Geothermal fluids

There are two general geothermal resourees, dry (steam) fields and wet (brine) fields. More than 800 MWe is being produeed from sueh dry geothermal steam fields in Northern California. The wet fields usually eannot be used in this manner and Rankine eyele-type systems, ealled binary plants, are being eonsidered at sueh loeations. At the wet fields found in the Imperial Valley of Southern California, the geothermal fluid is a 250°C brine, whieh does not lend itself for use in eonventional steam turbines. 

Browne, P.R.L. (1978) Hydrothermal alteration in active geothermal fluids. Ann. Rev. Earth Planet. Sci, 6, 229-250. 

White, D.E. (1965) Metal contents of some geothermal fluids Symposium on Problems of Postmagmatic Ore... 

Fig. 2.28. Plot of logarithm of activity product (log Q) versu.s reservoir temperature. Solid circles stand for activity products of Ca + and in geothermal fluids, and open circles for Ca + and CO. Small and...

Nitta, T., Adachi, M., Takahashi, M., Inoue, K. and Abe, Y. (1991) Heavy metal precipitation from geothermal fluid of 87N-15T production well in the Okuaizu geothermal field, Tohoku District, Japan. Mining Geology, 41, 231-242 (in Japanese). 

Their results show that the redox couples in a sample generally failed to achieve equilibrium with each other. For a given sample, the Nernst Eh values calculated for different redox couples varied over a broad range, by as much as 1000 mV. If the couples had been close to redox equilibrium, they would have yielded Nernst Eh values similar to each other. In addition, the authors could find little relationship between the Nernst values and Eh measured by platinum electrode. Criaud et al. (1989) computed similarly discordant Nernst Eh values for low temperature geothermal fluids from the Paris basin. 

Fig. 23.1. Variation in pH in a computer simulation of sampling, cooling, and then reheating a hypothetical geothermal fluid. Bold line shows path followed when system is held closed fine lines show variations in pH when fluid is allowed to degas CO2 as it cools.

Fig. 23.2. Changes in concentration of aqueous species H+, CC>2(aq), and HC1 with temperature during cooling of a geothermal fluid as a closed system. A positive value indicates an increase in concentration relative to 250 °C a negative value represents a decrease.

Fig. 23.4. Mineral saturation indices (log Q/K) over the course of simulating the reheating of a hypothetical geothermal fluid. Bold lines show indices for minerals assumed to be present in the initial formation fine lines show values for other minerals. Dashed line marks sampling temperature (250 °C).

For the Hveragerdi 4 well, we follow the same procedure, using the data in Table 23.2 and the calculations already shown. In this case, the model predicts that a number of minerals in the LLNL database are supersaturated near the inflow temperature of 181 °C. Close examination reveals that each of the supersaturated minerals contains either Mg++, Ca++, or Fe++, components that are characteristically depleted in geothermal fluids. The Mg++ concentration in this fluid, for example, is just 2 p,g kg-1. 

Criaud, A., C. Fouillac and B. Marty, 1989, Low enthalpy geothermal fluids from the Paris basin, 2 - oxidation-reduction state and consequences for the prediction of corrosion and sulfide scaling. Geothermics 18, 711-727. 

Table 9.13 Gas composition in geothermal fluids (from Henley, 1984). = pressure of...

The dissolved solids and gases in geothermal fluids have been divided into two types, reactive and conservative components (Giggenbach 1991). Temperature-dependent equilibria between solution and hydrothermal minerals fix the aqueous concentrations of the reactive components, at least if temperatures exceed some 100 C (e.g., Giggenbach 1980, 1981 Arnorsson et al. 1983). 

Methane. The concentrations of CH4 in geothermal fluids are quite variable (see Table 4). In some geothermal systems, the CH4 aquifer fluid concentration is apparently controlled by attainment of equilibrium for the reaction ... 

---