Page 113 - Geothermal Energy Systems Exploration, Development, and Utilization

2.5 Geochemistry 89
ascertained by accurate enthalpy data or by gas geochemistry (Giggenbach, 1980;
Bertrami et al., 1985).
The geothermal wells, which tap a single liquid phase at temperatures above
◦
100 C under reservoir conditions, obviously discharge two-phase liquid–vapor
mixtures, which are generated through boiling of the original single liquid phase.
Sampling and analysis of both liquid and vapor phases, separated at known
pressure and temperature conditions, are required to recalculate the composition
of the original single liquid phase (Drummond, 1981). The two phases can be
separated by means of a wellhead pressure separator. When samples of separated
liquid and vapor phases are collected from a geothermal well, it is necessary to
know the separation temperature and/or pressure and the well-bottom temperature
and/or pressure and total discharge enthalpy (steam to water ratios).
Where boiling (steam separation) occurs there is a partitioning of dissolved
elements between the steam and the residual liquid; dissolved gases and other
relative volatile components concentrate in the steam and nonvolatile components
become concentrated in the liquid in proportion to the amount of steam that
separates. When steam separation takes place, the less soluble gases (e.g., N 2 ,
H 2 ,CH 4 , and CO) enter preferentially the vapor phase, while the more soluble
gases (CO 2 ,H 2 S, and NH 3 ) are retained in part in the aqueous phase (Truesdell,
1975).
The physical mechanisms of boiling processes are extremely complicated. Two
limiting mechanisms of boiling can be recognized (Arnorsson, 2000; Tonani,
1970): single-step separation, where the steam, continuously produced by decom-
pression of the uprising liquid, remains in contact and in equilibrium with the
liquid until it is separated in a unique separation event; and continuous sepa-
ration, where the steam is continuously separated from the liquid as soon as it
forms. An infinite number of intermediate mechanisms can also exist (multistep
separation).
However, hot waters ascending from a geothermal reservoir may cool in upflow
zones not only conductively and/or by boiling due to depressurization but also by
mixing in the upflow with shallow, relatively cold water. Since cold waters are most
often lower in dissolved solids than geothermal waters, mixing is often referred to
as dilution. Large variations in the temperature and flow rates of thermal springs
in a particular field that can be linked with parallel variations in the concentrations
of nonreactive components in the water, such as Cl, usually constitute the best
evidence that mixing has occurred (Marini and Cioni, 1985).
Mixing models have been developed to allow estimation of the hot water
component in mixed waters emerging in springs or discharged from shallow drill
holes (Truesdell and Fournier, 1977). There are essentially three kinds of mixing
model:

• the chloride–enthalpy mixing model;
• the silica–enthalpy warm spring mixing model;
• the silica–carbonate mixing model.