Physics and modeling of gas flow in shale reservoirs

Gas transport in nanopores is a combination of several flow mechanisms: gas desorption, advection, molecular diffusion, and Knudsen / slip flow. The gas storage in the reservoir pores includes compressed free gas in larger pores and adsorbed gas on the organic and inorganic nanopore walls. While the conventional approach of modeling gas flow in unconventional reservoirs does not capture the relevant physics accurately, it has been successfully used by engineers to match reservoir history and predict future performance. Both single-porosity and dual-porosity approaches have been used in such efforts. For this research, both a dual-porosity and a triple-porosity finite-difference (FD) model for single-phase flow of a multicomponent gas were developed which include advective, diffusive, and Knudsen flow mechanisms. Numerical solution for a dual-porosity, single-component model with advective and Knudsen flow contribution was validated using a new analytical unsteady-state solution. The multicomponent models account for material balance for all components as well as for the molecular interaction during flow. The purpose is to evaluate chromatographic separation of components during the reservoir life. In this paper, however, only the single-component results are presented. The current results indicate that the models' performance are consistent with field observations, and favor the presence of inter-connected microfractures in the drainage volume surrounding horizontal production wells.