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

Concentrations of CO 2 at equilibrium between water and steam can be calculated using
thermodynamic data on the equilibrium partial pressures of dissolved CO 2 and a given temperature,
using Henry’s law constant. The expression used is:
mCO 2 = kh*Pi,
where mCO 2 = concentration of CO 2 in moles/kg,
Kh = Henry’s Law coefficient,
P i= Partial pressure of CO 2 at equilibrium in bars.
Arnόrsson et al (1996) describe the temperature dependence of Henry’s Law coefficient given by the
function:
CO 2 ,g = CO 2 ,aq = -59.612 + 3448.59/T - 0.68640×10 -6 T 2 + 18.847×log T
where T
= Temperature in Kelvin.
Upon extensive boiling of
geothermal water in producing
aquifers and wells, the CO 2 initially
dissolved in the aquifer water is
largely transferred to the steam
which forms. Figures 15 and 16
show how CO 2 concentrations in
the boiling water and the steam
change during one step adiabatic
boiling of water from selected wells
in the study areas.
The separated water at the surface
contains 0.5-5 ppm dissolved CO 2
and the separated steam 0.2-24
ppm. These are theoretically
calculated using Henry’s law
constants for CO 2 at equilibrium.
One might expect that the separated
water would cause corrosion of
mild steel due to its relatively high
CO 2 content. Such corrosion is,
however, generally not observed in
geothermal installations. The reason
is considered to be the formation of
protective sulphides, carbonates and
silicate coating (Jones, 1996).
Log PCO2 bar a
100
10
1
0.1
0.01
Olkaria East
Olkaria West
Olkaria North East
Olkaria Domes
Reykjanes
Svartsengi
Nesjavellir
5 6 7 8
pH
FIGURE 14: Aquifer pH vs CO 2 partial pressures
in the aquifer for selected wells in the study areas
Upon condensation of separated steam, CO 2 rich condensate with a relatively low pH will form
(Figure 17). The pH of condensate formed from the selected wells due to CO 2 in the Olkaria West
sector is lower than that formed from the rest of the selected wells in the study areas. A complication
that could arise is the influence of H 2 S. The problem with CO 2 corrosion is always linked to pH and
the only problem to worry about is corrosion by steam condensate. Instability of sulphide minerals
rendering them less protective will contribute.
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