Insights into shale gas adsorption and an improved method for characterizing adsorption isotherm from molecular perspectives

•The specific multiscale inter-connected pore models of shale are proposed.•The dependences of diverse media properties and pore size on CH4 adsorption behaviors are clearly examined by GCMC simulations.•An improved method for calibrating isothermal adsorption experimental result is proposed. Gas adsorption behavior in shale complex porous media is of great importance to gas in place (GIP) evaluation and exploration. At present, the mechanism of methane adsorption has not been adequately investigated in multi-size nanoscale space via experimental and simulation methods. In this work, the specific multi-size inter-connected pore models are proposed to study the adsorption isotherm and density profiles based on the veritable pore structure characterizations of shale samples. The grand canonical Monte Carlo (GCMC) simulations are utilized to give insight into methane adsorption behavior in organic matter and inorganic clay minerals with multi-size pores at reservoir conditions. Based on the proposed models, the effects on adsorption behaviors of methane molecules under the simultaneous action of diverse media surfaces and variable pore sizes are carefully examined. In addition, the adsorbent-adsorbate interactions, which involve the cation exchange and the isosteric heat variations with media surfaces, are carefully investigated to reveal complex adsorption behaviors in shale nanopores. Considering the identified factors influencing methane adsorption and the shortcomings of experimental test, an improved method for calibrating the gas adsorption isotherms in shale is proposed from a molecular perspective, which takes into account several factors, including i) the accessible volume of methane molecules, ii) the pore size distribution in shale, and iii) the composition of organic matter and clay minerals. The GCMC simulation results are integrated into the process of calibrating experimental results in a case study with a shale sample from the Longtan Formation in the Sichuan Basin, China. The findings of this study advance the in-depth understanding of gas adsorption mechanisms in complex nanoporous structure and provide a reliable approach to evaluate the GIP in shale.