Permeability anisotropy in fault gouges: From clay-rich to granular systems

Fluid injection activities associated with CO2 storage and wastewater disposal are not only linked to induced seismicity hazards, but they also pose a critical challenge related to fluid leakage along faults bounding the trap, or within the overlying top seal. In this context, understanding permeability and its anisotropy in fault gouges is essential in predicting fluid migration pathways and assessing fluid leakage hazard. In this study, we developed rock deformation experiments to investigate permeability anisotropy in monomineralic, bi-phase and three-phase synthetic fault gouges composed of quartz, calcite and illite. This selection was made because these are among the most abundant mineral phases in fault-bounded reservoirs, where clay smearing within sedimentary rocks can develop different rock fabrics that may strongly influence permeability. The experiments were performed in a double direct shear configuration under a constant effective normal stress (σ’n) of 20 MPa and a hydrostatic pore fluid pressure conditions. We characterized permeability, both perpendicular (k⊥) and parallel (k∥ ) to the fault plane to evaluate the role of mineralogy and the associated fabric development in the evolution of permeability and its anisotropy. Our results show that permeability decreases from granular (k⊥≈ 10-16 m2; k∥≈ 10-15 m2) to illite-rich gouges (k⊥≈ 10-18 m2; k∥≈ 10-16 m2). Across all compositions, permeability is anisotropic with the anisotropy that systematically increaseas with clay content and, to a lesser extent, with shear strain. Illite-rich mixtures show differences of up to two orders of magnitude between k⊥ and k∥, indicating that clay-rich fault gouges may strongly promote directional fluid flow during shear. These findings have direct implications for subsurface fluid injection scenarios, suggesting that deformation- driven anisotropy may promote preferential flow pathways and potentially enhance leakage risks along faults.