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05/21/2014

Integrated Solute Geothermometry

Source:  Nicolas Spycher and Dan Hawkes

Nic_Fig-1
Example multicomponent geothermometry using a geothermal water from Long Valley, California. The temperature is determined from the saturation indices of all minerals shown. Top: saturation indices as a function of temperature. Bottom: minimization of saturation indices using standard statistical functions (median, mean, root mean square and standard deviation). Results of classical geothermometers are also shown for comparison (using the reconstructed composition of the deep fluid).

Solute geothermometers have been successfully used for decades to infer the temperature of deep geothermal reservoirs (temperature being a key parameter in evaluating how productive a geothermal source could be) from analyses of spring or exploration-well fluid samples. However, traditional geothermometers relying on the concentrations of one or a few solutes have limitations, particularly when geothermal fluids ascending to the ground surface are affected by gas loss, mixing, or dilution with shallower waters, masking their deep geochemical signatures.

A team of ESD geochemists (with Nicolas Spycher and Loic Peiffer as lead, and also including Christoph Wanner, Eric Sonnenthal, Guiseppi Saldi, and Mack Kennedy) recently revisited a geothermometry method relying on the saturation indices of multiple minerals computed from full chemical analyses of geothermal fluids. The method was initially developed in the early 1980s by Dr. Mark Reed at the U. of Oregon, then Dr. Spycher’s Ph.D. advisor.  The objective of the ESD team was to simplify the application of this method and combine it with numerical optimization, for a more integral application using multiple water analyses simultaneously and from various locations. The reconstruction of the deep geothermal fluid compositions, and geothermometry computations, were implemented into a stand-alone program (Geo-T; http://esd.lbl.gov/research/projects/geot/ ), allowing unknown or poorly constrained input parameters to be estimated by numerical optimization using external parameter estimation software. The reservoir temperature was then estimated by numerically assessing the clustering of mineral saturation indices computed as a function of temperature.

This new geothermometry system was tested with geothermal waters from previous studies, and with fluids at various degrees of water–rock chemical equilibrium obtained from laboratory experiments and reactive transport simulations. The method was further tested and applied at the Dixie Valley geothermal system (see the paper by Peiffer et al.). Such an integrated geothermometry approach presents advantages over classical geothermometers for fluids that have not been fully equilibrated with reservoir minerals, or that have been subject to processes such as dilution and gas loss. The range of applications for this method and other solute geothermometers was further investigated, using a reactive transport model and simulated geothermal springs under various rates of fluid ascent and reaction with surrounding rocks (see the paper by Wanner et al.).  

To read further, go to: http://www.sciencedirect.com/science/article/pii/S0375650513001016

Citation: Spycher, N., L. Peiffer,, E.L. Sonnenthal, G. Saldi, M.H. Reed, and B.M. Kennedy (2014), Integrated multicomponent solute geothermometry. Geothermics, 51, 113-123; DOI: DOI:10.1016/j.geothermics.2013.10.012.

Funding: EERE, Geothermal Technologies Program