road with CNG and Biomethane, 2010, ed. Prague, The

[35] Poeschl M., Ward S., Owende P., Prospect for expanded utilization of biogas in Germany,

[36] Walsh J.C., Ross M., Smith S., Harper S., Wilkins W., Handbook on Biogas Utilization, 1998, In: Atlanta, GA, Georgia Institute of Technology, Engineering Technology Branch, U.S. Department of Energy, Southeastern Regional Biomass Energy Program, DOI:

[37] Federal Ministry for the Environment, Nature Conservation and Nuclear Safety. RES Legal. Swedish Gas Association, Department of Energy & Climate Change (DECC), Available from http://www.reslegal.de/en/search-for-countries [Accesed: 2017-02-06].

Patent 8669095 BE.

190 Advances in Natural Gas Emerging Technologies

Madagascar project.

Hydrogen, 1999, US Patent 6340382 B1.

S Removal, 2003, US

Jadwiga A. Jarzyna, Maria Bała, Paulina I. Krakowska, Edyta Puskarczyk, Anna Strzępowicz, Kamila Wawrzyniak-Guz, Dariusz Więcław and Jerzy Ziętek

Additional information is available at the end of the chapter

http://dx.doi.org/10.5772/67301

#### **Abstract**

An example of interpretation of the Silurian and Ordovician shale formations in the Baltic Basin in Poland regarding determination of potential sweet spots is presented. Short geological information shows the position of shale gas play. Description of the data—laboratory measurement outcomes (petrophysical and geochemical) and well logging—presents results available for analyses. Detailed elemental analyses and various statistical classifications show the differentiation between sweet spots and adjacent formations. Elastic property modelling based on the known theoretical models and results of comprehensive interpretation of well logs is a good tool to complete information, especially in old wells. Acoustic emission investigations show additional characteristic features of shale gas rock and reveal that acoustic emission and volumetric strain of a shale sample induced by the sorption processes are lower for shale than for coals.

**Keywords:** shale gas, petrophysics and well logging, statistical analyses, acoustic emission, Baltic Basin

### **1. Introduction**

Shale gas deposits belong to unconventional hydrocarbon resources. Nowadays, unconventional resources (tight gas, shale gas) are under careful and detailed consideration regarding cognitive works. The world is interested in prospection of unconventional deposits because of the necessity to increase energy resource production and geological limitations regarding conventional deposits. Also, economical aspects are important. Interest in shale gas arises when prices of hydrocarbons are high. In conventional oil and gas deposits, mature rock,

© 2017 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

reservoir and sealing rock are crucial. Also, geological traps enabling accumulation of hydrocarbons are important. Traditional prospecting by seismic methods and well logging is oriented to find traps and good reservoirs—high porosity and high permeability rocks enabling fluid flow. In shale gas, prospection source and reservoir rock are the same formation. Finding of shale formations is easier than conventional traps, but exploitation of hydrocarbons due to low porosity and very low permeability is more difficult. In shale gas, plays information on rock elastic properties is a crucial issue. An additional element complicating the unconventional shale gas reservoir model is dispersed organic matter (kerogen). New technologies are elaborated to make prospection of shale gas deposit more efficient.

In the paper, there are presented results of laboratory measurements and well logging obtained using conventional (i.e. resistivity and density and acoustic gamma ray logs) and selected modern methods (i.e. geochemical and NMR logs). The goal of the paper is presentation of available method application on contouring and characterization of sweet spots (differentiation between parts of the formation rich in hydrocarbons and surrounding rocks). An example of data interpretation of the Silurian- and Ordovician-age shale formations in the Baltic Basin in Poland is presented.

### **2. Selected geological information about Polish shale gas formations**

In the Polish sedimentary basins beginning with the Baltic Basin in the north (**Figure 1**) to the Lublin Basin in the southeast part of Poland, there are numerous siltstone and mudstone deposits, rich in oil-prone organic matter (Type II kerogen).

Shale rocks, rich in organic matter, may be the exploration targets in terms of unconventional hydrocarbon (oil and gas) reservoirs [3–5]. Silurian and Ordovician shales in Poland spread along the western margin of the East European Platform in Lublin, Podlasie and Baltic basins, reaching about 700 km in length [6]. On Łeba elevation where the exemplary boreholes (L-1, K-1, O-2, B-1 and W-1) are located, the sedimentation, organic-rich black shales where the graptolites are the main fossil, started in the Late Llanwirnian reaching Wenlock [7].

The object of the analysis covers two formations, being potentially resources of unconventional hydrocarbons: Ja Member of the Silurian Pa formation and Ordovician Sa formation. Silurian Pa Fm was firstly described by Tomczyk [8]. The formation lithostratotype is the part of the Ko IG-1 well profile. In analyzed wells, the Pa formation is placed between Pr Fm (lower boundary) and Pe Fm (upper boundary). The lower boundary is marked at the point of clear lithological changes involving the replacement of limestone or marl deposits of Ordovician series into clay sediments of the Pa formation. It is clearly visible on the well logs. The lower part of the formation is built by black, bituminous claystones passing to the top into dark grey claystones, laminated by greenish, grey-greenish and black claystones or brown calcareous claystones and bentonite laminas. These sediments contain very large and diverse taxonomic graptolite groups [7, 9]. Sedimentation environment of these strata may be compared with clays/marls formed in the central part of the open or periodically isolated epicontinental shelf with the dominance of environment where coarse-grained material was rarely delivered. An average thickness of the

reservoir and sealing rock are crucial. Also, geological traps enabling accumulation of hydrocarbons are important. Traditional prospecting by seismic methods and well logging is oriented to find traps and good reservoirs—high porosity and high permeability rocks enabling fluid flow. In shale gas, prospection source and reservoir rock are the same formation. Finding of shale formations is easier than conventional traps, but exploitation of hydrocarbons due to low porosity and very low permeability is more difficult. In shale gas, plays information on rock elastic properties is a crucial issue. An additional element complicating the unconventional shale gas reservoir model is dispersed organic matter (kerogen). New technologies are

In the paper, there are presented results of laboratory measurements and well logging obtained using conventional (i.e. resistivity and density and acoustic gamma ray logs) and selected modern methods (i.e. geochemical and NMR logs). The goal of the paper is presentation of available method application on contouring and characterization of sweet spots (differentiation between parts of the formation rich in hydrocarbons and surrounding rocks). An example of data interpretation of the Silurian- and Ordovician-age shale formations in the

**2. Selected geological information about Polish shale gas formations**

In the Polish sedimentary basins beginning with the Baltic Basin in the north (**Figure 1**) to the Lublin Basin in the southeast part of Poland, there are numerous siltstone and mudstone

Shale rocks, rich in organic matter, may be the exploration targets in terms of unconventional hydrocarbon (oil and gas) reservoirs [3–5]. Silurian and Ordovician shales in Poland spread along the western margin of the East European Platform in Lublin, Podlasie and Baltic basins, reaching about 700 km in length [6]. On Łeba elevation where the exemplary boreholes (L-1, K-1, O-2, B-1 and W-1) are located, the sedimentation, organic-rich black shales where the

The object of the analysis covers two formations, being potentially resources of unconventional hydrocarbons: Ja Member of the Silurian Pa formation and Ordovician Sa formation. Silurian Pa Fm was firstly described by Tomczyk [8]. The formation lithostratotype is the part of the Ko IG-1 well profile. In analyzed wells, the Pa formation is placed between Pr Fm (lower boundary) and Pe Fm (upper boundary). The lower boundary is marked at the point of clear lithological changes involving the replacement of limestone or marl deposits of Ordovician series into clay sediments of the Pa formation. It is clearly visible on the well logs. The lower part of the formation is built by black, bituminous claystones passing to the top into dark grey claystones, laminated by greenish, grey-greenish and black claystones or brown calcareous claystones and bentonite laminas. These sediments contain very large and diverse taxonomic graptolite groups [7, 9]. Sedimentation environment of these strata may be compared with clays/marls formed in the central part of the open or periodically isolated epicontinental shelf with the dominance of environment where coarse-grained material was rarely delivered. An average thickness of the

graptolites are the main fossil, started in the Late Llanwirnian reaching Wenlock [7].

elaborated to make prospection of shale gas deposit more efficient.

deposits, rich in oil-prone organic matter (Type II kerogen).

Baltic Basin in Poland is presented.

192 Advances in Natural Gas Emerging Technologies

**Figure 1.** Occurrence of the lower palaeozoic fine-grained rocks potentially accumulating shale gas. Small rectangle study area ([1] modified, [2]).

Pa Fm is 20–40 m increasing from east to west and not exceeding 70 m [10]. Ja Mb belongs to the lower part of the Pa Fm and is built of black bituminous claystones. It is characterized by high organic matter content; however, it may not ensure large gas reserves since the thickness of this bed does not exceed 12 m [10]. Geological profile of Gd IG-1 well is proposed as the member of lithostratotype. The lower boundary of the member is also the lower boundary of the Pa formation, while the upper boundary is marked on the profile in the place where grey and dark-grey claystones, laminated with grey-greenish and black claystones, occur. The discussed boundaries are clearly visible on the logs because of the sharp increase of natural radioactivity within the rocks. Ja Member is composed of black claystones containing commonly pyrite and high content of the oil-prone organic matter (total organic carbon (TOC) content up to 7.6 wt.% in O-2 well profile), with dark-grey calcareous laminas and few intercalations of dark-grey marly limestones.

The second object of the interest is the Sa formation, which was fully described by Modliński and Szymański [11, 12]. The formation lithostratotype is proposed as the part of the Za IG-1 well profile. The lower boundary runs on the Ko limestone formation within Llanwirnian and Llandeilian series, while the upper boundary is set by the Pr marl and shale formation within Ashgillian series. Lithology is mainly composed of black, dark grey and grey-greenish bituminous shales (TOC up to 7.2 wt.% in B-1 well profile). In some parts of the formation, bentonite intercalations are present. Moreover, dark grey, grey and grey-greenish marly limestone and marl intercalations are visible. Organic remains, in form of graptolites, are common in this formation [7]. The thickness of the Sa formation increases from the east to west and northwest, from 3.5 to 37 m in land part of the Baltic Basin and from 26.5 to 70 m on the Baltic Sea shelf.

The differences of mineral composition between shale rocks frequently presented in the literature [13] and shale rocks in the study area are shown in **Figure 2**. In L-1 borehole, there is presented division of the Silurian and Ordovician shales into selected formations. Distinctly visible differences are the reason of slightly different approaches applied to interpretation of the Polish shale gas formations in comparison to other shale plays in the world.

**Figure 2.** Mineral composition of shale gas formations: (a) well known from the literature (after Ref. [13]), black points— Thistleton reservoir; (b) Polish shales in L-1 well.

### **3. Petrophysical and geochemical laboratory and well-logging data to characterize shale gas formation**

The research material consists of the results of petrophysical and geochemical laboratory measurements on core samples and well-logging data [14]. Data were selected as the most representative for the Silurian and Ordovician shale formations in the Baltic Basin. Three of the wells (L-1, K-1 and O-2) are located in the north part of the study area (**Figure 1**), whereas B-1 and W-1 wells are located more to the south.

The results of petrophysical laboratory measurements composed the dataset of density, bulk density and total porosity from helium pycnometer. Additionally, effective porosity derived from the mercury or helium porosimetry was included. Nuclear magnetic resonance spectroscopy provided information about clay-bound water, capillary-bound water, free water and also total, effective porosity and irreducible water saturation. Absolute permeability was obtained using nitrogen permeameter [14]. Presented laboratory data set is typical for both conventional and unconventional hydrocarbon reservoir investigations.

The second object of the interest is the Sa formation, which was fully described by Modliński and Szymański [11, 12]. The formation lithostratotype is proposed as the part of the Za IG-1 well profile. The lower boundary runs on the Ko limestone formation within Llanwirnian and Llandeilian series, while the upper boundary is set by the Pr marl and shale formation within Ashgillian series. Lithology is mainly composed of black, dark grey and grey-greenish bituminous shales (TOC up to 7.2 wt.% in B-1 well profile). In some parts of the formation, bentonite intercalations are present. Moreover, dark grey, grey and grey-greenish marly limestone and marl intercalations are visible. Organic remains, in form of graptolites, are common in this formation [7]. The thickness of the Sa formation increases from the east to west and northwest, from 3.5 to 37 m in land part of the Baltic Basin and from 26.5 to 70 m on the Baltic Sea shelf. The differences of mineral composition between shale rocks frequently presented in the literature [13] and shale rocks in the study area are shown in **Figure 2**. In L-1 borehole, there is presented division of the Silurian and Ordovician shales into selected formations. Distinctly visible differences are the reason of slightly different approaches applied to interpretation of

the Polish shale gas formations in comparison to other shale plays in the world.

**3. Petrophysical and geochemical laboratory and well-logging data to** 

The research material consists of the results of petrophysical and geochemical laboratory measurements on core samples and well-logging data [14]. Data were selected as the most representative for the Silurian and Ordovician shale formations in the Baltic Basin. Three of the wells (L-1, K-1 and O-2) are located in the north part of the study area (**Figure 1**), whereas

**Figure 2.** Mineral composition of shale gas formations: (a) well known from the literature (after Ref. [13]), black points—

**characterize shale gas formation**

Thistleton reservoir; (b) Polish shales in L-1 well.

194 Advances in Natural Gas Emerging Technologies

B-1 and W-1 wells are located more to the south.

The organic geochemical analyses oriented to unconventional hydrocarbon deposits study are the same as for characterization of source rocks of conventional oil and gas accumulations. The most useful is Rock-Eval pyrolysis allowing determination inter alia contents of pyrolyzable (PC) and total organic carbon (TOC), free hydrocarbons (S1) and hydrocarbons generated through thermal cracking of non-volatile organic matter (residual hydrocarbons, S2). The temperature of hydrocarbon maximum release during kerogen cracking (Tmax) allows estimation of the thermal maturity and also calculated hydrogen (HI) and production (PI) indices may help in determination of kerogen genetic type and zones of epigenetic hydrocarbons saturation, respectively.

Shales of Sa Fm and Ja Mb are rich in organic matter with the median values of TOC amounting 3.1 and 3.0 wt.%, respectively. The highest TOC contents, 7.2 wt.% in Sa Fm and 7.6 wt.% in Ja Mb, were recorded in B-1 and O-2 well profiles, respectively. The highest TOC medians were recorded in wells located in the north-eastern part of the study area. In these profiles, hydrocarbon potential of analyzed rocks described by HI values was not high and mostly varied from 100 to 200 mg HC/g TOC for both formations. Hydrocarbon potential of Sa Fm and Ja Mb in profiles located south and west (in deeply buried parts of basin) was even lower—HI values usually did not exceed 100 mg HC/g TOC. Observed variability of Rock-Eval parameters and indices between individual profiles for both formations are result of thermal maturity changes: from middle in north-eastern to final stage of oil window in south-western part of the study area.

In the investigated wells, a set of standard logging curves along with more advanced measurements were available (i.e. cross dipole sonic and geochemical logs). Results of the comprehensive interpretation of logs were also included in the analyses [14]. Among all well-logging data, there were selected several that represented the most important properties in petrophysical description of shale formation. They were as follows: natural radioactivity represented by total (GR); spectral gamma ray logs (GRKT) (calibrated sum of the potassium and thorium energy windows); concentration of naturally occurring elemental sources (POTA, THOR, URAN); resistivity of invaded and uninvaded zones measured by shallow and deep laterologs (LLS, LLD); neutron porosity hydrogen index (NPHI); bulk density (RHOB); photoelectric absorption index (PEF) and velocity of compressional and shear waves in the formation expressed by slowness of P and S waves (DTP, DTS). Special meaning had information from geochemical logging: elemental weight fractions (Si, Ca, K, Mg, Al, Ti, Fe, Gd, S, Mn); mineral volume fraction (quartz (QRTZ), calcite (CALC), dolomite (DOLM), pyrite (PYRT), clay minerals (VCL) apart from illite (ILLI) and chlorite (MGCL) and kerogen (KERO)); volume of clay-bound water, gas and free water (CBW, VWF, VGAS); as well as total and effective porosity (PHIT, PHIE) and water saturation (SW). Described data set was not available in all wells; nevertheless, the detailed petrophysical and geochemical analyses of laboratory and well-logging data were carried out on shale gas formations.

Characteristics of the uranium content from spectral gamma ray log and TOC wt.% from laboratory measurements vs. depth in L-1 well are presented in **Figure 3a** including stratigraphy. It is distinctly visible that Ja Mb of Pa Fm and Sa Fm is different from the adjacent formations.

Transit interval time of P wave, DTP, vs. resistivity, LLDC, and bulk density, RHOB, plots confirmed different features of aforementioned beds (**Figure 3b, c**). Points from the adjacent formations (Pe Fm, Pa Fm and Pr Fm) and potential sweet spots (Ja Mb and Sa Fm) are composed of separate data sets. Resistivity and bulk density covered the bigger range in sweet spots in comparison to the surrounding formations. DTP decreased with the increase of resistivity in shale formations with limited content of organic matter. Data representing rock material of high volume of organic matter had similar and high DTP values and high resistivity (**Figure 3b**). In the same rocks, bulk density decreased due to lower density of kerogen (**Figure 3c**). Discussed formations rich in organic matter, i.e. Ja Mb of Pa Fm and Sa Fm, revealed also differences between each other.

**Figure 3.** Distinct well log anomalies in L-1 well: (a) very dense-point laboratory TOC results and U curve from the spectral gamma log; (b) transit interval time, DTP, vs. borehole-corrected resistivity, LLDC; and (c) transit interval time, DTP, vs. bulk density, RHOB.

### **4. Classification of shale gas formations by statistical methods on the basis of laboratory and well-logging data**

#### **4.1. Basic statistics of laboratory data**

Petrophysical and geochemical parameters from laboratory measurements were the object of analysis in order to create the model of shale gas formations in Poland. Statistical approach allowed to identify the regularities between the parameters in data set.

The average bulk density in L-1 well varied from 2.54 g/cm<sup>3</sup> in Ja Mb to about 2.67 g/cm<sup>3</sup> in Pr Fm. Distribution of bulk density in Pe Fm from B-1 well is presented in **Figure 4a**. In this case, bulk density concentrated in the range of 2.6–2.65 g/cm<sup>3</sup> in 45% of the samples. Average total porosity from pycnometer in L-1 well was equal to about 3.2% in Ko Fm to about 6.45% in Pa Fm, whereas from NMR spectroscopy from 3.28% in Ko Fm to about 8.62% in Sl Fm. Taking into consideration absolute permeability from permeameter in L-1 well in Pa Formation 75% of values was lower than 0.001 mD. In Pe Fm, the same percentage (75%) comprised plugs with permeability lower than 3.48 mD. Irreducible water saturation from NMR indicated lower values in B-1 well in comparison to L-1 well, where most of the samples were characterized with values above 85%. Average total organic carbon ranged from about 0.07 wt.% in Ko Fm to about 3.8 wt.% in Ja Mb in L-1 well (**Figure 4b**), while in B-1 well from about 0.20 wt.% in Pr Fm to about 3.37 wt.% in Ja Mb. The amount of free hydrocarbons in both wells was high in Ja Mb of Pa Fm and in Sa Fm (**Figure 4b**). Regarding mineral content in shale gas formations, Pe Formation was characterized by lower content of clay minerals and higher of quartz, calcite and dolomite than Ja Mb and Sa Fm in L-1 well. The higher amount of pyrite was observed in Pa formation, especially in Ja Mb. The presence of pyrite had a negative influence on measured parameters because it decreased the resistivity and increased density readings. Also, volume of pyrite influenced elastic properties of the rocks. The best deposit parameters (e.g. high porosity and total organic carbon) in all wells were observed in Sa Fm and Pa Fm within shale gas plays.

Characteristics of the uranium content from spectral gamma ray log and TOC wt.% from laboratory measurements vs. depth in L-1 well are presented in **Figure 3a** including stratigraphy. It is distinctly visible that Ja Mb of Pa Fm and Sa Fm is different from the adjacent formations. Transit interval time of P wave, DTP, vs. resistivity, LLDC, and bulk density, RHOB, plots confirmed different features of aforementioned beds (**Figure 3b, c**). Points from the adjacent formations (Pe Fm, Pa Fm and Pr Fm) and potential sweet spots (Ja Mb and Sa Fm) are composed of separate data sets. Resistivity and bulk density covered the bigger range in sweet spots in comparison to the surrounding formations. DTP decreased with the increase of resistivity in shale formations with limited content of organic matter. Data representing rock material of high volume of organic matter had similar and high DTP values and high resistivity (**Figure 3b**). In the same rocks, bulk density decreased due to lower density of kerogen (**Figure 3c**). Discussed formations rich in organic matter, i.e. Ja Mb of Pa Fm and Sa Fm, revealed also differences between each other.

**4. Classification of shale gas formations by statistical methods on the** 

**Figure 3.** Distinct well log anomalies in L-1 well: (a) very dense-point laboratory TOC results and U curve from the spectral gamma log; (b) transit interval time, DTP, vs. borehole-corrected resistivity, LLDC; and (c) transit interval time,

Petrophysical and geochemical parameters from laboratory measurements were the object of analysis in order to create the model of shale gas formations in Poland. Statistical approach

allowed to identify the regularities between the parameters in data set.

**basis of laboratory and well-logging data**

**4.1. Basic statistics of laboratory data**

DTP, vs. bulk density, RHOB.

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**Figure 4.** (a) Histogram of bulk density in Pe Formation, B-1 well. Symbols: N, number of samples; Av., average value' St. dev., standard deviation; Max, maximum value; Min, minimum value. (b) Total organic carbon (TOC) in wt.% and the amount of free hydrocarbons (S1) in % from Rock-Eval pyrolysis in various lithostratigraphic units, L-1 well.

#### **4.2. Elemental weight percent from geochemical logging in shale classification**

Geochemical logging was run over the Silurian and Ordovician intervals where geologists determined several shale formations. As a result, concentrations of 10 elements, Si, Ca, K, Mg, Al, Ti, Fe, Gd, S and Mn, were determined and utilized to characterize each formation. Statistical box plots clearly showed that the investigated shales could be grouped into three types of formation regarding distribution of the elements. The first group was composed of clayey sediments of Pu Fm and Pa Fm including Ja Mb and Sa Fm. Characteristic features of these shale formations were very high concentration of aluminium, reduced amount of calcium and slightly increased amount of iron (**Figure 5a**). Kc Fm, Pe Fm and Pr Fm formed the second group, where mudstones were present together with claystones. A bit higher content of calcium than in the first group was observed (**Figure 5b**). Both groups had a significant amount of silicon. The third group was represented only by calcareous mudstones of Re Mb of Kc Fm. It showed the highest concentration of calcium and magnesium with the lowest amount of aluminium and decreased amount of potassium and iron when comparing to the other fine-grained sediments (**Figure 5c**). It should be pointed that the distribution of elements was very similar to elemental distribution obtained for limestones of Ko Fm. Presented box plots excluded gadolinium content due to other orders of magnitude (ppm vs. percent). The separate box plot for this element (**Figure 5d**) revealed that two formations considered as sweet spots (Ja Mb and Sa Fm) were characterized by much higher amount of Gd.

 **Figure 5.** Box plots of elements distribution in shale formations from Baltic Basin in Poland: (a) Ja Mb and Pa Fm, (b) Pe Fm, (c) Re Mb and Kc Fm and (d) gadolinium content.

#### **4.3. Heterogeneity of shale formations confirmed by factor analysis**

Factor analysis (FA) was applied to logging data from three closely located wells in the Baltic Basin: L-1, O-2 and K-1. FA described a collection of observed variables (i.e. logs) in terms of a smaller collection of (unobservable) latent variables or factors. Achieving meaningful petrophysical interpretation of the logs through the factors helped to understand the complex geophysical responses of organic-rich Polish shales. The input data included parameters measured in wells, results of the comprehensive interpretation of well logs and results of elastic wave velocity and density estimations from Biot-Gassmann model [15]. FA was applied to data from each formation from each well independently and from all wells together. Results revealed very complicated nature of investigated sediments; however, some similarities were observed. For example, in Sa Fm, petrophysical properties represented by the factors were similar in each well, though a slightly different set of logs loaded the factors.

The first factor was always controlled mainly by clay and shale content, the second factor was expressed either by specific minerals (e.g. calcite or pyrite) or by elastic wave velocities and the third factor was represented by organic matter. When this formation was jointly analyzed in all the wells, results confirmed analyses done for the separate wells (**Figure 6**). However, more often diversity than similarity was observed within one formation between the wells. **Figure 7** shows results for Pa Fm from two wells. It can be seen that the appropriate factors were loaded by different logs and were represented by significantly different petrophysical parameters. The case of K-1 well showed that the most important parameters were clay content (first factor), properties of pore space, i.e. porosity and fluids volume (second factor), and organic matter (third factor). In L-1 well, petrophysical interpretation of the first factor was rather unclear, the second factor could be linked to sonic velocity controlled by shale and quartz volumes and the third factor was related to organic matter.

**Figure 6.** Factor analysis results for (a) Sa Fm in L-1 well and (b) all wells.

 **Figure 5.** Box plots of elements distribution in shale formations from Baltic Basin in Poland: (a) Ja Mb and Pa Fm, (b) Pe

these shale formations were very high concentration of aluminium, reduced amount of calcium and slightly increased amount of iron (**Figure 5a**). Kc Fm, Pe Fm and Pr Fm formed the second group, where mudstones were present together with claystones. A bit higher content of calcium than in the first group was observed (**Figure 5b**). Both groups had a significant amount of silicon. The third group was represented only by calcareous mudstones of Re Mb of Kc Fm. It showed the highest concentration of calcium and magnesium with the lowest amount of aluminium and decreased amount of potassium and iron when comparing to the other fine-grained sediments (**Figure 5c**). It should be pointed that the distribution of elements was very similar to elemental distribution obtained for limestones of Ko Fm. Presented box plots excluded gadolinium content due to other orders of magnitude (ppm vs. percent). The separate box plot for this element (**Figure 5d**) revealed that two formations considered as

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sweet spots (Ja Mb and Sa Fm) were characterized by much higher amount of Gd.

Fm, (c) Re Mb and Kc Fm and (d) gadolinium content.

These examples showed that FA was useful in determination of similarities and differences of shales. Investigated formations displayed significant heterogeneity between each other, between adjacent sediments and from one well to another, but some characteristic features were also observed. FA revealed that the most significant properties in characterization of Polish shales were mineral composition, porosity and fluid volume, mechanical properties and content of organic matter.

**Figure 7.** Factor analysis results for (a) Pa Fm in K-1 well and (b) Pa Fm in L-1 well.

#### **4.4. Principal component analysis**

Principal component analysis (PCA) was applied for laboratory and well-logging data derived from Silurian and Ordovician intervals drilled in wells L-1, B-1 and W-1. PCA is a multivariate statistical method used to reduce multidimensional data set into lower dimensions. This mathematical operation helps to extract unobservable variable hidden in the original measurements.

In each analyzed well intervals, results were similar. Three to five principal components (PC) were enough for 70–90% of variance explanation. PCA pointed out that about 50% in W-1 and L-1 wells and 70% in B-1 well of information about the data set were contained in spectral gamma logs, neutron logs and acoustic logs. It meant that the main reason for data diversification was shaliness, clay content and porosity of rocks. The next 20–30% of variance was expressed by density and resistivity logs. These parameters could be treated as porosity and saturation logs. The third PC provided information about the presence of organic matter. This was indicated by the high value of correlation coefficient between PC and uranium log. PCA applied to laboratory data showed that the main variance of the data set was associated with hydrocarbon indicators such as TOC, cation exchange capacity (CEC) and parameters from Rock-Eval measurement, i.e. free hydrocarbons content, S1, and hydrocarbons from cracking process, S2. In the second PC, the highest impact was connected with clay mineral content.

#### **4.5. Cluster analysis**

Cluster analysis (CA) was used for grouping data and classification according to natural physical features of rocks. CA pointed out preliminary formation classification and gas-bearing identification. It helped to define zones of interests based on well logs and laboratory data criteria and improved characteristic of shales with gas saturation. Several different hierarchical and nonhierarchical methods for cluster creating were applied. As a result, groups that corresponded to the gas-bearing intervals were selected. Diversification between sweet spots and surrounding beds was shown. Complex analysis showed internal diversification in each gas formation.

The input data included laboratory measurement results and well-logging data. In the first step of CA, data from all Ordovician and Silurian formations were included. Result showed that each cluster aggregated data associated mostly with one formation. It meant that all lithostratigraphic units had different physical properties and CA could be the first step in the formation identification.

In the second step of analysis, the CA was performed for Ja Mb and Sa Fm independently (**Figure 8**). These formations were treated as potential sweet spot intervals. In each formation, internal heterogeneity was found. Diversity was also observed between the same formations in different wells. For example, in L-1 well, the Ja Mb was divided into four groups, whereas in B-1 well it was divided into five groups. In Sa Fm, four clusters were selected in L-1 well and six in B-1 well. Despite this, in both wells clusters with corresponding parameters could be found. In all wells there were distinguished clusters: (a) with high (about 10% and more) kerogen content, small (0–3%) porosity, high (about 70%) clay minerals and small (about 30%) quartz content; (b) with small (about 3%) kerogen content, high (about 10%) porosity, high (about 70%) clay minerals and small (30%) quartz content; (c) with high (about 8%) kerogen content, porosity in range of 0–10%, comparable content of clay minerals and quartz (d) with high (about 8%) kerogen content, average porosity about 6%, small (about 30%) clay minerals and high (about 70%) of quartz content.

**4.4. Principal component analysis**

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**Figure 7.** Factor analysis results for (a) Pa Fm in K-1 well and (b) Pa Fm in L-1 well.

**4.5. Cluster analysis**

Principal component analysis (PCA) was applied for laboratory and well-logging data derived from Silurian and Ordovician intervals drilled in wells L-1, B-1 and W-1. PCA is a multivariate statistical method used to reduce multidimensional data set into lower dimensions. This mathematical operation helps to extract unobservable variable hidden in the original measurements. In each analyzed well intervals, results were similar. Three to five principal components (PC) were enough for 70–90% of variance explanation. PCA pointed out that about 50% in W-1 and L-1 wells and 70% in B-1 well of information about the data set were contained in spectral gamma logs, neutron logs and acoustic logs. It meant that the main reason for data diversification was shaliness, clay content and porosity of rocks. The next 20–30% of variance was expressed by density and resistivity logs. These parameters could be treated as porosity and saturation logs. The third PC provided information about the presence of organic matter. This was indicated by the high value of correlation coefficient between PC and uranium log. PCA applied to laboratory data showed that the main variance of the data set was associated with hydrocarbon indicators such as TOC, cation exchange capacity (CEC) and parameters from Rock-Eval measurement, i.e. free hydrocarbons content, S1, and hydrocarbons from cracking process, S2. In the second PC, the highest impact was connected with clay mineral content.

Cluster analysis (CA) was used for grouping data and classification according to natural physical features of rocks. CA pointed out preliminary formation classification and gas-bearing identification. It helped to define zones of interests based on well logs and laboratory data criteria and improved characteristic of shales with gas saturation. Several different hierarchical and nonhierarchical methods for cluster creating were applied. As a result, groups that corresponded to the gas-bearing intervals were selected. Diversification between sweet spots and surrounding beds was shown. Complex analysis showed internal diversification in each gas formation.

The input data included laboratory measurement results and well-logging data. In the first step of CA, data from all Ordovician and Silurian formations were included. Result showed that each cluster aggregated data associated mostly with one formation. It meant that all

 **Figure 8.** Box plots for porosity and kerogen content in each cluster in Ja Mb (left) and Sa Fm (right), well B-1. Symbols: middle point equals median value, box equals first and third quartile and whiskers equal range of non-outlier values.

Summarizing cluster characterization, CA was a good mathematical tool for fast preliminary Polish shale classification. Sweet spots in Polish shales are inhomogeneous; there are intervals with possible gas presence and without (or very poor) gas saturation.

### **5. Elastic property modelling of shale gas formations**

Clay minerals, important components of shales, influence elastic properties of rocks and their anisotropy [16]. Elastic properties depend not only on mineral composition and percentage of selected compounds but also on shape and orientation of grains. Jones and Wang [17] presented the example of the Cretaceous shales from Williston Basin and the results of the experimental measurements of five independent components of elasticity vector, C11, C33, C44, C66 and C13, which characterize the simplest case of anisotropy of hexagonal symmetry, i.e. transverse isotropy (TI) [18]. In the presented work, a synthetic model of the similar shales with organic matter on the basis of the published data was prepared [19, 20]. Elastic parameter modelling and bulk density calculations were done using the theoretical relations by Kuster and Toksöz [21] applying the differential effective medium (DEM) solution [19]. There were used theoretical models of porous formation by Biot-Gassmann [22, 23] and Kuster-Toksöz and the original software *Estymacja* [15, 24].

#### **5.1. Elastic parameters calculated by** *Estymacja* **software in L-1 well**

An estimation of P- and S-wave velocity and elastic moduli was based on known theoretical formulas of Biot-Gassmann or Kuster-Toksöz [21–23] which describe multiphase media corresponding to rocks with granular structure (grains of solid phase) with porous space saturated with medium (liquid phase or gas phase).

Elastic parameters of rocks are a result of an interaction of all phase components, rock matrix and medium and also depend on the anisotropy of a rock matrix. The *Estymacja* software allows elastic parameters of the rocks and bulk density to be determined from results of the comprehensive interpretation of well-logging data, i.e. volumes of mineral components, porosity and water, gas and oil saturation in the flushed zone or virgin zone [15, 25]. The final results of *Estymacja* software in the form of set of curves illustrating variability of P slowness and S slowness (DTP and DTS, respectively), together with neutron porosity, NPHI, bulk density, RHOB, water saturation, SW, natural radioactivity, GR, logs and lithology and VP/VS curves, are presented in **Figure 9**.

**Figure 9.** Calculated (red) and measured (black) curves in the Ordovician depth section in L-1 well. *Estymacja* software and Kuster-Toksöz model were used.

#### **5.2. Synthetic model of shale gas formation**

Elastic parameters were calculated for the synthetic model of shale gas formation with organic matter in mineral skeleton or in porous space and formation water and gaseous hydrocarbons in pore space. Passey [20] in his model of shale gas rock divided organic matter into three parts: rock matrix, organic matter component and medium in pore space. Increase of compaction caused that grains revealed tendency to horizontal position and organic matter composed subhorizontal lamellae. On the basis of thin sections, it was proved that organic matter (mainly kerogen) was plastic [20]. Primarily, organic matter was located in the matrix and did not fill the pores. When the maturity increased, kerogen was more mobile and could be intruded into pore space. Models of shale gas formation assumed in modelling are presented in **Figure 10**.

**Figure 10.** Models assumed in calculations.

**Figure 9.** Calculated (red) and measured (black) curves in the Ordovician depth section in L-1 well. *Estymacja* software

experimental measurements of five independent components of elasticity vector, C11, C33, C44, C66 and C13, which characterize the simplest case of anisotropy of hexagonal symmetry, i.e. transverse isotropy (TI) [18]. In the presented work, a synthetic model of the similar shales with organic matter on the basis of the published data was prepared [19, 20]. Elastic parameter modelling and bulk density calculations were done using the theoretical relations by Kuster and Toksöz [21] applying the differential effective medium (DEM) solution [19]. There were used theoretical models of porous formation by Biot-Gassmann [22, 23] and Kuster-

An estimation of P- and S-wave velocity and elastic moduli was based on known theoretical formulas of Biot-Gassmann or Kuster-Toksöz [21–23] which describe multiphase media corresponding to rocks with granular structure (grains of solid phase) with porous space saturated

Elastic parameters of rocks are a result of an interaction of all phase components, rock matrix and medium and also depend on the anisotropy of a rock matrix. The *Estymacja* software allows elastic parameters of the rocks and bulk density to be determined from results of the comprehensive interpretation of well-logging data, i.e. volumes of mineral components, porosity and water, gas and oil saturation in the flushed zone or virgin zone [15, 25]. The final results of *Estymacja* software in the form of set of curves illustrating variability of P slowness and S slowness (DTP and DTS, respectively), together with neutron porosity, NPHI, bulk density, RHOB, water saturation,

curves, are presented in **Figure 9**.

Toksöz and the original software *Estymacja* [15, 24].

SW, natural radioactivity, GR, logs and lithology and VP/VS

with medium (liquid phase or gas phase).

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**5.1. Elastic parameters calculated by** *Estymacja* **software in L-1 well**

and Kuster-Toksöz model were used.

In Model 1, it was assumed that clay, quartz and kerogen were present in skeleton; formation water was in pores. Porous spectra were as follows: αwater = 0.05 and C(αwater) = Φ = 0.05 (Φ, porosity) and water parameters KW = 2.6 GPa and RHOBW = 1.05 g/cm<sup>3</sup> . In Model 2, it was assumed that kerogen was in porous space with characteristic spectra, αkerogen = C(α<sup>k</sup> erogon) = Vkerogen (Vkerogen, volume of kerogen) and porous spectra of water, αwater = 0.05 and C(αwater) = Φ = 0.05. In Model 3, it was assumed that kerogen, formation water and gas were located in pores. Velocity of P-waves, VP , as a function of kerogen volume for Models 1 and 2 is presented in **Figure 11**. Decrease of VP was visible with increase of kerogen and decrease of volume of shale, Vsh, in rock matrix. Similar behaviour was observed for bulk elastic modulus, K, and shear elastic modulus, MI. Calculations were also made in the case when water and gas were present in porous space.

**Figure 11.** Velocity of P-wave vs. kerogen volume. Model 1 (blue curve) and Models 2 and 3 (other curves). Various kerogen volumes in pore space are marked by different colours.

In Variant 1, it was assumed that water and gas were present in separate pores and pore spectrum was the same: αwater = 0.05 and C(αwater) = Φ = 0.05 and αgas = 0.05 and C(αgas) = Φ = 0.05, what meant porosity Φ = 0.1. In Variant 2 it was assumed that water (SW = 0.8) and gas (SG = 0.2) were a mixture. In both cases, a linear decrease of VP and VS with kerogen volume increase was observed, but in Variant 2 it was higher. Variability of both waves, velocity V<sup>S</sup> = f(VP) (**Figure 12a**) and acoustic impedance AIS = f(AIP), was also analyzed (**Figure 12b**). Data in the plots were related to depth intervals with gas, formation water or saturated with gas and water from the Silurian and Ordovician formations in the L-1 well. Beds of different gas saturations were distinctly visible: yellow and red points meant high gas saturation, and blue points meant formation water saturation.

#### **5.3. Brittleness of shale gas formation**

Mechanical properties of shale gas formations are crucial in hydraulic fracturing for gas production in low permeability reservoirs. Such rocks are characterized by various brittleness indices [26] which are considered in aspects of mineral composition and elastic properties. Quartz and carbonates in rocks cause increase of brittleness in contrary to shaliness-rich formations where ductile deformations are more frequently observed and skeleton weakness is observed. On the other hand, carbonate cements may limit natural fracture flow ability. High volume of carbonates and presence of swelling clay minerals are the main reasons making difficult production from discussed gas deposits. Young's modulus or Poisson's ratio may be used as measures of brittleness index (BI) (**Figure 13**).

**Figure 12.** Relationship between velocity VS vs. VP and acoustic impedance AIS vs. AIP for the selected depth intervals in L-1 well.

 **Figure 13.** Poisson's ratio vs. Young's modulus, K-1 well.

**Figure 11.** Velocity of P-wave vs. kerogen volume. Model 1 (blue curve) and Models 2 and 3 (other curves). Various

In Variant 1, it was assumed that water and gas were present in separate pores and pore spectrum was the same: αwater = 0.05 and C(αwater) = Φ = 0.05 and αgas = 0.05 and C(αgas) = Φ = 0.05, what meant porosity Φ = 0.1. In Variant 2 it was assumed that water (SW = 0.8) and gas (SG = 0.2)

was observed, but in Variant 2 it was higher. Variability of both waves, velocity V<sup>S</sup> = f(VP) (**Figure 12a**) and acoustic impedance AIS = f(AIP), was also analyzed (**Figure 12b**). Data in the plots were related to depth intervals with gas, formation water or saturated with gas and water from the Silurian and Ordovician formations in the L-1 well. Beds of different gas saturations were distinctly visible: yellow and red points meant high gas saturation, and blue

Mechanical properties of shale gas formations are crucial in hydraulic fracturing for gas production in low permeability reservoirs. Such rocks are characterized by various brittleness

with kerogen volume increase

is presented in **Figure 11**. Decrease of VP was visible with increase of kerogen and decrease of volume of shale, Vsh, in rock matrix. Similar behaviour was observed for bulk elastic modulus, K, and shear elastic modulus, MI. Calculations were also made in the case when water and gas

kerogen volumes in pore space are marked by different colours.

points meant formation water saturation.

**5.3. Brittleness of shale gas formation**

were a mixture. In both cases, a linear decrease of VP and VS

were present in porous space.

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#### **5.4. Summary of results**

Kerogen influenced measured petrophysical parameters, but shaliness and pores geometry were also important in considering this influence. Changeability of elastic parameters was not distinct with the increase of kerogen volume when it composed mineral skeleton; however, when pores were filled with kerogen, variability of elastic parameters increased. Kerogen in pore space caused decrease of velocity of P- and S-waves as well as elastic moduli K and MI. Increase of kerogen volume caused increase of VP/VS ratio. VS vs. VP and AIS vs. AIP cross plots enabled separation between water-saturated and gas-saturated beds. Shales of higher Young's modulus values and lower Poisson's ratio revealed higher brittleness.

#### **6. Measurements of acoustic emission and volumetric strain during sorption and desorption of CH4 on shale sample**

Acoustic emission (AE) is an elastic wave generated and propagating in the medium as a result of dynamic processes. Generation of a seismo-acoustic signal is a mechanical process, brought about by diverse mechanisms associated with strain and breaking of the medium (opening and closing of pores, dislocation movements proceeding at variable rate, slippage and friction, plastic and nonplastic deformations), structural changes, phase transitions and chemical reactions and temperature changes.

The basic AE parameters include acoustic activity representing the number of impulses (AE counts) registered in an arbitrarily chosen time window, mean signal energy—the ratio of energy emitted within a given time interval to the number of impulses registered in the same time and cumulative energy—total energy of impulses registered from the beginning of record-taking. AE recordings are utilized for forecasting rock bursts and gas and rock outbursts in coal mines [27]. The test stands for acoustic emission measurement [27], and a single-channel device for seismic-acoustic measurements was designed and engineered as a prototype. The main component is a vacuum-pressure chamber, provided with a steel waveguide and six wires for strain measurements. The chamber is connected to gas bottles and a vacuum pump through a system of tubes and pressure-control valves. Measurement data are saved on a computer connected to the system.

The test facility has been modified since time of measurements made for coal [27]. At first, the integrating system in the seismic-acoustic instrumentation, which calculated the surface area of impulses over a given level of discrimination, would split the entire measurement range of 3 V into 10 identical channels, 300 mV each. When the interface was used, the measurement range was extended to 10 V and is now split into 4000 identical channels, 2.5 mV each. This solution provides a higher resolution energy data; hence, the results give more information about events and allow for detecting even low-energy events. The measuring path allows for monitoring AE impulses in the frequency range from 100 Hz to 1 MHz, with the option for frequency band setting. Within this band single events are detected, and the integral of the positive portion of the measured plot is obtained accordingly. The key component of the measurement circuit is a piezoelectric sensor. Four RL strain gauges were fixed on a shale sample, which was then attached to the waveguide in the pressure-vacuum chamber. Measurement cycles were conducted at the frequency of one per minute. The signal resolution was of the order of 0.001%. The observed acoustic emission patterns do not resemble those registered for coal samples (**Figure 14**) [28].

 **Figure 14.** Acoustic emission and volumetric strain registered on a shale sample during the sorption of CH4 .

In the case of coal samples, the process of AE data acquisition was much faster. Strains measured in the shale were lower by one order of magnitude than those registered for coal, and their variability pattern was different, too. Volumetric strains increased for about 150 h of the sorption process, including the stage of stabilization (around the 100th hour), followed by a strain decline, which was never observed for coal. The analysis of registered strain data revealed that they should be recorded for longer time during sorption. The general conclusion can be drawn that sorption processes in shale are much slower than in coals.

### **7. Final remarks**

**5.4. Summary of results**

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Increase of kerogen volume caused increase of VP/VS

**sorption and desorption of CH4**

chemical reactions and temperature changes.

saved on a computer connected to the system.

Kerogen influenced measured petrophysical parameters, but shaliness and pores geometry were also important in considering this influence. Changeability of elastic parameters was not distinct with the increase of kerogen volume when it composed mineral skeleton; however, when pores were filled with kerogen, variability of elastic parameters increased. Kerogen in pore space caused decrease of velocity of P- and S-waves as well as elastic moduli K and MI.

plots enabled separation between water-saturated and gas-saturated beds. Shales of higher

Acoustic emission (AE) is an elastic wave generated and propagating in the medium as a result of dynamic processes. Generation of a seismo-acoustic signal is a mechanical process, brought about by diverse mechanisms associated with strain and breaking of the medium (opening and closing of pores, dislocation movements proceeding at variable rate, slippage and friction, plastic and nonplastic deformations), structural changes, phase transitions and

The basic AE parameters include acoustic activity representing the number of impulses (AE counts) registered in an arbitrarily chosen time window, mean signal energy—the ratio of energy emitted within a given time interval to the number of impulses registered in the same time and cumulative energy—total energy of impulses registered from the beginning of record-taking. AE recordings are utilized for forecasting rock bursts and gas and rock outbursts in coal mines [27]. The test stands for acoustic emission measurement [27], and a single-channel device for seismic-acoustic measurements was designed and engineered as a prototype. The main component is a vacuum-pressure chamber, provided with a steel waveguide and six wires for strain measurements. The chamber is connected to gas bottles and a vacuum pump through a system of tubes and pressure-control valves. Measurement data are

The test facility has been modified since time of measurements made for coal [27]. At first, the integrating system in the seismic-acoustic instrumentation, which calculated the surface area of impulses over a given level of discrimination, would split the entire measurement range of 3 V into 10 identical channels, 300 mV each. When the interface was used, the measurement range was extended to 10 V and is now split into 4000 identical channels, 2.5 mV each. This solution provides a higher resolution energy data; hence, the results give more information about events and allow for detecting even low-energy events. The measuring path allows for monitoring AE impulses in the frequency range from 100 Hz to 1 MHz, with the option for frequency band setting. Within this band single events are detected, and the integral of the positive portion of the measured plot is obtained accordingly. The key component of the measurement circuit is a piezoelectric sensor. Four RL strain gauges were fixed on a shale sample, which was then attached to the waveguide in the pressure-vacuum chamber. Measurement cycles were conducted at the

 **on shale sample**

Young's modulus values and lower Poisson's ratio revealed higher brittleness.

**6. Measurements of acoustic emission and volumetric strain during** 

ratio. VS

vs. VP and AIS vs. AIP cross

Presented results show that Polish shale formations of the Silurian and Ordovician age are different as regards mineral composition, reservoir properties and elastic parameters. In each formation internal heterogeneity was found. Diversity was also observed within the same formations in different wells. Two selected formations were recognized as potential sweet spots, i.e. Ja Mb and Sa Fm. They are relatively rich in organic matter. Results of the analyses indicate distinctly visible differences between them and surrounding formations (Pe Fm, Pa Fm and Pr Fm).

Applied statistical analyses, i.e. simple statistics, histograms, box plots and also factor analysis, principal component analysis and cluster analysis show themselves as useful tools for proving diversity of shale formations in the study. Research based on the laboratory results, well logs and the outcomes of the comprehensive interpretation of well logging confirmed great diversification of formations in the study but also revealed some regularities. Factor analyses and principal component analyses enabled limitation of great number of logs saving the necessary information to make classification.

Elastic property modelling using theoretical models and results of the comprehensive interpretation of well logs provided estimated velocity of P- and S-waves and bulk density in all wells and helped in completing indispensable information about brittleness of shale gas formations.

Preliminary results of acoustic emission investigations show additional characteristic features of shale gas rock and revealed that acoustic emission and volumetric strain of a shale sample induced by the sorption processes are lower for shale than for coals.

Methodology of laboratory measurements, well data acquisition and processing was only slightly fitted to specific features of shale rocks. Majority of measurements were typical for prospection of conventional hydrocarbon reservoirs. In spite of it, selection of potential sweet spots among adjacent beds was a success.

### **Acknowledgements**

This study was financed by the National Centre for Research and Development in the programme Blue Gas project, 'Methodology to determine sweet spots based on geochemical, petrophysical and geomechanical properties in connection with correlation of laboratory test with well logs and generation model 3D' (MWSSSG) Polskie Technologie dla Gazu Łupkowego. Data for the study were delivered by Polish Oil and Gas Company, Warsaw, Poland. Statistica 12 software was used under the AGH UST grant from StatSoft. Plots and figures were prepared by Teresa Staszowska.

### **Author details**

Jadwiga A. Jarzyna\*, Maria Bała, Paulina I. Krakowska, Edyta Puskarczyk, Anna Strzępowicz, Kamila Wawrzyniak-Guz, Dariusz Więcław and Jerzy Ziętek

\*Address all correspondence to: jarzyna@agh.edu.pl

Faculty of Geology, Geophysics and Environmental Protection, AGH University of Science and Technology, Krakow, Poland

### **References**

[1] Kiersnowski H. Geological environment of gas-bearing shales. In: Nawrocki J., editor. Shale Gas as Seen by Polish Geological Survey. Polish Geological Institute – National Research Institute; Warsaw. 2013. pp. 26-31.

[2] Wawrzyniak-Guz K., Jarzyna J.A., Zych M., Bała M., Krakowska P.I., Puskarczyk E. Analysis of the heterogeneity of the Polish shale gas formations by Factor Analysis on the basis of well logs. In: Extended Abstract of the 78th EAGE Conference and Exhibition 2016; 30 May–2 June 2016; Vienna. 2016. p. Tu SBT3 07.

and principal component analyses enabled limitation of great number of logs saving the

Elastic property modelling using theoretical models and results of the comprehensive interpretation of well logs provided estimated velocity of P- and S-waves and bulk density in all wells and helped in completing indispensable information about brittleness of shale gas formations. Preliminary results of acoustic emission investigations show additional characteristic features of shale gas rock and revealed that acoustic emission and volumetric strain of a shale sample

Methodology of laboratory measurements, well data acquisition and processing was only slightly fitted to specific features of shale rocks. Majority of measurements were typical for prospection of conventional hydrocarbon reservoirs. In spite of it, selection of potential sweet

This study was financed by the National Centre for Research and Development in the programme Blue Gas project, 'Methodology to determine sweet spots based on geochemical, petrophysical and geomechanical properties in connection with correlation of laboratory test with well logs and generation model 3D' (MWSSSG) Polskie Technologie dla Gazu Łupkowego. Data for the study were delivered by Polish Oil and Gas Company, Warsaw, Poland. Statistica 12 software was used under the AGH UST grant from StatSoft. Plots and

Jadwiga A. Jarzyna\*, Maria Bała, Paulina I. Krakowska, Edyta Puskarczyk, Anna Strzępowicz,

Faculty of Geology, Geophysics and Environmental Protection, AGH University of Science

[1] Kiersnowski H. Geological environment of gas-bearing shales. In: Nawrocki J., editor. Shale Gas as Seen by Polish Geological Survey. Polish Geological Institute – National

induced by the sorption processes are lower for shale than for coals.

necessary information to make classification.

208 Advances in Natural Gas Emerging Technologies

spots among adjacent beds was a success.

figures were prepared by Teresa Staszowska.

Kamila Wawrzyniak-Guz, Dariusz Więcław and Jerzy Ziętek

\*Address all correspondence to: jarzyna@agh.edu.pl

Research Institute; Warsaw. 2013. pp. 26-31.

and Technology, Krakow, Poland

**Acknowledgements**

**Author details**

**References**

