corrosive species and scaling in wells at olkaria ... - Orkustofnun

other cases, casing corrosion has been caused by CO 2 rich waters in geothermal fields. These waters
form when ascending CO 2 rich steam dissolves in near surface shallow waters. In the Broadlands-
Ohaaki geothermal field, New Zealand, Hedenquist and Stewart (1985) reported severe external well
casing corrosion caused by CO 2 rich steam-heated water. Zarouk (2004) reviewed casing corrosion
occurrences in New Zealand geothermal fields. In all cases, these are related to CO 2 rich dilute
geothermal water. In Olkaria, Omenda (1998) and Karingithi (2002) report CO 2 rich fluids but the
extent to which they are corrosive is not known.
Modes of corrosion attack in geothermal installations have been disscussed by Conover et al. (1979),
Boulton and White (1983) and Corsi (1986). These are uniform (general) corrosion, pitting corrosion,
crevice corrosion, stress corrosion cracking (SCC), sulphide stress cracking (SSC), intergranular
corrosion, galvanic coupling, corrosion fatigue, microbiological by induced corrosion, erosion
corrosion and cavitation.
1.2 Scale formation
The main drawbacks in the utilization of geothermal resources arise from the precipitation of solid
scales from the geothermal fluid. In many cases the scales cause restriction in flow, e.g. in the
boreholes, two phase pipelines, the separators and waste water lines and steam pipelines. Their
formation often impedes the closing and opening of valves leading to leaks. Deposition on turbine
blades is common which results in the turbine chest pressures increasing. Three main areas of scale
deposition can be distinguished (Corsi, 1986). These are: deposition from a single phase fluid
(injection pipelines), deposition from flashing fluid (wells, separators, two phase-pipelines) and
deposition by steam carryover (separators, steam lines and turbines).
A great deal of work has been carried out on the nature of scales formed from geothermal fluids
(Thόrhallsson et al., 1975; Arnόrsson, 1981; Gallup, 1989; Gallup, 1998; Simmons and Christenson
1993; Simmons and Christenson, 1994). Amorphous silica and calcium carbonate scales are the most
extensively studied but metal sulphides and silicates to a lesser extent, although they are presently
receiving more attention (Simmons and Christenson 1993; Simmons and Christenson, 1994;
Ármannsson, 1989; Mecerdo et al., 1989; Benoit, 1989; Durak et al., 1993; Hardardόttir et al., 2001;
Weissberg et al., 1979; D’Amore et al., 1998; Karebalas et al., 1989; Gallup, 1993; 1998).
1.2.1 Silica
Amorphous silica deposition is probably the most commonly encountered and troublesome scale
formed from high-temperature geothermal water. Such scale has been studied by many workers
(Weres and Tsao, 1981; Hurtado et al., 1989; Thórhallsson et al., 1975; Arnόrsson, 1981; Gallup,
1989; Gallup, 1998; Mahon, 1966; Henley, 1983; Garcia et al., 1996, Yanagase et al., 1970; Itoi et al.,
1989; Kato et al., 2003; Corsi, 1986). A lot of effort has been devoted to the study of silica scale
formation. In the utilization of high-temperature geothermal resources the efficient extraction of
energy is limited by the silica scale that may form as a consequence of cooling.
It has been established that aqueous silica concentrations in high-temperature geothermal fluids are
controlled by close approach to equilibrium with quartz (e.g. Fournier and Rowe, 1966; Mahon, 1966;
Fournier, 1973; Fournier and Rowe, 1977; Fournier and Potter, 1982, Gίslason et al., 1997;
Gunnarsson and Arnόrsson, 2000). The quartz solubility constant has been the subject of thorough
experimental studies (Fournier, 1983; Fournier, 1985; Fournier and Potter, 1982; Fournier and Rowe,
1977). Quartz solubility increases with increasing temperature. Often quartz is not present as a primary
mineral in geothermal systems but forms by precipitation from the water. Silica scales are only known
to form if the extent of boiling and cooling of the aquifer water is sufficient to saturate it with
amorphous silica, the reason being the fast rate of deposition for this phase but slow rate for quartz,
particularly below 150°C. In contrast to calcite scale formation discussed below, amorphous silica
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