Bounding the localized to ductile transition in porous rocks: implications for geo-reservoirs

Porous rocks have long been the focus of intense research driven by their importance in our society as host to our most essential resources (oil, gas, water, geothermal energy, etc), yet their rheology remains poorly understood. With increasing depth, porous rocks transition from being brittle (dilational deformation leading to localized failure) to being ductile (homogeneous compactive flow, no failure). The transition between these two regimes is crucial for reservoir engineering. In fact, brittle, localized deformation of porous rocks is generally accompanied by permeability enhancement but also induced seismicity, while ductile deformation leads to aseismic permeability reduction. Decades of experimental work has shown that this transition is not sharp but rather spans a wide P, T domain, but to this day, no clear boundaries have been established. Here, we subjected pre-faulted samples of volvic trachyandesite to increasing confining pressure, deforming the samples each pressure step and recording strain partitioning between off-fault bulk deformation and on-fault slip. For the first time, we show that the localized-to-ductile transition (LDT) in porous rocks is bound by the stress criterion σy < σf < σflow. Additionally we show that, in this regime, once both fault sliding and bulk flow are active, the partitioning of strain between the two can be described by the empirical ratio: (σf – σy)/(σflow – σy). Finally, we propose a critical stress representation that takes into account the existence of the LDT in porous rocks.