We performed triaxial deformation experiments in Purbeck limestone (13.8% average porosity) across the brittle-ductile transition and monitored the evolution of permeability and wave velocities as a function of strain. In the brittle regime, the rock yields in dilation. In the ductile regime, the rock first yields in compaction and then undergoes net dilation at some critical level of strain. The permeability increases after failure in the brittle regime and decreases with increasing compaction in the ductile regime. The wave velocities decrease with increasing strain, and the material becomes transversely isotropic. At axial strains of the order of 5%, the anisotropy parameters (from Thomsen, 1986) are around ε ≈ −0.2 and δ ≈ decrease with increasing axial strain beyond the yield point−0.3. Under hydrostatic conditions, the rock also yields in compaction. The hydrostatic yield point is not marked by any significant drop or increase in wave velocity during loading, but wave velocities decrease (and therefore crack density increases) significantly upon unloading. In all our tests, the permeability change is proportional to the initial porosity change until the point of net dilation is reached. The strain at that point also acts as a scaling factor for the relative drop in P and S wave velocities during deformation. All our experimental data point to a disconnection between the evolution of permeability, porosity, and wave velocities during deformation in the ductile regime: permeability is controlled by a fraction of the micropore network, while wave velocities are mostly influenced by microcracks that do not contribute significantly to either the total rock porosity or fluid flow properties.
Bibliographical noteFunding Information:
This work was supported by the UK Natural Environment Research Council through grant NE/K009656/1 to N. B. Emmanuel C. David is gratefully acknowledged for his help in using the differential effective medium scheme. We thank Harry Lisabeth and Marco Scuderi for their review comments that helped clarify the paper. The data used in this paper are freely available from the UK National Geoscience Data Centre (http://www.bgs.ac.uk/services/ngdc/accessions/index.html#item110229) or upon request to the corresponding author.