Multi-step triaxial compressive creep behaviour and induced gas permeability change of clay-rich rock

Abstract

Clay-rich rocks are potential geological barriers for geological repositories of radioactive waste. The long-term safety of a repository requires a full characterisation of the time-dependent thermal hydro-mechanical behaviour of the clay-rich rocks. This paper investigates the creep deformation and induced permeability change of the Callovo-Oxfordian (COx) claystone by a series of multi-step triaxial compressive creep tests. The effects of confining pressure and deviatoric stress are both studied. Three values of effective confining pressure are 2 MPa, 6 MPa and 12 MPa, according to the in situ stress estimation at the Bure underground research laboratory in France. In each test, the deviatoric stress is progressively loaded in five steps so that the primary, secondary and tertiary stages of creep can be evaluated. In the last loading step, the tested sample may exhibit a creep failure process. During the creep tests, the gas permeability of the partially saturated COx claystone is measured by a transient pulse decay method in concurrence with the material creep deformation. The results obtained show that both the confining pressure and deviatoric stress have important effects on the time-dependent deformation and permeability evolution of the claystone. Analysis of creep strain rates during the last creep step indicates that there exists a threshold for the creep-induced failure of the COx claystone. The permeability evolution in the triaxial creep process is closely related to the microstructural evolution of rock, including the closure of pre-existing micro-cracks, the compaction voids and the creation, propagation and coalescence of micro-cracks.