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

A
B
FIGURE 28: Retractable coupon holders, a) Used for high pressure points e.g. at the wellheads;
and b) For low pressure points e.g. on the separated water line at the re-injection well
shown in Figures 28 a and b. In Figure 28 a a test holder for high pressure applications is shown and
this was used for test coupons at the wellheads. In Figure 28 b is shown coupon test holders for low
pressure applications as one that was used on the waste water line at the re-injection well. These have
test coupons attached to them.
The coupons were cleaned, dried and their weights taken before they were introduced into the test
environment. The coupons were mounted on retractable “slip in” specimen holders. These have the
advantage of being inserted into plant test environment without the need to shut down. The coupons
were first abrased with a soft sand paper then cleaned in hot distilled water before being degreased in
isopropyl alcohol. The method for cleaning and preparing the coupons is detailed in Appendix B.
7.2.2 Test procedures and selection of test sites for the study
The retractable specimen holders with the cleaned dried and weighed stainless coupons screwed onto
them were inserted in different fluid environments. During the tests the pressure at the wellhead of
one of the wells (NJ-14) was 25 bar and that of the other (NJ-22) was 34 bar. The discharge enthalpies
of NJ-14 and NJ-22 are 1400 kJ/kg and 1800 kJ/kg, respectively. Coupons were inserted at the
wellheads of each of these two wells. A third site was chosen just downstream of the heat exchangers
inside the plant which recieve separated water at about 188°C. At this study point the separated waste
water extruding from the heat exchangers has cooled to 60-95°C depending on the demand for hot
water in Reykjavik. The fourth site was at the point of entry to the retention tank and the fifth where
waste water flows into the injection well. The last two sites were chosen to investigate the effect of
silica polymerisation in the retention tank on the rate of amorphous silica deposition. The different
types of selected test sites are shown in Figures 29, 30 and 31 respectively.
The test coupons were introduced the test sites on 06 July 2005 at the retention tank and re-injection
well site and on 14 July 2005 at the wellheads of NJ-14 and NJ-22 and after the waste water leaves
the heat exchangers. The coupons were inspected after ~ 13 weeks on 14 October 2005 to check on
any signs of scale deposition. On this date one coupon was extracted from each test site and replaced
with a new set of cleaned and weighed test coupons. These new test coupons, together with the ones
that had been left intact 13 weeks were kept in place for an additional ~ 16 weeks to monitor the
deposition rates from fluids at all the sites. In all, the test thus lasted for ~ 29 weeks. The test
specimens were removed from the test sites on 30th January 2006. Incidentally, when we went to take
out the test coupons from all the sites on this date, it was realised that the coupons in the sites at wells
NJ-14, NJ-22 and the site after the heat exchangers were missing from the holders. It was hard to
establish what caused their removal but it could have been due to unexplained changes in the flow
patterns of the well fluids and the separated water downstream of the heat exchangers that could have
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