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

6. POTENTIAL CORROSION BY CO 2 AND HCl
6.1 Carbon dioxide
The pH of geothermal water is
largely controlled by silicate
hydrolysis equilibria. In some cases
high concentrations of dissolved
carbon dioxide (CO 2 ) in geothermal
fluids could have an influence on
the pH. Carbonate carbon occurs
mostly as dissolved CO 2 and
bicarbonate. The partial pressures of
CO 2 in the deep fluid of the study
areas have been evaluated with the
assistance of the WATCH computer
code (Arnόrsson et al., 1982;
Bjarnason, 1994) version 2.1A. At
Olkaria CO 2 partial pressures are
quite variable being very high in the
Olkaria West sector compared to the
other sectors of Olkaria or 10-100
bar (Table 4 of Appendix A). In
Olkaria North East they are 1-3 bar,
0.5 -5 bar in Olkaria East, and 2-4
bar in Olkaria Domes. In the
reservoirs at Reykjanes and
Svartsengi they are 0.5-0.9 bar and
0.6-1.7 bar respectively. At
Nesjavellir the CO 2 partial pressures
are even lower 0.07-0.09 bar for the
wells selected (Figure 13).
The carbon dioxide (CO 2 )
concentrations in high-temperature
Log PCO2
100
10
1
0.1
0.01
Olkaria East
Olkaria West
Olkaria North East
Olkaria Domes
Svartsengi
Reykjanes
Nesjavellir
180 200 220 240 260 280 300
Temperature °C
FIGURE 13: Aquifer temperature vs CO 2
partial pressures in the aquifer fluids
geothermal aquifer waters are highly variable. Essentially two factors control the aquifer water CO 2
concentrations, (1) the rate of supply of CO 2 to the water from magmatic heat sources and (2)
equilibration with specific mineral buffers. In the study areas these buffers include epidote + prehnite
+ calcite + quartz and epidote + grossular + calcite + quartz (Karingithi et al., 2006; Fridriksson et al.,
2006). In Olkaria West, the CO 2 in the reservoir water is considered to be controlled by the rate of
supply of CO 2 from the magmatic heat source. Another possibility is that Olkaria West is on the
periphery of the Olkaria geothermal system and the fluid is peripheral, i.e. it is in outflow from the
system and has developed an excess CO 2 concentration. In other parts of Olkaria and in the other
study areas, CO 2 aquifer water concentrations are on the other hand controlled by close approach to
equilibrium with one of the mineral buffers mentioned above.
Shallow groundwaters, rich in CO 2 , may form when CO 2 -rich steam rising from the deeper reservoir
condenses into such shallow water. Waters of this origin exist in Olkaria West, as evidenced by the
discharge of well OW-304D. The pH of such waters could be lowered by the high CO 2 concentrations
(Figure 14).
Waters of this type have also been reported from Broadlands in NewZealand (Hedenquist and Stewart,
1985). Shallow CO 2 -rich groundwaters may cause corrosion on the outside of casings, at least if
cementing of the casing is in adequate or if the water can dissolve the cement.
20