Micromechanical Investigation of Fault-Valving Cycles Using the Discrete Element Method

Fault zones play a fundamental role in controlling subsurface fluid circulation and mineralization. Their capacity to alternately behave as barriers or conduits—commonly conceptualized within the fault-valve model—results from complex interactions among multiphysical processes (thermal, hydraulic, mechanical, and chemical) acting across a wide range of spatial and temporal scales within the fault structure.

Focusing on the gouge scale, we investigate the hydromechanical behavior of faults using a three-dimensional pore-scale modeling framework that couples a Discrete Element Method (DEM) with a pore-scale finite volume (PFV) scheme. Building on the approach of Nguyen et al. (2021), the DEM is used to model the gouge material as a granular assembly, while the PFV method is used to model fluid flow within the evolving pore space.

Using this coupled approach, we simulate a sheared, fluid-saturated granular gouge subjected to hydromechanical loading through a controlled cyclic fluid injection protocol applied at constant shear stress. The DEM-PFV model captures emergent behaviors consistent with laboratory and in situ observations, and reveals pronounced cycle-to-cycle variability in both the onset of reactivation (Sarma et al., 2025) and the magnitude of slip under repeated pressurization and depressurization. In particular, some cycles produce large slip episodes whereas others exhibit comparatively small slip under similar loading conditions. To connect this macroscopic variability to micromechanics, we track the evolution of the force-chain population throughout the cycles using the characterization method proposed by Peters et al. (2005). The results provide grain-scale insights into how internal load-bearing structures reorganize across cycles and how these force-chain dynamics relate to the occurrence of large-slip events in fault-valving sequences.

References
Nguyen, H. N. G., Scholtès, L., Guglielmi, Y., Donzé, F. V., Ouraga, Z., & Souley, M. (2021). Micromechanics of sheared granular layers activated by fluid pressurization. Geophysical Research Letters. https://doi.org/10.1029/2021GL093222
Sarma, P., Aharonov, E., Toussaint, R., & Parez, S. (2025). Fault gouge failure induced by fluid injection: Hysteresis, delay and shear-strengthening. Journal of Geophysical Research: Solid Earth. https://doi.org/10.1029/2024JB030768
Peters, J., Muthuswamy, M., Wibowo, J., & Tordesillas, A. (2005). Characterization of force chains in granular material. Physical Review E. https://doi.org/10.1103/PhysRevE.72.041307