Fluid-flow anisotropies within carbonate fault damage zones: New insights from DFN-computed permeability under variable triaxial stress conditions

Fault damage zones might strongly influence subsurface fluid flow in carbonate aquifers, yet how their fracture networks respond to in-situ stress, and how that response varies with depositional architecture have rarely been quantified. We investigate how the equivalent permeability and its horizontal anisotropy vary with stress in the fractured carbonates exposed at Mt. Viggiano, which is bounded by high-angle faults of the East Agri Fault System (southern Apennines, Italy). These carbonates host an unconfined aquifer sealed at the bottom by the low-permeability Lagonegro unit. Four outcrops spanning a transect of decreasing fault influence, and contrasting depositional facies, are represented as outcrop-constrained discrete fracture networks. Two complementary workflows are used: MOVE , for geocellular Oda-tensor permeability including isolated fractures, and dfnWorks , for connectivity-based flow simulation and stress-dependent aperture modelling; matrix porosity is neglected, so flow is fracture-controlled. Fracture networks are evaluated at surface conditions and at stresses equivalent to −500 m burial, the shallowest estimate of the aquifer base across the study sites. Our study shows that increasing normal stress reduces the equivalent permeability, whereas shear dilation on fractures favourably oriented to σ Hmax locally enhances it. We find that proximity to both large- and small-scale, recorded in the fracture-area intensity (P 32 ) of fault-parallel sets, exerts a first-order control on horizontal permeability anisotropy under subsurface conditions. There, WNW–ESE to NW–SE striking fractures lie at low angle and sub-parallel to σ Hmax. At depth, the fault-related fractures remain open, dictating the flow direction, and producing the strongest anisotropy at the fault-proximal sites. This structural control is modulated by mechanical stratigraphy through fracture aspect ratio, which sets permeability magnitude and the network resistance to closure. Specifically, massive high-energy carbonates form vertically persistent but laterally weakly connected networks, whereas well-layered, strata-bound carbonates develop greater lateral connectivity and retain the highest permeability under stress. Overall, effective permeability decreases by close to one order of magnitude between surface and −500 m depth conditions. The predicted conductive directions coincide with the high-angle faults along which deep fluids reach the surface in the basin, consistent with independent structural and geochemical evidences. By accounting for the stress-dependent behaviour of fault-damage-zone fractures, the results of this study constrain flow direction and permeability magnitude in fault-bounded carbonate rocks, which are applicable to groundwater management, geothermal energy systems, and CO 2 storage.