Solute Geothermometry Applied to Low-Medium Enthalpy Geothermal Systems
Maria OLGUIN, Loic PEIFFER, Claudio INGUAGGIATO, Christoph WANNER, Patrick DOBSON, Nicolas SPYCHER, Thomas KRETZSCHMAR
One of the most useful tools to estimate the temperature of deep geothermal reservoirs and reduce the costs of exploratory drilling is solute geothermometry, which is based on the chemical analysis of a fluid sampled from a thermal spring or well. The basic assumption of solute geothermometry is that the geothermal fluid reaches chemical equilibrium with certain minerals within the reservoir, and subsequently flows toward the surface without substantially changing its chemical composition. Accordingly, the concentrations in dissolved elements reflect the thermal conditions at depth. The most commonly applied geothermometers (called “classical”) are based on the absolute concentration of one dissolved species, like silica (SiO2), or on the concentration ratio of several dissolved elements such as the sodium-potassium (Na-K), sodium-potassium-calcium (Na-K-Ca) or potassium-magnesium (K-Mg) ratio. In contrast to classical geothermometers, multicomponent geothermometry computes the saturation indices of selected minerals expected in the geothermal reservoir over a wide range of temperatures based on full water analyses. The reservoir temperature is then estimated by the clustering of these saturation indices around zero. For this contribution, we first perform reactive transport simulations to create a wide range of synthetic conditions and further test the response of solute geothermometers. Second, we evaluate the response of classical geothermometers and multicomponent geothermometry when applied to low–medium enthalpy geothermal systems (with reservoir temperatures below 180°C). Under such thermal conditions, the kinetics of dissolution/precipitation reactions are slower, and hence, the condition of equilibrium may not be achieved. Our evaluation of solute geothermometry was conducted using a database including water chemical compositions (182 analyses) and measured reservoir temperatures. Our results indicate that Na-K based geothermometers provide acceptable temperature estimates (Testimated – Tmeasured = ± 20°C) for only ~ 4% of the fluid compositions. Higher rates of acceptable estimates (up to 60%) are obtained with the multicomponent method when using fixed selected mineral lists for computing the saturation indices. We consider this result as promising considering that we did not perform a case-by-case analysis of each geothermal system contained within the database.
|        Topic: Geochemistry||Paper Number: 14134|