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

8.2 Water composition of well OW-34
Well OW-34 is water composition is shown in Table 10 of Appendix A together with that of a selected
few make up wells, i.e. wells OW-33 and OW-32 and some wells where discharge enthalpy is very
high, or wells OW-10 and OW-18 which have been under exploitation for long. Well OW-18 has
water flowrates and discharge enthalpy trends that resemble those of Olkaria well OW-34. The
concentration of the solute constituents Cl, F, K and Na are much higher in the separated water of well
OW-34 than that of well OW-18. The silica concentration is relatively high in this well water, higher
than in most separated waters of the other wells. The Cl - concentration in the separated water at
atmospheric pressure in well OW-34 is ~ 4000 ppm and Na ~ 2400 ppm. The silica concentration is
relatively high, ~ 900 ppm SiO 2 .
8.3 Chloride concentration as a function of vapour pressure at selected discharge enthalpies
Chloride concentration was modelled as a function of selected discharge enthalpies and vapour
pressures. Discharge enthalpies selected were 2450, 2500, 2550, 2600, 2650 and 2675 kJ/kg and the
vapour pressures ranged from 25 bars to 0.8 bar absolute. An initial chloride concentration of 100
ppm at a vapour pressure of 25 bars was selected.
Calculation of any solute constituent concentration e.g. chloride (Cl) in water at any pressure given the
initial concentration in the fluid is based on the following equation:
Cl = Cl
s
in
1 − X
1 −X
in
s
8.3.0
where X s and X in represent the steam fraction at any vapour pressure and 25 bar-abs, respectively.
Cl s and Cl in represent the Cl concentrations in water at vapour pressure P s and 25 bars, and:
100000
X
s
h − h
L
t s
= 8.3.1
s
Log chloride (ppm)
10000
1000
2675 kJ/kg
2650kJ/kg
2550 kJ/kg
2600kJ/kg
2500 kJ/kg
2450 kJ/kg
X
h − h
t in
in = 8.3.2
Lin
h t represent the selected discharge
enthalpy and h in and h s denote the enthalpy
of steam saturated water at vapour
pressures of 25 bar-abs, respectively. L in
and L s represent the latent heat of
vaporisation at 25 bar-abs vapour pressure.
100
0 5 10 15 20 25
Vapour pressure bars
FIGURE 40: Modelled chloride concentrations at
selected enthalpies with variations in vapour pressure
Calculated Cl concentrations as a function
of vapour pressure at the selected
enthalpies are shown in Figure 40.
At high enthalpies, especially enthalpies
close to those of dry steam i.e 2675 kJ/kg
and at low vapour pressures close to
atmospheric pressure the chloride
concentration increase exponentially and
by more than 2 orders of magnitude. By
lowering the enthalpy of the well by 25 - 2650 kJ/kg the chloride concentrations at the low vapour
pressures drops drastically. It drops by almost 2 orders of magnitude at vapour pressures close to
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