The Northwest Geysers Enhanced Geothermal System (EGS) demonstration project aims to create an EGS by directly and systematically injecting cool water at relatively low pressure into a known High Temperature (280-400°C) Zone (HTZ) located under the conventional (240°C) geothermal steam reservoir at The Geysers geothermal field in California. In this paper, the results of coupled thermal, hydraulic, and mechanical (THM) analyses made using a model developed as part of the prestimulation phase of the EGS demonstration project is presented. The model simulations were conducted in order to investigate injection strategies and the resulting effects of cold-water injection upon the EGS system; in particular to predict the extent of the stimulation zone for a given injection schedule. The actual injection began on October 6, 2011, and in this paper a comparison of pre-stimulation model predictions with micro-earthquake (MEQ) monitoring data over the first few months of a one-year injection program is presented. The results show that, by using a calibrated THM model based on historic injection and MEQ data at a nearby well, the predicted extent of the stimulation zone (defined as a zone of high MEQ density around the injection well) compares well with observed seismicity. The modeling indicates that the MEQ events are related to shear reactivation of preexisting fractures, which is triggered by the combined effects of injection-induced cooling around the injection well and small changes in steam pressure as far as half a kilometer away from the injection well. Pressure-monitoring data at adjacent wells and satellite-based groundsurface deformation data were also used to validate and further calibrate reservoirscale hydraulic and mechanical model properties. The pressure signature monitored from the start of the injection was particularly useful for a precise back-calculation of

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The presence of fl uid within a fault zone can cause overpressure and trigger earthquakes. In this work, we study the infl uence of fault-zone architecture on pore pressure distribution and on the resulting fault reactivation caused by CO 2 injection. In particular, we investigate the effect of the variation and distribution of lithological and rock physical properties within a fault zone embedded in a multi-layer sedimentary system. Through numerical analysis, we compare several models where the complexity of the fault-zone architecture and different layers (such as caprock and injection reservoir) are incrementally included. Results show how the presence of hydraulic and mechanical heterogeneity along the fault infl uences the pressure diffusion, as well as the effective normal and shear stress evolution. Hydromechanical heterogeneities (i) strengthen the fault zone resulting in earthquakes of small magnitude, and (ii) impede fl uid migration upward along the fault. We also study the effects of the caprock and aquifer thickness on the resulting induced seismicity and CO 2 leakage, both in heterogeneous and homogeneous fault zones. Results show that a thin caprock or aquifer allows smaller events, but a much higher percentage of leakage through the caprock and into the upper aquifer. The amount of leakage reduces drastically in the case of a multi-caprock, multi-aquifer system.

We performed a series of 3D thermo-hydro-mechanical (THM) simulations to study the influences of hydromechanical and thermal processes on the development of an enhanced geothermal system, strongly influenced by a network of short fault zones. The model we developed was calibrated by comparing the simulated THM responses to field observations, including ground-surface deformations, well pressure, and microseismic activity. Of particular importance was the comparison between the observed temporal and spatial distribution of microseismic activity, and the calculated shear reactivation of preexisting fractures inferred from simulated elasto-plastic mechanical responses in the short fault zone. Using this approach, we could identify when fault zones were reactivated (as manifested in the field by a surge of local microseismic activity within the fault zone), and we could back-calculate the in situ stress field as being close to the stress conditions required for shear reactivation. Our results show that the main mechanisms of inducing seismicity are related to injection-induced pressure increase and cooling. During injection, the reservoir expansion caused by the pressure increase led to mechanical stress transfer through the reservoir, which prevented or delayed the reactivation of preexisting fractures. After injection stopped, there was an inversion of the mechanical stress transfers that favored shear reactivation, which may explain why microseismic activity occurred after the cessation of the injection.

a b s t r a c tAn Enhanced Geothermal System (EGS) Demonstration Project is currently underway in the Northwest Geysers. The project goal is to demonstrate the feasibility of stimulating a deep high-temperature reservoir (HTR) (up to 400 • C, 750 • F). Two previously abandoned wells, Prati State 31 (PS-31) and Prati 32 (P-32), were reopened and deepened to be used as an injection and production doublet to stimulate the HTR. The deepened portions of both wells have conductive temperature gradients of 10 • F/100 ft (182 • C/km), produce connate native fluids and magmatic gas, and the rocks were isotopically unexchanged by meteoric water. The ambient temperature meteoric water injected into these hot dry rocks has evidently created a permeability volume of several cubic kilometers as determined by seismic monitoring. Preliminary isotopic analyses of the injected and produced water indicate that 50-75% of the steam from the created EGS reservoir is injection-derived.

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We present a three‐dimensional thermohydromechanical numerical study of the evolution and distribution of the stress tensor within the northwest part of The Geysers geothermal reservoir (in California), including a detailed study of the region around one injection well from 2003 to 2012. Initially, after imposing a normal faulting stress regime, we calculated local changes in the stress regime around injection wells. Our results were compared with previously published studies in which the stress state was inferred from inverting the focal plane mechanism of seismic events. Our main finding is that changes in stress tensor orientation are caused by injection‐induced progressive cooling of the reservoir, as well as by the seasonal variations in injection rate. Because of the gravity flow and cooling around a liquid zone formed by the injection, the vertical stress reduction is larger and propagates far below the injection well. At the same time, the horizontal stress increases, mostly because of stress redistribution below and above the cooling area. These two phenomena cause the rotation of the stress tensor and the appearance of a strike‐slip regime above, inside, and below the cooling area. The cooling and the associated rotation of the stress regime can play a significant role in the observed long‐term deepening of the microseismicity below active injection wells.