ASSESSING THERMO-HYDRODYNAMIC-CHEMICAL PROCESSES AT THE DIXIE VALLEY GEOTHERMAL AREA: A REACTIVE TRANSPORT MODELING APPROACH

摘要

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 multicomponent 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.