Simulation study on permeability characteristics of shale reservoirs considering multi-scale rough fracture network

The actual reservoir fracture network has multi-scale characteristics, is the main channel for gas flow in shale reservoirs, and determines the seepage capacity of the reservoir. Therefore, it is of great significance to conduct research on reservoir permeability characteristics considering multi-scale features of fractures. In this paper, a multi-scale fracture network generation algorithm is proposed. Then, considering millimeter-, centimeter-, and meter-scale multi-scale fractures in shale reservoirs, a multi-scale fracture network seepage simulation model is established to analyze the seepage characteristics of the fracture network under the influence of multi-scale features. The results show that both the fracture network structure and modeling parameters significantly affect the gas flow. The increase in mesoscale fractures has a more significant effect on the seepage capacity of the multiscale fracture network than that of small-scale fractures. The gas flux increases by about 1.5 times when the number of mesoscale fractures increases by a factor of 1, while the gas flux increases only by a factor of 0.1 when the number of small-scale fractures increases by a factor of 2. The increase in mesoscale and small-scale fractures enhances the topological connectivity of the multiscale fracture network, and the gas fluxes subsequently increase to —four to six times of the original ones. When the large-scale fractures are connected to the gas outlet boundary, the seepage performance of the multiscale fracture network is greatly improved. When the large-scale fractures are connected to the gas inlet boundary only, the seepage performance of the multiscale fracture network is not significantly improved or even decreased. Compared to when the boundary is unconnected, the gas flux of the multiscale fracture network after connecting the boundary is enhanced by about four times.