Inhomogeneous fault stability due to fluid injection

Forecasting and mitigating induced seismicity requires understanding of the underlying physical processes. Poromechanical and thermal effects on stresses and shear slip stress transfer play a non-negligible role that has challenged the classical interpretation in which induced seismicity is caused exclusively by pressure buildup (De Simone et al., 2017). In this contribution, we analyze how the stress changes induced as a result of fluid injection affect fault stability. We perform fully coupled hydro-mechanical simulations of fluid injection into a saline aquifer bounded above and below by low-permeable clay-rich rock and intersected by a low-permeable steep fault. Simulation results show that maintaining a constant injection rate leads to a progressive reservoir pressurization on the side of the fault where injection takes place (Fig. 1a). Given the low-permeability of the fault core, pressure buildup is negligible on the other side of the fault. These pore pressure changes cause strong variations in the total stresses controlled by rock stiffness around the fault. Deviatoric stress changes are controlled by stress balance from the two sides of the fault: the upper part of the reservoir, juxtaposed to the stiffer reservoir on the right, has a lower increase in the deviatoric stress than the lower part, which is juxtaposed to the more compliant caprock. This implies increased fault stability in the upper part and decreased fault stability in the lower part (Fig, 1d). As highlighted by our results, fault stability is: i) non-homogeneous within the whole fault and ii) controlled by poromechanical stress changes as much as by pressure buildup.V.V. would like to acknowledge funding from the European Research Council (ERC) under European Union’s Horizon 2020 research and innovation programme (grant agreement No 801809), and CSIC through the Intramural project 201730I100Peer reviewe