Study of CO2 Injection into Sumatran Shale Layers to increase Hydrocarbon Gas Productivity of The Shale Gas Reservoir

Injecting CO2 into a reservoir has some important reasons. CO2 injection can enhance oil and gas recovery by reducing the capillary pressure, increasing the pressure gradient, and changing the phase behavior of the fluids. It can reduce greenhouse gas emissions by capturing and storing CO2 underground. It can also create economic benefits by utilizing CO2 as a valuable resource and generating revenue from carbon credits. Therefore, injecting CO2 into a reservoir benefits the environment and the industry. We can inject CO2 in shale gas reservoirs to increase productivity because CO2 has a stronger adsorption capacity on shale surfaces than hydrocarbon gas. When CO2 is injected into shale reservoirs, it can displace the adsorbed CH4 flow out of the micropores and free up more space for gas flow. Injecting CO2 can also reduce the viscosity and density of shale gas, improving its mobility and transport. Moreover, injecting CO2 can provide environmental benefits by reducing carbon emissions and storing CO2 underground. Therefore, CO2 injection is a promising technique for enhancing shale gas recovery and mitigating climate change. We characterized several types of shale from Sumatra using XRD to determine the mineral content. We injected the shale sample with the inert gas and CO2 gas. The characteristics of pressure build up after CO2 injection seem lower than one after inert gas injection. The volumetric of released gas after injection shows the same phenomena as pressure build up’s phenomena which shows clearly that shale rock released less of CO2 gas than the one of inert gas (CH4). These phenomena show that the CH4 can be released easier than the CO2 in the shale rock. Therefore, CO2 can be utilized as material for enhancing the gas recovery in shale reservoir.


Introduction
Injecting CO2 into shale is important for two main reasons: enhancing shale gas recovery and storing CO2 underground to mitigate climate change.Shale gas is a type of natural gas that is trapped in the tiny pores of shale rocks, which are very low permeability formations.To extract shale gas, hydraulic fracturing (or fracking) is used to create cracks in the rocks and release the gas.However, fracking only recovers a fraction of the gas, and the production rate declines rapidly after a few years.Therefore, enhanced gas recovery (EGR) methods are needed to increase the gas recovery and prolong the production life of shale wells.The importance of CO2 Injection into shale gas reservoir caused some authors to research these topics, i.e. [1]- [6].
One promising EGR method is CO2-enhanced shale gas recovery (CO2-ESGR), which injects CO2 into shale reservoirs.This method can provide dual benefits by enhancing shale gas recovery and securing geological sequestration of CO2.Theoretically, based on the stronger adsorption capacity of CO2 on shale surface, injected CO2 can displace adsorbed CH4 out of micropores.Moreover, CO2 can also dissolve in the formation water and reduce its viscosity, which can improve the mobility of both water and gas.Additionally, CO2 can weaken shale, decrease its brittleness, and increase its plasticity and toughness, which can create more complex fracture networks.
On the other hand, injecting CO2 into shale can also help mitigate climate change by storing CO2 underground while increasing the production of hydrocarbons.There are several studies of Indonesian shale characteristics, i.e. [7]- [10].CO2 is one of the major greenhouse gases that contributes to global warming and its anthropogenic emissions need to be reduced.Carbon capture and storage (CCS) is a technique that captures CO2 from large point sources (such as power plants) and transports it to a suitable geological site for long-term storage.Shale formations can be potential sites for CCS because they have low permeability caprocks that can prevent CO2 leakage.Furthermore, CO2 can be stored in shale in different ways, such as adsorption, dissolution, mineralization, and trapping.
Therefore, injecting CO2 into shale is important for both enhancing shale gas recovery and storing CO2 underground.However, there are also some challenges and uncertainties associated with this method, such as the optimal injection parameters, the long-term fate of CO2, the environmental impacts, and the economic feasibility.More research and field experiments are needed to address these issues and optimize this technique.Shale, smectite, and illite are different types of clay minerals that can be found in caprocks or reservoirs for CO2 storage.They have different structures, compositions and properties that affect their ability to adsorb CO2.Shale is a sedimentary rock composed of clay minerals and other fine-grained materials.It has low porosity and permeability, which makes it a good seal for trapping CO2 in the reservoir.Shale can adsorb CO2 in its pores and on its surface, depending on the pressure, temperature, and composition of the gas.The amount of CO2 adsorption in shale varies depending on the type and content of clay minerals in the shale.
Smectite is a type of clay mineral that has a layered structure with negative charges on the layers and positive charges on the edges.It can expand and contract by exchanging water and cations between the layers.Smectite has a high cation exchange capacity (CEC) and a high specific surface area (SSA), which means it can adsorb a lot of CO2 on its surface and in its interlayer spaces.Smectite also swells when it adsorbs CO2, which can generate swelling pressures that may affect the integrity of the caprock or the reservoir.
Illite is another type of clay mineral that has a similar layered structure as smectite, but with less negative charges on the layers and more potassium ions between the layers.Illite is less expandable and less reactive than smectite and has a lower CEC and SSA.Illite adsorbs less CO2 than smectite, but more than kaolinite or chlorite, which are other types of clay minerals.
Based on these characteristics, smectite has the highest CO2 adsorption capacity among the three types of clay minerals, followed by illite and then shale.However, this does not necessarily mean that smectite is the best for CO2 storage, as its swelling behavior may also pose some risks for leakage or damage.Illite may be more stable and less prone to swelling, but it may also undergo some chemical reactions with CO2 that may alter its structure and properties.Shale may have the lowest CO2 adsorption capacity, but it may also have the highest sealing efficiency due to its low permeability and porosity.Therefore, the optimal type of clay mineral for CO2 storage depends on various factors such as pressure, temperature, gas composition, mineral composition, pore structure, stress state, and geochemical interactions.Figure 1 shows the illustration of CO2 injection to increase hydrocarbon production in shale gas field.Our research is to analyze the effectiveness of CO2 injection to increase gas production and to investigate the mineralogy of shale which may be causing these phenomena.The illustration shown in Figure 1 is described by a chemical reaction between carbon dioxide and methane as expressed below, see (1).

Methods 2.1 Sampling Position and Mineralogy of Sumatran Shale
The samples used in this research were taken from the South Sumatra Basin, especially from Lahat formation that has an epoch of Eocene as can be seen in Figure 2 (a) while the geological location of the sampling position itself is shown by Figure 2 (b).In addition, a picture of Sumatran Lahat shale sample collected from Lengkiti, Baturaja, South Sumatra can be seen in Figure 3.The mineralogy of carbonate is analyzed by XRD spectra which show the domination of clay minerals, as shown in Figure 4. Sample 1 consists of 38% clay minerals, including illite, kaolinite, and chlorite.Sample 2 consists of 54% clay minerals, including illite, kaolinite, and chlorite.

Volumetric Gas Released Measurements After CO 2 and Inert Gas Injected Into Shale Sample
The CO2 vs CH4 gas released volumetric measurements diagram from shale rock after the injection of CO2 gas compared with one of inert gas is shown by Figure 5.The measurement procedure can be illustrated as following: the gas is injected into the shale rock until the specified time while the pressure build up is measured for the volumetric calculation of CO2 vs CH4 gas absorbed.The measurement is continued by opening the valve to calculate how much volumetric CO2 vs CH4 gas can be released as a function of time.
The characteristics of pressure build up after CO2 injection seem lower than one after inert gas injection, see Figure 6.In addition, the volumetric of released gas after injection shows the same phenomena as pressure build up's phenomena which shows the shale rock released less of CO2 gas than the one of inert gas (CH4), see Figure 7.

Conclusions
In this study we characterized several types of shale from Sumatra using XRD to determine the mineral content.We also injected the shale sample with the inert gas and CO2 gas.The characteristics of pressure build up after CO2 injection seem lower than one after inert gas injection.
The characteristics of pressure build up after CO2 injection seem lower than one after inert gas injection.The volumetric of released gas after injection shows the same phenomena as pressure build up's phenomena which is shale rock released less of CO2 gas than the one of inert gas (CH4).These phenomena show that the CH4 can be released easier than the CO2 in the shale rock.Therefore, CO2 can be utilized as material for enhancing the gas recovery in shale reservoir.

Figure 1 .
Figure 1.Illustration of CO2 injection to increase hydrocarbon gas production.

Figure 4 .
Figure 4. XRD spectra show the dominance of clay minerals in samples of Sumatran shale.

Figure 5 .
Figure 5. Diagram of gas volumetric measurement after injected into the shale sample [12].

Figure 6 .
Figure 6.Characteristics of pressure build up after CO2 vs inert gas injection.

Figure 7 .
Figure 7. Gas volume released after the injection, inert gas vs CO2 gas.

Table 1 .
Mineral identification of Sumatran shale samples as results of XRD.