Perspective Chapter: Geothermics and Thermogenesis in Gas Reservoirs

*Yasir Ali and Yasir Yousif*

### **Abstract**

Studies on terrestrial heat flow, particularly in oil and gas reservoir systems, have gained substantial attention. While the traditional focus was on igneous and metamorphic activities, this chapter focuses on geothermics and thermogenesis in gas reservoirs, emphasizing the fundamental concepts of heat and temperature, subsurface conditions related to heat, and responses of reservoir materials to temperature changes. Geothermics, at its core, explores the source and destiny of terrestrial heat, with "geo-" denoting the Earth and "thermos" signifying heat. It is the study of heat transport and thermal conditions in the Earth's interior. In practical terms, geothermics extends to the assessment of geothermal resources, examining heat distribution in the Earth's outer layers and the potential for heat extraction. Moreover, this science has evolved into an applied field, with geothermal energy being a notable application that harnesses the Earth's heat. In this context, "thermogenesis" encompasses all physical and chemical reactions in the reservoir, including gas generation, thermal gas cracking, and mineral alteration. In essence, this chapter delves into the intricate dynamics of heat and temperature within gas reservoirs, providing valuable insights into geothermics and thermogenesis, and their significance in the energy industry.

**Keywords:** earth's heat, heat flow, oil and gas reservoir, geothermic, thermogenesis, thermal gas cracking

### **1. Introduction**

Since its early formation, the earth planet has evolved thermally, and is layered according to density. Earth is subjected to both internal and external source of heat, from the sun and from subsurface. Other source of heat on earth is the heat that originates from the subsurface, which is known later as the ground heat, which is quite important for life on earth, as well as for the earth itself. Volcanoes and seismic activity are induced by such ground heat. Formation of the rocks that form the lithosphere would not have resulted without the interaction between earth matter and earth temperature [1]. One important effect of heat is that the landscape and earth morphology depend to a large extent on internal as well as external heat or thermal stress. In the case of the former, the physical weathering process that acts on reshaping the earth surface is induced by earth temperature as expansion and contraction processes are solely temperature-controlled. Indirectly, temperature effect physical weathering (abrasion process) and deposition (eolian deposits) through wind motivation as wind—the second factor in physical weathering—is influenced by temperature differences from one region to another. Even chemical weathering potential in rocks is enhanced by temperature through the catalyzation of the reaction rate, as well as providing extra surfaces for chemical reactions on rock. An account of the possible temperature-induced geomorphic changes in the land surfaces is given in [2].

### **2. Heat flow and geothermal gradient**

The difference between heat and temperature is almost well-established for everybody. Heat is the vital source of energy, while temperature is the measure of the status of bodies in terms of how cold or hot are they. Therefore, units used for temperature are those of energy (Joule, calorie, … etc.), and for temperature numerical units such as Celsius, Fahrenheit, and Kelvin degrees are applied. The main source of heat on earth's surface is the Sun, through its radiation which is known as solar radiation. The surface of the earth is subjected to solar radiation during the daytime, and this radiation is lost at night in a continuous reversible process. Another characteristic of such radiation is that it varies over almost all time scales, from daily, through monthly, annually, to century. Spectral composition of this radiation showed that the solar radiation "falls into visible short-wave part of the spectrum, while the other half is mostly in the near-infrared part with a small part" [3]. As geothermal gradient describes the variation in earth's temperature with depth, different formulas have been used to find temperature at any depth based on the gradient/slope. The most famous formula for finding out temperature at any depth is by adding the surface temperature (or sea bottom temperature in case of offshore temperature) to the required depth multiplied by the geothermal gradient at that region. An example of geothermal gradient is shown in **Figure 1**, at a depth of up to 5 km.

The internal heat of the earth has been observed a long time ago and evidenced by the occurrence of volcanic phenomenon [4]. The internal heat is inferred also by the observation of elevated temperature with depth that is associated with subsurface

#### **Figure 1.**

*Shallow geothermal gradient for five different regions. Note variations in surface heat flow based on variations in different geothermal gradients for each region.*

### *Perspective Chapter: Geothermics and Thermogenesis in Gas Reservoirs DOI: http://dx.doi.org/10.5772/intechopen.108350*

drilling for underground mining operations. In such a case, temperature of these tunnels and mines becomes a major problem, which requires the ventilation of the subsurface environment. Trials to understanding this temperature go back to the eighteenth century, when [5] started some discussions on that matter. One important quote of his discussion was reported "because of my being particularly subject to the offended by anything that hinders, full freedom of respiration, I was not solicitous to •oe down into the deep mines"; he, therefore, collected information "(by diligent. inquiry purposely made) from the credible relations of several eye-witnessed suffering in nation, and for the most part unacquainted with each oilier."

However, one cannot talk about the source of earth's heat without talking about the origin of the earth. The most acceptable theory on the formation of the planet state that the earth's matter is condensed by the gravitational force. The earth has become differentiated into different compositional zone-based density, that is, heavy materials sank down to the mantle and lighter materials to the surface forming the earth's crust.

Heat of the earth, which can be classified as external heat and internal is acquired by two means. The exterior heat is acquired during the separation of the earth as young planet, where commits and other floating bodies in the atmosphere hit the earth's surface. The major source of the earth's interior heat is the decay of radioactive minerals [6].

The earth's center is believed to have a temperature of around 6000 C, while the mantle typically ranging from about 1000°C to 3700°C, while the Earth's crust has lower temperatures, ranging from around −40°C to 1000°C, with the highest temperatures in areas with active volcanic activity [7]. Heat or geothermal zonation of the earth is similar to the rock/lithological zonation (**Figure 2**).

The origin of the interior heat is attributed to two main processes that occur inside the earth, namely: radioactivity and earth cooling. Radioactivity contributes 80% of the earth's heat, and heat normally originates as a result of nuclear transformations of radioactive minerals. Radiogenic heat is basically created by the decay of the

three minerals, namely: potassium, thorium, and uranium (K, Th, and U), which accounts for an estimated 30–40% of heat loss through four continents [8–11]. The most famous transformation is that one in which uranium 238 U converts into lead. Radioactive minerals are generally associated with igneous rocks, particularly granitic rocks. Cooling contributes 20% of the earth's interior heat. Radiogenic heat flow is useful in the study of metamorphic rocks and have been applied in the Chinese metamorphic belts. The mean heat production was found to be 0.76 μW m − 3) which is estimated to contribute 24 mW m − 2 to the surface heat flow [12].

Heat production and transport in the earth are illustrated in **Figure 3** for each earth zone. Heat production from radiogenic activity appeared to characterize the

#### **Figure 3.**

*Geothermal sources of the earth: A: Heat sources of the earth and heat transport in the earth with respect to different parts of the earth, and b: earth's heat flow from inside to the surface. The color scale (in watts) shows the distribution from the minimal of 23–45 mW/m<sup>2</sup> (dark blue) to the maximal flux 150–450 mW/m2 \*reddish).*

*Perspective Chapter: Geothermics and Thermogenesis in Gas Reservoirs DOI: http://dx.doi.org/10.5772/intechopen.108350*

#### **Figure 4.**

*Demonstration of convection, comparison between soup pot and rock mantle: Soup convects when heated from the bottom of the pot (after [14].*

core and lithosphere. Heat due to cooling is referred to as to grow from the solidification of the outer core. Vertical heat transport is shown to be dominant, with minor lateral heat transport. Small portion of the heat generated at the top of the lithosphere is reflected and transferred inward.

As known from fundamental physics, heat is conducted via three modes; namely: radiation, conduction, and convection. In the earth's subsurface, heat moves from the center of the earth outwards through the sedimentary crust into the ocean or atmosphere, where it is lost as radiant energy. The subsurface temperature distribution is influenced by conductive heat transport [13]. Convection, the most important in heat conduction is defined as the process of heat conduction through fluids. The general concept of convection is the movement of material when heated and density lost. The best demonstration for this phenomenon is a comparison an analogy between mantle convection and soup pot, where the content of the soup convects and rises upon heating (**Figure 4**) [14].

### **3. Thermal properties of rocks**

Several recent applications depend on temperature basically induced by thermal properties of rocks; these include high-temperature applications, such as maturation of organic matter for oil generation, oil migration, and enhanced thermal oil recovery, where combustion or hot fluids and steam are injected to the subsurface, geothermal reservoirs, nuclear waste disposal and storage, and groundwater heat storage where hot fluids might be stored and recovered later. On the other hand, there are some processes that are considered low-temperature, such as Perma frost, in the cold regions at low depths, however, these are considered out of the scope of this book. For many constructions, such as underground mines, knowledge of the geothermal gradient is mandatory for the mine design.

There are several thermal properties that are normally considered, including thermal conductivity, heat capacity, and heat diffusivity. Thermal conductivity describes heat flow in steady state flow where no change in temperature with time. In transient heat flow, thermal diffusivity describes the heat flow [15]. Only thermal conductivity and heat capacity will be discussed in the context of thermal imaging heat is conducted from the interior of the earth outward mainly by conduction.


#### **Table 1.**

*Thermal of rock-forming minerals after [20].*

Heat capacity will be discussed as the process of storing or releasing heat energy will significantly affect the detection of infrared, and hence, the process of resulting thermal imagery.

Thermal conductivity (TC) defines how much heat flows in a rock [16–19]. TC is a vector quantity, unlike density, it depends on the direction of measurement.TC is crucial for the heat flow modeling required for basin thermal history and hydrocarbon generation and migration [13]. Quantification and characterization of heat flow are done through the coefficient TC, which is considered an intrinsic and important petrophysical property.

Thermal conductivity is defined as the capacity of a substance to conduct or transmit heat. This is the coefficient (1) in Fourier's Law of heat conduction:

$$\mathbf{q} = -\mathbf{h}\,\mathbf{grad}\,\mathbf{T}(\mathbf{V})$$

where q = heat flux, watts/m2; grad T = temperature gradient, Klm. The standard unit of thermal conductivity is W/m-K. Other units include cakec-cm-"C and Btu/ hr-ft-"F.

Thermal conductivity of rocks depend on thermal conductivity of individual minerals constituting that rock. **Table 1** gives values of thermal conductivity of some common rock-forming minerals; in which quartz is the highest conductive mineral and micas are the lowest ones.

### **4. Heat capacity**

Heat capacity is derived from heat content with respect to temperature. Water is taken as the standard material with heat capacity of 1.00 cal/g-"C at 15°C (4.184 kJ/ kg-K in SI units), and other substances are compared to that value. Heat capacity is measured experimentally using calorimeters. Review of heat capacity of rocks with respect to their thermal conductivity, it is clear that there is an inverse relationship

*Perspective Chapter: Geothermics and Thermogenesis in Gas Reservoirs DOI: http://dx.doi.org/10.5772/intechopen.108350*


**Table 2.**

*Calculated and measured heat content of some sedimentary rocks. Values in cal/g; temperature base 298 k.*

between both properties, that is, the higher the thermal conductivity of the rock type, the lower its heat capacity. This is simply interpreted as that conductive rocks cannot keep heat, however, the range of variation in the heat capacity of rocks is not as wide as in thermal conductivity. **Table 2** show heat capacity of common sedimentary rock types, including siliciclastic rocks (sandstone, siltstone, and shale), and carbonate ones. Heat capacity property in reservoirs is important in storing thermal energy by injecting fluids and restoring them later when needed.

Heat capacity for different rock types is proved to be of almost similar values with no signficat variations, where the range was from 20 to 22. In higher temperatures, the range is from 120 to 130 J/g.

### **5. Thermal diffusivity**

Thermal diffusivity describes the heat flux inside a certain volume of the material, while the out flux between the rock and surrounding is the thermal conductivity. In other words, thermal diffusivity controls the rate at which temperature rises inside a uniform block of the material. If, on the other hand, heat capacity reflects the stored heat in a volume that causes the rapid increase of its temperature, we can find that there is a genetic relation between the three rock properties, where rock thermal diffusivity is the ratio of thermal conductivity to heat capacity [21]. The thermal diffusivity is demonstrated in **Figure 5**.

### **6. Reservoir geothermics**

As reservoirs are basically a system of rocks, pore systems, and fluids, the heat flow in such systems is complicated and is the resultant of all components of the system. This section discusses the subsurface conditions in relation to heat and temperature.

**Figure 5.** *Demonstration of thermal diffusivity using a block of uniform material.*

### **7. The subsurface PT conditions**

In reservoirs, the subsurface is composed of the rock material as a framework, subsurface fluids, and the acting processes such as overburden pressure and temperature. With increasing depth, the pressure, temperature, and salinity increase. Many processes are temperature dependant as well as pressure dependant. Temperature-dependant processes are affected basically by the pre-mentioned thermal properties of reservoir rocks such as thermal conductivity, heat capacity, and thermal diffusivity. Such processes include thermal oil recovery, geothermal reservoirs,

Formation pressure is defined as that pressure other than hydrostatic pressure [22]. Among other factors, including the concentration of salts in formation water, subsurface heat is a major factor in increasing formation pressure. The Daltons law relates temperature to pressure in pressurized systems. In the subsurface, the same law applies where overburden pressure increases with an increase in reservoir temperatures. This relationship is shown in the pressure-temperature-density diagram **Figure 6**.

The figure shows that with increasing depth, temperature and overburden pressure increase.

Thermal conductivity of reservoir rocks is measured for dry, solvent-saturated, and brine saturated to simulate thermal conductivity of the reservoir system. All results showed that TC of brine-saturated sandstone is highest, followed by the solvent-saturated, and the lowest value recorded is for the non-saturated ones. This agrees with the models that consider TC of rocks as a summation of the individual minerals that makeup the rock, and the thermal conductivity of brine is higher than solvent, which is higher than air. Within the saturated sandstone samples, TC recorded for the medium-grained samples show relatively higher values as compared to coarse ones (**Figure 7**).

*Perspective Chapter: Geothermics and Thermogenesis in Gas Reservoirs DOI: http://dx.doi.org/10.5772/intechopen.108350*

#### **Figure 6.**

*Pressure-temperature-density diagram for water after [23]. Excess pressure is the higher pressure due to the temperature increase from T 50 to T 60.*

In low permeability reservoirs or tight reservoirs, permeability is minimal and fluid may be considered as stationary. However, fluids normally flow under different mechanisms, of which fluids under thermal conditions are to be considered. One of the flow motions is simple convection where fluid moves under temperature/density gradient where hot fluids move upward, while the cold one moves down. Another effect occurs under high temperature where fluid evaporates and the vapor escapes to colder zones where it condenses again. Further details are present in Ref. [18], and initial tests and observations on the effect of vapor pressure and partial saturation of rocks on thermal conductivity were conducted by Ref. [24].

One of the expected subsurface conditions related to temperature is the rock expansion. Study of expansion phenomenon was done for rock forming mineral by

#### **Figure 7.**

*Thermal conductivity of air-saturated, solvent saturated, and brine-saturated sandstones of oil sands (note the distribution of the points scattered around the curve for each type, where medium-grained ones mostly above the curve, being the highest values for TC.*

Ref. [25]. Where gradual expansion with increasing temperature was found to reach up to 2% along the crystal axis. Dry rock expansion was documented by Ref. [26] where direct expansion in rocks was reported as in all natural materials. In saturated rocks, tedious and intensive experiments have been conducted by Ref. [27] who measured experimentally the strain of fluid-saturated rocks using highly sensitive equipments. They concluded that thermal stress on saturated rocks under temperature conditions similar to the subsurface results in the contraction of pore space and increase in fluid expansion. Dry rock expansion, and the comparison of dry sandstone and saturated sandstone is shown in **Figure 8**.

### **8. Reactions and alteration induced by temperature**

Temperature results into thermal stress in almost most materials including earth materials. The response of rocks to thermal expansion is a reflection of responses of the minerals that make up the rock. Mineral alteration by temperature is a well-known *Perspective Chapter: Geothermics and Thermogenesis in Gas Reservoirs DOI: http://dx.doi.org/10.5772/intechopen.108350*

#### **Figure 8.**

*Thermal expansion of rocks: A: Volumetric thermal expansion of three dry sandstone rocks, and b: Thermal expansion of saturated sandstone rocks versus dry ones.*

phenomenon as temperature damages the mineral structure through the differential thermal expansion of the minerals. The thermal expansion of minerals varies from one mineral to another where the term coefficient of expansion appears. The coefficient of expansion not only varies from one mineral to another, but it also varies depending on the crystallographic direction.

In most cases, the effects of temperature are evidenced by an alteration of the mechanical properties and rheological behavior of rocks by making fractures similar to that of cooling joints in igneous rocks at the surface. The phenomenon of making fractures in reservoir rocks is in favor of thermal fracing of reservoir rocks in enhanced oil recovery. However, according to [28], the effect of high overburden pressure inhibits the thermal fracking of rocks due to the high weight rocks. Experiments on coalbeds regarding the impact of thermal cracking on the formation of artificial cracks [29] showed that thermal cracks formed proportional to thermal stress in terms of crack size. According to the above-mentioned two cases, the lightweight of coalbeds might make thermal frocking possible as compared to thermal fracking of siliciclastic reservoir rocks.

Diagenesis is another phenomenon that is significantly affected by temperature. In formations under trapped radiogenic heat and high pressures cause diagenesis. Of montmorillonite which decomposes into illite. The former contains compositional water which is released as released freshwater of crystallization either remains in the transformed clay under high pressure because the adjacent sand beds are already geopressured or flows to and dilutes normally pressured aquifers.

The effect of heat in decreasing the viscosity of oil, and subsequently in its movement and migration, is documented in many literature sources; however, scanning electron microscopy techniques were used to demonstrate this effect are shown in **Figure 9**.

While the mantle typically ranging from about 1000°C to 3700°C, while the Earth's crust has lower temperatures, ranging from around −40°C to 1000°C, with the highest temperatures in areas with active volcanic activity [30].

### **8.1 Occurrence of gas in a gas reservoir**

Excluding the secondary occurrence of hydrocarbons in fractured igneous rocks [31], all geologists agree that hydrocarbons do not form in igneous or metamorphic zones, but are generated and retained in sedimentary rocks [32].

The temperature range of formation of both oil and gas from organic matter is called the oil and gas window, respectively. The oil window normally takes place between 60 and 120 ~ gas generation occurs between about 120 and 220 ~ above which the kerogen has been reduced to inert carbon (**Figure 10**).

The subject of oil source rocks is covered in far greater depth in the textbooks [32, 34]. Oil generation normally takes place between 60 and 1R mineral reactions.

Some mineral reactions are generally catalyzed by temperature where they absorb heat. For reversible reactions, absorbed heat is released again and the mineral phase is restored [27, 35]. Experiments have been conducted to demonstrate such reactions by increasing the temperature to simulate the subsurface reservoir (**Table 3**).

### **9. Reservoir thermogenesis**

The fact that source rock evolves thermally is well established [34]. Because of the fact that hydrocarbons in normal cases migrate upward [32], the source rock is subjected to heat flow firstly; and the hydrocarbons migrated from affected shales *Perspective Chapter: Geothermics and Thermogenesis in Gas Reservoirs DOI: http://dx.doi.org/10.5772/intechopen.108350*

#### **Figure 9.**

*Conventional SEM images of residual oil droplets in the (a) horn river. (B) Eagle ford, and (C) Woodford shales. The oil migrated into matrix pores, and microfractures upon heating to 350°C for four days in hydrous pyrolysis apparatus (experiments were only run on the Eagle Ford and Woodford formations). The Horn River example, which was not heated, demonstrates that hydrocarbons can occur within matrix mineral pores and not be solely confined to organopores [30].*

#### **Figure 10.**

*Photomicrograph of a heat-affected lower Permian Barakar formation shale from Raniganj basin, India, showing the development of bireflectance in vitrinite [33].*


#### **Table 3.**

*Heats of reaction for several minerals (after Barshad 1972, [36]).*

to reservoir. The abnormal heat flow is generally a heat plume resulting from intrusion. In such cases, hydrocarbon matter volatiles to the reservoir and vitrinite in the affected shale reflects birefringence as shown in the photomicrograph by [33]. However, maturity of hydrocarbons in a reservoir might not reflect the thermal status of the source rock, where immature oils can occur in reservoirs of thermal history, and vice versa. For gas formation, gas is formed either by the effect of bacteria, the process known as biogenesis, at relatively shallower depth below 550 C [37]; or, at higher depths, is formed by the effect of temperature, known as thermogenic gas. In unconventional petroleum systems where the source rock is the reservoir itself, the thermal effect causes the formation of secondary porosity [38]. Such secondary porosity significantly increases the storage capacity of the system.

Geothermal reservoirs represent one of the applications of the responses of reservoir fluids to reservoirs geothermic. Geothermal reservoirs provide clean renewable energy, [39] and the research in that field are going on for decades [40] according to [41] the geothermal system can be defined as a reservoir in a certain area that provides the opportunity to extract heat economically. Generally, the geothermal system can be divided into three main groups [42, 43]. The first one is the hydrothermal system which is formed when heat is transferred from a source by conductivity to porous media and the porous fluid within it. Moreover, the hydrothermal system can be classified into liquid-dominated and vapor-dominated (or dry steam/steam-alone) systems, depending on the existence of water or vapor. The second one is the Geopressuredgeothermal system formed when the water is trapped in permeable media (rock) surrounded by impermeable or low permeable rock [42]. The last one is the artificial geothermal reservoir system called the hot dry rock system (HDR), formed in which boreholes are drilled and water is injected into the hot igneous rock [41]. Furthermore, the geothermal systems can be classified upon the equilibrium state into static and dynamic systems, the static is characterized by contentious recharge and discharge of water, while the dynamic is dominated by low or no recharge [42].

In terms of temperature conditions, or reservoir geothermic, geothermal reservoirs are classified into low, medium, and high geothermal reservoirs [41, 42, 44, 45]. The low-temperature geothermal reservoirs (at temperature less than 90°C) can provide waters source for industrial use and other uses but are not sufficient for electricity [44]. The intermediate temperature (the temperature is <150°C and ≥ 90°C), and

the high temperature (the temperature is ≥150°C). More details on the subject matter can be found in [46].

### **10. Thermal fracking**

One of the thermal applications of reservoir geothermic also is thermal oil recovery [47–50]. In conventional hydrocarbon systems, heat is used in oil recovery to enhance the oil recovery by reducing the oil viscosity allowing it easily to flow (Reference). The process depends totally on reservoir thermal properties. However, in unconventional shale gas and shale oil recovery, heat is used as a fracking agent instead of hydraulic fracking which requires injecting a large volume of water [51].

### **11. Conclusion**

The geothermics of reservoir have been studied and investigated where sources of terrestrial heat, means of heat transport, and thermal properties of rocks were discussed in detail and in relation to subsurface conditions such as overburden pressure and fluid content. The subsurface heat and subsequent temperature were proven to affect the reservoir system and many processes that take place in it. Porosity changes by the effect of temperature and similarly, fluid phase changes also. Some minerals show alterations and reactions have been described also. The applications of reservoir geothermics have also been addressed such as geothermal reservoirs. In conclusion, this chapter provides deep insight into the heat regime and temperature in reservoirs and, especially, reactions between different subsurface heat and components of reservoir systems.

### **Acknowledgements**

The authors are willing to acknowledge the Red Sea University as their parent university where several facilities are provided for authoring this chapter.

### **Conflict of interest**

The authors declare no conflict of interest.

*Topics on Oil and Gas*

### **Author details**

Yasir Ali1 \* and Yasir Yousif<sup>2</sup>

1 Red Sea University, Port Sudan, Sudan

2 Pan African University, Ibadan, Nigeria

\*Address all correspondence to: yasirovic2009@gmail.com

© 2023 The Author(s). Licensee IntechOpen. 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.

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[37] Shurr GW, Ridgley JL. Unconventional shallow biogenic gas systems. American Association of Petroleum Geologists Bulletin. 2002;**86**(11):1939-1969

[38] Loucks RG, Reed RM, Ruppel SC, Hammes U. Spectrum of pore types and networks in mudrocks and a descriptive classification for matrix-related mudrock pores. American Association of Petroleum Geologists Bulletin. 2012;**96**(6):1071-1098

[39] Washington D. Geothermal Energy | National Geographic Society [Internet]. 2015. pp. 1-4. [cited 2022 Sep 15]. Available from: https://education.

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[40] Manzella A. Geothermal energy. EPJ Web of Conferencs. 2017;**148**:1-26

[41] Gupta HK, Roy S. Geothermal Energy: An Alternative Resource for the 21st Century. Elsevier; 2006

[42] Ganguly S, Kumar MSM. Geothermal reservoirs - a brief review. Journal of the Geological Society of India. 2012;**79**(6):589-602

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[46] Brown DW, Duchane DV, Heiken G, Hriscu VT. Mining the earth's Heat: Hot Dry Rock Geothermal Energy. Springer Science & Business Media; 2012

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[48] Hascakir B. Introduction to thermal Enhanced Oil Recovery (EOR) special issue. Journal of Petroleum Science and Engineering. 1 Jun 2017;**154**:438-441

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[50] Nian Y-L, Cheng W-L. Insights into heat transport for thermal oil recovery. Journal of Petroleum Science and Engineering. 2017;**151**:507-521

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## **Chapter 6** Coking

## *Jafar Ramezanzadeh and Hossein Moradi*

### **Abstract**

Currently, conventional oil is used as the main source for the petrochemical industry. However, conventional oil's capacity is declining, and that source will probably be exhausted in the near future. Heavy oil and petroleum residues have become a suitable alternative source to meet global energy demand. However, heavy oil and oil residues require many upgrading processes before turning to be valuable products. Among the various upgrading processes, delayed coking, which is capable of processing any residue at a low investment cost, garnered tremendous importance. Petroleum coke is one of the coking products that is divided into three types: shot coke, sponge coke and needle coke, depending on the feed properties and operating conditions of the process. Needle coke is used as a valuable product in the production of graphite electrodes used in electric arc furnace (EAF) for melting scrap metal and producing steel.

**Keywords:** heavy oil, petroleum residue, upgrading process, delayed coking, needle coke, graphite electrodes, electric arc furnace

### **1. Introduction**

The petroleum industry provides most of the world's energy needs and has been the world's most important energy source since the mid-1950s because of its high energy density, easy transportability and relative abundance [1]. Due to rapid population growth, the consumption of fuels, energy, and petrochemical products has increased sharply [2]. At present, light crude oil reserves are the main source of energy that meets global energy demand due to high quality and low production costs. Nevertheless, light crude oil reserves are declining. Such a rapid decline in light crude oil reserves poses great challenges to meeting the world's energy needs. Over the past few decades, renewable, nuclear and bioenergy have been developing rapidly; however, these resources are costly and insufficient in meeting energy demands, especially for transportation [3]. Therefore, refineries have to depend increasingly on unconventional feedstocks such as heavy oils, oil residues, and bitumen to supply the increasing demand for fuels [1]. The fundamental characteristics of heavy crude oil are low American Petroleum Institute gravity (API), low economic value, high viscosity, and high asphaltenes content which makes it more difficult to transport and process than conventional crude oil [4]. This fact leads to an emphasis on the upgrading of heavy and residual oil. The purpose of upgrading heavy oil and residues

is to convert feedstock with high boiling point and low H/C ratio to low boiling point distillate fractions and higher H/C ratio and to eliminate hetero atoms such as sulphur, nitrogen, and metals to Environmentally acceptable levels. To achieve this goal, hydrocarbon molecules are exposed to thermal and catalytic cracking reactions during the upgrading processes [5]. According to the approaches to achieve higher H/C ratios, upgrading technologies can be divided into carbon rejection and hydrogen addition processes. Carbon rejection rejects the carbon into carbonaceous product (coke) to obtain lighter products (with a high H/C ratio) in these processes. On the other hand, hydrogen addition processes such as hydrocracking involve the reaction of raw materials with an external source of hydrogen in the presence of a catalyst, which leads to an overall increase in the H/C ratio [6]. Hydrogen addition processes have higher quality and yield of desired products. However, these processes require the participation of hydrogen and catalysts, which leads to higher investment and operating costs compared to carbon rejection processes. In contrast, carbon rejection processes are superior to hydrogen addition processes in terms of simplicity and operating costs, and therefore have many units in the world [7, 8]. Petroleum residues processing capacity indicates that the major portion (approximately 63 wt.%) of petroleum residues are upgraded by thermal processes such as visbreaking and delayed coking [2].

### **2. Carbon rejection processes**

Carbon rejection technologies have been used by refineries since 1913 to upgrade various hydrocarbon feeds. These technologies include visbreaking, gasification, and coking processes. visbreaking and coking technologies can be generally applied to all residual feeds because they are not limited to constraints such as metal content and coke-forming tendencies as in the case of catalytic processes for upgrading. In a carbon rejection process, the feeds (larger molecules) are heated under inert atmospheric pressure to fracture them into smaller molecules [2, 9, 10].

### **2.1 Visbreaking**

Visbreaking remains the oldest and least costly of the upgrading option and is only used in areas where heavy fuel oil is used to generate electricity and fuel ships. Visbreaking is a process in which residues are slowly cracked to reduce viscosity, and its main product is fuel oil, which has a dwindling market and provides low margins. This is a very low conversion process, and 15–20 wt. % residues are converted into lighter fractions. The yield of gas and gasoline together is generally limited to a maximum of about 7 wt. % as the cracking reactions are arrested to prevent asphaltene flocculation. Current interest in visbreaking is in those areas where motor fuel demand is relatively low. Vacuum residue and atmospheric residue can be used as feedstock for the visbreaking process [2, 6, 8, 10].

### **2.2 Gasification**

The Texaco Gasification Process (TGP) was developed in the late 1940s. This process involves the complete cracking of residues into gaseous products, which has received less attention than other processes. Residual gasification is done at high temperatures (>1000°C) and synthetic gas (hydrogen and carbon monoxide), carbon

### *Coking DOI: http://dx.doi.org/10.5772/intechopen.106190*

black, and ash are the major products. It was modified in the 1950s for heavy oil feeds, in the 1970s for solid feeds such as coal and in the 1980s for petroleum coke. Almost from the beginning, this process has been attractive for hydrogen production. gasification can be used by refineries to produce hydrogen, increase the yield of high-value products, eliminate the production of high sulphur fuel oil, minimize the environmental effects of refineries (reduce the emission of NOx and SOx pollutants) and process a wider range of crude oil [2, 9, 11].

### **2.3 Coking**

Coking is a process in which raw materials are thermally decomposed into products with lower boiling points. Different types of coking processes include delayed coking, fluid coking, and flexicoking. Delayed coking is the most common technology used in petroleum refineries to produce petroleum coke. More than 90% of petroleum coke is produced by this process. The main reasons are the relatively low investment cost and the claims of a better quality of liquid products compared with the fluid or flexicoking process [12].

### *2.3.1 Delayed coking*

In the delayed coking process, the general goal of such a technology is to maximise liquid product yield while minimising coke production. The inherent flexibility of the delayed coking process for handling various feedstocks gives the refinery a promising solution to the problem of decreasing residual fuel demand and takes advantage of the attractive economics of upgrading it to more valuable lighter products. A refinery with a delayed coker is called a 'zero resid refinery' that can convert various feedstocks to valuable engine fuels while eliminating unsold refinery flows that are environmentally unfriendly. Disadvantages of this technology can be the abundant production of coke, low yield of liquid products, and highly aromatic products which require post-treatment. Another disadvantage of delayed coking is that it is a more expensive process than solvent deasphalting. Environmental pollution from coke particles is also a concern. In this process, 20–30 wt.% coke is also produced as a byproduct. Although coke is accepted as a by-product of coking processes, excessive coke formation is economically disadvantageous because the value of coke is much lower than that of distillates. Even considering these disadvantages, delayed coking is the most frequently preferred process for refiners to residue processing because of the low investment cost [2, 6, 8–10].

Delayed coking is a severe form of thermal cracking process that operates at low pressures, without the use of hydrogen and catalysts, and falls in the temperature range of 450–500°C. Delayed coking is highly efficient in rejecting mineral solids and metals as well as some organic nitrogen and sulphur in the coke. The name 'delayed' derives from the fact that cracking reactions are given enough time (long) to form coke in coke drums. The first commercial delayed coker was started in 1930 at Standard Oil's Whiting refinery [12, 13].

The global trend of processing heavy raw materials in delayed cokers, in order to obtain maximum yield of liquid products, has led to the production of coke with fuel grade that contains large amounts of sulphur and metals. Fuel grade coke, once considered a by-product of waste, is now an important fuel for the cement industry and electricity generation [6].

#### *2.3.1.1 Process description of delayed coking*

A schematic flow diagram of the delayed coking is shown in **Figure 1**. The process includes a fractionator, furnace, two coke drums, and stripper. the feedstock is charged directly to the fractionator, where it is heated, and the lighter fractions are removed as middle distillates. The bottom of the fractionator is pumped to the coking furnace and then heated to the temperature range of 485–500°C. The heated feedstock (liquidvapour mixture) enters one of the pairs of coking drums, where the cracking reactions continue. The energy obtained in the furnace passages is sufficient to perform the cracking reaction when the coking drum is filled. In the furnace, steam is injected to prevent the formation of premature coking. In addition, to prevent the formation of coke in the furnace, short residence time and high mass velocity in the furnace are required. Overhead stream in the coking drum; gases, naphtha, middle distillates and coker heavy gas oil are sent to the fractionator for separation, then separated and sent to downstream units for post-treatment and coke deposits on the inner surface. For continuous operation, two coke drums are used; while one is onstream, the other is decoking. The typical volume of a modern coke drum is about 1000 m<sup>3</sup> , with a size range of 5–9 meters in diameter and a height range of 20–45 meters. The temperature in the coke drum ranges from 415 to 465°C and the pressure varies between 2 and 6 bar. Coker heavy gas oil is recycled as a coker feed and combined with fresh preheated feed and fed to the furnace, or used in other refining processes such as hydrocracker or gas oil hydrotreater or as a catalytic fluid cracking feed. The Coke drum is usually onstream about 24 hours before filling with porous coke. **Figure 2** shows a section of a coke drum and shows how coke forms during a delayed coking operation. The material at the bottom of the coke drum is fully carbonised, creating a porous structure through which gases and liquids can pass. The top layer is not fully carbonised until it is exposed to heat for a long time. Some foam forms on the top of the drum, so foam forming can be prevented by injecting antifoam materials (silicone oil) into the coke drums during the last 5 or 6 hours of the coking cycle. It is important to prevent the carryover of foam into vapour lines. Level

**Figure 1.** *Flow sheet of delayed coking [modified from 12].*

**Figure 2.**

*Coke formation in coke drum of a delayed coking unit [6].*

indicators are useful for detecting the position of liquid or foam in the drum. After steaming and cooling the coke drum, the coke is removed by drilling and cutting with high-pressure (up to 340 bar) water jets [6, 12–17].

Decoking operation of the drum (**Figure 3**) involves the following steps:


Most cokers were originally designed for a 20- to 24-hour coking cycle. In the late 1980s and early 1990s, the coking cycle time was reduced to 16–20 hours. In the late 1990s, it dropped to 14 hours. A typical time cycle in delayed coking is shown in **Table 1**.

#### **Figure 3.**

*Steps of decoking operation [14].*


#### **Table 1.**

*Time cycle for delayed coking [6, 14].*

### *2.3.1.2 Delayed coking process variables*

Delayed coking process variables include process operating variables, feedstock properties and engineering variables. Furnace outlet temperature, coke drum pressure and recycle ratios are the main operating variables that affect not only the coke yield but also its properties. Increasing the drum pressure leads to a higher coke yield and a slight increase in gas yield, because more molecules, even in the gas oil range, contribute to coke formation by remaining in the liquid phase. It also reduces the sulphur content of coke. However, refinery economics requires operating at minimal coke formation.


#### **Table 2.**

*Effect of operating variables on the yield and quality of coke [6].*

As the temperature of the furnace and drum increases, due to the removal of more volatile matter, the yield of coke reduces and the higher quality and harder coke is produced. However, it can cause cutting problems during decoking. Lower temperatures produce more coke, but lower quality. Therefore, the temperature at the furnace outlet must be optimized to form a minimum amount of coke in the furnace coils. To reduce the formation of coke in furnace coils, steam is injected into the furnace before the critical decomposition zone. However, the coke produced by steam injection in this process is more isotropic, that is, of lower quality. The recycle ratio has the same pressure effect as in delayed coking units, which varies from 1.03 to 1.30. The highest values are used in commercial units that produce premium coke, while the lowest values are used in delayed coking units where the goal is to maximise distillate yields. In addition, reducing the recycle ratio causes low-quality coke because the concentration of asphaltenes in the reaction mixture is higher [6, 12–17].

Delayed coking units for processing vacuum residues are designed to operate under operating conditions that maximise liquid distillates yield and minimise coke production. These operating conditions include lower pressures, higher temperatures, and a lower recycle ratio. Feedstock variables are characterization factors and conradson carbon that affect product yields. Engineering variables also affect process performance, including mode of operation, capacity, and equipment used in coking and handling equipment. Operating variables have practical constraints that prevent further changes. Also, the constraints for each will be different with the type of feed consumed [14]. The effect of operating variables on coke yield and quality is shown in **Table 2**.

### *2.3.1.3 Delayed coking feedstock*

The delayed coking process can be applied to all residues in general, as they are not limited to constraints such as metal, sulphur, and asphaltene content. Heavy residues such as atmospheric and vacuum residue usually enter the delayed cokers, however, there are many raw materials that have been used as delayed coker feedstock for years. These feedstocks include:


#### *2.3.1.4 Delayed coker yield prediction*

In general, the products of the delayed coking process (based on vacuum residue feed) include gas (approximately 13 wt. %), naphtha (approximately 11 wt. %), middle distillate (approximately 45 wt. %), and green petroleum coke (approximately 31 wt. %).

The yield of products from delayed coking depends on the feed composition, in particular the amount of micro carbon residue (MCR) or Conradson carbon residue (CCR) content. Product yields can be estimated using the correlation based on the weight percentage of Conredson carbon residue (wt. % CCR) in the vacuum residue [14].

> *Gas C*� 4 *wt:*% <sup>¼</sup> <sup>7</sup>*:*<sup>8</sup> <sup>þ</sup> <sup>0</sup>*:*144<sup>∗</sup> ð Þ *wt:*%*CCR* (1)

$$
\mathfrak{N} \mathfrak{a} \mathfrak{h} \mathfrak{h} \mathfrak{a} \mathfrak{w}. \mathfrak{w}. \mathfrak{W} = \mathfrak{11.29} + \mathfrak{0}. \mathfrak{Z43}^\* \left( \mathfrak{w} \mathfrak{t}. \mathfrak{W} \mathfrak{C} \mathbf{C} \mathbf{R} \right) \tag{2}
$$

$$\text{Light NaOH } wt.\text{96} = 0.3322^\* \left( \text{Naphtha } wt\% \right) \tag{3}$$

$$\text{Heavy } \text{Naphtha } wt.\text{\(\mathfrak{G}\)}{\mathfrak{G}\mathfrak{G}7\mathfrak{F}^\*(\text{Naphtha } wt\mathfrak{G})} \tag{4}$$

$$\text{Coke } wt.\text{\textquotedblleft} = \text{1.6}^\* \left( wt.\text{\textquotedblright} \text{CCR} \right) \tag{5}$$

$$\text{Gas Oil } wt.\text{\textquotedblleft} = \text{100} - (\text{Gas wt.}\text{\textquotedblleft} + \text{Nap}\\\\text{\textquotedblright} + \text{Coke } wt.\text{\textquotedblleft})\end{aligned}\tag{6}$$

$$\text{Light Cycle Gas Oil } wt.\text{\textquotedblleft} = \text{0.645}^{\*} \left(Gas \text{ Oil } wt.\text{\textquotedblright}\right) \tag{7}$$

$$\text{Heavy Cycle Gas Oil } wt.\text{\(\% = 0.35\) (Gas Oil } wt.\text{\(\%\)}\text{\(\%\)}$$

The gaseous compounds from the delayed coking process typically include methane, ethane, propane, butane, carbon monoxide, carbon dioxide, hydrogen, nitrogen, hydrogen sulphide and ammonia, the composition of which depends on the type of feed and the operating conditions.

#### *2.3.1.5 Types of coke and their properties*

Depending on the properties of feedstock and the operating conditions of the delayed coking process, different types of the coke can be produced. Coke can be distinguished by its morphology. Typically, coke can be divided into spherical shot coke (isotropic, amorphous, with almost no pores), sponge coke (semi-isotropic), and needle coke (anisotropic, regular crystalline structure, containing numerous fine pores and crystal sizes in the order of 4–7 nm). Either, according to its use, can be divided to fuel grade coke (cement industry and power generation), anode grade coke (aluminium production) or electrode grade coke (steel production). The differences between these types of coke are not always very clear. Due to the heterogeneity within the coke drum, one coke type may contain certain values of another coke type. Therefore, sponge coke may contain some shot coke and needle coke may contain some sponge coke [6, 15, 19]. Types of coke resulting from the delayed coking process with their optical structure are shown in **Figure 4**.

Petroleum coke can be in two forms, green petroleum coke and calcined petroleum coke. Petroleum coke obtained without calcination is called green coke. Coke calcination is done in a furnace to remove remaining hydrocarbons by heating green coke to about

*Coking DOI: http://dx.doi.org/10.5772/intechopen.106190*

#### **Figure 4.**

*Delayed coke types and optical textures. a: Needle coke, b: sponge coke, c: shot coke [20].*


#### **Table 3.**

*Typical properties for different types of coke [12].*

1300–1500°C. During calcination, the coke decomposes further, and the carbon to hydrogen ratio increases from about 20 in green coke to 1000 for calcined coke [18].

Typical properties for different types of coke are shown in **Table 3**:

#### *2.3.1.5.1 Shot coke*

Shot coke comprises dense low porosity spherical clusters with 2–10 mm diameters, frequently present as agglomerates up to the size of basketballs. These large agglomerates are fragile and can be broken easily; however, the small spheres are very hard. Shot coke is obtained from petroleum precursors with high resin and asphaltene and low API gravity, and it is less valuable than sponge coke. High velocities in the reactor are required to produce shot coke with spherical particles. Given that a very turbulent condition is required for the formation of shot coke, shot-coke production in the laboratory is difficult, because surface velocities are very low [14, 19].

#### *2.3.1.5.1.1 Variables affecting shot-coke formation*

The variables which impact coke structure are the quality of the feedstock and the operating variables including pressure, temperature, vapor velocity, and recycle ratio.

#### • Feedstock quality:

Different authors agree that the feedstock properties associated with the production of shot coke are asphaltene content and Conradson carbon residue content. Researchers claim that the tendency to produce shot coke increases when the ratio between the asphaltene content and the Conradson carbon residue content approaches 0.5. Moreover, the characterisation of vacuum residues from different heavy oil sources shows that this ratio (asphaltene content/Conradson carbon content) is equal to or higher than 0.5; therefore, if the operating conditions are favourable, the formation of shot coke is likely when these feedstocks are processed.

Another fact that shows that the feedstock quality has an important impact on the coke structure is the use of decanted oil mixed with vacuum residue. Decanted oil is the residual product from the fluid catalytic cracking (FCC) process. This hydrocarbon stream is highly aromatic (more than 70% aromatics) and its incorporation into the coker with the feedstock (between 15% and 20% of the total feedstock) suppresses shot-coke formation. This suppressing action can be related to the solubility effect of the aromatics on the asphaltenes, although, this has not been shown experimentally [6, 14, 19].

#### • Operating variables

Operating variables refer to the pressure, temperature, vapour velocity, and recycle ratio within the coker.

Pressure: Reduction of the coker pressure favours the formation of shot coke.

Temperature: Higher temperatures favour shot-coke formation, and temperature change of 5°C or less can either suppress or promote shot-coke formation. In a commercial delayed coking unit, the heater outlet temperature varies between 490 and 500°C. However, scaling down of these units is reached by operating the small-scale units at lower temperatures, which may vary between 417 and 450°C.

Vapour Velocity: The feedstock flow is not an important variable that affects product yields in delayed coking technology, but this variable is an important parameter for shot-coke formation because it impacts the vapour superficial velocity, which is thought to give a spherical shape to shot-coke particles. The vapour superficial velocities in commercial delayed coking units are between 0.12 and 0.21 m/s. These vapour velocities are so high that they are not achieved in laboratory-scale units.

Recycle Ratio: It is calculated with the following expression:

$$RR = \text{HF}/\text{FF} \tag{9}$$

HF is the flow of the heater. After mixing the recycling flow with fresh feed at the bottom of the main fractionator, it is measured at the heater inlet. FF is the fresh feed stream that is measured before pumping the processed feedstock into the main fractionator. Both flows are measured in barrels per day.

The recycle ratio in delayed coking units varies from 1.03 to 1.30. The highest values are used in commercial units that produce needle coke, while the lowest values are used in delayed coke units where coke yields should be minimised [6, 14, 15, 18, 19].

#### *2.3.1.5.2 Sponge coke*

Sponge coke is the most common form of green coke. Sponge coke is a friable solid material with pores on the surface and internal cavities connecting the pores,

### *Coking DOI: http://dx.doi.org/10.5772/intechopen.106190*

which is due to the evolution of gas from the liquid in the coke drum. The structure of this coke causes good drainage of water from the coke drums and easy cutting of the coke bed with water jets. This coke is typically derived from crude oil, which contains numerous cross-linkages. The diffusion of gas bubbles into the coke drum may also cause some spongy coke. In fact, sponge coke is a combination of sponge and shot structures. Most sponge coke is used to fuel boilers. Some low-sulphur, low-metal sponge coke can be used to make anodes used in aluminium production [6, 14].

### *2.3.1.5.3 Needle coke*

Using the proper feedstocks, optimal design techniques, and operating parameters, delayed coking can be used to produce needle coke, a specialized and rare product in the refining and coke production industry.

Producing good quality needle coke is not easy, because the control of several parameters is necessary to control the production process. In other words, it is a control process of several parameters. Needle coke is a premium coke made from special petroleum feedstocks. The needle coke has a silvery-grey appearance that has a broken crystalline needle-like structure, highly ordered, microcrystalline, under a light microscope. The observed optical texture is called flow domain. Needle coke has anisotropic components such as fine fibrous and leaflet structure. This coke has long, thin cavities that result from the gas bubbles released by the solid coke itself. This high-quality coke can only be produced from feedstocks of high purity (low metals and sulphur) and with high aromatic compounds, such as cycle oil from the fluid catalytic cracking unit. In addition, a long filling time is required for the solid coke in the coke drum to react and release the gases. This type of coke cannot be produced from vacuum residue [6, 14, 15, 21].

### *2.3.1.5.3.1 Needle coke applications*

Natural graphite is a limited source. It is estimated that 800 million tons can be mined worldwide. Only 10 to 15% of natural graphite is actually graphite carbon. Most of it is amorphous and contains minerals or silicate metals. In contrast, needle coke is continuously produced with high graphitizable content and low impurity concentration [12].

It was generally accepted that needle coke can be divided into two types according to the different feedstocks and named coal-based needle coke and petroleum-based needle coke. Excellent physical and chemical properties of needle coke such as high mechanical strength, high electrical conductivity (strong oxidation resistance), high thermal conductivity, high density as well as low thermal expansion coefficient (good abrasion resistance/heat shock resistance), low ash and sulphur content, low volatility, low energy consumption and easy graphitizable make needle coke an excellent raw material to obtain high-quality artificial graphite [12, 22].

There are two methods, basic oxygen furnace (BOF) and electric arc furnace (EAF), for steel production. Coal, iron, and limestone are used to produce steel in the BOF method. However, in the EAF method, an electric current passes through the graphite electrodes to convert the steel scrap into molten steel. Approximately 70% of world steel is produced by the BOF method and 30% by the EAF method. EAF has historically been the fastest growing sector of the global steel industry, with EAF steel production amounting to about 20 million tonnes per year in 1950, and EAF steel


**Table 4.**

*Typical calcined needle coke specification [6, 12].*

production expanded rapidly after 1950, and it exceeded 100 million tons in the 1970s. Needle coke, produced in the delayed coking process of petroleum oil refineries, was later developed in 1960 and commercialised in 1970. Finally, EAF steel production in 2020 reached about 550 million tons [12].

Inputs/initial costs of steel production through the EAF method include scrap steel, electricity, and graphite electrodes. There is no known alternative to graphite electrodes used in the EAF method of steel production. Needle coke is a major component in the production of graphite electrodes. The main application of needle coke is in the graphite electrode industry, and it can be purchased for 1500–3000 \$/ton. In addition, needle coke is also used in the production of graphite cathodes in the aluminium industry. Electrodes made of needle coke need to withstand temperatures above 3000°C. Global steel production on the EAF is expected to grow. This has led to a similar increase in consumption of graphite electrodes. It is expected to eventually increase the consumption of needle coke [6, 12].

Needle coke is now widely used as a carbon filler for the production of graphite electrodes in the steel industry for smelting scrap metal for recycling in an electric arc furnace (EAF), cathodes required for smelting aluminium, anodes for commercial lithium-ion batteries, electric machines and some inherent parts of mobile phones, electrode materials for high energy density supercapacitors, anode materials for highperformance sodium-ion batteries, adsorbents, isotropic graphite, nuclear graphite, perovskite solar cell, carbon substitute super-activated carbon, graphene precursors, aerospace and other functional materials are used. Graphite electrodes have a low coefficient of thermal expansion (CTE), which is defined as an increase in length per unit temperature increase. Low CTE values indicate anisotropic needle coke, while high values indicate an isotropic shot coke [6, 12, 22, 23].

In terms of grade, needle coke is divided into an intermediate, premium, and super premium needle coke. As shown in **Table 4**, their difference is in the amount of thermal expansion coefficient and sulphur content.

#### *2.3.1.5.3.2 Feedstocks quality for needle coke production*

Precursors for needle coke production have historically been limited to available residues whose aromatic molecular composition naturally predisposes them to form highly anisotropic carbon during carbonisation. However, further requirements of the feedstock include:

### *Coking DOI: http://dx.doi.org/10.5772/intechopen.106190*


Coal-based needle coke is made from Coal Tar Pitch, refined coal tar pitch, refined coal liquefied pitch, and coal extraction. Petroleum-based needle coke is usually obtained by delayed coking of residual oil, petroleum bitumen, oxidized petroleum bitumen, and Fluidised Catalytic Cracker Decant Oil [18].

The chemical and physical properties considered in choosing a proper feedstock for the production of needle coke are summarised as follows:


### *2.3.2 Fluid coking*

Although the delayed coking process has been selected for large-scale operations, they are more attractive for processing the small volumes of residues due to the safety issues involved in decoking the drums at the end of each cycle. In addition, by reducing the retention time of cracked vapours, the yields of coking distillation products can be improved. To simplify the handling of the coke and to enhance product yields, Exxon developed a continuous process in the mid-1950s called fluidized bed coking (or fluid coking), in which the residence time was shorter, with more liquid and less coke. However, in this process, the products have lower quality. Fluid coking is a fluidized bed process developed by fluid catalytic cracking (FCC) technology, except that no catalysts are used and heavy feedstocks such as atmospheric and vacuum residues, residues of catalytic cracking units and oil sand bitumen turn into light products. In fluid coking, about 6% of the coke is burned to provide heat to the process, while the net coke yield is 70 to 75% of delayed coking. The yields of products resulting from fluid coking are determined by feed properties, fluidized bed temperature, and residence time in the bed [12, 14–17].


#### **Table 5.**

*Yield of fluid coker process [14].*

An example of the material balance for fluid coking of Arab light vacuum residue is given in **Table 5**.

### *2.3.2.1 Process description of fluid coking*

Fluid coking is a thermal cracking process consisting of a fluidized bed reactor and a fluidized bed burner. A flow diagram is shown in **Figure 5**. Vacuum residue is preheated and fed to a scrubber that operates at 370°C above the reactor for coke fine particle recovery. The heavy hydrocarbons in the feed are recycled with the fine particles to the reactor as slurry recycle. The heavy vacuum residue feed is injected through nozzles to a fluidized bed of coke particles. Cracking reactions take place in the reactor at a temperature of 500–550°C, and the feed is converted to vapour and lighter gases, which enter the scrubber after passing through the cyclones at the top of the reactor and go to the fractionator column. Steam enters from the bottom of the reactor to remove heavy hydrocarbons from the coke surface. The evolution of vapour from the cracking of the feed, and the addition of steam, gives intense mixing of the

**Figure 5.** *Flow sheet of fluid coking [modified from 12].*

coke particles within the reactor. The coke formed in the reactor flows continuously to the burner, where it is heated to 593–677°C and burns with partial combustion of 15–30% of the coke by injecting air into the burner. Coke combustion produces flue gases with low heating value (20 BTU/SCF), which are rich in CO and H2. Parts of the heated coke particles are returned to the reactor to provide energy for the endothermic cracking reactions and to maintain the reactor temperature. After cooling, the remaining coke is removed from the process as a stream of fine particles of 'petroleum coke' and is burned in power plants or cement industries. This coke is very isotropic, rich in ash and sulphur and therefore not used in the carbon and graphite industry [12, 16, 17].

The lower limit on operating temperature for fluid coking is set by the behaviour of the fluidized coke particles. If the conversion to coke and light ends is too slow, then the coke particles become sticky and agglomerate within the reactor. This phenomenon occurs in localised zones of the reactor, likely near the nozzles that inject the (colder) liquid bitumen feed, giving rise to chunks of coke that fall to the bottom of the bed. For this reason, optimising the method for introducing feed into the reactor is crucial. In addition, excellent heat transfer in the fluidized bed helps to reduce hotspots, which allows the reactor to operate at a higher temperature to cause more cracking of volatile matters. These factors generally reduce coke yields and increase the yields of gas oil and olefins compared to the delayed coking process. One disadvantage of the fluid coking process is the high rate of coke accumulation inside the unit. The reactor operates in a fouling mode, so coke deposits continuously on the interior surfaces during operation. The reactor must be shut down for a month or more every 2 or 3 years to remove the accumulated coke, which can grow to be as thick as 1 meter on the interior walls of the coker. The second disadvantage is the emission of significant amounts of hydrogen sulphide and sulphur dioxide from the reactor burner [16, 17].

At first, it was thought that the fluid coking process would replace the delayed coking process in the market, but so far this has not happened.

### *2.3.3 Flexicoking*

The decline in coke markets derived from delayed coking and fluid coking due to constraints in sulphur emissions encouraged the development of flexicoking. Burning coke to generate process heat (**Figure 6**) liberates the sulphur in the coke as hydrogen sulphide and sulphur dioxide gases. The off-gas stream from the coke burner also contains CO, CO2 and N2. An alternate approach is to use a coke gasifier which can convert the carbonaceous solids to a mixture of CO, CO2 and H2 without producing SO2. Flexicoking was designed by ExxonMobil as a fluid coking modifier that was introduced in 1976 in Japan. This process combines fluid coking with coke gasification, which, similar to fluid coking, is a fluidized bed process developed from catalytic fluid cracking technology. A fluidized bed is added to the process, which acts as a gasifier in which coke from the heater is reacted with steam and air in a fluid-bed gasifier to produce a gas of low heating value (20–40 BTU/sCF) and significantly reduces coke production. Yields of liquid products are the same for flexicoking and fluid coking because the coking reactor is unaltered, but up to 97% of the coke can be converted to gas by steam and air in a gasifier. Air is injected into the gasifier to maintain temperatures of 830–1000°C, but injected air is not enough to burn the entire coke. Under these conditions, the sulphur in the coke is converted to hydrogen sulphide, which can be scrubbed from the gas prior to combustion elsewhere.

#### **Figure 6.** *Flow sheet of flexicoking [modified from 12].*

After removal of the hydrogen sulfide, a typical gas product contains 18% CO, 10% CO2, 15% H2, 51% N2, 5% H2O and 1% CH4. Petroleum coke is removed, and economical fuel gas is available for use at the refinery. Due to the high initial investment and mechanical cost, only seven units were built worldwide. The main drawback of gasification is the requirement for a large additional reactor, especially if the high conversion of the coke is required [12, 14–17].

### *2.3.3.1 Process description of flexicoking*

In the process, the viscous feedstock enters the scrubber for direct-contact heat exchange with the overhead product vapours from the reactor. Lower-boiling overhead constituents in the scrubber go to a conventional fractionator and also to light ends recovery. The feedstock is thermally cracked in the reactor fluidized bed to a range of gas and liquid products and coke. The typical bed temperature is 510–540°C. Vapour products resulting from the conversion reactions in the bed pass through the cyclone separators, which remove most of the entrained coke and return it to the reactor bed. The cyclone outlets discharge the vapor product directly into a scrubber, where the heavy liquid is used to scrub out the remaining coke dust and condense unconverted high-boiling fractions. The dust-laden liquid is recycled as 'a slurry cycle' to the reactor with the feed. The scrubbed vapour is sent to the coker fractionator, where the stream is split into gas, naphtha, distillate and heavy gas oil streams.

The heater is located between the reactor and the gasifier, and it serves to transfer heat between the two vessels. The heater temperature is controlled by the rate of coke circulation between the heater and the gasifier. Adjusting the air rate to the gasifier controls the unit inventory of coke, and the gasifier temperature is controlled by steam injection into the gasifier. Excess coke is converted to a low-heating value gas in a fluid-bed gasifier with steam and air. The air is supplied to the gasifier to maintain temperatures of 830–1000°C, but is insufficient to burn all the coke. The heater transfers heat from the gasifier overhead gas to coke, which in turn supplies the heat of reaction in the reactor. The heater bed temperature is approximately 610°C. Coke is continuously circulated between the three vessels to transfer heat and maintain vessel inventories. A typical gas product, after the removal of hydrogen sulfide, contains carbon monoxide (CO, 18%), carbon dioxide (CO2, 10%), hydrogen (H2, 15%), nitrogen (N2, 51%), water (H2O, 5%) and methane (CH4, 1%) [12, 14–17].

In the oxidation zone of the gasifier, the following reactions take place very rapidly [14]:

$$\text{C} + \text{0.5O}\_2 \rightarrow \text{CO} \tag{10}$$

$$\text{CO} + \text{0.5O}\_2 \rightarrow \text{CO}\_2 \tag{11}$$

In the reduction zone, the following reactions take place slowly:

$$\text{C} + \text{H}\_2\text{O} \rightarrow \text{CO} + \text{H}\_2\tag{12}$$

$$H\_2O + CO \rightarrow CO\_2 + H\_2\tag{13}$$

Delayed coking is the most commonly used process among all commercial coking processes. More than 92% of petroleum coke is produced in the delayed coking process; About one-third of feed streams are produced in the form of petroleum coke. Due to the reaction conditions, net coke production from fluid cokers and flexicokers is only about 5–10 wt.% of the feed material. About 20–25% of 700 refineries worldwide are equipped with delayed cokers. Of the 140 US refineries in operation, 55 have delayed coker units. Most of the petroleum coke is produced in the United States, followed by China, South America, Canada, India, the Middle East and Western [6, 12].

Coke produced by delayed coker is a marketable product, while coke produced by fluid coker and flexicoker is burned to meet the reactor heat needs and feed preheat.

### **3. Conclusion**

At present, light crude oil reserves are the main source of energy that meets global energy demand due to high quality and low production costs. Decline in light crude oil reserves poses great challenges to meeting the world's energy needs. Heavy oil and oil residues have become a suitable alternative source to meet global energy demand. According to the approaches to achieving higher H/C ratios, upgrading technologies can be divided into carbon rejection and hydrogen addition processes. However, the cost of hydrogen addition processes is much higher than carbon rejection processes, because the production of hydrogen and the catalysts used in hydrogen addition processes are very expensive. Carbon rejection technologies have been used by refineries since 1913 to upgrade various hydrocarbon feeds. In a carbon rejection process, raw materials are heated to high temperatures to crack large hydrocarbons into

smaller ones. Coking (delayed, fluid and flexi) is one of the types of carbon rejection processes. Delayed coking has been chosen by many refineries as an upgrading process due to its low investment cost and the inherent flexibility of the process to process any residuals. In this process, 20–30 wt.% coke is produced as a by-product. Depending on the properties of the raw materials and the operating conditions of the delayed coking process, different types of the coke can be produced. Typically, coke can be divided into spherical shot coke, sponge coke, and needle coke. Using the proper feedstocks, optimal design techniques, and operating parameters, delayed coking can be used to produce needle coke, a specialized and rare product in the refining and coke production industry. Needle coke is a premium coke made from special petroleum feedstocks. There are two methods, BOF and EAF, for steel production. Coal, iron, and limestone are used to produce steel in the BOF method. However, in the EAF method, an electric current passes through the graphite electrodes to convert the steel scrap into molten steel. There is no known alternative to graphite electrodes used in the EAF method of steel production. Needle coke is a major component in the production of graphite electrodes. The main application of needle coke is in the graphite electrode industry. Global steel production on the EAF is expected to grow. This has led to a similar increase in consumption of graphite electrodes. It is expected to eventually increase the consumption of needle coke.

## **Author details**

Jafar Ramezanzadeh\* and Hossein Moradi University of Tehran, Catalysis and Nanostructured Materials Research Laboratory, School of Chemical Engineering, College of Engineering, Tehran, Iran

\*Address all correspondence to: jafarramezanzadeh@ut.ac.ir

© 2022 The Author(s). Licensee IntechOpen. 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.

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## *Edited by Ali Ismet Kanli and Tye Ching Thian*

Reservoir characterization is the model that characterizes reservoirs based on their ability to store and produce hydrocarbons. Reservoir modeling is the process of creating a three-dimensional representation of a given reservoir based on its petrophysical, geological, and geophysical properties. Reservoir engineering is the formulation of development and production plans that will result in maximum recovery for a given set of economic, environmental, and technical constraints. It is not a one-time activity but needs continual updating throughout the production life of a reservoir. Reservoir management is often defined as the allocation of resources to optimize hydrocarbon recovery from a reservoir while minimizing capital investments and operating expenses. The oil and gas industry is divided into upstream and downstream sectors. In general, the upstream sector involves activities carried out at a reservoir, while the downstream sector involves all petroleum processing activities carried out in a refinery. Petroleum processing engineering involves all downstream crude oil processing operations, from crude oil as the feedstock to final products such as gasoline, kerosene, diesel, gas oil, and lubricating oil. Petroleum processing technology continues to develop in consideration of all types of sustainability-related challenges. This book provides a comprehensive overview of the oil and gas industry, presenting the most recent research in the field.

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Topics on Oil and Gas

Topics on Oil and Gas

*Edited by Ali Ismet Kanli and Tye Ching Thian*