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Origin of the Co‐Seismic Variations of Elastic Properties in the Crust: Insight From the Laboratory

Mateo Alejandro Acosta, Federica Paglialunga, François Xavier Thibault Passelègue, Marie Estelle Solange Violay

Seismological observations highlighted that earthquakes are often followed by changes in elastic properties around the fault zone. Here, we studied the origin of these variations using stick-slip experiments on saw-cut granite samples presenting different degrees of bulk damage (i.e., microcracks). Stick-slip events were induced under triaxial compression configuration with continuous active ultrasonic measurements at confining pressures representative of upper crustal conditions (15–120 MPa). Both the P-wave velocity (urn:x-wiley:00948276:media:grl62539:grl62539-math-0001) and amplitude (urn:x-wiley:00948276:media:grl62539:grl62539-math-0002) showed drops, concurrently with stress drops, and had a non-monotonic dependence toward the fault's stress state. Our experimental results suggest that co-seismic changes in urn:x-wiley:00948276:media:grl62539:grl62539-math-0003 were mostly controlled by the elastic re-opening of microcracks in the bulk, rather than by co-seismic damage or the formation of fault gouge. Co-seismic changes in urn:x-wiley:00948276:media:grl62539:grl62539-math-0004 were controlled by a combination of elastic re-opening of microcracks in the bulk and inelastic processes (i.e., co-seismic damage and gouge formation and dilation).

2021

Effect of normal and shear loading on the hydraulic transport properties of calcite bearing faults with customized roughness

Mateo Alejandro Acosta, Marie Estelle Solange Violay

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 normal stresses from 20 to 50 MPa and shear stresses from 0 to 30 MPa. An open source numerical procedure was developed to simulate normal contact and fluid flow through fractures with complex geometries. It 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 surface geometry ii) the standard deviation of heights (hrms) 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 roughness. Reversible elastic shear loading has little effect on transmissivity, it can increase or decrease depending on contact geometry and overall stress state on the fault. Irreversible shear displacement (up to 1 mm offset) slightly decreases transmissivity and its variation with irreversible shear displacements can be predicted numerically and 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. Generally, reduction of a carbonate fault’s effective stress increases its transmissivity while inducing small shear displacements doesn’t.

2020

Experimental studies of hydro-mechanical couplings in Enhanced Geothermal Reservoirs

Mateo Alejandro Acosta

Enhanced Geothermal Systems (EGS) allow for worldwide geothermal electricity production. They target deep (3-5 km), fractured rock reservoirs whose permeability is artificially increased through hydraulic stimulations (fluid injections). The injections modify the effective stresses, leading to new fracture creation and the reactivation of pre-existing ones, along with permeability changes. Today, the main issues with this technique are i) understanding and predicting the final fluid transport capacity of the reservoir and ii) avoiding the seismicity associated with the stimulation treatment. This thesis aims at understanding the two-way coupling between mechanical rock deformation and fluid transport in EGS reservoirs by reproducing them in the laboratory. Complex experimental protocols were developed and applied to various lithologies typically found in EGS. The experimental observations, coupled with analytical and numerical models, and with natural observations, allowed for the unravelling of some fundamental processes at play during EGS stimulations. A first part focused on how mechanical deformation influences fluid flow in i) anisotropic rocks and ii) fractures with customized roughness. In anisotropic rocks, the foliation orientation towards the stress field controls its mechanical and hydraulic transport properties. With ongoing deformation, anisotropic micromechanical models accurately predict the onset of damage, ultimate strength, porosity evolution, and fracture structure towards foliation orientation. Permeability is controlled by foliation orientation, fracture structure, and the applied stress. This shows that the full permeability tensor needs to be used with care on failed anisotropic rocks. In rock fractures with customized roughness, increasing the normal load decreases fracture transmissivity and thus fluid transport capacity. The transmissivity decrease is controlled by the contact geometry, as predicted by numerical models. Further, reversible shear loading and irreversible shear displacements (up to 1 mm offset) have little effect on transmissivity. Transmissivity evolution with shear displacement can be predicted by simple models only at low normal stress, where wear is not too prominent. These results question the concept of hydro-shear stimulations with small fault displacements. A second part targeted the effect of fluids across the earthquake cycle (nucleation and propagation) to approach induced seismicity. During nucleation, slight changes in fluid pressure drastically change the temporal evolution of earthquake precursors (slip and seismicity). Under all fluid pressure conditions, a semi empirical scaling relationship links the total moment (energy) released before the mainshock to its magnitude, in compatibility with several natural and anthropogenic earthquakes. This observation could help with real-time estimation of an impending earthquake's magnitude. During propagation, fluid pressure has a strong control on the thermal evolution of frictionally-heated asperity contacts. At low pressures, fluids vaporize and allow for contact melting, reducing shear resistance and leading to large magnitude quakes. At pressures close to the liquid-supercritical transition, water is an efficient thermal buffer, to the point that contact melting is impeded, leading to smaller quakes with little dynamic weakening. Consequently, injection strategies in EGS could be improved through careful extrapolation of the results.

EPFL2020