MULTICOMPONENT EQUILIBRIUM GEOTHERMOMETRY APPLIED TO THE RAFT RIVER GEOTHERMAL AREA, IDAHO: PRELIMINARY RESULTS

The Raft River Geothermal (RRG) area in Idaho is a designated Known Geothermal Resource Area (KGRA) with a temperature in the range of 135-150 ºC. It has been a site for intense exploratory and production activities since late 1970s, thus the RRG area offers a well characterized geothermal system to test a multicomponent equilibrium geothermometry (MEG) approach. We have developed a new MEG optimization approach to find a common equilibrium temperature for geothermal fluid for a likely reservoir mineral assemblage by minimizing the weighted sum of squares of the mineral saturation indices through modification of temperature and volatile components (e.g., water and carbon dioxide gain or loss). An automated MEG tool called RTEst (Reservoir Temperature Estimator) that couples PEST and Geochemist’s Workbench is applied to RRG area. Water compositions from four RRG wells (RRG1, RRG2, RRG3, and RRG5) using several alternate mineral assemblages consisting of clays, zeolites, carbonates, feldspars, and silica-polymorphs were considered in the development of temperature estimates. The results indicate that RRG waters are affected by composition altering processes (such as boiling, mixing, and CO₂ degassing). For example, in most cases, our results suggest that RRG waters have lost significant amounts of CO₂ due to degassing. In general, results for RRG wells indicate that MEG will lead to an erroneous temperature estimates if composition altering processes are not taken into account. When these effects are taken into account, RTEst estimated an average reservoir temperature of 158 ºC with 95% confidence limit of ±28 ºC for the RRG. Compared to the measured production well temperatures, the RTEst optimized temperature estimate for the reservoir is slightly higher. Similarly, when compared, the RTEst temperature estimates are similar in accuracy with quartz (no steam loss) temperature estimates (136 – 164 ºC) but lower than Na-K-Ca (169 – 187 ºC) and Na-K (183 – 208 ºC) temperature estimates. However, unlike traditional geothermometers, the RTEst’s capability to predict temperatures up to 300 ºC with associated uncertainties in the temperature estimates improves our ability to predict and interpret reservoir temperatures for geothermal resource exploration.