Effects of light hydrocarbons and extractable organic matter on the methane sorption capacity of shales

Abstract

High-pressure methane (CH4) sorption measurements at 30°C and up to 20 MPa have been conducted on four carbonaceous shales with total organic carbon contents ranging from 8.52 to 11.73 wt. % and different maturities (0.53%–1.45% vitrinite reflectance). Excess sorption isotherms were measured on all four samples in the “dry,” “solvent-extracted,” “hexane-equilibrated,” and “moisture-equilibrated” states. The isotherms of all samples, irrespective of thermal maturity, showed consistent effects of extraction, preadsorbed hexane, and moisture on methane sorption capacity. Removal of bitumen by solvent extraction generally increases the methane sorption capacity of the shales (at 1 MPa) by up to 63% compared to the dry state, most likely due to enhancing the accessibility of sorption sites. Moisture consistently reduces methane sorption capacity by approximately 23% to 48% as compared to the dry (unextracted) state. The effect of preadsorbed hexane on methane sorption capacity is strongly pressure dependent: At low pressures, its influence is negative and at high pressures positive. The significant increase of sorption capacity at high pressures is attributed to the almost linear increase of methane solubility in hexane with pressure, whereas methane adsorption on the organic and mineral surfaces reaches saturation. The preadsorbed hexane reduces methane sorption capacity by approximately 20% to 40% if solubility effects are excluded. In view of these findings, the methane adsorption capacity of shales at the “wet gas” maturity level should be reconsidered. Our observations contribute to a better understanding of natural gas occurrence and producibility in liquid-bearing unconventional petroleum systems and a more accurate estimation of gas-in-place of shale gas reservoirs.