Validation of multicomponent geothermometer with sensitivity analysis in basaltic geothermal setting

Reservoir temperature estimation is a key technique in successful geothermal reservoir exploration. Since often the errors in reservoir temperature estimations are high, statistical approaches are used. The focus on this study lies in the application of multicomponent geothermometry to estimate the reservoir temperature in Krafla and Námafjall geothermal fields, Iceland.
In quantitative geothermometry the mineral saturation and element ratios of the geothermal fluid are used to obtain the reservoir temperature based on the geochemical equilibrium between mineral phases and the reservoir rock. Thereby, the saturation state of different minerals serves as a geothermometer and hence the coupling of several geothermometers allows for statistically robust estimates. Within this study the reservoir temperatures of the geothermal system are determined by in-situ measurements. The objective is to define a typical mineral set for basaltic reservoir rocks by calibration with the measured data.
Krafla and Námafjall are high-enthalpy geothermal reservoirs located in the neo-volcanic zone of Northeast Iceland. Magma heated geothermal brines are expected to reach 350°C at a depth of 2000 m. ... mehrThe geochemical data of the well discharges were collected by Guðmundsson and Arnórsson (2002) as well as Arnórsson et al. (1983).
Geochemical equilibrium calculations are done using PhreeqC, while the statistical evaluation in form of box-plots is conducted in Matlab. The evaluation of individual datasets from Krafla and Námafjall allows the calibration of a specific set of minerals allowing a most accurate temperature estimation in basaltic reservoirs. As in-situ measurements of pH, aluminuium concentration and redox potential do not reflect reservoir conditions, a further sensitivity analysis is conducted to improve the estimated reservoir temperatures. Therefore, the variations of the value yield a minimization of estimations errors reflecting the most plausible valuation. It can be shown that in all cases the temperature estimations match with the measured temperature ranges of the reservoirs. While the variations of pH and aluminium concentration effect the estimated temperatures, the redox potential has only negligible effect and thus can be discarded. To test the developed and calibrated method the procedure is applied for natural spring water of the geyser Uxahver. The calculated reservoir temperature matches the measured reservoir temperature with an error of ±10%. In conclusion the developed method tends to be 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 multicomponent geothermometry uses secondary mineralization it can be adapted to different geothermal settings yet requiring further calibration.