Evolution of permeability during the process of shale gas extraction

The sensitivity of shale permeability to effective stress has been reported widely in the literature but the relation between them is not always consistent with the principle of effective stress. This knowledge gap defines our specific research goals: (1) to resolve the inconsistency between the evolution of shale permeability and the effective stress principle, and (2) to incorporate the evolution of shale permeability into the evaluation of shale gas production rate. In this study, we define shale blocks bounded by artificially introduced hydraulic fractures. At block scale the system comprises two components represented by inorganic and organic components. Unlike previous studies, we assume that the organic system is embedded within the inorganic system and both systems are dual porosity media. We define the shale permeability as a function of strains representing the stress transfer between fractures and matrix. These permeability-strain relations (shale permeability models) are then used to couple the governing equations of shale deformation and gas flow. The composite suite of equations are then solved by a commercial PDE solver. We use the coupled model to generate typical evolution profiles of shale permeability under stress-controlled conditions for the cases of both gas injection and gas desorption. These profiles are consistent with experimental observations as reported widely in the literature. We also use the coupled model to examine how the shale permeability evolves during the extraction of shale gas. Model results show that the evolutions of both shale permeability and gas production rate at field scale are primarily controlled by the contrasts in both the transport and deformation properties of the reservoir and the interfacial dynamics of mass and effective stress transfer between matrix and fractures.