Abstract
The apparent permeability function in kerogen is developed by combining effects of viscous flow, Knudsen diffusion, and surface diffusion, in which the surface diffusion is defined as the transport of adsorbed gas due to the difference of adsorbed gas concentrations. The concentration of the adsorbed gas is quantified by the monolayer adsorption coverage and the specific surface area in shale. Weighting coefficients of viscous flow and Knudsen diffusion are determined based on the probabilities of collisions frequency between gas molecules and between gas molecules and pore walls. Analyzing the effect of surface diffusion, we found that the mass flux contribution of surface diffusion was dependent on many parameters that include pressure, temperature, pore size, specific surface area, etc. The apparent permeability function is further incorporated into the generalized lattice Boltzmann method (LBM) for porous media. A 3D nanometer-scale kerogen digital rock including macro-pores and larger-scale digital rocks including mineralogical components are reconstructed based on the focused ion beam-scanning electron microscopy (FIB-SEM) and Nano-CT experiments, respectively. The kerogen structures (1.53μm3) from FIB-SEM experiments distinguish macro-pores (>10nm) and kerogen solids. Kerogen solid is permeable as micro/meso-pores (1-10nm) exist in nature but are lost by the FIB-SEM experiment. The apparent permeability function is assigned locally on permeable kerogen solids. In Nano-CT digital rocks (413μm3), 4 different mineralogical components have been differentiated. Rock/fluid properties of shales in nanometer scale and micrometer scale are obtained by performing LBM simulations on digital rocks. The relations of methane permeability in kerogen with pressure, temperature, average pore diameter, and specific surface area have been investigated for the FIB-SEM digital rock. In addition, we have investigated the effects of organic contents on methane permeability in shale by comparing the simulation results of 3 different Nano-CT digital rocks.
