Effect of rock matrix permeability on fluid-injection-induced hydro-mechanical coupled behaviour of rock discontinuities

Summary

Abstract

Fluid injection into rock masses and the associated hydro-mechanical coupled behaviour are of paramount importance in deep underground engineering projects. Although numerous studies have been undertaken to investigate the fluid behaviour in rock discontinuities, most of the studies neglect the possibility of fluid leakage to the surrounding rock matrix, which may lead to an overestimation of the risk for instability leading to shear failure and/or induced seismicity. To evaluate its influence, the present study conducted fluid injection experiment and simulation whilst varying the permeability of the rock matrix surrounding the discontinuity. The experiment employed rectangular parallelepiped rock specimens with a through-going discontinuity inclined at 60°. The experiment indicated the significant effect of the rock matrix permeability on the increase rate and peak value of the pore pressure in the discontinuity as well as the occurrence of slip. Specifically, when fluid is injected at the rate of 0.03 mL/min, slip occurs only for the intact specimen, whilst slip took place for the specimen heated at 500 ℃ at an injection rate of 0.3 mL/min, implying the existence of a threshold above which the discontinuity stability drastically decreases. This hydro-mechanical coupled behaviour was further confirmed through the numerical simulation, demonstrating the non-linear evolution of the pore pressure and shear displacement. This study provides important insights into the role of the rock matrix in the hydro-mechanical coupled behaviour of rock discontinuities and contributes to the improvement of evaluation methods for the risk of induced seismicity.