**6. CO2 injection in unconventional reservoirs**

Conventional reservoirs are oil and gas reserves that could be found in discrete accumulation of pools. Therefore, the hydrocarbon can be easily recovered through classic exploration techniques and vertical or deviated wells. Unconventional reservoirs on the other hand could be defined as a reservoir that requires out-ofthe-ordinary and hence complicated techniques of recovery as compared to the conventional oil and gas reservoirs [57, 58]. The main reasons why such reservoirs are getting considerable attention are the depletion of conventional sources and huge energy demand. **Figure 9** shows the unconventional reservoirs that can be potentially produced for recovery of hydrocarbon. Tight-gas sands, gas and oil shales, coalbed methane, heavy oil, tar sands, and gas-hydrate deposits are among the most anticipated reservoirs. These reservoirs often necessitate complex recovery

**Figure 9.** *Common unconventional hydrocarbon reservoirs.*

solutions such as stimulation treatments or thermal recovery methods and particular process facilities. Moreover, those requirements should be technically and more importantly economically viable [59].

### **6.1 Shale reservoirs**

Shale gas reservoir is referring to unconventional reservoirs that produce natural gas. Shale gas reservoir has received a lot of attention due to the potential reservoir in supplying clean burning energy and the way it copes with the depletion of conventional reservoirs [58]. However, at a certain time, the production of shale gas well decreases rapidly; thus, an enhanced gas recovery method has been aiming to improve the recovery from shale gas reservoirs.

In shale reservoir, methane (CH4) is adsorbed initially onto the surfaces of matrix particles and natural fracture faces and is stored in the matrix limiting its effective extraction [59]. Although large amounts of adsorbed gas exist, the ultra-low permeability of the shale matrix limits its effective extraction. CO2 injection is one of the methods that are largely implemented for EOR purposes due to the availability of CO2, the economics of operation, specific properties of CO2 gas, and positive environmental impact. CO2 can be used for enhanced gas recovery as well [60]. The process of EGR (enhanced gas recovery) using CO2 is mainly dominated by pressurizing effect. The pressurizing effects can cause CO2 injection to increase the rock permeability. The amount of CO2 injected into the well will be divided into two amounts; about 1% of injected CO2 will be produced, while 99% of injected CO2 will be stored in the reservoir. Therefore, tight shale gas reservoirs potentially make excellent repositories for CO2 sequestration purposes as well [60].

Various factors affect the recovery from tight shale reservoirs such as matrix porosity and permeability, hydraulic fracture half-length, and well spacing [61]. CO2 injection in shales is often conducted using huff and puff method. The supercritical carbon dioxide injection repressurizes the reservoir after the initial production period. Once the injected gas soaks from the fractures into the shale's organic matrix through diffusion and convection, methane is released by the competitive adsorption since the shale has a stronger affinity for carbon dioxide than for methane. Then, during the second production period, the methane partial pressure is lowered and the shale gas production rate increases [62].

One example of such reservoirs is Chattanooga shale in Missouri, USA. The main objective of this project was to inject 500 tons of CO2 to survey the injection and storage potential of CO2 in a natural shale development while checking for enhanced gas recovery. The roads leading up to the wellpad were regraveled and graded to facilitate CO2 delivery by truck to the injection site. The wellpad was cleared and graveled prior to moving equipment on site. A 70-ton CO2 storage vessel was located permanently on site and refilled periodically by 20-ton tankers. The skid pump with all the controls and meters, as well as the propane tank and heater to heat the CO2, was also located on site (**Figure 10**). The well head of the injection well was converted to accommodate the CO2 injection by adding a gate valve, an inlet for the CO2 line, as well as a tee for additional tests and monitoring [63].

The injection of 510 tons of CO2 during this test exhibits the first successful injection of CO2 in an organic shale formation to monitor for storage and enhanced gas recovery potential in Central Appalachia. This productive injection and monitoring of a CO2 infusion in an organic shale reservoir are extraordinary

**Figure 10.** *Injection well site layout [63].*

*Enhanced Oil Recovery Processes - New Technologies*

importantly economically viable [59].

improve the recovery from shale gas reservoirs.

**6.1 Shale reservoirs**

purposes as well [60].

increases [62].

monitoring [63].

solutions such as stimulation treatments or thermal recovery methods and particular process facilities. Moreover, those requirements should be technically and more

Shale gas reservoir is referring to unconventional reservoirs that produce natural gas. Shale gas reservoir has received a lot of attention due to the potential reservoir in supplying clean burning energy and the way it copes with the depletion of conventional reservoirs [58]. However, at a certain time, the production of shale gas well decreases rapidly; thus, an enhanced gas recovery method has been aiming to

In shale reservoir, methane (CH4) is adsorbed initially onto the surfaces of matrix particles and natural fracture faces and is stored in the matrix limiting its effective extraction [59]. Although large amounts of adsorbed gas exist, the ultra-low permeability of the shale matrix limits its effective extraction. CO2 injection is one of the methods that are largely implemented for EOR purposes due to the availability of CO2, the economics of operation, specific properties of CO2 gas, and positive environmental impact. CO2 can be used for enhanced gas recovery as well [60]. The process of EGR (enhanced gas recovery) using CO2 is mainly dominated by pressurizing effect. The pressurizing effects can cause CO2 injection to increase the rock permeability. The amount of CO2 injected into the well will be divided into two amounts; about 1% of injected CO2 will be produced, while 99% of injected CO2 will be stored in the reservoir. Therefore, tight shale gas reservoirs potentially make excellent repositories for CO2 sequestration

Various factors affect the recovery from tight shale reservoirs such as matrix

One example of such reservoirs is Chattanooga shale in Missouri, USA. The main objective of this project was to inject 500 tons of CO2 to survey the injection and storage potential of CO2 in a natural shale development while checking for enhanced gas recovery. The roads leading up to the wellpad were regraveled and graded to facilitate CO2 delivery by truck to the injection site. The wellpad was cleared and graveled prior to moving equipment on site. A 70-ton CO2 storage vessel was located permanently on site and refilled periodically by 20-ton tankers. The skid pump with all the controls and meters, as well as the propane tank and heater to heat the CO2, was also located on site (**Figure 10**). The well head of the injection well was converted to accommodate the CO2 injection by adding a gate valve, an inlet for the CO2 line, as well as a tee for additional tests and

The injection of 510 tons of CO2 during this test exhibits the first successful injection of CO2 in an organic shale formation to monitor for storage and enhanced gas recovery potential in Central Appalachia. This productive injection and monitoring of a CO2 infusion in an organic shale reservoir are extraordinary

porosity and permeability, hydraulic fracture half-length, and well spacing [61]. CO2 injection in shales is often conducted using huff and puff method. The supercritical carbon dioxide injection repressurizes the reservoir after the initial production period. Once the injected gas soaks from the fractures into the shale's organic matrix through diffusion and convection, methane is released by the competitive adsorption since the shale has a stronger affinity for carbon dioxide than for methane. Then, during the second production period, the methane partial pressure is lowered and the shale gas production rate

**140**

achievements and points of reference for CO2-EGR and additionally geologic CO2 storage in unconventional reservoirs. Once the well was brought back online after the soaking period, a significant increase in gas production occurred. During the first month of flowback, the average daily production rate was ~124 Mcf/day, which is over 8 times the average production for the last month before the well was taken offline for injection [64]. After 2 years of flowback, the well was still flowing at an increased production rate but is close to the projected historical production rate. The similar behavior of the injection well has been reported for modeling CO2 'huff-and-puff ' test in shale-oil reservoirs. The CO2 concentration in the product gas has steadily declined during the flowback of the injection well and 41% of the injected CO2 had been produced by the end of 2015 (17 months after flowback started). If the rate held constant, it would take over 8 years to produce all of the CO2 injected [63, 64].
