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We analyzed relative contributions to the cause and mechanism of injection-induced seismicity at The Geysers geothermal field, California, using coupled thermal-hydrological-mechanical modeling. Our analysis shows that the most important cause for injection-induced seismicity is injection-induced cooling and associated thermal-elastic shrinkage that changes the stress state in such a way that mechanical failure and seismicity can be induced. Specifically, the cooling shrinkage results in unloading and associated loss of shear strength in critically shear-stressed fractures, which are then reactivated. Thus, our analysis shows that cooling-induced shear slip along fractures is the dominant mechanism of injection-induced seismicity at The Geysers.

In this paper, we present progress made in a study aimed at increasing the understanding of the relative contributions of different mechanisms that may be causing the seismicity occurring at The Geysers geothermal field, California. The approach we take is to integrate: (1) coupled reservoir geomechanical numerical modeling, (2) data from recently upgraded and expanded NCPA/Calpine/LBNL seismic arrays, and (3) tens of years of archival InSAR data from monthly satellite passes. We have conducted a coupled reservoir geomechanical analysis to study potential mechanisms induced by steam production. Our simulation results corroborate co-locations of hypocenter field observations of induced seismicity and their correlation with steam production as reported in the literature. Seismic and InSAR data are being collected and processed for use in constraining the coupled reservoir geomechanical model.

While the purpose of geologic storage in deep saline formations is to trap greenhouse gases underground, the potential exists for CO2 to escape from the target reservoir, migrate upward along permeable pathways, and discharge at the land surface. In this paper, we evaluate the potential for such CO2 discharges based on the analysis of natural analogs, where large releases of gas have been observed. We are particularly interested in circumstances that could generate sudden, possibly self enhancing release events. The probability for such events may be low, but the circumstances under which they occur and the potential consequences need to be evaluated in order to design appropriate site-selection and risk-management strategies. Numerical modeling of hypothetical test cases is suggested to determine critical conditions for large CO2 releases, to evaluate whether such conditions may be possible at designated storage sites, and, if applicable, to evaluate the potential impacts of such events as well as design appropriate mitigation strategies.

In this paper we present results of a numerical simulation of the potential for fault reactivation and hydraulic fracturing associated with CO2 injection in a multilayered reservoir-caprock system, and discuss its implications on site characterization. The numerical simulation is performed using the coupled processes simulator TOUGH-FLAC (Rutqvist et al. 2002, Rutqvist and Tsang, 2003), and is an extension of earlier numerical studies of a single caprock system (Rutqvist and Tsang, 2002). In this study, CO2 is injected for 30 years in a 200 meter thick permeable saline water formation located at 1600 meters depth (Figure 1). The injection formation is overlaid by several layers of caprocks, which are intersected by a permeable fault zone allowing upward migration of the CO2 within the multilayered system (see Table 1 for material properties). The potential for fault slip or fracturing are calculated, based on the time-dependent evolution and local distribution of fluid pressure and the three-dimensional stress field, including important poro-elastic stresses.The numerical results are discussed with respect to the site-characterization strategy that would be recommended for evaluation of maximum sustainable injection pressure at an industrial CO2 injection site.

A numerical study of hydrological and geomechanical processes during and after injection of supercritical CO2 into a deep brine formation is conducted. The simulations are performed using the coupled computer code TOUGH2-FLAC3D, which considers multi-phase flow (in this case supercritical CO2 and water with dissolved NaCl) together with rock deformation. A hypothetical geologic setting is assumed, with the injection formation situated below a sequence of aquitard and aquifer layers, creating a multi-barrier system. Various scenarios are analyzed; in particular, those cases where the aquitard sequence is intersected by subvertical conductive high-permeability (fault) zones. These zones may be pre-existing, or may be created or enhanced by rock failure in response to the pressure increase during injection. The hydrological, mechanical, as well as coupled hydromechanical responses caused by the injection are investigated to study the behavior of the system with and without mechanical coupling. The study aims at (1) understanding the upward migration of CO2 in a multi-layer aquifer-aquitard system as a function of formation characteristics and fault properties, and (2) evaluating the possibility of mechanical damage in the injection aquifer as well as in the overlying geologic units.

While the purpose of geologic storage in deep saline formations is to trap greenhouse gases underground, the potential exists for CO2 to escape from the target reservoir, migrate upward along permeable pathways, and discharge at the land surface. In this paper, we evaluate the potential for such CO2 discharges based on the analysis of natural analogs, where large releases of gas have been observed. We are particularly interested in circumstances that could generate sudden, possibly self-enhancing release events. The probability for such events may be low, but the circumstances under which they occur and the potential consequences need to be evaluated in order to design appropriate site-selection and risk-management strategies. Numerical modeling of hypothetical test cases is suggested to determine critical conditions for large CO2 releases, to evaluate whether such conditions may be possible at designated storage sites, and, if applicable, to evaluate the potential impacts of such events as well as design appropriate mitigation strategies.

A numerical investigation is conducted on the impacts of the thermal loading history on the evolution of mechanical response and permeability field of a fractured rock mass containing a hypothetical nuclear waste repository. The geological data are extracted from the site investigation results at Sellafield, England. A combined methodology of discrete and continuum approaches is presented. The results of a series of simulations based on the DFN-DEM (Discrete Fracture Network-Distinct Element Method) approach provide the mechanical and hydraulic properties of fractured rock masses, and their stress-dependencies. These properties are calculated on a representative scale that depends on fracture network characteristics and constitutive models of intact rock and fractures. In the present study, data indicate that the large scale domain can be divided into four regions with different property sets corresponding to the depth. The results derived by the DFN-DEM approach are then passed on to a large-scale analysis of the far-field problem for the equivalent continuum analysis. The large-scale far-field analysis is conducted using a FEM code, ROCMAS for coupled thermo-mechanical process. The results show that the thermal stresses of fractured rock masses vary significantly with mechanical properties determined at the representative scale. Vertical heaving and horizontal tensile displacement are observed above the repository. Observed stress and displacement fields also shows significant dependency on how the mechanical properties are characterized. The permeability changes induced by the thermal loading show that it generally decreases close to the repository. However, change of permeability is small, i.e., a factor of two, and thermally induced dilation of fracture was not observed. Note that the repository excavation effects were not considered in the study. The work presented in this paper is the result of efforts on a benchmark test (BMT2) within the international co-operative projects DECOVALEX III and BENCHPAR.

In December of 2003 a large amount of water from the Santa Rosa wastewater project began being pumped to The Geysers for injection. Millions of dollars are being spent on this injection project in the anticipation that the additional fluid will not only extend the life of The Geysers but also greatly increase the net amount of energy extracted. Optimal use of the injected water, however, will require that the water be injected at the right place, in the right amount and at the proper rate. It has been shown that Microearthquake (MEQ) generation is a direct indicator of the effect of fluid injection at The Geysers Majer and McEvilly 1979; Eberhart-Phillips and Oppenheimer 1984; Enedy et al. 1992; Stark 1992; Kirkpatrick et al. 1999; Smith et al. 2000). It is one of the few, if not only methods, practical to monitor the volumetric effect of water injection at The Geysers. At the beginning of this project there was not a detailed MEQ response, Geysers-wide, to a large influx of water such as will be the case from the Santa Rosa injection project. New technology in MEQ acquisition and analysis, while used in parts of The Geysers for short periods of time had not been applied reservoir-wide to obtain an integrated analysis of the reservoir. Also needed was a detailed correlation with the production and injection data on a site wide basis. Last but not least, needed was an assurance to the community that the induced seismicity is documented and understood such that if necessary, mitigation actions can be undertaken in a timely manner. This project was necessary not only for optimizing the heat recovery from the resource, but for assuring the community that there is no hazard associated with the increased injection activities. Therefore, the primary purpose of this project was to develop and apply high-resolution micro earthquake methodology for the entire Geysers geothermal field such that at the end of this project a monitoring and process definition methodology will be available to: a. Optimize the economic development of The Geysers (as well as other areas) by providing improved information on fluid flow and reservoir dynamics. b. Aid in the mitigation of environmental impacts of increased fluid injection by improving the understanding between fluid injection and seismicity. c. Provide a cost-effective blueprint such that the technology can be applied on a routine basis in the future.