Spatio-temporal evolution of permeability during quasi-static fault growth in granite

In tight crystalline basement rocks such as granite, faults are known to be substantially more hydraulically conductive than the rock matrix. However, most of our knowledge of rock permeability in the laboratory and in the field relies on indirect inference, static measurements, or before/after datasets, and the spatio-temporal evolution of the permeability field during faulting remains unknown. Specifically, we would like to determine at which stage of the faulting process does permeability change most, and the degree of permeability heterogeneity along shear faults.

We conducted a series of triaxial deformation experiments in initially intact Westerly granite, where faulting was stabilised by monitoring the acoustic emission rate. At many stages from pre- to post-failure states, we paused deformation and imposed macroscopic fluid flow to characterise the overall permeability of the material. In addition, we measured the pore pressure distribution in the sample, and estimated apparent permeability at different locations along the fault, from the intact ligaments to damaged regions. We monitored the propagation of the macroscopic shear fault by locating acoustic emissions.

We find that average permeability increases dramatically (by around 3 orders of magnitude) near the peak stress, where the fault (as seen by acoustic emission locations) is not yet through-going. Post-peak evolution shows a more gradual increase in overall permeability, with local heterogeneities remaining along the fault, primarily controlled by small-scale fault geometry and the existence of undamaged regions as imaged by acoustic emission locations.

We conclude that permeability change and fluid flow focussing occurs at very early stages of faulting, and do not require substantial slip. Our results highlight the key role of fault geometry in the fine-scale permeability structure of basement rocks.