Fluid Injection Induced Seismicity: A Numerical Study of Aseismic Cascade Slip Events in Fault Damage Zones

Fault damage zones consists of intricate networks of microcracks and subsidiary faults surrounding a primary fault core, which are highly sensitive to changes in stress conditions [1]. These zones are particularly susceptible to activation due to fluid injection into the main fault, a common practice in geothermal energy extraction and other sub-surface engineering applications. Once fluid is injected, elevated pore pressure reduces the effective normal stress, lowering frictional resistance and promoting the nucleation and propagation of microcracks. These microcracks weaken the rock matrix and can coa- lesce into larger fractures, reducing fault stability. The slip of individual microcracks can induce a chain reaction, triggering further slips in adjacent fractures—a process known as cascading rupture. This phenomenon can propagate through the fault damage zone, potentially leading to significant dynamic or seismic events [2]. Therefore, understanding these processes is crucial for assessing and mitigating the seismic risks associated with hydraulic stimulation and optimizing its effectiveness in enhancing reservoir permeability.

In this study, we simulate these aseismic cascade slip events within fault damage zones induced by fluid injection by constructing a 2D discrete fracture network (DFN) model across a primary fault. We leverage a fully coupled hydro-mechanical solver (PyFracX) developed at the Geo-energy Lab of EPFL, based on the boundary element method (BEM) for our simulations. By injecting fluid at a constant pressure into the main fault, we aim to elucidate how fluid pressure diffusion and stress perturbations can initiate a sequence of aseismic slip events in the damage zone. The DFN model allows us to capture the complex interactions between microcracks and to analyze how pressure changes propagate through the network. By simulating various injection scenarios, we can identify critical parameters that govern activation. The orientation relative to the initial stress state and percolation number of the DFN, along with the initial distance to failure (residual shear strength) and fluid overpressure, play crucial roles in the propagation of these cascade aseismic slip events. This study will contribute to the broader understanding of induced seismicity and provide valuable insights for mitigating the risks associated with fluid injection in geothermal and other subsurface projects. Ultimately, our findings will help develop more effective strategies for managing induced seismicity and improving the safety and efficiency of geothermal energy extraction.

References [1] D.R. Faulkner et al. “A review of recent developments concerning the structure, mechanics and fluid flow properties of fault zones”. In: Journal of Structural Geology 32.11 (2010). Fault Zones, pp. 1557–1575. issn: 0191-8141. doi: https://doi.org/10.1016/j.jsg.2010.06.009. url: https://www.sciencedirect.com/science/ article/pii/S019181411000101X. [2] Kadek Hendrawan Pa