Fundamental Physical Mechanisms Leading to Initiation of Fault Rupture, With Application to Induced Seismicity at the Geysers Geothermal Field

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This research project is a suite of laboratory experiments examining the dynamic evolution of motion of a contact junction on a fault surface which results in gross rupture - using what is called nano-seismology. In order to understand the mechanisms leading to fluid-injection induced seismicity (IS), the mechanisms leading to rupture initiation must be defined. This project's novel sensors will allow measurement, at a scale approaching shear of nanojunctions, of evolving frictional behavior. Waveform inversion and forward synthetics will model measured behaviors, tying together the multiple scales across which there are self-similar frictional mechanisms, leading up to actual "slip." The first stage of the experimental campaign is the investigation of the effects of thermoelastic strain and cementation on fault strength; the second stage will investigate the effects of contact melting on rupture initiation mechanisms. The third set of experiments will take place in a steam driven true-triaxial geothermal reservoir simulator. This set of tests will investigate the effects of thermal contraction, and injected water flashing to steam, as proximate causes of fault weakening. Both through-going and growing hydraulic fractures will be modeled.Enhanced geothermal systems (EGS) are potentially a major contributor to the nation's supply of clean, sustainable energy. An EGS is an engineered subsurface heat exchanger designed to either improve a conventional hydrothermal reservoir, or to create a circulation system where hot reservoir rocks have low permeability. This is most often done by injection of pressurized fluids. This injection has been shown to induce seismicity, so understanding the mechanisms of why induced seismicity occurs is fundamental for utilizing this resource. Understanding the mechanisms of induced seismicity is also necessary for the exploitation of the country's rich deposits of shale gas, and for carbon sequestration. The Geysers hydrothermal field, which is the largest such electricity producer in the world, will serve as the field test bed; laboratory results being compared to actual seismic behavior from the Geysers through partnership with Lawrence Berkeley National Laboratory.