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

deposition does not occur at depth in production wells but characteristically in wellheads, surface
pipings and injection wells.
The control of silica concentrations in high-temperature geothermal waters by quartz solubility implies
that aqueous silica concentrations in producing aquifers increase with increasing temperature of the
water. As a consequence, the temperature at which amorphous silica saturation is attained for
particular well water depends on the temperature of the source aquifer. Thus amorphous silica scale
formation is not a problem unless the reservoir temperature exceeds 250˚C.
The factors that affect the rate of amorphous silica precipitation and colloidal formation
polymerisation) include the degree of supersaturation, pH, temperature and salinity. Aeration may also
contribute. Reactions between silica molecules in amorphous silica oversaturated solutions may react
between themselves to form colloidal silica or deposit from solution to form amorphous silica. The
kinetics of amorphous silica precipitation and silica polymerisation have been studied by several
workers (Rothbaum et al., 1979; Rimstidt and Barnes, 1980; Weres and Tsao, 1981; Gunnarsson and
Arnόrsson, 2005).
The solubility of pure amorphous silica has been the subject of many studies (Fournier and Marshall,
1983; Rimstidt and Barnes, 1980; Gunnarsson and Arnόrsson, 2000; Marshall and Chen, 1982; Weres
and Tsao, 1981; Rothbaum et al., 1979). [Amorphous silica solubility is dependent on ions that affect
its surface charge]. Marshall and Warakomski (1980) and Chen and Marshall (1982) have studied the
effects of dissolved salts of varying concentrations on the solubility of amorphous silica. Generally,
they found that the effect of the cations of the salts had a decreasing effect on amorphous silica
solubility in the order Mg 2+ > Ca 2+ > Sr 2+ > Li + > Na + > K + . Yokoyama et al. (1989) showed
specifically that aluminium ion could have a strong influence on silica polymerisation rates. These
effects are likely to be caused by complex formation between silica and cations in the salts but the
salts will also affect the value of the activity coefficients taken by aqueous silica species. The
presence of cations in solution may also affect the composition of and the precipitated amorphous
silica and therefore its solubility. Thus aluminium and iron silicates deposition has been described in
other saline geothermal systems such as Milos, Nissyros, Asal., Reykjanes, Salton sea, e.g. Karabelas
et al. (1989), Virkir-Orkint (1990), Hardardόttir et al. (2001, 2004, 2005), and Gallup (1989, 1993,
1998).
Several methods have been adapted to reduce or eliminate deposition of amorphous silica in
geothermal installations (Yanagase et al., 1970; Weres and Tsao, 1981; Kiyota et al., 2000; Arnórsson,
2000; Gunnarsson and Arnórsson, 2005). A method commonly adapted to reduce amorphous silica
scaling in production wells and wellhead equipment is to maintain steam separation pressures (and
temperatures), above amorphous silica saturation. Many other methods have been practiced including
removal of silica from solution by its precipitation, silica polymerisation and by raising the pH of the
separated water even at elevated temperature sufficiently to cause some of the silica to ionize.
Acidification has also been applied. It reduces deposition rates. Polymeric silica has less tendency to
precipitate out of solution than monomeric silica. The relative rates of the two reactions, amorphous
silica deposition and silica polymerisation, and the rate at which polymeric silica settles from solution,
determine how successful polymerisation treatment is in reducing amorphous silica deposition from
spent geothermal waters.
Deposition of silica scales has been observed in Svartsengi, Reykjanes, Olkaria and Nesjavellir. In all
the cases it is reported in wellhead equipment, separated water and in the plant. In Svartsengi,
Thórólfsson (2005) reported maintenance problems associated with silica scale formation in heat
exchangers, brine pipes and on 1 st stage turbine nozzles. In Reykjanes amorphous silica is deposited
from separated water at lower temepratures and exists as a mixed scale (Hardardόttir et al., 2001,
2004, 2005). In Olkaria, Opondo and Ofwona (2003) reported intense deposition of silica in the
wellhead equipment of one well. At Nesjavellir studies on scale formation by Haukson (1996),
Gíslason and Gunnlaugsson (1994-1995) and Kjartansson (1996) indicated problems in pilot heat
exchangers. Little indication of silica polymerization or scaling was observed on fluidized heat
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