Page 124 - Geothermal Energy Systems Exploration, Development, and Utilization

100 2 Exploration Methods
2.5.7.4 Gas (Steam) Geothermometers
There are essentially three types of steam geothermometers:
• those based on gas–gas equilibria that require only data on the relative abundance
of gaseous components in the gas phase;
• those based on mineral–gas equilibria involving H 2 S, H 2 ,and CH 4 , but assuming
CO 2 to be externally fixed according to empirical methods;
• those based on mineral–gas equilibria that require the information on gas
concentrations in steam.
Steam geothermometry is more difficult to handle than water geothermometry.
Gas concentrations in geothermal reservoir fluids are affected by the ratio of steam
to water of that fluid (Chiodini et al., 1991a). The gas content of fumarole steam
is affected by the boiling mechanism in the upflow, steam condensation, and the
separation pressure of the steam from parent water (D’Amore and Panichi, 1980).
The flux of gaseous components into geothermal systems from their magmatic heat
source may be quite significant and influence how closely gas–gas and mineral–gas
equilibria are approached in specific aquifers. The gas geothermometers are useful
for predicting subsurface temperatures in high temperature geothermal systems.
They are applicable to systems in basaltic to acidic rocks and in sediments with
similar composition, but should be used with reservation for systems located in
rocks, which differ much in composition from the basaltic to acidic ones (Arnorsson
and Gunnlaugsson, 1985).

2.5.7.5 Isotope Geothermometers
Several isotope-exchange reactions can be used as subsurface temperature indi-
cators. It may not be possible to apply gas isotope geothermometers in some
liquid-dominated geothermal systems during the exploration phase because of lack
of natural gas manifestations. The exchange of 18 O between dissolved sulfate and
liquid water is most useful because of rapid equilibrium at reservoir tempera-
◦
tures greater than 200 Cand pH < 7, conditions that favor sulfate ion–water
exchange (Arnorsson, 2000). The sulfate water 18 O geothermometer gives correct
reservoir temperature prediction. Water isotopic composition may also be affected
by evaporation or mixing with shallow groundwater.

2.5.8
Forecast of Corrosion and Scaling Processes

The solids and gases occurring in geothermal fluids are thermodynamically capable
of causing corrosion by attacking metal surfaces or scaling during the utilization
cycle (Vetter and Kandarpa, 1987). During geothermal reservoir exploitation, tem-
peratures may range from 350 C at the bottom of the well to 60 Corsoin the
◦
◦
reinjection lines, and associated pressures can vary from 300 to 0.08 bar. Of course,
geothermal fluids may contact air in the condenser and when reinjected. However,
the chemical composition of these fluids will range from almost pure water to hot
brine with a total dissolved solid content of 200 g l −1 or more. Consequently, it is