A poroelastic model for evolution of fractured reservoirs during gas production

Coupled processes between matrix deformation and fluid flow in fractured reservoirs such as gas shale are important for predicting fracture permeability change and reservoir production behavior. In contrast to sandstone reservoirs, the coupling between gas desorption and rock matrix/fracture deformation must be accounted for in poroelastic response of gas shale. For this purpose, a constitutive relation is developed and used in this work. The continuity equation of dry gas considers the mass conservation including both free and absorbed phases. The absorbed gas content and the sorption-induced volumetric strain are described through a Langmiur-type equation. A porosity model is developed and utilized in the governing equations, and a discrete system of fractures is included in flow and rock deformation analysis. The resulting field equations are solved using a finite element model based on the dual permeability method (DPM). The model is then used to investigate coupled desorption/diffusion-rock deformation phenomena including fracture deformation (both normal and shear deformation with dilation), and its impact on gas flow in naturally fractured reservoir. Simulation results show that the time evolution of gas pressure, effective stresses, fracture aperture, and permeability are strongly affected by gas desorption during production, especially in the near-wellbore region. Gas desorption retards the influence of the effective stress increase associated with pore pressure reduction during production. The combination of in-situ stress condition and gas desorption mechanism governs the fracture deformation behavior in fractured gas shale reservoirs.