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

The solubility of CO 2 in water changes
with temperature, being at minimum
around 200°C. Degassing during the
early stages of boiling is accordingly
most effective for aquifer waters with
temperatures around 200°C. For aquifer
waters with temperatures around 300˚C,
or higher, early degassing with respect
to CO 2 is slow due to its relatively high
solubility in water at these high
temperatures.
2
1
OW-30 Olkaria East
OW-34 Olkaria East
OW-306 Olkaria West
OW-301 Olkaria East
OW-714 Olkaria North East
OW-709 Olkaria North East
OW-20 Olkaria East
OW-901 Olkaria Domes
OW-304D Olkaria West
SV-05 Svartsengi
SV-11 Svartsengi
RN-10 Reykjanes
RN-11 Reykjanes
NJ-14 Nesjavellir
NJ-22 Nesjavellir
The shape of the curves in Figure 24
can essentially be explained by a
combination of three factors, (1) CO 2
degassing during boiling, (2) the
solubility of CO 2 in water as a function
of temperature and (3) the retrograde
solubility of calcite with respect to
temperature. The concentration of CO 2
in the initial aquifer fluid also has an
influence. Aquifer waters with
temperatures below about 280˚ C show
an increase in the value of SI cal during
the early stages of boiling and greater
the lower the aquifer temperature. Such
an increase is not observed for the
~300˚ C aquifer waters at Nesjavellir.
Maximum SI cal values are attained some
20-40 °C below the initial aquifer
temperature. At maximum the waters
have largely been degassed and at
temperatures below the maximum, SI cal
Calcite Saturation Index
0
-1
-2
-3
80 120 160 200 240 280 320
Temperature °C
FIGURE 24: Changes in calcite saturation produced
by several steps of single-step adiabatic steam loss
for selected well fluids at Olkaria, Reykjanes,
Svartsengi and Nesjavellir
values become successively lower due to increasing calcite solubility with decreasing temperature. For
the relatively hot aquifer waters at Nesjavellir, SI cal values decrease progressively with decreasing
temperature due to the combined effects of increasing calcite solubility, limited CO 2 degassing during
the early stages of boiling due to the relatively high CO 2 solubility in water at the high temperatures
and relatively low CO 2 concentrations in the initial aquifer waters. Continued high SI cal values for
wells OW-301 and OW-304 during boiling all the way down to atmospheric pressure is due to the
high CO 2 content of the initial aquifer fluid which requires extensive steam formation for extensive
degassing of the aquifer water.
The results discussed above and presented in Figures 23 to 25 assume adiabatic boiling and the
equilibrium distribution with respect to CO 2 is attained between the liquid water and steam phases.
Such equilibrium may not be attained due to rapid steam formation and insufficient time for the CO 2 to
be transferred from the boiling water to the forming steam which is required for attainment of CO 2
distribution equilibrium between the fluid phases. Incomplete CO 2 transfer would change the shape of
the curves in Figure 24 in such a way that the maximum would be depressed and the overall change in
SI cal would decrease more gradually with decreasing temperature.
Calcite scale formation is not expected to be a problem at Nesjavellir and Olkaria East, Olkaria, the
reason being the dilute nature of the waters and the high temperatures at Nesjavellir. However, in
Olkaria West calcite scale formation in wells may be a problem, depending on the depth level of first
boiling. If it is within the well it may be relatively severe but not so if extensive boiling starts in
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