CH4 adsorption and diffusion in shale pores from molecular simulation and a model for CH4 adsorption in shale matrix

The adsorption and diffusion behaviors of CH4 in montmorillonite (MMT) slit pores were investigated. Generally, the pore space can be divided into three parts namely inaccessible zone, adsorption zone and free zone. The length of inaccessible zone was found to be independent of pore size and pressure. A critical pore diameter of 2 nm was obtained to investigate the characteristics of adsorption and free zones. When the pore diameters <2 nm: there was no free zone. When the pore diameters >= 2 nm: the length of adsorption zone was independent of pore diameter and pressure; the average density of adsorption zone didn't change with pore diameter. The critical pore diameter of 2 nm was also explored during gas diffusion analysis, and when the pore diameters <= 2 nm, the influence of the pore size on in-plane self-diffusion is more significant. Based on these findings, a model for CH4 adsorption in shale matrix was proposed. It indicated that the adsorption quantity obtained by our model agrees well with experiments. From molecular simulations, much more detailed information can be found compared with experiments, however, the most difficult problem is the concise linkage between simulation and experimental results. Our work provides a concise and intuitive approach to solve this problem. Our model can predict not only adsorption quantity which can be obtained by experiments, but also space distribution properties such as gas number distribution, adsorption zone and free zone distributions, gas density distribution, proportion of adsorbed and free gas, and et al. These details may help to estimate gas storage potential, select gas sites and design optimum recovery protocols. (C) 2019 Elsevier Ltd. All rights reserved.