Laboratory investigation of hydraulic properties of deformable rock samples subjected to different loading paths

Abstract

This study experimentally investigated the internal fracture geometries and hydraulic properties of deformable rock samples subjected to various loading paths. Three loading paths, uniaxial compression, conventional triaxial compression, and triaxial prepeak unloading, were administered to granite samples. A high-precision X-ray microfocus-computed-tomography scanning system was adopted to explore the internal failure modes of the deformable samples, and a series of flow tests with different hydraulic pressures and confining stresses were then performed. The results show that the samples after uniaxial compression exhibit a typical splitting failure mode; however, for conventional triaxial compression and prepeak unloading, the samples generally show tensile-shear failure modes. The relationship between the flow rate and pressure gradient of the sample after uniaxial compression can be best described using the Forchheimer law. Both the linear and nonlinear coefficients in the Forchheimer law increase with increasing confining stress. As the confining stress increases from 4 to 20 MPa, the critical hydraulic gradient that quantifies the onset of nonlinear flow increases from 5.33 to 56.74, but the transmissivity decreases due to the fracture closure. For water flow through the samples after conventional triaxial compression and prepeak unloading, two representative types of nonlinear flow behaviors induced by inertial effects and fracture dilation were observed. Different loading paths lead to different failure mechanisms and thus different fluid-flow responses, and the samples subjected to prepeak unloading with a high confining pressure possess a more significant flow capacity.