Competitive adsorption of methane and ethane on organic-rich shale at pressure up to 30 MPa: Experimental results and geological implications

Abstract

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 ambiguous. In this study, the adsorptions of methane, ethane, and their binary mixture on twelve dried mature lacustrine organic-rich shale samples were measured at 60 °C and variable pressures ranging from 0.001 to 30 MPa. The Langmuir and supercritical Dubinin–Radushkevich (SDR) models combined with the ideal adsorbed solution theory (IAST) and Extended Langmuir (EL) model were used to describe the raw adsorption isotherms. By contrast, the Langmuir model with consistent adsorbed phase volume can better describe the ethane excess adsorption. The IAST and EL models matched binary gas excess adsorption equally well. The adsorption capacity of pure ethane was about 1.6 times as large as pure methane, while the Langmuir pressure for ethane was lower. For the adsorption of pure and binary gas, organic matter was the primary contributor, followed by clay minerals. The adsorption selectivity of ethane over methane ranged from 3.49 to 8.85, which strongly depended on total organic carbon (TOC), and weakly negatively correlated with clay mineral content. The estimated gas-in-place (GIP) of binary gas ranged from 4.57 cm3/g to 8.08 cm3/g, which was about 30% larger than that of pure methane. A two-stage trend, first falling and then rising, was observed in relationship between binary gas GIP and TOC. This phenomenon could be attributed to the opposite effects of residual bitumen on shale porosity and gas adsorption. This study provides better insight in the storage mechanisms of shale gas and a basis for the reliable estimation of shale gas potential.