PROCEEDINGS, Thirty-Eighth Workshop on Geothermal Reservoir Engineering Stanford University, Stanford, California, February 11-13, 2013 SGP-TR-198 ASSESSING THERMO-HYDRODYNAMIC-CHEMICAL PROCESSES AT THE DIXIE VALLEY GEOTHERMAL AREA: A REACTIVE TRANSPORT MODELING APPROACH Christoph Wanner 1 , Loic Peiffer 1 , Nicolas Spycher 1 , Eric Sonnenthal 1 , Jon Sainsbury 2 , Joe Iovenitti 2 , B. Mack Kennedy 1 1 Lawrence Berkeley National Laboratory, 1 Cyclotron Road, Berkeley, CA, 94720, USA 2 AltaRockEnergy Inc., 2320 Marinship Way, Sausalito CA, 94965, USA e-mail: cwanner@lbl.gov ABSTRACT A 2D reactive transport model of the Dixie Valley, Nevada, geothermal area was developed to assess fluid flow pathways and fluid rock interaction processes. Setting up the model included specification of the mineralogy of the different rock units, the formulation of the corresponding mineral dissolution and precipitation reactions, the explicit definition of two major normal faults and the specification of a dual continuum domain along the uppermost 1 km of one of these normal faults. The model was run using a range of permeabilities for the dual continuum fault, whereas bulk rock fluid flow and thermal parameters were defined according to a previous flow simulation study performed by others. Model results were tested against available field data such as chemical analysis of thermal springs, isotherms inferred from geothermal wells, and results of the previous modeling study. Moreover, simulated chemical compositions for the geothermal spring were combined with multicompo- nent geothermometry to assess whether the model reflects the observation that geothermal springs often are out of chemical equilibrium. Simulation results reveal that a minimum permeability of 10 -12 m 2 for the spring-feeding fracture is needed to preserve the geochemical signature of the reservoir. The simulations also suggest that the presence of such small-scale spring-feeding fractures having an elevated permeability can significantly alter the shallow fluid flow regime of geothermal systems. INTRODUCTION Numerous geothermal systems are present in the Basin and Range province of the western US. One of them, the Dixie Valley geothermal area, 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). Typical field scale observations include the presence of geothermal springs with temperatures of 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 heat flux on the order of 90 mW/m 2 that is typical for the Basin and Range province (McKenna and Blackwell, 2004). Recent investigations also showed that near surface groundwater temperatures can be on the order of 100°C for isolated locations. The general understanding of Basin and Range geothermal systems is that meteoric water infiltrates via the range top or valley floor, heats up during deep circulation and ascends along the highest permeable pathway such as range-bounding normal faults (McKenna and Blackwell, 2004). To what depth the fluid circulation goes, however, is still under debate. Helium isotopic studies revealed that ca. 7.5% of the He in the Dixie Valley system is derived from mantle sources, requiring fluid input from below the brittle-ductile transition (Kennedy and van Soest, 2006). McKenna and Blackwell (2004) postulated a large scale fluid convection cell where infiltrating meteoric water reaches a depth of up to 8 km depth before it finally ascends to the surface. Moulding and Brikowski (2012) on the contrary, argued that such deep fluid infiltration seems unrealistic considering that the lithostatic stress at this depth reduces the rock permeability needed to establish significant advective fluid flow (e.g., 5 x 10 -17 m 2 , McKenna and Blackwell, 2004). In this paper we present a reactive transport model of the Dixie Valley geothermal area to assess and improve the current understanding of thermo- hydrodynamic-chemical processes. Compared to classical fluid flow and heat transfer simulations, reactive transport modeling bears the advantage of having additional model constraints by comparing chemical compositions observed in geothermal wells and springs with simulated fluid compositions. An additional model constraint was obtained by combining reactive transport modeling with multicomponent chemical geothermometry.