Quantitative assessments on fluid flow through fractures embedded in permeable host rocks: Experiments and simulations

Abstract

An accurate understanding on hydraulic behaviors of fluid flow through fractured rocks is of central importance for both underground resources exploitation and CO2 geological sequestration. Despite its importance, fluid flow through fractures surrounded by permeable matrix is rarely investigated. Here, this study aims to explore fluid flow through fractures surrounded by permeable host rocks using a combined experimental and numerical approach. In particular, core-flooding experiments were sequentially conducted on both intact and fractured cores subjected to different rock types, under varying confining stress and fracture roughness. According to the experimental results, fracture permeability decreases rapidly with increasing confining stress and fracture roughness, while matrix permeability is less affected by confining pressure, resulting in an increment in contributions of matrix flow to the overall flow with increased confining pressure. In addition, numerical simulations were performed on digital fractured rocks with a broadened range of roughness from 2.58 to 18.2, and under confining pressure ranging from 2 MPa to 53 MPa. The fluid flows within matrix and fracture were assumed to follow linear Darcy's law and nonlinear Forchheimer's law, respectively. The simulated results also show that Km/Kf increases with the increase of confining pressure, reflecting the enhancement in contributions of matrix permeability to the overall transportation. If Km/Kf = 0.1 is defined as a threshold over which matrix permeability should not be ignored, then the critical confining pressure of 18 MPa, 20 MPa, and 46 MPa is determined for mudstone, sandstone and limestone, respectively. In addition, the fluid flow through rough-walled fractures is observed to be susceptible to normal loading and fracture roughness, which directly alters flow paths within the fractures. With an increment in confining pressure and fracture roughness, the contact area between rough fracture surfaces grows, promoting flow tortuosity and generating eddies inside fracture, which consequently increases flow resistance and hence weakens fracture permeability. Ultimately, by fitting the simulated data, an empirical equation linking fracture permeability with confining stress, fracture roughness and hydraulic gradient is provided. This study provides quantitative characterizations on fluid flow through fractures accounting for permeable matrix, and thus can potentially improve the accuracy in predicting hydraulic properties of fractured rocks.