Permeability Evolution of Granite Gneiss During Triaxial Creep Tests

Abstract

Permeability is an important factor for seepage analysis of rock material, and a key factor in ensuring the safety of underground works. In this study, the permeability evolution of granite gneiss during triaxial creep tests was investigated. In the context of an underground oil storage cavern in China, a series of hydro-mechanical coupling creep tests were conducted on rock cores of granite gneiss at three different pore pressures to reveal the effect of pore pressure on the permeability evolution and to investigate the correlation between the permeability and volumetric strain during the creep process. During the creep tests, the permeability decreases in the initial loading phase. At all deviatoric stress levels, the permeability remains stable in the steady creep stage and increases rapidly in the accelerated creep stage. Based on the test data, the initial permeability, steady permeability and peak permeability at various stress levels are defined. The effect of pore pressure on the permeability is captured by a linear model. In addition, the relationship between permeability and volumetric strain can be described as a process divided into three phases, with different functions in each phase.

On the Relationship between Creep Strain and Permeability of Granite: Experiment and Model Investigation

“…Correspondingly, the measured permeability almost remained stable in volumetric compression, see Figure 2, once volumetric dilation occurred, the permeability went up obviously, especially in the last creep step, the permeability increased dramatically. The measured results in permeability of Liu [3] by steady-state method show the similar tendency to our cases, permeability increased slightly and then promptly following the volumetric dilation of tested specimens.…”

“…Granite has been recognized as a good option of host rock for HLW repositories, due to its high performance of strength and low permeability. While during such a long duration of nuclide decay, although the creep strain of granite is small due to its brittle performance, evidence from previous creep experiments of brittle rocks suggest that, the creep damage change the permeability of brittle rocks obviously [2][3][4][5][6]. Therefore, a good understanding of the relationship between creep strain and permeability evolution of granite, in terms of the evolution characters of creep strain, permeability and creep damage, mechanism of permeability evolution and models of the creep strain and permeability, is essential to the safety assessment of repositories.…”

“…A review on the principal experiments of the creep behaviors of granite by Hashiba [12] suggests that, previous studies on the creep behavior of granite mainly focused on the creep mechanism, long-term strength, evolution of creep strain and microcracks, damage characters, effect of confining pressure and temperature, etc. While previous studies on the influence of creep process on the permeability evolution are rare [3].…”

“…Precisely because of the microcracks initiation and propagation, which provide potential channels for fluid. Liu [3] indicated that both the creep strain and permeability of granite keep steady in the steady creep stage but increases rapidly in the accelerated creep stage. By application of acoustic emission (AE) technology, Chen [7] observed the AE hits and creep strain of granite increase slowly in the steady creep stage but dramatically in the accelerated creep stage.…”

Effects of injection parameters on heat extraction performance of supercritical CO2 in enhanced geothermal systems

“…Moreover, raising the injection pressure will increase the pore pressure (as shown in Figure ), which can reduce the effective stress of the reservoir . Meanwhile, it results in corresponding increases in porosity and permeability . So it can be seen from the stabilization stage that the mass productivity and net HER increase by 17.5 kg/s and 1.57 MW, respectively, when the injection pressure is raised from 43.7 to 51.7 MPa.…”