Page 138 - Geothermal Energy Renewable Energy and The Environment

124 Geothermal Energy: Renewable Energy and the Environment

The decision on what cut-off temperature to use in an assessment will be determined by the
technology that it is assumed will be available for power generation. As discussed in Chapter 9,
power generation at temperatures as low as 150°C or less can be accomplished with binary power
generating systems. However, such systems must also be utilized in settings in which there is suf-
ficient cooling capacity to meet the thermodynamic aspects of maximizing the delta T between
production fluid temperature and the cooling capacity of the system. In the assessment performed
by Williams et al. (2008a), a temperature of 150°C is utilized to define the lower temperature bound
for defining the reservoir.
Establishing a reservoir temperature structure is rarely possible, except in instances where wells
have been drilled and the thermal profile for each well monitored. In the absence of such data, the
most useful means for estimating the reservoir temperature comes from geothermometers, such as
those described in Chapter 6. Reed and Mariner (2007) and Williams et al. (2008a) conclude that
the most reliable geothermometers are the silica, K–Mg, and Na–K–Ca geothermometers. The
silica geothermometer provides good results, particularly if the formulation of the geothermom-
eter considers the various effects of different silica polymorphs, as does the method developed
by Giggenbach (1992). In the temperature range from 90 to 130°C the K–Mg geothermometer
(Giggenbach 1988) works well, according to the review of results considered by Williams et al.
(2008a). However, it is often difficult to obtain Mg analyses of sufficient quality and consistency,
since Mg concentrations can be quite low and are difficult to do at high precision. In the absence
of Mg analyses, or in waters that are Cl-rich, the Na–K–Ca geothermometer (see Chapter 6 for a
discussion) is utilized.
When using such data it is important to keep in mind the limitations of computed temperatures.
It is generally assumed that the temperature that is obtained from a geothermometer reflects the
equilibrium chemical composition of the water at the highest temperature the water experienced,
and that there was, therefore, very little reequilibration of the water as it ascended through the rock
mass on its way to the surface. The assumptions that equilibrium was achieved and that reequilibra-
tion did not occur are not wholly justified, since reaction rates do not go to zero once the fluid mass
passed beyond the thermal maximum. Recall Equation 5.8,

R = S × k × T × α × ϕ × ∏a × (1 − Q/K) (5.8)
ω
fac
A
i
2
where R is the rate (moles/s), S is the effective surface area exposed to the fluid (cm ), k is the
A
far-from-equilibrium rate constant (moles/cm -s), T is the temperature correction factor for the
2
fac
rate constant k (usually an Arrhenius function), α is a power function that accounts for changes
in the rate close to equilibrium conditions, ϕ is a function that modifies the rate for precipitation
relative to that for dissolution that is based on experimental data, a accounts for the dependence
i
of the rate on the activities of specific components in solution, and ω is power dependence based
on experimental data that accounts for the particular dissolution or precipitation mechanism.
Note that in this formulation, R is temperature dependent and thus will not go to zero unless
equilibrium is achieved at the T of interest. Once equilibrium is achieved, the term k becomes
zero and R goes to zero, but only for that specific T. As the fluid migrates to lower tempera-
tures, k is no longer zero, and increases in magnitude as the delta T between the equilibrium
temperature and the T along the flow path increases. The extent to which reequilibration will
occur will depend upon the residence time of the fluid in the lower temperature regime and is
thus dependent on the flow rate—the higher the flow rate, the lower will be the extent to which
reequilibration takes place.
These considerations suggest that the computed temperature from a geothermometer is likely to
underestimate the actual reservoir temperature. The extent to which this is the case is not readily
determined. For that reason, most geothermometer temperatures used in an assessment should be
considered conservative.