High-Pressure Methane Adsorption and Desorption in Shales from the Sichuan Basin, Southwestern China

Adsorbed methane is an important component of shale gas. Methane desorption, commonly ignored, can affect gas production. Gaining a better understanding of methane sorption in shale is important for evaluating gas reserve and predicting gas production. This paper presents the results from a study of the methane adsorption and desorption of four shale samples from the Sichuan Basin, southwestern China. Adsorption experiments were performed on dry samples at temperatures of 40-100 degrees C to a maximum equilibrium pressure of 30 MPa. The desorption measurements were conducted at a temperature of 60 degrees C. Slight sorption hysteresis was observed over the entire pressure range for every sample. Both the three-parameter Langmuir model and the supercritical Dubinin-Radushkevich (sD-R) model were applied to describe the adsorption data. Model-fitted methane maximum absolute adsorption capacities (n(0)) ranged from 1.83 to 3.04 mg/g at 40 degrees C, and n(0) increased linearly with increasing total organic carbon content. The decrease in n(0) with increasing temperature can also be represented by a linear model. The isosteric heat of adsorption ranged from 16.54 to 22.03 kJ/mol. A possible mechanism for sorption hysteresis is the irreversible swelling of micropore throats caused by methane adsorption. The swelling reduces the size of the pore throats and makes them smaller than the kinetic diameter of methane molecules so that when the pressure is reduced, more energy is required for the methane molecules in the pores to escape. Thus, fewer methane molecules can desorb from the material than were previously adsorbed at the same pressure.