Pore scale study of gas sorption hysteresis in shale nanopores using lattice Boltzmann method

Abstract

Tremendous amount of gas can be adsorbed in shale nanopores as a result of the high surface-to-volume ratio, and capillary condensation might occur in mesopores (2–50 nm) for subcritical gases, leading to sorption hysteresis. However, the sorption (adsorption and desorption) behavior in different pore systems in shale is not very well understood. Herein we use a D3Q19 lattice Boltzmann model (LBM) that incorporates the mesoscopic fluid-fluid and solid-fluid interactions to study sorption hysteresis in nanopores. The model is developed based on our previous work using a D2Q9 LBM, and is calibrated to lattice density functional theory (LDFT). Improved thermodynamic consistency is observed using exact difference method and tunable interparticle forcing scheme. Sorption simulation in an open and semi-closed slit pore shows different extent of hysteresis, which is in good agreement with Grand Canonical Monte Carlo simulation (GCMC) results. We then reconstruct two types of nanopore systems that are commonly observed in shale, namely the interparticle and intraparticle pores, and study and compare nitrogen sorption hysteresis. The spatial distribution of the condensed nitrogen is visualized and two distinct types of sorption hysteresis loops based on the classification by International Union of Pure and Applied Chemistry (IUPAC) are observed, which indicates that the experimentally measured nitrogen sorption curves can be used to distinguish the dominant pore type in shale. The 3D LBM developed in this study can be used to explore a wide variety of thermodynamic processes that happen at nanoscale, and it can also be incorporated with hydrodynamics to study coupled problems of fluid adsorption/transport in shale.