Page 95 - Geothermal Energy Renewable Energy and The Environment

80 Geothermal Energy: Renewable Energy and the Environment

Figure 5.4 illustrates several key points relevant for understanding the geochemistry of geo-
thermal systems. Foremost is that the geochemistry of any particular geothermal system is not
likely to be a reflection of an equilibrium state. The time it takes to achieve equilibrium for any
specific reaction can vary from seconds to millions of years, depending upon the conditions at a
specific location. The dissolution rates, at 25°C, for quartz and halite, for example, are 1.26 e-14
2
2
moles/cm -s and 1.0 e-5 moles/cm -s, respectively. Given the extremely heterogeneous distribu-
tion, on the meter scale, of mineralogy, grain size, pore volume, permeability, exposed surface
area, fluid composition, and so on, many competing reactions will be occurring simultaneously,
each approaching an equilibrium condition at a different rate. Hence, when considering the char-
acteristics of a geothermal site, it is wise to view those conditions as simply a point along an
evolutionary pathway.
The figure also illustrates the relative sensitivity of reaction progress to perturbations in tem-
perature and changes in surface area along a flow path. Note that a few tens of degrees difference in
the temperature, relative to the equilibrium temperature of a reaction, can change the time it takes
to achieve complete reaction by thousands of years. Changes in the exposed surface areas will also
affect the time required to achieve complete reaction, but the effect is less pronounced than for
temperature, reflecting the fact that reactions scale approximately linearly with surface area, but
exponentially with temperature, as is evident from Equation 5.8.
Although the results in Figure 5.4 are specific to the methodology outlined by Lasaga (1986),
other approaches and other reactions would give qualitatively similar results, in the sense that larger
surface areas and higher temperatures (relative to the equilibrium temperature) result in shorter
reaction times. However, the absolute changes in reaction times will depend on the amount of min-
eral present, its surface area, and the reaction rate for the assemblage of minerals involved in the
reaction at the temperature of the system. Clearly, to obtain an accurate understanding of a specific
geothermal system requires knowledge of the mineralogical characteristics throughout the flow
regime the geothermal system has affected.

Gases In GeoThermal FlUIds

Geothermal fluids always contain a dissolved gas component that can play an important role in
establishing the chemical characteristics of the fluid. The source and influence of that component
is multifaceted.
As previously mentioned, the heat source for geothermal systems is the underlying magmatic
system that has perturbed the local geology. When magmas are generated, usually many kilometers
underground, they include components of the rocks that melted to form them. Since the melting
process involves large volumes of many different minerals, the melts that are generated are complex
chemical systems that contain virtually every element in the periodic table. As the melts migrate to
shallow levels in the crust, cool and start to crystallize, a fractionation process occurs in which some
elements are preferentially incorporated into the newly forming minerals, and others are excluded
because they do not fit into the crystal structure of the new minerals. Many of the excluded ele-
ments and compounds have a tendency to form volatile compounds, including H O, CO , H S, O ,
2
2
2
2
CH , and Cl . As the magma crystallizes, the diminishing volume of molten material that con-
4
2
tains these compounds as a dissolved load eventually reaches saturation in them and they begin
to exsolve, migrating from the crystallizing magma into the surrounding rock. As these volatiles
migrate upward, they interact with geothermal fluids and groundwater. Consequently, geothermal
fluids can contain elevated concentrations of these constituents.
The host rocks through which heated geothermal fluids migrate also can contribute to the devel-
opment of a dissolved gas component. If, for example, the rocks contain calcite (CaCO ), dissolu-
3
–
tion of that mineral will increase the amount of carbonate (CO ) and bicarbonate (HCO ), which
=
3
3
ultimately will increase the dissolved CO content via the reactions listed below.
2