Competitive Adsorption of Methane and Ethane on Organic-Rich Shale at Pressure Up to 30 Mpa : Experimental Results and Geological Implications

Methane and ethane are the two most abundant hydrocarbon species in shale gas, while the adsorption behavior of their binary mixture in shale is still not well understood. In this study, adsorption of methane, ethane, and their binary mixture on twelve dried shale samples collected from the Yanchang formation in the Ordos Basin were measured at 60 °C with pressure up to 30 MPa. The Langmuir-based and supercritical Dubinin–Radushkevich (SDR)-based excess adsorption models combined with the ideal adsorbed solution theory (IAST) and Extended Langmuir (EL) model were used to describe the raw adsorption isotherms. The IAST and EL model-fitted binary gas excess adsorption were both in good agreement with experimental results. The adsorption capacity of pure ethane was about 1.6 times larger than which for pure methane, while the Langmuir pressure of ethane was significantly lower. For both pure and binary gas adsorption capacities, organic matter was the primary contributor, meanwhile clay minerals provided minor contributions. Adsorption selectivity of ethane over methane ranged from 3.49 to 8.85 and was positively correlated with total organic carbon (TOC) but weakly negatively correlated with minerals, indicating a higher adsorption selectivity in organic matter than in minerals. The estimated gas-in-place (GIP) for binary gas ranged from 4.65 cm3/g to 7.70 cm3/g and was about 30% larger than the value for pure methane. A two-stage trend of falling followed by rising in relationship between GIP for binary gas and TOC was observed. This occurrence was attributed to the dual effects of residual bitumen on GIP. This study provides better insight in the storage mechanisms of shale gas and sheds light on the reliable estimation of shale gas potential