Gaseous Components in Geothermal Processes
Gas concentrations in fluids encountered during drilling of geothermal fields range from 0.05 wt% (Wairakei, Ahuachapan) up to about 1 wt% (Ngawha, Broadlands). We discovered in Chapter 2 that carbon dioxide is the dominant gas in geothermal systems and, as we shall see later, plays an important role in controlling the pH of the aquifer fluid. The ratios of the principal gases (e.g., CO2, H2, CH4) are controlled by reactions such as and may therefore be used as geothermometers in the same way as we have used alkali ion ratios. The development of gas geothermometers is discussed in a later chapter; at this stage we will examine the behaviour of gases when phase separation occurs from an initially singlephase geothermal fluid. This is important when we consider the recalculation of analyses of steam samples separated at the surface to determine aquifer dissolved gas compositions.
Gas pressures are also important in reservoir modelling studies as well as in a number of engineering problems associated with geothermal field development; in studies of fossil hydrothermal systems — ore deposits — the constraints imposed by gas contents are just as important and deserve much more attention.
Figures & Tables
This text is designed to introduce you to the practical concepts and calculations involved in interpreting the chemistry of high-temperature fluids in geothermal systems and hydrothermal ore-forming environments. It is intended that the energetic reader will learn to understand chemical principles, handle routine calculations and follow specialized chemical studies involved in geothermal exploration and exploitation and in ore genesis.
Although the emphasis of the text is on the interpretation of the chemistry of active geothermal systems, the principles involved are equally relevant to the interpretation of fossil hydrothermal ore-forming environments. Many gold-silver ore deposits, for example, have been shown to have formed in the near-surface region of hydrothermal systems similar in fluid chemistry and setting to those active today (White, 1981; Henley and Ellis, 1983). Combination of a knowledge of the principle processes within the active geothermal systems, the thermodynamics of complex ion formation, mineral-fluid equilibria and stable isotope systematics provide a framework which may assist in reconstruction of the hydrological regime within a fossil hydrothermal system where ore deposition occurred. This in turn may become useful in ore search. A chapter dealing with the hydrothermal chemistry of magmatic systems is included later in order to encompass a wider range of ore depositing environments and perhaps the root zones of the active geothermal systems.
After a short introduction to the types of geothermal fluids and chemical calculations, successive chapters will address the interpretation of water and gas analyses from geothermal wells. When we understand the reservoir compositions of some geothermal fluids and their relations to rock chemistry and temperature, we will consider the chemical and isotopic changes that occur in the natural transport of this fluid to the surface, derive and use chemical geothermometers and mixing relations, and map the surface chemistry of a hot spring system. After these studies of natural fluids at depth and at the surface, we will study chemical changes that occur during the exploitation of geothermal fluids and how to anticipate and avoid some of the problems of scaling and corrosion.