Mechanical and hydraulic properties of fractured Bentheim sandstone at different laboratory-simulated depths
Abstract
Understanding how rock properties change with depth is crucial for a variety of geoengineering applications. Even rocks that are homogenous at both micro and macro scales, such as Bentheim sandstone, lose this characteristic once fractured. While recent studies have shown how concomitant changes in stress, temperature and pore pressure affects the evolution of intact sample permeability at depths, an equivalent study on fractured material is missing. Therefore, by combining a multi-methodological approach consisting of rock deformation experiments simulating depth conditions up to 4 km, thin section analysis and fluid composition analysis of water samples, the evolution of permeability of fractured Bentheim sandstone is investigated in this study. Results suggests that fine particles produced by the fracturing and the movements along these fractures play a crucial role in permeability evolution. When these particles are removed, the fracture constitutes a preferential pathway and, together with the chemical processes occurring on the rock–fluid system, lead to a 3–7 times reduction in permeability followed by a complete recovery of it after a simulated burial and exhumation path. On the contrary, when these particles are still present within the fracture zone, they impede fluid flow. This causes a slightly reduction of permeability during the burial path followed by almost constant values of permeability throughout the exhumation path. These findings provide crucial information for georeservoir applications and the transfer of results from laboratory experiments to in situ conditions for a correct prediction of hydraulic properties.