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

7. SCALING IN WELLS AND SURFACE INSTALLATIONS
7.1 Theoretical aspects of calcite scale formation
Calcite is abundant as a secondary
mineral in many hydrothermal systems.
Many studies have indicated that >100˚
C waters are close to being calcite
saturated (e.g. Ellis and Mahon, 1977;
Arnórsson, 1978). In some exploited
fields, water in producing aquifers may
be calcite undersaturated. The
calculated state of calcite saturation in
the initial aquifer water of wells at
Olkaria, Reykjanes, Svartsengi and
Nesjavellir is presented in Figure 23.
Calcite Saturation Index
2
1
0
Olkaria East
Olkaria West
Olkaria North East
Olkaria Domes
Reykjanes
Svartsengi
Nesjavellir
The data show some scatter. This is
more likely due to errors in calculating
the calcite activity product rather than a
reflection of departure from
equilibrium. In calculating the aquifer
water composition with the aid of the
WATCH speciation program
(Arnórsson et al., 1982; Bjarnasson,
1994) Version 2.1A, it was assumed
that excess well discharge enthalpy was
caused by phase segregation in
producing aquifers and that no
-1
-2
180 200 220 240 260 280 300
Temperature °C
FIGURE 23: Aquifer temperature vs calcite saturation
for selected wells at Olkaria, Reykjanes, Svartsengi
and Nesjavellir
equilibrium steam was present. If such steam is present in the initial aquifer fluid, the calculated
aquifer water pH is too low and the aquifer water CO 2 concentration too high with the result that too
low values are obtained for the calcite saturation index (SI cal ), i.e. calcite undersaturation, if the initial
aquifer water is truly just calcite saturated. Also, some calcium may have been removed from the
aquifer water upon its boiling as a consequence of calcite precipitation leading to a low calcium
concentration in the water samples collected at the wellhead relative to the initial aquifer water. This
effect will also tend to give low values for SI cal . Removal of calcium from solution by calcite
precipitation is expected to affect the calculated SI cal of the dilute waters significantly at Olkaria and
Nesjavellir because of their low calcium content but not the calcium-rich saline waters at Reykjanes
and Svartsengi. Other errors that may contribute significantly to departure from calculated calcite
equilibrium include erroneous values obtained in the measurement of pH of water samples and by
contamination of these samples by condensed steam. Such contamination is particularly prone to occur
when sampling water from wells with high discharge enthalpy, such as well OW-34 at Olkaria.
Figure 24 shows how the calculated saturation index for calcite varies during adiabatic boiling of
aquifer water of selected wells from the study area. The shapes of the SI cal curves are more accurately
defined than the absolute SI cal value at each temperature. The effect of depressurization boiling upon
the calcite saturation state is essentially twofold. Firstly, the water is degassed with respect to CO 2 ,
which leads to an increase in the pH of the boiling water (Figure 25) which in turn brings about an
increase in the carbonate ion concentration and hence in the [Ca +2 ][CO 3 -2 ] solubility product.
Secondly, depressurization boiling causes cooling of the water. The solubility constant for calcite
increases with decreasing temperature. Degassing by pressurization boiling causes an initially calcite
saturated solution to become supersaturated whereas the cooling by this boiling has the opposite
effect.
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