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

8. EVALUATION OF SCALES DEPOSITED AT OLKARIA WELL OW-34, KENYA
Olkaria well OW-34 was drilled as a make up well together with wells OW-33, OW-32, OW-31,
OW30 and OW-29 after steam decline was experienced in the Olkaria East Production Field during
the first five years of production. Well OW-34 was connected to the steam gathering system in
March, 2001 and was disconnected in September, 2002 after its output, as measured by the pressure
drop across the orifice plate, indicated a substantial drop. On dismantling the flow pipes and wellhead
equipment, a thick deposit of scale which was intense, almost 1 inch thick, was found inside the two
phase pipe line. The scale also formed in the wellhead separator and in the separated water flowline.
Since the commissioning of the Olkaria I plant and production from the Olkaria East Production Field,
until 2002 no other well had experienced this kind of intense scale deposition. Well OW-34 is
anomalous. The concentration of chloride in the water is high because of its discharge enthalpy. This
was unusual and what caused the scale to form was not known. The general location of wells in
Olkaria East Production Field with well OW-34 is shown in Figure 39.
8.1 Output characteristics of well OW-34
Three flow tests have been conducted on this well, in 1993, 1996 and 2003. The first flow test was
carried out upon completion of drilling, the second to monitor tracer tests and the effect of cold water
injection in well OW-R3 and the third to investigate the causes of decline in steam output. The well
was discharged under different throttle conditions using different “lip pressure” pipe sizes i.e. 8”, 6”,
5”, 4” and 3”. During the first test enthalpy varied between 2640 kJ/kg and 2680 kJ/kg and the water
flow rate between 0.85 and 1.55 t/hr. In the second test, the discharge enthalpy was about 2650 kJ/kg
and water flow rate between 2.2
and 3.3 t/hr, while in the third test
enthalpy ranged from 2670 kJ/kg to
2675 kJ/kg and water flow rate from
1.5 t/hr to 2.1 t/hr. A summary of
these tests is shown in Table 9a, in
Appendix A (Opondo and Ofwona,
2003).
FIGURE 39: Location of wells in Olkaria East
production field
When the well was connected to the
production system, steam output
well declined from ~42.8 t/h
(Kariuki and Opondo 2001) in June
2001 to 7.2 t/h in March 2002. The
output of well OW-34 with time
when it was connected to the
production system is shown in
Table 9 b, in Appendix A.
While the enthalpy differences may
appear small, at such high
enthalpies close to the enthalpy of
dry steam these small differences
have tremendous effects on the
solute content of the water discharged at atmospheric pressure. Over all the discharge tests, the
discharge enthalpy varied between 2640 kJ/kg and 2680 kJ/kg at the throttle conditions and the steam
fractions calculated at atmospheric pressure (X s ) are high, or between 0.9973 and 0.9996.
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