Impact of Supercritical CO₂ on Shale Reservoirs and Its Implication for CO₂ Sequestration

Abstract

Hydraulic fracturing has transformed the international energy landscape by becoming the go-to method for the exploitation of natural gas from unconventional shale reservoirs. However, in the recent years, the search for an alternative method of shale-gas exploration has intensified, because of various problems (e.g., contamination of ground and surface water, overexploitation of precious water resources, air pollution, etc.) associated with the usage of water-based fracturing techniques. The use of CO2 for shale gas exploitation has emerged as a better alternative to aqueous-based gas exploration techniques. CO2 when injected into deep shale reservoirs, transitions into supercritical CO2 (SC-CO2) when temperature and pressure condition exceeds the critical point, i.e., 31.1 °C and 7.38 MPa. In this paper, we comprehensively review the impact of SC-CO2 on shale gas reservoirs during the different stages of shale-gas exploration, i.e., (i) drilling, which involves the superiority of SC-CO2 over water-based drilling fluids, in terms of achieving under-balanced well condition, higher rates of penetration, and resistance to formation damage; (ii) fracturing, which involves factors affecting the tortuosity of fractures created by SC-CO2 fracturing, breakdown pressure, and proppant-carrying capacity; and (iii) injection, which involves the twin-headed benefit of enhanced recovery due to CO2/CH4 competitive adsorption and geological sequestration, CO2 vs CH4 excess sorption as a function of pressure, etc. Several research works have indicated discrepancies on how SC-CO2 impacts different shale properties. Some studies show low-pressure N2-gas-adsorption-derived surface area and total pore volume to be increasing with SC-CO2 imbibition, while others show a decreasing trend for the same. Similarly, for some shales, the quartz content, along with the clay mineral contents, decreased as the exposure to SC-CO2 increased, while in some other studies, with similar long-term exposure to SC-CO2, the quartz content was observed to increase along with the decrease in clay content and vice versa. Essentially, the increased exposure to SC-CO2 results in the dissolution of primary porous structures and fractures, and reformation of newer porous structure and conduits in shales. Nonetheless, these changes in the mineralogy weaken the microstructure of the rock bringing significant changes in the mechanical properties of the shales with implications on the wellbore stability and fracturing efficiency. The mechanical properties such as uniaxial compressive strength (UCS), Young’s modulus, and tensile strength decrease as the SC-CO2 saturation period increases. However, some studies have shown factors like bedding angle and phase-state of CO2 having varying effect on the strength behavior of the shales. Moreover, changes in the structure of shales caused by the creation of fractures and the reduction of their strength can also pose major risks, because of potential leakage of CO2 through these created pathways. How these processes would i...

Recent Advances in Geological Storage: Trapping Mechanisms, Storage Sites, Projects, and Application of Machine Learning

“…While the geological characteristics of shale are relatively stable, cracks generated by hydraulic fracturing may provide a pathway for CO 2 leakage. Furthermore, as the duration of CO 2 injection increases, notable modifications occur in the microstructure of shale, leading to consequential alterations in its mechanical properties . The exact amount of CO 2 that can be stored in shale formations globally is unknown because shale reservoirs have better injectability after transformation.…”

“…Furthermore, as the duration of CO 2 injection increases, notable modifications occur in the microstructure of shale, leading to consequential alterations in its mechanical properties. 174 The exact amount of CO 2 that can be stored in shale formations globally is unknown because shale reservoirs have better injectability after transformation. With the development of technology, shale is expected to become a popular site for storing CO 2 .…”

A Minireview of the Influence of CO2 Injection on the Pore Structure of Reservoir Rocks: Advances and Outlook

“…The effect of CO 2 on pore enlargement is not unidirectional but first inhibits and then promotes pore size. Hazra et al 102 suggest that increased SC−CO 2 exposure results in the dissolution of native pore structures and fractures, as well as the reformation of new pore structures in shales.…”

Microscale Damage Induced by CO2 Storage on the Microstructure of Sandstone Coupling Hydro–Mechanical–Chemical Processes

“…For example, CO 2 can be sequestrated in deep saline sandstone aquifers , or deep-sea sediments . In addition, it can be used to enhance coalbed methane, − shale gas , and oil recovery, , improve heat extraction from an enhanced geothermal system, and facilitate rock fracturing. − Large-scale storage of CO 2 in deep formations, which involves thermo–hydro–mechanical–chemical processes, may damage the microstructure of the rock formation, causing considerable fluid pressure perturbation, altering the in situ stresses, and then increasing the leak-off risk. , The damage induced by CO 2 storage to sandstone can be attributed to physical and chemical factors. In the physical case, the pore fluid (such as pore water and CO 2 ) can be absorbed on the pore surface, causing water-sensitive minerals to swell and generate microfractures.…”

“…6 For example, CO 2 can be sequestrated in deep saline sandstone aquifers 7,8 or deep-sea sediments. 9 In addition, it can be used to enhance coalbed methane, 10−12 shale gas 13,14 and oil recovery, 15,16 improve heat extraction from an enhanced geothermal system, 17 and facilitate rock fracturing. 18−22 Large-scale storage of CO 2 in deep formations, which involves thermo−hydro−mechanical− chemical processes, may damage the microstructure 23 of the rock formation, causing considerable fluid pressure perturbation, altering the in situ stresses, and then increasing the leakoff risk.…”