Mark Walters

Seismic and aseismic deformations and impact on reservoir permeability: The case of EGS stimulation at The Geysers, California, USA

Author(s): Jeanne, P; Rutqvist, J; Rinaldi, AP; Dobson, PF; Walters, M; Hartline, C; Garcia, J | Abstract: ©2015. American Geophysical Union. All Rights Reserved. In this paper, we use the Seismicity-Based Reservoir Characterization approach to study the spatiotemporal dynamics of an injection-induced microseismic cloud, monitored during the stimulation of an enhanced geothermal system, and associated with the Northwest Geysers Enhanced Geothermal System (EGS) Demonstration project (California). We identified the development of a seismically quiet domain around the injection well surrounded by a seismically active domain. Then we compare these observations with the results of 3-D Thermo-Hydro-Mechanical simulations of the EGS, which accounts for changes in permeability as a function of the effective normal stress and the plastic strain. The results of our modeling show that (1) the aseismic domain is caused by both the presence of the injected cold water and by thermal processes. These thermal processes cause a cooling-stress reduction, which prevent shear reactivation and favors fracture opening by reducing effective normal stress and locally increasing the permeability. This process is accompanied by aseismic plastic shear strain. (2) In the seismic domain, microseismicity is caused by the reactivation of the preexisting fractures, resulting from an increase in injection-induced pore pressure. Our modeling indicates that in this domain, permeability evolves according to the effective normal stress acting on the shear zones, whereas shearing of preexisting fractures may have a low impact on permeability. We attribute this lack of permeability gain to the fact that the initial permeabilities of these preexisting fractures are already high (up to 2 orders of magnitude higher than the host rock) and may already be fully dilated by past tectonic straining.

Geomechanical simulation of the stress tensor rotation caused by injection of cold water in a deep geothermal reservoir

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.

Numerical simulation study of the Northwest Geysers geothermal field, a case study of the Coldwater Creek steamfield

Abstract Commercial exploitation of the Northwest Geysers began in early 1988. To date little has been published about the connectivity of this area to the rest of the geothermal field, the interaction between the so-called “typical” reservoir and the underlying high-temperature zone, or the combined potential of both producing horizons. To investigate these issues, a dual-porosity numerical model of The Geysers geothermal system with special emphasis to the NW Geysers was developed. The model was calibrated using production data from 1957 to early 1991. Simulation results and the measured data suggest that: the typical reservoir in NW Geysers is connected to the rest of the field; significant migration of fluids from this part of the reservoir to the rest of The Geysers has occurred; and there still are some undeveloped areas in the NW Geysers with production potential. The production/injection scenarios analyzed with the model suggest that the existing wells are not sufficient to sustain a 60 MWe operation for 30 years starting in 1991. However, the addition of new wells within the licensed boundaries of the Coldwater Creek steamfield, drilled over a period of years, could sustain the steam production required to generate 60 MWe for 30 years. A 130 MWe development is not feasible within the mid-1993 licensed boundaries.