Effects of an adsorbent accessible volume on methane adsorption on shale

An adsorbed gas amount obtained from excess adsorption isotherms is of great importance to an estimation of reservoir reserves and prediction of gas production in shale reservoirs. Generally, excess adsorption amounts are calculated by a void volume of an adsorbent measured by helium expansion tests. However, helium atoms can closely approach a pore wall and get into small pores due to their small size. Therefore, a helium-based volume is always larger than the accessible volume for a specific adsorbate. In this work, we use GCMC (Grand Canonical Monte Carlo) simulation to investigate methane adsorption on shale with pore sizes of 10, 20 and 30 angstrom at a temperature of 323 K. Five void volumes, including an apparent volume determined by helium expansion tests, a helium-probed volume, a simulated methane-based volume, an analytical methane based volume, and a physical volume, are recognized. Use of a physical volume and a helium-based volume leads to negative excess adsorption at high pressure, which is not consistent with local density profiles at such pressure. It is thus necessary to calibrate a physical volume or use a methane-based volume as a theoretical value for calculating excess isotherms. Excess isotherms determined by a calibrated volume or methane-based volume are more accurate and keep positive even at high pressure. Appropriate calculations of excess isotherms also influence an estimation of proportions of excess adsorbed and free gas amounts in shale gas reservoirs, further having a great impact on the determination of a development strategy. (C) 2020 Elsevier B.V. All rights reserved.