Geothermal Controls

From the corrosion point of view, it is very important to control the deposition of scale. Removal of deposited scale by mechanical means is the first step. Standard, industrial water-treating techniques can be used to control scale deposition in general. In deep, hot wells or geothermal wells it is best to avoid untreated makeup water (i.e., geothermal brines). 

Carracedo, J. C. 1994. The Canary Islands an example of structural control on the growth of large oceanic-island volcanoes. 1. Volcan. Geotherm. Res. 60 225-241. 

Shikazono (1984) summarized analytical data of the epidote from geothermal areas to consider the relationship between the composition of epidote and that of the original fresh rocks and to inspect the other factors controlling the compositional variations in epidote. The discussion on the epidote composition by Shikazono (1984) is described below. 

Sumi, K. (1968b) Structural control and time sequence of rock alteration in the Matsukawa geothermal area, with special reference to comparison with those of Wairakei. J. Jpn. Geotherm. Energy Assoc., 17, 80-92. (in Japanese with English abst.). 

Chemical compositions of geothermal waters controlled by hydrothermal alteration mineral assemblage... 

The above discussions are based on the assumption of constant temperature. However, temperature varies widely. The chemical compositions of geothermal waters intimately relate to temperature. For example, the correlation between Na/K ratio in geothermal waters and temperature has been interpreted as indicating that this ratio is controlled by albite and K-feldspar (White, 1965 Ellis, 1969, 1970). 

The Na/Li ratio of geothermal waters decreases with increasing temperature and has been used as a geothermometer (Fig. 2.10) (Fouillac and Michard, 1981), suggesting that this ratio is controlled by feldspar-solution equilibrium (Shikazono, 1978a). 

These results indicate that the chemical composition of geothermal water at 250°C is largely controlled by such minerals commonly occurring in geothermal area as albite, K-feldspar, sericite, calcite, wairakite and quartz. 

The CO2 concentrations of present-day geothermal waters in terrestrial environment have been also interpreted in terms of the interaction of hydrothermal solutions with country rocks (Giggenbach, 1981 Shikazono, 1978,1985). For example, as noted in section 2.4.3, Shikazono (1985) estimated /CO2 for epithermal Au-Ag and base-metal vein-type deposits in Japan which formed in terrestrial environments at Miocene-Pliocene age and showed that fco2 controlled by the alteration minerals (Fig. 3.6). Estimated /coi" temperature range for epithermal Cu-Pb-Zn vein-type deposits are clearly similar to those for the Kuroko and back-arc deposits in which base metals (Cu, Pb, Zn) are concentrated. 

Among the minerals mentioned above calcite and kaolinite may be important for controlling /CO2 of terrestrial geothermal waters. It was cited by Giggenbach (1981) that /CO2 (or Xco2> mole fraction of CO2) of terrestrial geothermal waters is controlled by plagioclase -1- CO2 = calcite -H kaolinite . 

M. L. Allan and L. E. Kukacka. Calcium phosphate cements for lost circulation control in geothermal. Geothermics, 24(2) 269-282, April 1995. 

D. A. Glowka, G. E. Loeppke, P. B. Rand, and E. K. Wright. Laboratory and field evaluation of polyurethane foam for lost circulation control, volume 13 of The geysers—three decades of achievement A window on the future, pages 517-524. Geothermal Resources Council, Davis, CA, 1989. 

Arnorsson, S., E. Gunnlaugsson and H. Svavarsson, 1983, The chemistry of geothermal waters in Iceland, II. Mineral equilibria and independent variables controlling water compositions. Geochimica et Cosmochimica Acta 47, 547-566. 

The initial methodology used assumed that all excess calcite from the solution would be deposited in a predetermined volume of the geothermal pipe. This approach is conservative considering that calcite deposition is an instantaneous process. However, the literature frequently states that calcite deposition is kinetically-controlled where the process of deposition could either be slower or faster (Sjoberg ... 

The Ralph M. Parsons Company Regional Water Quality Control Board, Central Valley Region, California Republic Geothermal Research and Development Associates... 

Methane. The concentrations of CH4 in geothermal fluids are quite variable (see Table 4). In some geothermal systems, the CH4 aquifer fluid concentration is apparently controlled by attainment of equilibrium for the reaction ... 

---