Differences between geothermal and experimentally derived fluids: How well do hydrothermal experiments model the composition of geothermal reservoir fluids?

Abstract

Fluid parameters from a diverse set of dilute water/rock experiments (80–300°C) were compared with reservoir data from several geothermal fields. The comparisons show that experiments of this type can reproduce many properties of geothermal reservoir fluids. Compositional parameters for most species in experimentally derived fluids are temperature dependent but are relatively independent of rock type, initial fluid composition, and water/rock mass ratio. At lower temperatures (≤250°C), many experimental parameter-temperature trends agree with geothermal trends. However, at higher temperatures (≤250°C), the geothermal fluids have calculated high-temperature pH values 1 to 2 pH units lower than do the experimentally derived fluids. This produces a consistent offset between values of experimental and natural cation/proton activity ratios. Two possible experimental reasons for offsets are pervasive metastable mineral formation in experiments; and absence, in experiments, of the equivalent of a magmatic or metamorphic gas input. Results from current experiments suggest that, while metastability may be partially responsible for offsets, observed geothermal fluid parameters can be duplicated by addition of CO 2. Furthermore, maintenance of high ƒCO 2, low pH conditions requires a CO 2 flux. Finally, in geothermal fields, addition of exotic CO 2 or other acid gases during upflow or sampling of wet steam discharges may also contribute to such offsets by producing lower than actual reservoir pH values. Thus the offsets could suggest that some geothermal fluid pH values are more basic than commonly calculated and closer to those pH values produced during dilute water/rock experiments.