Extended Finite Element Modeling of Multi-scale Flow in Fractured Shale Gas Reservoirs

Abstract

AbstractAn economic production scheme for gas shale demands a better understanding of gas flow behavior and a proper reservoir simulator. The complex fracture network and multi-scale flow channel intensify the complexity of gas flow behavior. This paper integrated an improved shale-gas transport model with the extended finite element method (XFEM) to characterize the main flow mechanisms and discrete fracture network. The gas shale was viewed as the dual permeability porous media with discrete fractures. The discrete fractures are not required to be meshed, which can be placed anywhere with given location, length, and orientation. Rock deformation is implicitly coupled with gas flow to reflect stress sensitivity of gas shale. Furthermore, the displacement and matrix pore pressure across fractures are treated as discontinuous by using enrichment approximation functions. The computer coding of proposed model was developed. Terzaghi's problem for dual permeability media was considered to validate the code. The results show that pressure field is disturbed by the discrete fractures obviously, which is compared with regular pressure field obtained from the continuous fracture model. Therefore, it is important to account for discrete fractures for the fractured porous media. One case study of shale gas reservoir was presented to improve the model application. Two patterns of fracture networks were simulated in the rectangular reservoir. It is evident that the orthogonal fracture network is an ideal pattern contrast to the oblique fractures, since the former makes the pore pressure field depleted symmetrically. Furthermore, the stimulated area is the main factor to control the pressure depletion. The results confirm that the presented model and code is capable and flexible to simulate shale gas reservoir with discrete fracture network. This work provides an alternative workflow for parameter designs of hydraulic fracture and production scheme.