Page 112 - Geothermal Energy Systems Exploration, Development, and Utilization

88 2 Exploration Methods
In order to calculate the saturation index, the chemical speciation in the aque-
ous solution has to be reconstructed first, taking into account all ion complexes
(Bowers, Jackson, and Helgeson, 1984). Due to these effects of ion complex-
+
+
ing, total (analytical) concentration ratios of compatible cations (Na ,K ,Ca ,
2+
2–
–
–
2+
and Mg ) and compatible anions (SO ,F ,and HCO ) diverge, to variable
4 3
amounts, from free ions activity ratios, which are uniquely fixed, at a given temper-
ature or temperature–P CO2 condition, by mineral–solution equilibrium (Helgeson,
Kirkham, and Flowers, 1981). This is a great step forward with respect to simple
geothermometers (Guidi et al., 1990). Calculations have to be carried out with the
aid of a computer program, specifically implemented for this purpose (Reed and
Spycher, 1984; Reed, 1982; Pitzer, 1981). Since most hydrothermal minerals are
aluminum silicates, the aluminum concentration in the aqueous solution has to
be introduced in the computations. However, the absence or the poor analytical
quality of Al data is not a major problem and it can be circumvented assuming that
aluminum concentrations are constrained by equilibrium with a given aluminum
silicate (Pang and Reed, 1998).
In addition to geothermal waters, analysis of gases escaping from geothermal
areas or in the fluid produced by the geothermal wells is useful in understanding
subsurface conditions (D’Amore and Panichi, 1987; D’Amore and Nuti, 1977).
Geothermal gas samples are usually analyzed for H 2 O, CO 2 ,H 2 S, NH 3 ,He, Ar,
O 2 ,N 2 ,H 2 ,CH 4 , and CO; in addition to these constituents, HCl, HF, and oxidized
Sspecies (mainlySO 2 ) have to be taken into account too in high temperature
volcanic gases (Giggenbach, 1996).
In general, the major component of geothermal gases is H 2 O, which is followed
by CO 2 and H 2 S in order of decreasing importance. Other gas species present
in lower concentrations are N 2 ,H 2 ,CH 4 ,CO,NH 3 , Ar, and He. Strongly acid
gases, that is, SO 2 , HCl, and HF, which are typical of fluids degassed from magma
bodies (Chiodini et al., 1992), are virtually absent in geothermal fluids (Giggenbach,
1980). Sulfur dioxide was detectable only in some fumarolic discharges from active
volcanic areas.
Similar to what is done for waters, for which a first classification step is needed
before investigating mineral–solution equilibria, also for gases it is convenient
to carry out an initial evaluation involving the less reactive constituents, with the
aim to get information on the possible origin of fluid components, on the main
processes controlling their distribution and on the secondary processes possibly
interfering with gas equilibria evaluations (Chiodini and Marini, 1998). The most
obvious constituents to use are N 2 , Ar, and He, as suggested by Giggenbach
(1991a).

2.5.4.1 Boiling and Mixing
Depending on pressure and temperature conditions, the main component of
geothermal fluids, H 2 O, can be present in different physical states. The presence
of a single liquid phase in the geothermal reservoir is the most frequent situation,
but it is not the only one, since either a two-phase liquid–vapor mixture or a
single vapor phase can also be present in the reservoir. These possibilities can be