Assessing the Effects of Hydraulic Stimulation Strategies on Fault Reactivation and Induced Seismicity Using Coupled Hydro-Mechanical Models

Abstract

Hydraulic stimulation is often used to enhance the permeability of natural fracture networks in deep low-permeability reservoirs. The downside of hydraulic stimulation is that it may trigger or induce felt seismicity on pre-existing fractures or faults. Key to a successful development of enhanced geothermal systems is stimulation of rocks in order to sufficiently enhance flow rates, whilst keeping magnitudes of induced earthquakes at acceptable levels - so-called ‘soft’ hydraulic stimulation. The hydro-mechanical model in TOUGH-Flac3d was used to simulate the effect of hydraulic stimulation on the potential of fault reactivation and associated seismicity. Using the TOUGH-Flac3d simulator, we take into account the full coupling between the hydraulic and mechanical processes affecting the evolution of permeability and flow through the reservoir rocks and the mechanical response of the fault system. We model injection into a single well, at close distance to a steeply dipping fault, which is bounded by a fault damage zone and embedded in a low-permeability rock matrix. We analyze the impact of the transmissivity of the fault core on fault stresses and loading rates, both during the injection period and after shut-in of the injection well. We discuss the effect of fault properties on the evolution of induced seismicity through the rate-and-state model, both during and after hydraulic stimulation. This way, we build a further understanding of physical mechanisms which drive fault reactivation and induced seismicity, which may help in the future design of soft hydraulic stimulation strategies. The project leading to part of the results in this article received funding from the European Unions Horizon 2020 research and innovation programme under grant agreement No 691728.