Pore-scale geometry effects on gas permeability in shale

One main challenge for prediction of gas permeability in shale is the geometrical complexity at pore scale of shale. Shale structure is highly anisotropic and heterogeneous, which cannot be well described by packing of spheres or bundle of tubes. Besides, there are abundant nanoscale pores in shale so that the Knudsen number of gas flow is high, leading to failure of the conventional Darcy's law. Aiming at these challenges, we have studied the influences from pore-scale anisotropy and heterogeneity of shale microstructures on gas permeability including the high Knudsen number effect (or Klinkenberg effect for Darcy scale). First, a geometry-based method is proposed to quantify the pore-scale anisotropy and heterogeneity of shale. Then we reconstruct three-dimensional shale structures by the random generation-growth algorithm and use the lattice Boltzmann method to predict its permeability. To reveal the high Knudsen number effect, both intrinsic permeability and apparent permeability are evaluated. Our results suggest that the intrinsic permeability increases with the anisotropy of pore geometry in parallel direction to the bed, while decreases in perpendicular direction. The slip factor for Klinkenberg correction also exhibits anisotropy when high Knudsen effect is considered. On the other hand, the heterogeneity of pore distribution may have positive influences on intrinsic permeability for given porosities.