Numerical simulation of shale gas reservoirs considering discrete fracture network using a coupled multiple transport mechanisms and geomechanics model

Shale has typical characteristics of multi-scale space for gas storage and migration. It is suitable for description by a triple medium model composed of double continuous medium and discrete fracture model, and need to consider multiple migration mechanisms. Besides, during gas production, effective stress and desorption will cause the deformation of shale, leading to the dynamic changes in porosity and permeability. In this paper, a fully coupled model was established to evaluate the shale gas production dynamics, including the governing equation of shale deformation and the governing equation of gas migration in the matrix, micro-fractures, and hydraulic fractures, respectively. All processes are coupled by the dynamic models of porosity and permeability for matrix and micro-fractures, and by the gas exchange between different systems. The fully coupled model was solved by the finite element method through the PDE solver software named COMSOL Multiphysics. The rationality of this fully coupled model is verified by the production data of Barnett shale. The validated model was applied to study the effects of adsorption and geomechanics parameters on dynamic changes of porosity and permeability, and gas production. Moreover, the impact of primary fracture geometry and secondary fracture distribution parameters on production were also analyzed. Simulation results show that the adsorption parameters have essential effects on both gas flow and production; that the geomechanics parameters have important influences on the evolution of porosity and permeability; that optimizing the distribution and development of hydraulic fracture and secondary fracture is beneficial to the efficient development of shale gas.