Kinetic and Thermodynamic Characteristics of Supercritical Methane Adsorption on Organic-Rich Shale: A Case Study of Ordovician Wufeng and Silurian Longmaxi Gas Shale

The kinetic and thermodynamic characteristics of supercritical methane adsorption are beneficial to understanding the methane adsorption mechanism in gas reservoirs, which is poorly documented. In this work, nitrogen (N-2) physical adsorption and methane isothermal adsorption are used to analyze the pore structure and methane adsorption characteristics of the Wufeng and Longmaxi shales. The energy alterations of methane adsorption process for gas shale were analyzed from the aspects of adsorption thermodynamics and kinetics. Results have shown that high temperature reduces the adsorption capacity of methane molecules on the pore surface, but high pressure increases the interaction force between methane molecules and the solid surface, thereby promoting adsorption on shale. Regardless of the temperature effect, the adsorption potential decreases and becomes stable as pressure increases, and adsorption decay rate curves generally show an S-shape. In the early period of methane adsorption, methane molecules occupy high-energy sites, and surface free energy begins to decrease and then stabilize, indicating a decrease in adsorption potential. As pressure increasing, the actual adsorption heat shows a decreasing trend on shale. In the low-pressure stage (<10 MPa), isosteric adsorption heat is relatively close under different temperatures. In the high-pressure stage (>10 MPa), adsorption heat will cross with increasing pressure, showing that higher temperatures can change methane molecules' sensitivity on the substrate surface. As the burial depth of shale increases, adsorption ability increases first and then decreases. When the burial depth of shale is smaller than approximately 1000 m the pressure has a positive effect on the adsorption capacity. When the depth of shale is between 1000 and 3500 m, the pressure and temperature have comparable effects on the adsorption of shale. In contrast, when the depth is greater than 3500 m, the negative effect of temperature exceeds the influence of pressure on adsorption ability, which is not conducive to the adsorption of shale gas.