Analysis of a fully coupled gas flow and deformation process in fractured shale gas reservoirs

摘要

Coupled gas flow and solid deformation in porous media have received considerable attention because of their importance in shale gas transport. The existence of propped and un-propped fractures makes simulating the complex flow more difficult in fractured shale reservoirs. The effect of matrix deformation on production has been ignored in most previous studies. Moreover, the influence of fracture conductivity loss has not been well studied in shale reservoirs with discrete fractures. In this study, a general porosity model and a correlation of fracture permeability loss are introduced to develop a fully coupled gas flow and deformation model that contains the shale matrix and discrete fractures. The numerical hydra-mechanical model is implemented using a finite element method and it is validated using an analytical solution with the simplified condition. The coupled model with discrete fractures is verified by reducing the fracture conductivity and comparing the results with the continuous model. Then the effects of the stress-dependent permeability of the matrix and fracture on cumulative production are analyzed. The numerical results indicate that the apparent permeability of shale gas is determined by both the effects of pore-compressibility and the non-Darcy flow. The intrinsic permeability decreases as the effective stress increases, while the apparent permeability can be enhanced because of the non-Darcy flow effect as the gas pressure is depleted. The ignorance of geomechanics about the matrix will lead to an overestimated cumulative production. The loss of fracture conductivity, including both the propped and un-propped fractures, impairs the production distinctly only when the dimensionless conductivities are small. Improving the fracture conductivity can offset the negative effect of conductivity loss on the cumulative production. (C) 2015 Elsevier B.V. All rights reserved.