Impact of variable permeability in fault networks on fluid-induced seismicity dynamics

The connection between fluid pressure, reservoir permeability evolution, slow-slip events, and the triggering of larger earthquakes remains a crucial but unresolved issue in the study of fluid-induced seismicity. Understanding these interactions is essential for seismic hazard mitigation and optimizing subsurface fluid injection productivity.

Discrete Fracture Networks (DFNs) are commonly used to study hydraulic diffusion and seismic activity within fault systems. However, traditional DFN models often rely on quasi-static assumptions and a simple Mohr-Coulomb criteria for earthquake triggering. These limitations hinder their ability to capture dynamic phenomena, such as self-propagating slow-slip events, and they provide little insights into the earthquake dynamics.

This study  addresses these gaps by developing a 2D DFN model capable of simulating both fluid-induced slow-slip events and the potential for earthquake triggering. The model integrates hydraulic diffusion and slip processes governed by rate-and-state friction across several interacting faults within an impermeable, elastic rock matrix. A key innovation of this model is the dynamic evolution of fault permeability, which depends on normal traction changes and accumulated slip, consistent with laboratory and in-situ experiments.

The model was applied to two scenarios, both with and without permeability evolution: (1) fluid injection along a primary rough, rate-strengthening fault, where slow slip events occur and subsequently triggers microseismicity on secondary, smaller faults; and (2) fluid injection within a network of rate strengthening intersecting faults, where fluid diffusion reactivates slip throughout the network. In both cases, the simulated slow-slip events propagate faster than the fluid pressure diffusion front.

Interestingly, the migration patterns of microseismicity in the first case and slow slip in the second resemble diffusion processes, yet exhibit diffusivity values distinct from the imposed fault’s hydraulic diffusivity. This finding suggests that estimates of hydraulic diffusivity based solely on microseismicity front migration may not be accurate, in line with previous experimental and modeling studies.

These results highlight the influence of variable permeability and stress transfer caused by slow slip transients, offering valuable insights into induced seismicity within crustal reservoirs.