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

secondary steam not sampled is free of gas. When collecting water and steam samples from wet-steam
well discharges using a Webre separator good phase separation is essential. Pipes from wells with low
discharge enthalpy (less than 900 kJ/ kg) may experience slug flow and separation can be a problem.
When the steam fraction is high, i.e. the discharge enthalpy approaches that of dry steam (above 2500
kJ/kg), the water flow rate into the Webre separator may not be sufficient for collecting a steam free
water sample and when this is the case, the only alternative may be the collection of a water sample at
atmospheric pressure (from the weirbox) or from a wellhead steam separator.
The sampling methods in case 3 were applied in Olkaria since the inception of the Olkaria East
Production Field, but may change with total re-injection of separated water being a requirement in
current power development schemes. This will require in the future that both water and steam are
collected at the same pressure.
Steam samples were collected into two gas-sampling flasks, which had been evacuated in the
laboratory after introducing 10 ml of freshly prepared 50 % w/w KOH in the case of Olkaria and the
others 50 ml of 4M NaOH solution. The gas bulbs were weighed before and after sampling to record
the amount of steam condensate collected. Water samples were filtered through 0.2 µm millipore
membrane (cellulose acetate) into low density polyethylene bottles using a propylene filter holder. The
entire filtration apparatus was thoroughly rinsed with deionised water before collecting a sample. A
100 ml sample was acidified with 1 ml of suprapure concentrated HNO 3 for ICP-AES analysis for Si,
B, Na, K, Ca, Mg, Al, Fe and S(SO 4 ). Another 100 ml sample was collected for SO 4 analysis to which
1 ml of 1% zinc acetate solution was added to remove H 2 S. A third 100 ml sample was collected and
for the determination of Cl and F by ion chromatography only, 250 ml glass bottles with special caps
that prevent entrapment of air were used to collect water samples for the determination of pH and total
carbonate carbon. These two water samples, which were not filtered, were cooled to < 40˚C by passing
them through a stainless steel cooling coil to prevent degassing of the sample that would otherwise
occur upon storage due to thermal contraction of the water prior to analysis.
3.2 Analysis of water and steam samples
Steam samples were analysed for CO 2 , H 2 S, H 2 , CH 4 , N 2 , and O 2 . The non-condensable gases (H 2 ,
CH 4 , N 2 and O 2 ) were analysed by gas chromatography, while CO 2 and H 2 S were determined by
titration of the NaOH-condensate solution with 0.1 M HCl and 0.001M Hg(CH 3 COO) 2 standard
solutions respectively (Arnόrsson et al., 2000). Both CO 2 and H 2 S disssolve quantitatively in the
alkaline solutions. In this way, the non-condensable gases become concentrated in the gas phase
making analysis for them more precise.
In water samples, H 2 S was analysed for in the same way as in steam samples and determined on site to
obtain its own concentration and again at the time of the total carbonate (TCC) titration for the
purpose of subtraction. Total carbonate carbon (TCC) and pH were determined in the laboratory as
soon as possible (1-2 hours) after sampling by titration with 0.1 M HCl. Interference from other bases
was corrected for by back titration with 0.1M NaOH following the bubbling of N 2 through the solution
to remove CO 2 and H 2 S (Arnόrsson et al., 2000). By this method, the difference of the two titrations
gives the sum of TCC and total sulphide. The value for TCC was obtained by difference from
independent measurements of total sulphide. The major aqueous cations (Na, K, Ca, Mg, Al, Fe,) plus
Si, SO 4 (as S) and B were analysed for on a Thermo Jarrel Ash ICP-AES. Ion chromatography was
used to determine SO 4 , Cl, F.
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