Geothermics 51 (2014) 130–141 Contents lists available at ScienceDirect Geothermics journal h om epa ge: www.elsevier.com/locate/geothermics Reactive transport modeling of the Dixie Valley geothermal area: Insights on flow and geothermometry Christoph Wanner a,∗ , Loïc Peiffer a,1 , Eric Sonnenthal a , Nicolas Spycher a , Joe Iovenitti b , Burton Mack Kennedy a a Earth Sciences Division, Lawrence Berkeley National Laboratory, 1 Cyclotron Road, Berkeley, CA 94720, USA b AltaRockEnergy Inc., Sausalito, CA, USA a r t i c l e i n f o Article history: Received 6 September 2013 Accepted 16 December 2013 Keywords: Reactive transport modeling Solute geothermometry Dixie Valley Fracture flow Geothermal springs a b s t r a c t A 2D reactive transport model of the Dixie Valley geothermal area in Nevada, USA was developed to assess fluid flow pathways and fluid rock interaction processes. The model includes two major nor- mal faults and the incorporation of a dual continuum domain to simulate the presence of a small-scale thermal spring being fed by a highly permeable but narrow fracture zone. Simulations were performed incorporating fluid flow, heat conduction and advection, and kinetic mineral-water reactions. Various solute geothermometry methods were applied to simulated spring compositions, to compare estimated reservoir temperatures with “true” modeled reservoir temperatures, for a fluid ascending the simulated fracture and cooling on its way to the surface. Under the modeled conditions (cooling but no mixing or boiling), the classical Na–K(–Ca) geothermometers performed best because these are least affected by mineral precipitation upon cooling. Geothermometry based on computed mineral saturation indices and the quartz geothermometer were more sensitive to re-equilibration upon cooling, but showed good results for fluid velocities above ca. 0.1 m/d and a reactive fracture surface area 1–2 orders of magnitude lower than the corresponding geometric surface area. This suggests that such upflow rates and relatively low reactive fracture surface areas are likely present in many geothermal fields. The simulations also suggest that the presence of small-scale fracture systems having an elevated permeability of 10 -12 to 10 -10 m 2 can significantly alter the shallow fluid flow regime of geothermal systems. For the Dixie Valley case, the model implies that such elevated permeabilities lead to a shallow (less than 1 km) convection cell where superficial water infiltrates along the range front normal fault and connects the small-scale geothermal spring through basin filling sediments. Furthermore, we conclude that a fracture permeabil- ity on the order of 10 -12 m 2 may lead to near surface temperature >100 ◦ C whereas a permeability of 10 -10 m 2 is not realistic because this permeability led to extreme upflow velocities and to a short-circuit of the regional fault zone. © 2013 Elsevier Ltd. All rights reserved. 1. Introduction Although simulations of geothermal systems have in some cases incorporated reactive transport (Xu and Pruess, 2001; Dobson et al., 2003, 2004) most large-scale (2–3D) models for geothermal areas have only taken into account fluid flow and heat transfer (e.g., Clearwater et al., 2012; McKenna and Blackwell, 2004; Moulding and Brikowski, 2012). Fully coupled reactive transport models of field scale geothermal systems are rarely found in the litera- ture. Exceptions are simulations of enhanced geothermal systems ∗ Corresponding author. Tel.: +1 5104958147; fax: +1 5104865686. E-mail addresses: cwanner@lbl.gov, christoph.wanner@geo.unibe.ch (C. Wanner). 1 Current address: Instituto de Energías Renovables, Universidad Nacional Autónoma de México, Temixco, Morelos 62580, Mexico. (EGS) (Bachler and Kohl, 2005; Sonnenthal et al., 2012; Taron and Elsworth, 2009), formation of scale within geothermal wells (Alt- Epping et al., 2013; Xu et al., 2004) or the simulation of shallow hydrothermal systems (Jones and Xiao, 2006; Xu and Pruess, 2001). In this study, a 2D reactive transport model of the Dixie Valley geothermal area (Nevada, USA) was developed to assess fluid flow pathways and fluid rock interaction processes. The Dixie Valley geothermal field, located in the Basin and Range province of the western US, was chosen as an example study because it has been used for power production (ca. 63 MW) over the last two decades and has been extensively characterized (Blackwell et al., 2007, and references therein). Our reactive transport model specifically benefits from the availability of an extensive geochemical and isotopic dataset (Goff et al., 2002). Field scale features include geothermal springs with temperatures up to 84 ◦ C (Goff et al., 2002), subsurface temperatures in excess of 280 ◦ C at 3 km depth, the absence of known magmatic heat sources and an elevated basal 0375-6505/$ – see front matter © 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.geothermics.2013.12.003