Summary
The physics of gas transport through shale systems remains ambiguous. Although several theoretical and experimental studies have been reported, most concentrate only on the permeability of shale kerogen. Shales, however, are composed of various proportions of organic matter and inorganic minerals (e.g., calcite and clay). Inorganic pores are larger than organic pores, thus affecting apparent permeability. To accurately predict the apparent permeability of shale, we couple molecular dynamics (MD) and a pore-network model (PNM) to develop a multiscale framework for gas flow through shales. First, we use nonequilibrium MD (NEMD) to study the pressure-driven flow behavior of methane (CH4) through organic, calcite, and clay [montmorillonite (MMT)] nanopores under reservoir conditions, from which, using the slip-corrected Poiseuille equation, we propose a mass-transport model accounting for the contributions of both the adsorbed-phase fluid and bulk fluid. Then, we incorporate these formulations into a shale PNM in which the influences of shale composition and bimodal pore-size distribution (PSD) are taken into account. We also develop an analytical model for the apparent permeability of shale matrix using the bundle-of-capillaries approach. In comparison with previous methods, our proposed models highlight the effect of relatively greater pore sizes in inorganic matrices. This work provides an efficient tool for better understanding gas transport through shale systems at both molecular and pore scales.
Validation of pore network model for hygric property calculation