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

ABSTRACT
The Olkaria geothermal system in Kenya is located within the Okaria volcanic complex in the central
sector of the Kenya Rift Valley. Reykjanes, Svarstengi and Nesjavellir geothermal fields are located
in southwest Iceland and fall on a continuous earthquake epicentric line extending through the
Reykjanes Penninsula that stretches northeast to Langjökull. These four geothermal fields are all
high-temperature. Measured temperatures in Olkaria are as high as 350°C, Reykjanes 320°C,
Svartsengi 240°C and Nesjavellir > 380°C. The reservoir waters in the four fields vary. The water type
at Olkaria is mainly dilute near neutral pH sodium -chloride and sodium-bicarbonate waters with
chloride ranging between 50 and 4000 ppm at atmospheric pressure. At Reykjanes and Svartsengi
these are saline sodium-chloride waters with chloride being 20,000 and 13,000 ppm, respectively,
while at Nesjavellir they are very dilute sodium chloride waters with chloride of ~ 150 ppm.
Gas concentrations in the fluids of all the four fields are low, except for fluids of the Olkaria West
sector in Olkaria. Speciation calculations indicate that in Olkaria CO 2 partial pressures range between
0.5 bar and 5 bar except for fluids in the Olkaria West sector with > 90 bars a. At Reykjanes,
Svartsengi and Nesjavellir the CO 2 partial pressures fall between 0.0867 to 1.66 bars a. The high CO 2
partial pressures cause CO 2 rich waters to develop when CO 2 in steam encounters shallower ground
waters. This becomes corrosive in the process as the pH of the water is lowered. At Reykjanes and
Svartsengi, well fluids are low in pH but this is little influenced by the partial pressures of CO 2 . The
pH of condensates that form due to dissolution of CO 2 at separation pressures range between 4.55 and
5.51 for Olkaria wells while at Reykjanes, Svartsengi and Nesjavellir these are between 5.20 and 5.55.
Thermodynamic calculations of HCl concentrations using pH, chloride concentrations and aquifer
temperature indicate high concentrations of HCl in the aquifer water of Reykjanes and Svartsengi
(0.81 and 0.083 ppm) and low HCl concentrations in the Olkaria and Nesjavellir well fluids. In steam
high HCl concentrations are derived for Reykjanes well fluids due to high wellhead pressures. Low
pH and high chloride concentration coupled with high temperatures contribute to high HCl
concentrations in fluid in the Reykjanes and Svartsengi fluids. In dry steam conveyed in steam
gathering systems HCl becomes corrosive as steam condenses due to formation of H + and Cl - ions.
Studies of scales formed during tests at Nesjavellir using the binocular microscope, FTIR, XRD, SEM,
ICP and UV indicate that scales formed at the wellheads of wells NJ-14 and NJ-22 consisted mainly
of sulphides at well NJ-14 and mixed sulphides and oxides at well NJ-22. In separated water after the
heat exchangers, entry to retention tank and at injection well the scales consisted mainly of amorphous
silica with some indication of clays. Crystalline phases were not prominent in the scales but traces of
chalcopyrite were identified in scales formed at the wellhead of well NJ-14 and at the entry to the
retention tank. Traces of clays formed in scales at the wellhead of well NJ-22. The highest amount of
scale deposited at the entry to the retention tank, with a deposition rate of ~0.261mm/yr. At the
injection well the rate was lower ~0.0168 mm/yr.
Olkaria well OW-34 has an enthalpy close to that of dry steam of 2672 kJ/kg and anomalous
chemistry. Chloride concentration in the separated water is ~ 4000ppm at atmospheric pressure. The
high solute content in well OW-34 fluids is influenced by evaporative effects due to the high discharge
enthalpy. Scales formed in the wellhead equipment and studied by the same method above indicated
they were prominently amorphous silica scales. Crystalline phases were absent from the scales.
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