Lattice Boltzmann modeling of gaseous microflow in shale nanoporous media

Gaseous microflow in nanoporous media of ultratight shale strata was reexamined. Hundreds of nanoscale pore network samples of the Wufeng and Longmaxi shale matrix were reconstructed by ultra-precise FIB-SEM experiments and the Quartet Structure Generation Set (QSGS) method. An OpenMP-based, high-performance, parallelized MRT-LBM model to predict shale gas flow in reconstructed nanoporous media was developed accordingly. The MRT-LBM model was validated against the analytical solutions, molecular dynamics simulations, and discrete velocity method. The MRT-LBM accurately predicted gaseous microflow in the continuum, slip, and early transition flow regimes. Using this solver, extensive pore-scale simulations of the gas transport in nanoscale pore networks were performed to study the effects of pore heterogeneity and slippage effects on the apparent permeability of shale gas. It has been known that gas slippage and Knudsen diffusion dramatically improve gas flow in a single capillary. Hence, permeability models based on a bundle of capillaries followed the increasing gas flow predictions. However, our modeling results revealed that gas slippage is suppressed by pore tortuosity, wall curvature, and surface roughness. The proportionality factor of the Klinkenberg model decreases with increasing tortuosity and decreasing porosity. We develop an apparent permeability model appropriate for low-porosity and large-tortuosity shale nanoporous media, with a corrected proportionality factor.