Permeability models for carbonate fault cores

The present contribution focuses on carbonates fault cores exposed in central and southern Italy, which crosscut Mesozoic limestones and dolostones, pertain to 10’s of m- to 100’s of m-throw extensional fault zones, and include two main domains named as inner and outer fault cores, respectively. The inner fault cores are made up of main slip surfaces (MSS), matrix-supported cataclasites, and fault breccia. Cement-supported cataclasites, if present in the limestone-hosted fault cores, localize around the MSS. The outer fault cores mainly include grain-supported cataclasites, subsidiary slip surfaces, and lithons of fragmented host rocks. In order to assess the fluid flow properties of the carbonate fault cores, the results of microstructural, petrophysical, and ultrasonic studies are first presented, and then discussed in terms of pore type, geometry, textural anisotropy, and poro-perm relationships. Overall, the documented pore distribution is mainly function of both deformation micro-mechanisms and diagenetic processes, which took place in the carbonate fault cores during faulting and fault rock exhumation from depth. In the limestone-hosted fault cores, the experimental results show that the cross-fault fluid flow properties are affected by the the irregular geometry of the cement fronts. These fronts, which depart from the MSS, are due to calcite precipitation in vadose environments from meteoric-derived fault fluids. At depths of about 1 km, these fluids merge with the local freshwater aquifers, and cement the whole fault cores. Overall, these fault cores include a stiff pore networks, and are thought to behave like a granular medium. There, it is proposed that the cross-fault permeability can be computed by applying the Kozeny-Carmen correlation. For any given value of effective porosity, the value of permeability is therefore proportional to the average value of the pore throat, which characterize the aperture of capillary tubes with a geometrical tortuosity of ca. 2.5. On the contrary, the dolostone-hosted fault cores include a soft pore network made up of elongated pores, and are thought to behave like an elastic cracked medium. Accordingly, it is proposed that the cross-fault permeability can be computed by following percolation theory by considering the values of dynamic elastic moduli measured during ultrasonic tests at Pc=30 MPa, and almost isotropic fracture networks. Results of this work could be helpful during appraisal and development operations of hydrocarbon reservoirs, for freshwater aquifer protection, and activities of CO₂ storage in depleted carbonate reservoirs.