Modeling the hydromechanical responses of sandwich structure faults during underground fluid injection

Injection or withdrawal of fluid at depth may trigger felt seismicity. At a site with natural faults or artificial fracturing cracks, understanding how to avoid triggering felt earthquakes plays a crucial role in the success of underground anthropogenic activities, such as subsurface energy storage and CO₂ geological sequestration. With the application of a hydromechanical coupling finite element model, we examine the potential of injection-induced pore pressure change and the responding fault slippage along the fault surface in two injection scenarios: multi-layer injection (MLI) and single-layer injection (SLI). The results indicate that fault slippage buildup is mainly located along the portion of the fault intersecting the reservoir. MLI leads to a relatively lower level of induced seismicity compared with SLI. In the fault zone, the external zone and core of the fault exert a connection effect and barrier effect on the hydraulic connectivity, respectively. After 20 years of injection, the pore pressure in the core presents a smaller change than that in the internal and external zones. Furthermore, an uncertainty analysis was performed to evaluate the statistical significance of permeability in the caprock, reservoir, and fault (core, internal zone, and external zone), and only the permeability in the reservoir significantly affects the level of fault slippage based on the fault zone hydromechanics associated with subsurface energy storage and CO₂ geological sequestration.