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

product may contribute as well as the presence of equilibrium steam in the aquifer. In calculating
aqueous species distribution in the aquifer water such equilibrium steam was assumed not to be
present. Calculated variation in the calcite saturation state upon adiabatic boiling is considerably more
accurate than calculation of the absolute saturation state. Upon boiling degassing of the water with
respect to CO 2 generally causes an initially calcite saturated water to become supersaturated. This is
particularly the case with waters high in CO 2 such as in Olkaria West and where aquifer temperatures
are around 200°C but CO 2 solubility in water at minimum around 200°C. In relatively high
temperature and low CO 2 waters, such as at Nesjavellir, early boiling may only cause limited
degassing and not calcite supersaturation. Prolonged depressurization boiling, which leads to a
decrease in fluid temperature, decreases the calcite saturation index due to the retrograde solubility of
calcite with respect to temperature. This often causes boiled geothermal waters to be calcite
undersaturated. Calcite scaling is not expected to be a problem at Olkaria and Nesjavellir and neither
at Svartsengi and Reykjanes if the first depth level of boiling is outside wells i.e. in the producing
aquifers.
Scaling tests at Nesjavellir showed that at high temperatures of the wellheads of wells NJ-14 and NJ-
22, the two phase fluids deposited scales that were dominantly sulphide phases in well NJ-14 fluids
and mixed sulphides and oxides in the case of well NJ-22. The scales were essentially amorphous to
X-rays. There were however, some indications of some mineral phases in the scales such as
chalcopyrite and clayey material. Separated waters after the heat exchangers, entry to the retention
tank and at the injection well deposited mainly amorphous silica but there are also some indications of
the presence of clays in the scales. The rate of deposition of ~0.261 mm/yr was highest at the entry to
the retention tank and much lower of ~ 0.0168 mm/yr at the injection well. Silica polymerisation
occurs in the retention tank and this reduces the rate of silica deposition at the injection well. At entry
to the retention tank, the high rate of deposition could be due to supersaturation by monomeric silica
which causes a higher rate of precipitation (Yanagase et al., 1970; Rothbaum, 1979). An intermediate
test site between the steam separators and the wellheads could be chosen to investigate the type of
scales that would form.
The solute content of the water discharged from the atmospheric silencer of Olkaria well OW-34 is
high The discharge enthalpy of the well fluids is close to that of dry steam leading to extensive
evaporation of the water fraction in the discharge by depressurization boiling to a low pressure. This
extensive evaporation is thought to be the cause of the high salt content of the water discharged at
atmospheric pressure as well as the extensive amorphous silica scale formation.
A solution to this scaling problem involves mixing of the discharge of well OW-34 with separated
brine or condensate from another well e.g. well R-3 where separated brines from the Olkaria II plant
are being re-injected to lower the discharge enthalpy of OW-34. This will reduce the amount of
evaporation of water in well OW-34. Theoretical calculations based on mass balance equations,
chloride concentration of spent brine being re-injected in well R-3 and that of well OW-34 for
purposes of diluting the water have been tried. In this way aqueous silica concentrations will not build
up to the same extent and amorphous silica deposition will be reduced or inhibited.
48