An Integrated Sensitivity analysis for the Basalt Specific Multicomponent Geothermometer for High Temperature Settings

Abstract

For a successful geothermal reservoir exploration, an in-situ temperature estimation is essential. Since geothermometric reservoir temperature estimations often entail high uncertainties, statistical approaches are used. The focus is on the application of sensitive analyses on a basalt specific mineral set as multicomponent geothermometer to estimate the reservoir temperatures in Krafla, high-temperature geothermal field, Iceland. In quantitative geothermometry, the element ratios and mineral saturation of the geothermal fluid serve as single geothermometers. The geochemical equilibrium between mineral phases and the reservoir rock are used to obtain the reservoir temperature. The coupling of several minerals serving as a multicomponent geothermometer allows to get statistically robust temperature estimations. Herein, we set up a specific mineral set for basaltic reservoir rocks, which are calibrated by in-situ measurements of the reservoir temperature in Krafla. The developed method uses IPhreeqc to determine the geochemical equilibrium conditions, followed by the statistical evaluation conducted with a Matlab-based in-house tool called MulT_predict. The results are presented via box plots. The evaluation of the dataset from Krafla allows the calibration of a basalt specific mineral set for the most accurate reservoir temperature estimation. As surface measurements of pH, aluminum concentration and steam loss do not reflect reservoir conditions, further sensitivity analyses are combined to back calulate these parameters in order to improve the temperature estimation. This statistical evaluation reflects the most plausible reservoir conditions. It is shown that, the variation of the redox potential, iron and magnesium concentration have only negligible effects and thus can be discarded, the correct determination of the in-situ pH, aluminum concentration and steam loss are essential for a robust temperature estimation. The calculated reservoir temperature matches the measured in-situ reservoir temperature with an overall spread of 1.7% of the total measured median temperature. In conclusion, the developed method is a promising tool for the estimation of reservoir temperatures. In addition, it is an economical exploration tool that allows a high precision temperature estimation. Since the developed basalt specific multicomponent geothermometer also uses secondary mineralization it could be adapted to different geothermal settings, yet requiring further calibration and validation.