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Numerical Model Assesses Nevada Geothermal Area

Source:  Nicolas Spycher and Dan Hawkes

Example steady state temperature distribution and fluid flow field computed for one of the various tested cases of fracture permeability and fluid flow velocity. Temperature contours refer to the rock matrix temperature. Computed temperatures (as a function of elevation) within the main modeled fracture, and rock matrix surrounding that fracture, are shown in the graph on the right. Modeled compositions of the synthetic spring fed by such fracture were used to test various chemical geothermometry approaches.

To investigate the Dixie Valley geothermal system (in Nevada), and the application of solute geothermometry in similar systems, a team of ESD geochemists (with Christoph Wanner and Eric Sonnenthal as lead, and also including Loic Peiffer, Nicolas Spycher, and Mack Kennedy) developed a new reactive transport model of this geothermal region, relying in part on recent site data provided through the courtesy of Alta Rock Energy (Joe Iovenitti). Specifically, the ESD group aimed to assess fluid flow pathways and fluid rock interaction processes at this site. Their model included two major, normal 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 suggest that the presence of small-scale fracture systems having an elevated permeability of 10−12 to 10−10 m2 can significantly alter the shallow fluid flow regime of geothermal systems. For the Dixie Valley case, the model implies that such elevated permeability leads to a shallow convection cell where superficial water infiltrates along the range front normal fault and connects the small-scale geothermal spring through basin filling sediments.  The model also accurately reproduced the chemical composition observed in geothermal springs at Dixie Valley. 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 cooling conditions without mixing or boiling, some traditional geothermometers performed best because these are least affected by mineral precipitation upon cooling (also see paper by Peiffer et al.). Temperature estimates based on mineral saturation indices were more sensitive to re-equilibration upon cooling, but showed good results for fluid velocities above ~0.1 m/d and reactive fracture surface areas 1–2 orders of magnitude lower than the corresponding fracture geometric surface area. This suggests that such upflow rates and relatively low reactive fracture surface areas are likely present in many geothermal fields.

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Citation: Wanner, C., L. Peiffer, E. Sonnenthal,  N. Spycher, J. Iovenitti, and B.M. Kennedy (2014), Reactive transport modeling of the Dixie Valley geothermal area: Insights on flow and geothermometry. Geothermics, 51, 130-141; DOI: 10.1016/j.geothermics.2013.12.003.

Funding: EERE, Geothermal Technologies Program