Stress-induced permeability anisotropy and fluid flow dynamics in a Mesozoic fractured carbonate aquifer of southern Italy

Understanding the impact of local stress states on computed permeability for fractured carbonates or any other lithotype is crucial to better assess the modalities of fluid flow in the subsurface. Considering Mesozoic fractured carbonates exposed along the flanks of the Viggiano Mt. of southern Italy, we investigate the control exerted by the local fracture networks on the output of DFN modelling of geocellular volumes whose dimensions are like those of the studied outcrops. Specifically, the following four sedimentary units are considered:

• Scarrone La Macchia II, (SLM II), well-layered, Sinemurian–Pleinsbachian carbonate succession of wackestone-packstones to grainstones arranged in discrete bed packages originally deposited in a low-energy, open lagoon environment.
• Scarrone la Macchia I, (SLM I), Toarcian oolithic carbonates characterized by bed amalgamation originally formed in a ramp setting rimmed by oolithic sand shoals.
• Piana del Buon Cuore (PBC), Lower Cretaceous - Upper Jurassic limestones whose clasts consist of oolites, oncolites and intraclasts deposited in a high-energy platform margin environment.

• The Il Monte (ILM), massive, amalgamated, Cretaceous carbonate rudstones and grainstones originally deposited along the paleo-slope of the carbonate platform.

By employing existing field data (Abdallah et al., 2023, 2024), we carried out Discrete Fracture Networks (DFN) modelling of 5 m-sided geocellular volumes including internal sub-volumes representative of single carbonate beds. This work was conducted by means of  high-resolution computational meshes provided by the dfnWork ® code, which is capable of non-reactive solute transport simulation, and constrain of imposed depth-equivalent stresses to assessing effective horizontal permeability (effk_xx, effk_yy).

Focusing on the results achieved for the Viggiano Mt. aquifer, we simulated depth conditions of 500m coupled with principal stress axes of ~13MPa (sv), 10 MPa (shmax, NW-SE), and 7.58 MPa (shmin, NE-SW). The theoretical aperture data were hence modulated by the local stress state conditions. SLM I, exhibits an increase in permeability anisotropy ratio as a function of the stresses, hence, leading to flow channelling within the network. Lagrangian solute transport simulation supports the afore-mentioned results by marked changes in primary flow path, increase in path tortuosity, as a function of stress, and delay in breakthrough time. Similar results were achieved for PBC and ILM units. Differently, SLM II undergoes the opposite effect, where the permeability ratio seems to reduce drastically at a depth of 500m and stabilizes after that. We infer this behaviour as due to non-linear-fluid-flow behaviour as a function of aperture closure or dilation, as seen in highly connected systems of fractures including both stratabound and non-stratabound elements. Accordingly, SLM II is characterized by efficient mechanical units made up of bed interfaces, which were able to compartmentalize the vertical growth of high-angle fractures.

This research highlights the complex behaviour of permeability anisotropy in fractured carbonate rocks, in response to depth-equivalent stresses, and the importance of building realistic geomechanically coupled-DFN models to estimate fluid-flow and storage properties of fractured rocks at depth.