Mechanical and hydraulic transport properties of transverse-isotropic Gneiss deformed under deep reservoir stress and pressure conditions

In central Europe, many geo-energy reservoirs have revealed to be hosted in transverse isotropic crystalline rock, where the rock's mechanical and hydraulic transport properties are poorly constrained. Here, we performed triaxial experiments on Cresciano Gneiss samples under realistic stress (25-40 MPa) and fluid pressure (5 MPa) conditions. We tested 5 different foliation orientations towards the major principal stress (0, 30, 45, 60, 90 degrees). During deformation, we measured the porosity evolution and acoustic emission activity of the samples. In addition, we measured the axial permeability and P-wave velocity of the samples both during isostatic confinement and after sample failure. Our results show that the mechanical and hydraulic transport properties of transverse isotropic tight crystalline rocks can be separated into two classes. First, the mechanical properties such as onset of dilatancy, yield stress, peak strength and residual strength, follow a "U-type" anisotropy towards foliation angle, with maximum values at 0 and 90 degrees and minima between 30 and 45 degrees. These properties, as well as the porosity variation during deformation which follows an inversed "U-type" shape can be explained by anisotropic wing crack models. Second, the volumetric physical properties (permeability and P-wave velocity) follow a "decreasing order" shape towards foliation angle, with maximum values at 0 degrees decreasing to the minimum at 90 degrees. These properties show a high dependence on the stress state and the wave path. We discuss the implications of these results for deep geothermal energy prospection, and for reservoir stimulation and operation.