Brittle faulting induced by pore fluid pressure increase in ductile rock mass

Abstract:

Within the upper crust, deformations are primarily brittle, accommodated by fracturing and friction along localised faults. At greater depth, deformations are diffuse (i.e. ductile) and accommodated through crystal plasticity, diffusion mass transfer or cataclastic flow. One of the main parameters that controls this transition is the pore fluid pressure (P_f), which is expected to reduce the effective stresses and favour brittle deformations. Indeed, P_f can rise due to magma build-up in volcanoes, fluid release from mineral phase change or anthropogenic fluid injection.

To investigate this phenomenon, we performed triaxial deformation tests to characterize the pressure sensitivity of the brittle-ductile transition (BDT) of Tavel limestone. Then, we investigated the effect of P_f increase during the ductile deformation of samples. Three injection rates were tested: 1, 5 and 10 MPa/min. Samples were equipped with 4 strain gages and 12 piezo-electric sensors to investigate the mechanical and physical processes at hand. Our results showed that Tavel limestone exhibits BDT at room temperature and P_c – P_f = 80 MPa. The injection experiments were then performed at P_c – P_f = 110 MPa (i.e. ductile domain). A reference experiment at P_c – P_f = 110 MPa without injection showed that after a linear elastic phase, Tavel limestone deformed by compaction and exhibited a quasi-constant strain hardening rate with increasing strain.

The injection experiment showed that: 1) When P_f was increased, the sample passed immediately from compaction to dilation and strain hardening rate decreased. Ultimately, and always at P_c – P_f ≈ 70 MPa, the differential stress (Q) reached a peak value after which slow softening was observed. The strain needed to reach this point depended on the injection rate. 2) When injection stopped (at P_c – P_f = 20 MPa), a constant Q (corresponding closely to the peak Q of experiment at P_c – P_f = 20 MPa) remained. During this phase, dilation continued and seismic velocities slowly went down. 3) This phase led to a fast stress drop and sample failure through shear fracturing. For all of the experiments, the stress drop occurred when samples reached ≈ 1% of dilation. We demonstrated that brittle deformation can occur due to P_f increase in a ductile domain. However, to reach shear fracturing of the rock mass, a dilatancy threshold was needed.