Pore-scale prediction of transport properties in reconstructed nanostructures of organic matter in shales

Size, morphology and distributions of pores in organic matter of shale matrix are discussed based on high resolution images from experiments in the literature. 250 nanoscale structures of the organic matter are then reconstructed by randomly placing pore spheres with different diameters and overlap tolerances. Effects of porosity, the mean diameter and the overlap tolerance on void space connectivity and pore size distribution are studied. Furthermore, a pore-scale model based on the lattice Boltzmann method developed in a previous study is used to predict the Knudsen diffusivity and permeability of the reconstructed organic matter. The simulation results show that the mean pore diameter and overlap tolerance significantly affect the transport properties. The predicted Knudsen effective diffusivity is compared with Bruggeman equation and it is found that this equation underestimates the tortuosity. A modified Bruggeman equation is proposed based on the simulation results. The predicted intrinsic permeability is in acceptable agreement with Kozeny-Carman (KC) equation. In addition, the apparent permeability is determined based on Knudsen diffusivity and intrinsic permeability predicted. The apparent permeability is compared with that obtained with various correlations in the literature. Knudsen's correlations match best with our numerical results and are recommended for calculating the apparent permeability.