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 in