Preliminary Assessment of Fluid Mineral Equilibria in Thermal Waters from Sedimentary Formations: Implications for Geothermal Resources

Abstract

The states of fluid-mineral equilibria in hot sedimentary aquifers were determined for thermal waters (100-220°C) from sites in the USA (San Joaquin, California; Grant Canyon-Bacon-Flat, Nevada; East Texas; Thermo Hot Springs, Utah) and New Zealand (Ngawha). In these examples, host rocks comprise siliciclastic and carbonate units. All the waters are neutral pH, and most are rich in chloride (100-35,000 ppm) relative to bicarbonate and sulfate. The results indicate that at reservoir conditions thermal waters are close to saturation in quartz or chalcedony. Although calcium and bicarbonate concentrations range widely, all the waters are calcite saturated and many are dolomite saturated. The hottest thermal water (New Zealand, 220 °C) is the only one that is close to “full equilibrium” with a mineral assemblage comprising quartz, albite, K-feldspar, K-mica, and Mg-chlorite. All the other thermal waters show strong departures from equilibria involving these minerals, which implies there will be problems in the application of Na-K and K-Mg chemical geothermometers. Equilibration temperatures computed using the Na-Li and Mg-Li empirical chemical geothermometers show a modest correlation in comparison to well temperature. Reaction path calculations were computed to assess scaling potential due to conductive cooling in a closed loop circuit in which pressures exceed vapor/gas saturation. The results show that mineral deposition is unlikely, mainly because carbonate and sulfate minerals have reverse solubility, and quartz deposition is impeded by reaction kinetics. Heating on injection, assuming no gas loss, has the opposite effect, and the solution will return to calcite saturation if the original reservoir temperature is achieved.