Recent experiments systematically explore rock friction under crustal earthquake conditions revealing that faults undergo abrupt dynamic weakening. Processes related to heating and weakening of fault surface have been invoked to explain pronounced velocity weakening. Both contact asperity temperature $T_a$ and background temperature $T$ of the slip zone evolve significantly during high velocity slip due to heat sources (frictional work), heat sinks (e.g. latent heat of decomposition processes) and diffusion. Using carefully calibrated High Velocity Rotary Friction experiments, we test the compatibility of thermal weakening models: (1) a model of friction based only on $T$ in an extremely simplified, Arrhenius-like thermal dependence; (2) a flash heating model which accounts for evolution of both $V$ and $T$; (3) same but including heat sinks in the thermal balance; (4) same but including the thermal dependence of diffusivity and heat capacity. All models reflect the experimental results but model (1) results in unrealistically low temperatures and models (2) reproduces the restrengthening phase only by modifying the parameters for each experimental condition. The presence of dissipative heat sinks in (3) significantly affects $T$ and reflects on the friction, allowing a better joint fit of the initial weakening and final strength recovery across a range of experiments. Temperature is significantly altered by thermal dependence of (4). However, similar results can be obtained by (3) and (4) by adjusting the energy sinks. To compute temperature in this type of problem we compare the efficiency of three different numerical solutions (Finite differences, wavenumber summation, and discrete integral).

Understanding fluid flow in rough fractures is of high importance to large scale geologic processes and to most anthropogenic geo-energy activities. Here, we conducted fluid transport experiments on Carrara marble fractures with a novel customized surface topography. Transmissivity measurements were conducted under mechanical loading conditions representative of deep geothermal reservoirs (normal stresses from 20 to 70 MPa and shear stresses from 0 to 30 MPa). A numerical procedure simulating normal contact and fluid flow through fractures with complex geometries was validated towards experiments. Using it, we isolated the effects of roughness parameters on fracture fluid flow. Under normal loading, we find that i) the transmissivity decreases with normal loading and is strongly dependent on fault geometry ii) the standard deviation of heights (RMS) and macroscopic wavelength of the surface asperities control fracture transmissivity. Transmissivity evolution is non-monotonic, with more than 4 orders of magnitude difference for small variations of macroscopic wavelength and RMS roughness. Reversible shear loading has little effect on transmissivity, it can increase or decrease depending on the combined contact geometry and overall stress state on the fault. Finally, irreversible shear displacement (up to 1 mm offset) slightly decreases transmissivity contrary to common thinking. The transmissivity variation with irreversible shear displacements can be predicted geometrically at low normal stress only. Finally, irreversible changes in surface roughness (plasticity and wear) due to shear displacement result in a permanent decrease of transmissivity when decreasing differential stress. We discuss the implications for Enhanced Geothermal Systems stimulation.

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 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). 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 and minima between 30 and 45. 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 decreasing to the minimum at 90. 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.