Predicting the spatial extent of injection-induced zones of enhanced permeability at the Northwest Geysers EGS Demonstration Project

Predicting the Spatial Extent of Injection-Induced Zones of Enhanced Permeability at the Northwest Geysers EGS Demonstration Project Rutqvist, J., Oldenburg, C.M. , and Dobson, P.F. Lawrence Berkeley National Laboratory, Berkeley, California, USA Garcia, J., and Walters, M. Calpine Corporation, Middletown, California, USA This paper was prepared for presentation at the 44 th US Rock Mechanics Symposium and 5 th U.S.-Canada Rock Mechanics Symposium, held in Salt Lake City, UT June 27–30, 2010. ABSTRACT: We present the results of coupled thermal, hydraulic, and mechanical (THM) modeling of a proposed stimulation injection associated with an Enhanced Geothermal System (EGS) demonstration project at the northwest part of The Geysers geothermal field, California. The project aims at creating an EGS by directly and systematically injecting cool water at relatively low pressure into a known High Temperature (about 280 to 350C) Zone (HTZ) located under the conventional (240C) steam reservoir at depths below 3 km. Accurate micro-earthquake monitoring from the start of the injection will be used as a tool for tracking the development of the EGS. We first analyzed historic injection and micro-earthquake data from an injection well (Aidlin 11), located about 3 miles to the west of the new EGS demonstration area. Thereafter, we used the same modeling approach to predict the likely extent of the zone of enhanced permeability for a proposed initial injection in two wells (Prati State 31 and Prati 32) at the new EGS demonstration area. Our modeling indicates that the proposed injection scheme will provide additional steam production in the area by creating a zone of permeability enhancement extending about 0.5 km from each injection well which will connect to the overlying conventional steam reservoir. 1. INTRODUCTION The Geysers is the site of the largest geothermal electricity generating operation in the world and has been in commercial production since 1960. It is a vapor dominated geothermal reservoir system, which is hydraulically confined by low permeability rock units. As a result of the high steam withdrawal rates, the reservoir pressure declined until the mid 1990s, when increasing water injection rates resulted in a stabilization of the steam reservoir pressure. If The Geysers were produced without simultaneously injecting water, reservoir pressures and flow rates from production wells would decline fairly rapidly to uneconomical levels. In a portion of the northwestern part of The Geysers, exploratory drilling in the early 1980’s discovered a relatively shallow High Temperature (about 280 to 350C) Zone (HTZ) in low permeability rock below the Normal Temperature (240C) steam Reservoir (NTR). A number of steam production wells were drilled, but later abandoned because of problems caused by high concentrations of non-condensable gases (NCG) and highly corrosive hydrogen chloride gas in the steam. As result, the Northwest Geysers, which contains a significant portion of the recoverable geothermal energy in the Geysers system, is currently underutilized. In the ongoing Northwest Geysers EGS Demonstration project (funded by US Department of Energy’s Geothermal Technologies Program and Calpine Corporation), the objective is to develop and demonstrate the technology required to extract energy from this type of low- permeability HTZ that typically underlies any high- temperature geothermal system. One of the motivations for the project is the ample evidence that a large EGS was inadvertently created in the late 1970’s below the oldest production area in the central part of The Geysers when injected water reached the HTZ several kilometers below the deepest wells [1]. Micro-earthquake (MEQ) and geochemical monitoring of this EGS area indicated on-going reactivation of fractures in the HTZ, and a temporal correlation of sustained steam production and lower NCG concentration since injection of wastewater from the Santa Rosa Geysers Recharge Project began in 2003. If a similar type of EGS can be created and successfully demonstrated at the Northwest Geysers, then large untapped resources can be utilized with the potential to increase the production of geothermal energy at The Geysers.