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05/21/2014

Optimized Geothermometry for Geothermal Exploration

Source:  Nicolas Spycher and Dan Hawkes

Spycher_paper2
Example geochemical and isotopic correlation plots for Dixie Valley thermal waters (springs and wells) showing distinct groups of water compositions that were analyzed by optimized multicomponent geothermometry (DMWL, Dixie Valley Meteoric Water Line; WMWL, World Meteoric Water Line).

A team of ESD geochemists (with Loic Peiffer and Nicolas Spycher as lead, also including Christoph Wanner, Eric Sonnenthal, and Mack Kennedy) recently explored a new geothermometry approach to infer geothermal reservoir temperatures (http://esd.lbl.gov/research/projects/geot/). This approach incorporates multicomponent geothermometry coupled with numerical optimization to provide more confident estimates of geothermal reservoir temperatures (see paper by Spycher et al.). They applied this approach to geothermal well and spring waters from the Dixie Valley geothermal area (Nevada), to evaluate the influence of salt brines mixing and dilution of geothermal fluids on calculated temperatures. By grouping spring and well waters according to their geochemical characteristics and processing each group of waters simultaneously, a conceptual model of two main geothermal reservoirs was proposed for Dixie Valley. The first reservoir was inferred to be located along the Stillwater range normal fault system (240–260°C), with a second reservoir (175–190°C) inferred further to the northeast.

Because the chemical evolution of deep geothermal fluids is a combination of multiple time-dependent processes that take place when these fluids ascend to the surface, the group also used reactive transport modeling to assess constraints on the application of solute geothermometers.  This was done initially using simple one-dimensional models and then, in a separate study (see paper by Wanner et al.) using a more complex vertical two-dimensional model of Dixie Valley. Simulation results reveal that Al and Mg concentrations of ascending fluids are sensitive to mineral precipitation–dissolution affecting reservoir temperatures inferred with multicomponent geothermometry. In contrast, simulations show that the concentrations of major elements, such as Na, K, and SiO2, are less sensitive to re-equilibration. Geothermometers based on these elements give reasonable reservoir temperatures in many cases, except when dilution or mixing with saline waters has taken place. Optimized multicomponent geothermometry yields more representative temperatures for such cases. All in all, the results of this study indicated that this new approach holds promise for evaluating geothermal reservoir temperatures in an integrated manner.

To read further, go to: http://www.sciencedirect.com/science/article/pii/S037565051300117X

Citation: Peiffer, L., C. Wanner, N. Spycher, E.L. Sonnenthal, B.M. Kennedy, and J. Iovenitti (2014), Optimized multicomponent vs. classical geothermometry: Insights from modeling studies at the Dixie Valley geothermal area. Geothermics, 51, 154-169; DOI: 10.1016/j.geothermics.2013.12.002.

Funding: EERE, Geothermal Technologies Program