Supercritical Methane Sorption on Organic-Rich Shales over a Wide Temperature Range

Methane sorption on organic-rich shales over wide temperature and pressure ranges (30-120 degrees C, up to 25 MPa) is analyzed by the pore filling/potential theory. The supercritical Dubinin Astakhov (SDA) sorption model using a density term instead of the pseudosaturation vapor term is extended to methane sorption isotherms of shales with high accuracy. A modified adsorption potential approach is suggested to analyze the temperature dependence of supercritical methane sorption on shales. The temperature-invariant characteristic curves are obtained using the modified adsorption potential approach. A characteristic curve equation derived from the SDA model is provided to predict sorption isotherms at other temperatures using one isotherm. The physical meaning of the characteristic curve has been discussed, and it comprehensively reflects the available pore space for methane sorption and the affinity between methane molecules and organic matter. According to methane characteristic curves of shales and clay minerals, shales in the gas window show higher affinity than shales in the oil window and clay minerals, though the clay minerals may provide comparable adsorbed volume. The adsorption characteristic energy shows a parabola-like shape with a minimum of approximately R-eq = 1.1%, which is related to the evolution of the porosity of the shales. This study advanced the fundamental understanding of the dynamic process of methane sorption on shales.