Page 101 - Geothermal Energy Renewable Energy and The Environment

86 Geothermal Energy: Renewable Energy and the Environment

waters listed in Table 5.1 are supersaturated with respect to quartz. At temperatures below 100°C,
all of the waters are supersaturated with respect to all of the polymorphs shown in Figure 5.7. As
a result, deposits of silica on geothermal generating equipment that is in contact with geothermal
fluids have the potential to acquire deposits of silica (silica scale). Indeed, many geothermal power
facilities have in place programs for remediating this problem. This issue is discussed in detail in
Chapters 9 and 12.
The rate at which silica precipitation occurs is not readily predicted, despite the work that has
been done to evaluate silica polymorph precipitation and dissolution kinetics. Carroll et al. (1998)
have shown that the precipitation rate of amorphous silica in New Zealand geothermal systems
is influenced by, among other things, the concentration of aluminum in solution. This and other
chemical properties of the solution can be a factor influencing reaction rates, and argues for the
importance of acquiring high quality chemical analyses for geothermal fluids in order to fully
understand the properties of any specific geothermal site (Reed and Mariner 1991).

synopsIs
Water acts as a solvent when it interacts with rock systems. As a result, it contains chemical signa-
tures of processes, conditions, and compositions that reflect its history. This information is useful
for evaluating the properties and conditions of geothermal reservoirs. The chemical processes that
determine the signatures contained in a solution, and which must be considered when interpreting
analytical data, include the chemical potentials of the components in a system of phases, the phases
that are present, and the activities and Gibbs energies of components and phases. Also important
are the activity coefficients and the influence they have on the equilibrium constant and resulting
affinities and ion exchange processes. Together, this collection of basic chemical attributes of a
system establish the chemistry of the solution, and allow extraction of such information as reservoir
temperature, mineralogy, gas compositions, and reaction paths.

Problems

5.1 The dissolution of calcite into water is represented by the reaction
–
+
CaCO 3 + H < = > Ca + HCO 3 . The log of the equilibrium constant for this reaction
++
is –0.5838 at 200°C. Write the equilibrium constant expression for this reaction.
5.2 If calcite is dropped into a solution at 200°C with a pH of 2.0, a Ca concentration of
++
0.003 and an HCO 3 - concentration of 0.0001 molal, will it dissolve?
++
5.3 If the concentration at 200°C of Ca is 0.02 molal and the pH is 8.0, what is the con-
–
centration of HCO 3 if calcite is in equilibrium with the solution?
5.4 If the value of the natural log of the equilibrium constant for a reaction was equal to
–3.7, what would be the Gibbs energy of the reaction at 300°C?
5.5 What would be the concentration of CO 2 (aq) in water at 25°C if the water was in con-
++
tact with calcite, and he same pH, Ca and bicarbonate concentrations were present as
in problem 5.2?
5.6 What would be the amount of CO 2 in a coexisting gas phase? Assume that the density
of water at its critical point is 0.32 gm/ml.
5.7 Using the data in Figure 5.2, determine whether any of the fluids listed in Table 5.1 are
in equilibrium with quartz at 200°C.
5.8 Compute the value of the log Q for the alkali feldspar reaction for each of the waters in
Table 5.1. Assuming a temperature of 200°C, which waters are in equilibrium with the
potassium end member and which are in equilibrium with the sodium end member.
5.9 If a geothermal water at 150°C had an aqueous silica concentration of 0.006 molal,
would it be in equilibrium with any silica polymorphs.
5.10 Compute the charge balance on the analysis for the Iceland geothermal fluid in Table 5.1
and decide whether this analysis is sufficiently accurate for geothermetric work.