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

9. CONCLUSIONS
Olkaria well fluids generally exhibit a discharge enthalpy that ranges from slightly less than 1000
kJ/kg to a saturated steam enthalpy of about 2700 kJ/kg. Most wells have “excess enthalpy”.
Enthalpy for selected wells from Svartsengi, Reykjanes, and Nesjavellir is variable. In Svartsengi the
enthalpy of the fluids is fairly homogenous and apart from wells that tap into the shallow steam cap,
average enthalpy for the fluids is ~ 1030 kJ/kg. Enthalpy of well fluids in Reykjanes is slightly higher,
between 1300 kJ/kg and 1400 kJ/kg for the selected wells. At Nesjavellir the discharge enthalpies of
the wells are high and most wells have enthalpy >1600 kJ/kg. For the selected wells, the enthalpy
varied between ~ 1400 kJ/kg and ~ 1800 kJ/kg. “Excess enthalpy” in all wells considered for the
present study is adequately explained by phase segregation in producing aquifers.
The average of tqtz and tNa/K for the wells studied in Olkaria and Nesjavellir were taken to represent
the aquifer temperatures. Quartz equilibrium temperatures were calculated using the phase segregation
model. Large differences are observed between tqtz and tNaK for some wells in Olkaria. These wells
have discharge enthalpy close to that of steam saturated water at aquifer temperature so aquifer water
composition is not the source of error. Measured downhole aquifer temperatures in thermally
stabilised wells were used for Reykjanes and Svartsengi.
The phase segregation model does not take into account changes in composition of the flowing fluid
between the wellhead and initial conditions by precipitation or dissolution of minerals. When
geothermal water degasses by boiling, many reactions tend to occur. Changes in temperature occur as
a consequence of depressurisation boiling and may cause changes in saturation with respect to many
minerals. Speciation calculations may thus indicate significant departures from equilibrium between
minerals and the water in the aquifer beyond the depressurisation zone, even when such equilibrium is
closely approached in the initial fluid. Large errors in SI values can arise when potential mineral and
precipitation reactions are ignored, especially for minerals which contain chemical components
present in low concentrations in the initial water. The effect is much smaller for components, which
are abundant in the fluid such as reactive gases (CO 2 , H 2 S and H 2 )
The concentration of CO 2 in the aquifer water of Olkaria, Reykjanes, Svartsengi and Nesjavellir is
generally in the range of 60-5000 ppm except in Olkaria West where it is as high as 43,026 ppm. In
Olkaria West the aquifer fluid CO 2 concentrations are considered to be much affected by its rate of
supply from the magmatic heat source where in other parts of Olkaria and the other study areas aquifer
water CO 2 is fixed by equilibration with a specific mineral buffer. The CO 2 -rich waters in Olkaria west
could cause corrosion on the outside of well casing. This should be investigated. Condensates that
form due to dissolution of CO 2 would be lower for Olkaria West than in the rest of the study areas.
The relatively low concentrations of CO 2 in the other sectors of Olkaria, as well as Reykjanes,
Svartsengi and Nesjavellir do pose a low risk in developing CO 2 rich water that would be corrosive.
HCl concentrations in the aquifer waters and steam are controlled by pH, temperature and chloride
concentrations. The Olkaria and Nesjavellir aquifer waters contain very low HCl concentrations
(1.65×10 -5 and 2×10 -3 ppm in Olkaria and 1×10 -5 ppm in Nesjavellir). In Reykjanes and Svartsengi,
the HCl concentration is higher (0.32 and 0.81 ppm and 0.056 and 0.083 ppm, respectively) due to a
lower pH and higher chloride concentrations in the fluids. Much higher HCl concentrations are
present in the steam at the wellheads at Reykjanes (0.65 ppm) than at Svartsengi due to high steam
separation pressures. The HCl in the steam at Reykjanes would produce a pH of about 5.5 in a
condensate. It is accordingly concluded that HCl in steam (0.65 ppm) at the wellheads at Reykjanes is
an unlikely corrosion candidate and even much less likely in the other study areas.
The undisturbed aquifer waters in all the study areas are expected to be close to equilibrium with
calcite. Calculated saturation indices for calcite show, however, considerable scatter. The cause of this
scatter is partly considered to be analytical imprecision, particularly with respect to pH but also
removal of calcium from solution between initial aquifer conditions and wellhead by calcite
precipitation. Further the quality of the thermodynamic data used to calculate the calcite activity
47