Carbon dioxide and methane sorption characteristics of Damodar Valley and Upper Assam shale, India

Abstract Shale gas has become one of the most significant unconventional hydrocarbon resources. The gas is adsorbed in shale micropores, more precisely in the pores of clay minerals and organic matter. In this work, studies were done on the shale samples of Damodar valley and Upper Assam Basin. High pressure adsorption isotherms of methane (CH4) and carbon dioxide (CO2) were analysed to assess the gas storage capacity. The organic geochemistry, thermal maturity and mineralogy are analysed by Rock-Eval pyrolysis, petrographic study and XRD study respectively. The purpose of this study was to understand the factors controlling the variations in the nature of high pressure CH4 and CO2 sorption capacities. The factors include organic-inorganic constituents, and pore characteristics. Results show that the shale samples have a higher CO2 sorption capacity compared to that of methane. However, the CO2 sorption capacity shows a sharp decrease after 70 bar. Under similar temperature and pressure condition CO2: CH4 ratio increases with increasing pressure up to a pressure of 70 bar and then shows a sharp decrease. Petrographic study shows that vitrinite is the most dominant maceral group. Unlike clay minerals, organic matter shows a robust control over gas sorption in the studied shale samples. Sorption potential of carbon dioxide and methane increases positively with vitrinite content but, decreases with rising mineral matter and thermal maturity. Vitrinite content also controls the CO2: CH4 ratio, and the ratio bears no relation with thermal maturity. Micropores, rather than mesopores, mostly influence the gas sorption.