High Temperature Calculations in Geothermal Development
The large scale development of geothermal resources usually involves low to medium pressure steam separation from production wells where flashing to steam and water also occurs within the wells themselves. The temperature changes due to such flashing processes and the pH changes attendant on gas removal in the steam phase are the principal processes leading to mineral deposition in wells, separator plants, and waste water disposal pipelines or channels. In this chapter we will examine the Chemistry of waters resulting from steam (+ gas) separation with special reference to the common problems of silica and calcite scaling. In earlier chapters the effects of boiling on the deposition of metals and metal sulfides were examined. We shall also briefly touch on the calculation of the pH and composition of steam condensate; these data are frequently required in geothermal development studies where problems of pipeline corrosion, disposal of condensate or control of H<sub>2</sub>S emission are under consideration.
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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.