Permian shales of Barakar formation in India were investigated to study their pore structure to understand their potential for natural gas production and possible CO 2 sequestration. The studied shale samples with variable clay content were of early mature stage and contained low (<2%) total organic carbon. Initially, a combination of small-angle neutron scattering (SANS) and low-pressure gas adsorption (LPGA) was used to identify the pore sizes and fractal dimensions of Indian shales. It was found that the quenched surface density functional theory model in the LPGA method gave better pore size distribution (PSD) estimates over the nonlocal density functional theory model. The micropores and smaller mesopores contribute the most to the total pore volume and the surface area of the studied shale samples. The average pore size decreased with an increase in pore volume. The fractal studies using SANS reveal that all studied shales possess similar fractal dimension despite being different in mineralogy, maturity, and total pore volume. The PSD and its possible relation with the mineral composition and the accessibility of the pores in terms of gas storage have been elucidated. Pore morphology was analyzed using image analysis of field emission scanning electron microscopy and low-pressure adsorption, corroborated by SANS results. The effects of dissolution and deposition probability on the fractal dimension of the shale were interpreted using the Monte Carlo-based computer modeling. The fractal dimension was higher in the case of shales that underwent simultaneous dissolution and deposition processes.

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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...