Permeability Anisotropy in Fractured Mesozoic Platform Carbonates under Variable Triaxial Stress Conditions

This study investigates how local stress state governs permeability magnitude in fractured carbonate aquifers. By using outcrop-constrained Discrete Fracture Network (DFN) modelling from Mt. Viggiano of the southern Apennines, Italy, we investigate the control exerted by 500 m-depth tri-axial local stress state on computed horizontal permeability anisotropy. Fractured carbonate systems commonly exhibit strong permeability anisotropies that evolve with depth as fractures respond to changes in both normal and shear stresses. Accurately capturing this behaviour remains challenging due to the combined effects of fracture geometry and connectivity, as well as primary depositional architecture and stress-dependent aperture modification.
Field-derived fracture datasets from four carbonate outcrops representing two contrasting paleo depositional settings are used to construct three-dimensional DFN models at the bed-package scale. Two DFN-based modelling workflows are employed to explore how different representations of fracture connectivity and flow influence predicted permeability. One approach estimates bulk permeability from fracture population statistics within distinct geocellular volumes. Differently, the other one explicitly simulates steady-state fluid flow through hydraulically connected fracture networks within a fully meshed computational domain. This integrated strategy allows evaluation of how modelling assumptions related to connectivity, aperture scaling, and flow representation affect permeability predictions without implying a preferred modelling tool.
The results of this study show that increasing normal stress generally reduces horizontal permeability anisotropy, although local increases in permeability occur where favourably oriented fractures undergo shear induced dilation. Result are also consistent with the permeability response varying systematically with depositional architectures: (i) massive, high-energy carbonates dominated by non-strata bound fractures exhibit vertically persistent but weakly connected networks; (ii) on the contrary, layered, low-energy carbonates containing abundant strata-bound fractures display enhanced lateral connectivity and higher hydraulic effective transmissivity.
The main outcomes of this work demonstrate that permeability anisotropy in fractured carbonates evolves with stress through its interaction with fracture orientation, connectivity, and stratigraphic architecture. Incorporating stress dependent behaviour and explicit connectivity into DFN workflows therefore improves predictions of subsurface fluid flow relevant to groundwater resources, CO₂ storage, and geothermal systems.