How transport properties of a shale gas reservoir change during extraction: A strain-dependent triple-porosity model

Abstract In this study, we consider a shale gas reservoir as a triple-porosity media consisting of organic matrix, inorganic matrix, and fractures. A strain-dependent triple-porosity model is established by accurately expressing the deformation of each medium to reveal the change in the transport properties of a shale gas reservoir during production and to predict the reservoir recovery. The deformation of each medium is based on a combination of mechanical deformation and desorption-induced shrinkage behavior, and fracture aperture as well as the difference of effective stress in each medium were considered. We find that the transport properties of shale gas reservoirs are related to the porosity and diffusion coefficient of the organic matrix, and the porosity and permeability of the fracture system and inorganic matrix. The triple-porosity model is then incorporated into the governing equations for gas flow. The finite element method is used to solve the governing equations. By comparing the simulation result with field data, analytical solution, and previous simulation result, we are able to verify the accuracy of our model. Then, we analyze and reveal in detail how the transport properties of shale gas reservoirs change during extraction through the combined evolution of gas pressure, volumetric strain, and strain induced by the adsorption/desorption of gas in the organic matrix. Finally, we conduct sensitive analysis focusing on how the initial transport properties affect their evolutions. We find that the relationship between mechanical deformation and strain induced by adsorption/desorption is not always competitive during gas extraction.