Geothermics 51 (2014) 240–252 Contents lists available at ScienceDirect Geothermics jo ur nal homep age: www.elsevier.com/locate/geothermics A 3D hydrogeological and geomechanical model of an Enhanced Geothermal System at The Geysers, California Pierre Jeanne a,∗ , Jonny Rutqvist a , Donald Vasco a , Julio Garcia b , Patrick F. Dobson a , Mark Walters b , Craig Hartline b , Andrea Borgia a a Lawrence Berkeley National Laboratory, Earth Science Division, Berkeley, CA 94720, USA b Calpine Corporation, 10350 Socrates Mine Road, Middletown, CA 95461, USA a r t i c l e i n f o Article history: Received 4 September 2013 Accepted 20 January 2014 Keywords: Enhanced Geothermal Systems The Geysers Induced seismicity TerraSAR-X satellites Shear zones Thermo-hydromechanical simulation a b s t r a c t In this study, integrated coupled process modeling and field observations are used to build a three- dimensional hydrogeological and geomechanical model of an Enhanced Geothermal System (EGS) in the northwestern part of The Geysers geothermal field, California. We constructed a model and characterized hydraulic and mechanical properties of relevant geological layers and a system of multiple intersecting shear zones. This characterization was conducted through detailed coupled process modeling of a one- year injection stimulation with simultaneous field monitoring of reservoir pressure, microseismicity, and surface deformations. The analysis of surface deformations was found to be particularly challenging as the subtle surface deformations caused by the injection taking place below 3 km depth are inter- mingled with deformations caused by both tectonic effects and seasonal surface effects associated with rainfall. However, through a detailed analysis of the field data we identified deformations associated with injection. Hydraulic and mechanical properties of relevant rock layers and shear zones were deter- mined using a 3D hydrogeological and geomechanical model. Hydraulic properties were determined using inverse analysis by fitting the pressure evolution in monitoring wells surrounding the injection well. Mechanical properties were estimated by comparison of the predicted microseismicity potential with the observed microseismicity and by fitting the predicted vertical displacement with the surface deformations measured by satellite. The results show the critical importance of considering the regional fault system, especially reservoir-level faults and shear zones that modify injection water flow and steam pressure diffusion. In the vicinity of the EGS Demonstration Project, fluid flow pathways and pressure diffusion fronts appears to be at a maximum along N130 oriented shear zones and at a minimum along N50 oriented shear zones. Evidence for this comes from microseismic event hypocenters which extend several kilometers horizontally from the injection well and deep into a recent granitic intrusion that underlies the high temperature reservoir. Published by Elsevier Ltd. 1. Introduction An Enhanced Geothermal System (EGS) is a technology for extracting geothermal energy under circumstances and locations where conventional production is not economic (Tester et al., 2006). Generally, EGS would be applied for extracting geother- mal energy at sites where the reservoir rock is hot (has sufficient heat content) but has insufficient permeability for economic pro- duction. If an EGS could be created and successfully managed, then large untapped geothermal resources could be utilized. The current strategy is to increase permeability by reactivating shear- ing fractures through water injection at a relatively low rate ∗ Corresponding author. Tel.: +1 510 486 6261. E-mail addresses: pjeanne@lbl.gov, pierrejeanne06@yahoo.fr (P. Jeanne). and a bottom-hole pressure much less than the estimated mini- mum principal compressive stress at injection depth. The aim is to avoid the propagation of a single hydraulic fracture set and to create a more pervasive stimulation zone by dilating a net- work of pre-existing fractures though shear reactivation, known as hydroshearing (Cladouhos et al., 2009). This process is frequently accompanied with a strong increase in microseismic activity, and characterizing mechanisms of inducing seismicity can increase the understanding about their role in enhancing permeability. The scientific challenges of EGS are to build accurate hydro- logic and geomechanical models to investigate injection strategies, their effects upon EGS development and to predict the extent of the stimulation zone. To address such challenges it is desirable to develop reliable hydrogeological and geomechanical models to assist in designing and optimizing the exploitation of the EGS. Such models require reliable input data in the form of large scale 0375-6505/$ – see front matter. Published by Elsevier Ltd. http://dx.doi.org/10.1016/j.geothermics.2014.01.013