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

pH on adiabatic steam loss
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OW-901 Olkaria Domes
OW-301 Olkaria West
OW-306 Olkaria West
OW-20 Olkaria East
OW-30 Olkaria East
OW-714 Olkaria North East
OW-34 Olkaria East
OW-709 Olkaria North East
OW-304D Olkaria West
SV-05 Svartsengi
SV-11 Svartsengi
RN-10 Reykjanes
RN-11 Reykjanes
NJ-14 Nesjavellir
NJ-22 Nesjavellir
80 120 160 200 240 280 320
Temperature °C
FIGURE 25: Calculated aquifer water pH for selected
Olkaria, Reykjanes, Svartsengi and Nesjavellir wells
and changes in pH during several steps of
single-step adiabatic steam loss
producing aquifers because the pore
spaces in the rock are expected to be
able to cope much longer with calcite
precipitation than the wellbore (see
Arnórsson, 1978). The extent of CO 2
degassing is also important it is
anticipated that calcite scaling will be
less for good producers than for poor
wells.
At Svartsengi calcite scaling was
troublesome in shallow wells during
the early years of production when the
first level of boiling occurred within
the producing wells. Pressure
drawdown in the reservoir by
exploitation caused extensive boiling in
producing aquifers of these shallow
wells with the result that calcite scale
formation was no longer observed. It is
that Calcite is still considered to be
deposited but in the aquifer outside the
wells. At Reykjanes calcite scaling has
not been observed as a problem. The
cause is considered to be high reservoir
temperatures and high yield of wells
which tends to cause incomplete CO 2
degassing during boiling.
7.2 Scaling tests at Nesjavellir
It is common practice in the geothermal industry to carry out scaling and corrosion tests to aid material
selection and quantify the rate of scale deposition which is not possible theoretically. This involves
insertion of plates into the stream of the geothermal fluids and the study of their corrosive nature and
any deposits that may form on the plates. The present scaling study at Nesjavellir involved the
insertion of stainless steel coupons into the two phase flow pipelines at two wellheads and into the
stream of separated water upstream and downstream of a retention tank. This water is significantly
supersaturated with respect to monomeric silica (i.e. it is present mostly as single SiO 2 molecules).
Monomeric silica grows on surfaces which act as active sites as mechanism of silica deposition.
Downstream of the retention tank, the water is diposed of into an injection well. In the tank most of
the silica in excess of amorphous silica solubility polymerizes. Polymeristion goes through a stage
process i.e. monomer → dimers → trimers → tetramers → polymers → gel etc. Polymerisation
continues until silica saturation is reached. It is known that (Yanagase et al., 1970; Rothbaum et al.,
1979) fully polymerized silica precipitates much more slowly from solution than monomeric silica, at
least from dilute water as such that at Nesjavellir. The purpose of the retention tank is thus to reduce
amorphous silica deposition in the injection well and the receiving aquifers. The purpose of the scaling
tests up and downstream from the retention tank was thus to observe the extent to which silica
polymerisation reduced the rate of amorphous silica precipitation. The purpose of the tests at the
wellheads was principally to observe the rate and nature of sulphide scale formation and to identify
whether phases other than sulphides formed in significant amounts.
A simplified process flow diagram for the Nesjavellir co-generation power plant is shown in Figure
26. The diagram shows the process flow from the geothermal wells being separated into steam and
water at ~ 14 bar absolute pressure in a central separation station. The steam is piped to power plant
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