Page 116 - Geothermal Energy Systems Exploration, Development, and Utilization

92 2 Exploration Methods
immature unstable waters and gives an initial indication of mixing relationships or
geographical groupings. The diagram distinguishes several types of thermal water
including immature waters, peripheral waters, volcanic waters, and steam-heated
waters. It gives a preliminary statistical evaluation of groupings and trends.
The Cl–Li–B triangular diagram is useful for evaluating the origin of geothermal
fluid. The alkali metal probably least affected by secondary processes is lithium
(Li). It may therefore be used as a tracer for the initial deep rock dissolution process
and as a reference to evaluate the possible origin of two important conservative
constituents of geothermal waters. The boron (B) content of thermal fluids is likely
to reflect to some degree the maturity of a geothermal system; because of its
volatility, it is expelled during the early heating up stages. It is striking that both
Cl and B are added to the Li containing solutions in proportions close to those in
crustal rocks. The Cl/B ratio is often used to indicate a common reservoir source
for the waters.
Four types of waters circulating in high enthalpy geothermal systems are
generally described (Ellis and Mahon, 1977; Henley et al., 1984; Giggenbach, 1988;
Truesdell, 1991). It must be underscored, however, that each of these waters may
mix with each other giving rise to hybrid water types.

2.5.5.1 Sodium–Chloride Waters
Waters circulating in deep, high enthalpy geothermal reservoirs usually have
sodium–chloride composition and chloride contents ranging up to 10 000 mg
−1
kg .pHofthese waters is close (±1 or 2 units) to the neutral pH at depth (e.g.,
◦
5.5–5.6 at 200–300 C). Silica, potassium, lithium, boron, fluoride are much higher
than in cold waters. The high chloride waters also contain appreciable calcium.
Magnesium is instead much lower than in cold waters. The main dissolved gases
are CO 2 and H 2 S.
In general, the waters circulating in deep, high enthalpy geothermal reservoirs
are mainly of meteoric origin, but in some systems connate or other saline waters
may be present. In geothermal systems with close volcanic–magmatic association
and located along convergent plate boundaries, the deep, magmatic heat source
may add acid gases such as HCl, HF, SO 2 ,H 2 S, and CO 2 as well as some andesitic
water. The ration of chloride to sulfate is high.
Conversion of the initially acid aqueous solutions to neutral sodium–chloride
waters requires extensive rock–water interaction and virtually complete removal
of magmatic sulfur species in the form of sulfates and sulfides. The deep
sodium–chloride waters may flow directly to the surface and discharge from
boiling, high chloride springs, whose pH ranges from near neutral to alkaline;
alternatively, they may mix with shallow, low-salinity waters to give relatively diluted
chloride waters.

2.5.5.2 Acid–Sulfate Waters
Acid–sulfate waters are typically found above the upflow part of the geothermal
systems, where steam separation takes place. Boiling results in the transfer of gas
species, mainly CO 2 and H 2 S, into the vapor phase. This vapor phase can reach