Page 84 - Geothermal Energy Renewable Energy and The Environment

5 Chemistry of

Geothermal Fluids

Geothermal waters exhibit a broad range of chemical compositions (Tables 5.1 and 5.2), from very
dilute (a few hundred parts per million, by weight, of total dissolved constituents) to very concen-
trated (solutions containing tens of percent, by weight, of dissolved constituents). This dissolved
load can provide important information about the characteristics of a reservoir including its tem-
perature, mineralogy, and history. However, the dissolved load can also impact the performance of
machinery in a geothermal power plant, or in direct use or heating and cooling applications. This
chapter considers the basic chemical processes that influence the chemical properties of geothermal
fluids, and how to utilize chemical analyses to gain information about the geothermal resource.
Later, in Chapters 6 and 7, we will apply these principles to evaluate specific aspects of geothermal
resources. In Chapter 12 we will consider how the chemical composition of geothermal fluids can,
in some instances, be mitigated.

why The GeochemIsTry oF GeoThermal FlUIds maTTers
Water occurs virtually everywhere in the subsurface, as we previously noted in Chapter 4. For appli-
cations that require temperatures of a few tens of degrees centigrade or less, such as heat for direct-
use applications or for heating-ventilation-air conditioning (HVAC) purposes, meteoric waters that
occur in shallow groundwater systems are usually sufficient to provide the needed energy. Heat
pumps, as will be discussed in Chapter 10, can efficiently move heat in such systems. Since such
systems lead to a relatively small temperature drop in the fluids, and because the chemistry of the
fluid is of little direct consequence for these uses, it is not necessary to pursue detailed knowledge
of the fluid chemistry for such applications. That is not the case for instances in which higher tem-
perature fluids are required.
The high temperature fluids used in geothermal power production are often associated with
magma bodies, or are in regions where igneous activity has occurred in the recent past. In those
cases where recent igneous activity is not part of the geological history, geothermal fluids can form
where deep fluid circulation, often facilitated by faulting and fracturing, allows water to reach depths
where sufficient heat is available to raise the temperature above 150°C. In either case, the water will
interact with the surrounding rocks, taking on chemical characteristics that are influenced by the
local geology. These interactions impart a chemical signature to the water. Deciphering the implica-
tions of that signature for the quality of the resource and the potential economic and environmental
impacts that may need to be addressed, depends upon a detailed understanding of the aqueous
geochemistry (Brook et al. 1979).
The geochemistry of natural waters is also important for a different, but just as crucial, aspect of
geothermal energy considerations; namely, exploration for the resource. In many instances, surface
evidence that a high temperature resource is present at depth may not be obvious, thus making the
resource difficult to find. These “hidden” resources are often detectable through chemical signatures
in surface waters. Some of these signatures can be specific chemical constituents that indicate the pres-
ence of a heat anomaly at depth, in other cases it may be shifts in the distribution patterns of certain ele-
ments, isotopes or compounds, or changes in the ratios of elements. Understanding the processes that
control these geochemical signatures provides the ability to assess the value and quality of a resource.

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