SCALE FORMATION ALONG THE INJECTIONWATER
PATH IN WATER-FLOOD OPERATIONS
At the injection wellhead, injection water temperature is usually
much lower than reservoir temperature. When it travels
down the injection well-string, the water cools the surrounding
formations, and its temperature and pressure increase. If the
water is saturated at surface conditions with salts whose solubility
decreases with increasing temperatures (e.g. anhydrite),
scale may form along the well-string.
Scale precipitation from the injection water may happen
behind the mixing zone as a consequence of temperature and
pressure changes. This is particularly true of waters containing
salts whose solubility decreases with increasing temperature
and decreasing pressure. Forward of the mixing zone only reservoir
brine (with oil) is present in the rock pores. Behind the
mixing zone, only injected water in equilibrium at local temperature
and pressure (with residual oil) exists. In the mixing
zone, precipitation of insoluble salts may occur due to the interaction,
at local temperature and pressure, of chemical species
contained in the injection water with chemical species present inthe reservoir brine.
Nevertheless, at a different pressure, the remaining clear
water moves ahead mix again with reservoir brine and scale
precipitation may again take place. This cycle is repeated
until the remaining clear water reaches a production well.
Pressure and temperature decrease along the flow string up to
the surface in the production well, and further changes in
thermodynamic conditions occur in the surface equipment.
This may again result in scale formation. Normally, these scales
do the most damage in the well-bore when there are major
falls in pressure but hardly any temperature changes [16].
There are three principal mechanisms by which scales
form in both offshore and onshore oil field system [17, 18]:
a) Decrease in pressure and/or increase in temperature of
a brine, goes to a reduction in the solubility of the salt
(most commonly these lead to precipitation of carbonate
scales, such as CaCO3).
Ca (HCO3 )2