Study on seepage characteristics of 3D fracture network of rock mass based on equivalent pipe network

The lower rock mass is filled with fissures,which is of great significance for oil and gas resource development,groundwater resource utilization,pollutant migration,carbon dioxide storage,and nuclear waste geological disposal. Nuclear waste repository has extremely strict requirements for geological environment. However,the natural fractures in rock mass may be the potential channel for nuclide migration. Therefore,it is necessary to evaluate the impact of fractures on rock permeability. In general,there are three methods to analyze the fracture connectivity and seepage characteristics through the disc-shaped fracture network seepage model:the finite element method; the discrete element method; the DFN is converted into a plane crack with similar network pipes with effective connectivity and hydraulic characteristics are also called "pipe network models". In this paper,the granite outcrop of Xinchang high-level radioactive waste repository in China is studied. Variable-section equivalent pipes to simulate the permeability characteristics of rock fractures is used. The complex fissure disk network is equivalent to a pipe network model. An equivalent path permeability coefficient is proposed to objectively study the permeability characteristics of the seepage path of the fractured rock mass. When the fresh fractures are exposed to a certain depth by cutting the surface rocks,some fractures in the strike range belong to open type,others belong to the cemented closed type. The calculation results of equivalent path permeability coefficient under different conditions show that the average values of equivalent path permeability coefficient are 10-7 m/s and 10-3 m/s respectively when the fracture is cemented and partially cemented. On this basis,the representative unit volume(REV) of fissured rock mass in the study area is determined to be about 25 m. Through the matrix in the rotation model,the spatial permeability tensor(including permeability value and main direction) of the area is obtained,and the predicted results are within the range of borehole data,which can provide reference for related projects.