Injection Induced Seismicity in Hot Resevoirs

University Of California-Berkeley, Berkeley CA

Investigators

Abstract

Enhanced geothermal systems are potentially a major contributor to the nation's supply of clean, sustainable energy. An enhanced geothermal reservoir is an engineered subsurface heat exchanger designed to 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. Understanding why injection-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. Because the in situ mechanisms of injection induced seismicity are so complex, there still is a lack of deep understanding. The project will carry out a set of experiments in very specialized equipment that can model the Geysers geothermal field. This will bring the understanding needed to engineer for the mechanisms behind induced seismicity so that subsurface injection of fluids for energy resources can be used without causing earthquakes The project is based on a carefully designed sequence of tests, each informing the next. The first stage is the investigation of the effects of fault unloading vs. increased shear loading on foreshocks and rupture. Preliminary work by Glaser shows that the dynamics are quite different. These experiments will take place in a laboratory earthquake machine. The next set of experiments will take place in the Glaser lab's true-triaxial geothermal reservoir simulator. Integral to this device is a high pressure boiler that floods 250 mm cubes of rock with 2 MPa steam, duplicating the Geysers field. Specimens will be fractured to represent a rock mass rather than a solid cube of rock. We 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. The dynamic asperity-level displacement "seeds" that lead to macro-rupture will be studied through nano-seismic imaging. This is made possible by absolutely-calibrated nanoseismic sensors, which have a noise floor of 0.2 pm. Over many years, laboratory analysis methodologies have been developed based on tools used by seismologists, to accurately measure absolute metrics of rupture kinematics, including the source-time function to mN sensitivity. Because similar tools are used to study seismicity at the Geysers, it is possible to reasonably scale our results.