Titolo della tesi: Laboratory experiments of fluid injection–induced seismicity: insights on fault deformation using Vp/Vs, dilation, and fluid pressure monitoring
Over the last decades, deep geological repositories in phyllosilicate-rich formations have been considered promising candidates for radioactive waste disposal, and potential caprock for carbon storage and sequestration. The success of these projects relies on the long-term retention and sealing capacity for radionuclides and large volumes of injected CO2 within the repository. However, given the shallow depth of these geological deposits, the presence of faults and fractures can compromise their insulation capabilities. In Switzerland, the Opalinus Clay (OPA) formation, studied in the context of the Mont Terri Underground Laboratory (MTL) project, presents advantageous sealing properties. At the MTL, large scale fluid-injection experiments and recent excavation activities caused mechanical instability in the nearby faults, posing at risks the long-term confinement capabilities of the OPA Clay formation.
In this context, the objective of this thesis is to study the frictional and hydromechanical properties of OPA Clay, through laboratory experiments at relevant boundary conditions for the study of deep geological repositories, induced seismicity, and active fault zones.
The first section of this work reveals that the increase in relative humidity weakens the OPA Clay fault gouge, decreasing its coefficient of sliding friction. Furthermore, the OPA Clay is suitable for stable aseismic slip, characterized by velocity-strengthening behaviour and low healing rates. Both frictional stability parameter (a-b) and fault dilatancy increase with sliding velocity at low normal stresses. As the normal stress increases, (a-b) and dilatancy are little to no dependent on sliding velocity. This change in mechanical response correspond to a change of deformation: from distributed to localized, with increase in normal stress.
The second section of this work reveals that OPA Clay is extremely weak under fluid pressure conditions. For the first time the permeability is measured perpendicular to the fault slip direction on OPA Clay gouge, with extremely low values in line with previous measurements on gouge and intact core samples. Furthermore, either the frictional strength, stability parameter (a-b), and permeability of OPA Clay decrease in chorus with increase in effective normal stress, with only minor influence due to the pressurization conditions of the fault.
The third and last section of this work reports ultrasonic measurements performed during triaxial deformation experiments, on porous sandstones. Here, the anomalously high values of P- to S-wave velocity ratio (Vp/Vs), typically observed in subduction zones, can result from crack-induced anisotropies during non-hydrostatic deformation, added to the effect of fluid overpressure.
This Ph.D. work could bring relevant insights for studies applied to carbon capture and storage, nuclear waste disposal, caprocks integrity, induced seismicity, and active fault zones.