Page 122 - Geothermal Energy Systems Exploration, Development, and Utilization

98 2 Exploration Methods
deeper reservoirs. Under some conditions, mixing models may be used to estimate
reservoir temperatures and salinities in deeper reservoirs. Geochemistry can be
used to estimate reservoir temperatures encountered by newly drilled wells long
before temperature logs (Rybach and Muffler, 1981). Fluid–mineral equilibrium
modeling helped the selection and improved the use of conventional chemical
geothermometers.

2.5.7.1 Geothermometric Methods for Geothermal Waters
The ratios of the cations in geothermal fluids are controlled by temperature-
dependant water–rock interactions with chemical equilibrium attained in high
temperature reservoirs when fluid residence times are relatively long (years). Water
geothermometers can be classified into two groups (Nicholson, 1993):
• those based on temperature-dependent changes in the solubility of individual
minerals, such as the silica geothermometers;
• those based on temperature-dependent ion exchange reactions, which involve at
least two minerals and the aqueous solution, thus fixing the ratios of suitable
dissolved constituents (e.g. the ionic solutes geothermometers).
It must be stressed that all geothermometers are used assuming that (Arnorsson,
2000)
• the geothermal liquid is in equilibrium with relevant hydrothermal minerals in
the reservoir;
• the pore fluid pressure in the reservoir is fixed by coexistence of liquid and steam;
• the geothermal liquid cools either conductively or adiabatically, through steam
separation at 100 C;
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• the geothermal liquid does not mix with cold, shallow waters during the ascent
toward the surface;
• the geothermal liquid does not precipitate any relevant mineral phase along the
upflow path.

2.5.7.2 Silica Geothermometer
The rate of quartz dissolution and precipitation depends strongly on temperature
and is relatively fast at high temperatures and very slow at low temperature (Rimstidt
and Barnes, 1980). This explains why in the geothermal reservoirs of constant, high
◦
temperature (generally >180 C), liquids attain saturation with respect to quartz,
after relatively long water–rock interaction, and little dissolved silica polymerizes
and precipitates during the relatively fast ascent of geothermal waters, even
though saturation with respect to quartz is largely exceeded. On the other hand,
amorphous silica precipitates relatively fast when saturation is exceeded, although
the lack of solubility and polymerization data at high temperatures, pH, and in
multicomponent solutions limits the understanding of amorphous silica behavior
(Chan, 1989).
◦
At temperatures below 300 C, and at depth generally attained by commercial
drilling for geothermal resources, variations in pressure at hydrostatic conditions