Page 251 - Formation Damage during Improved Oil Recovery Fundamentals and Applications

222 Xingru Wu

scale type at low temperatures. Anhydrate is the stable form of calcium

sulfate scale at temperatures above 100 C. Hemihydrate scale formation

occurs at temperatures between 100 Cand 120 C in brines with high

ionic strength and in nonturbulent environments. Among these calcium
sulfate scales, gypsum is more likely to form in the oil and gas reservoir
conditions. Given right conditions, gypsum and anhydrate can transform
from one to the other (Raju and Atkinson, 1990). As an example, the
reaction of forming barium sulfate is as follows:
22
Ba 11 1 SO -BaSO 4 k
4
Fig. 5.3 shows the solubility of different scales at different conditions.
Given that barium sulfate has a very low solubility in brine and even in
mineral acids such as hydrochloric acid, the near wellbore formation
damage caused by sulfate scale precipitation cannot be effectively reme-
diated without a well workover. Field experience indicates that well pro-
ductivity can be severely impacted by sulfate scale in a matter of days or
weeks if left untreated in sea waterflooded fields. The solubility of barium
sulfate slightly increases with temperature and the presence of mineral
salts such as NaCl.

5.2.3 Other inorganic solids
Other inorganic solids can also precipitate during scaling process. For
example, usually the formation brines contain a low concentration of
iron, but some iron scales such as iron oxides and, hydroxides, and sulfides
can occur as a result of corrosion when dissolved gases such as CO 2 and
H 2 S contribute to tubular corrosion, which leads to the formation of var-
ious iron compounds. Some organic scales such as calcium napthenates
may also be generated in some particular conditions. Some other inor-
ganic solids include lead sulfide, zinc sulfide, and calcium fluoride.
Some naturally occurring radioactive material (NORM) may also pre-
cipitate in reservoirs or wells. For example, Low Specific Activity (LSA)
226
( Ra) can be coprecipitated with BaSO 4 and CaSO 4 scale, and it emits
alpha, beta, and gamma radiations. Therefore, if this type of scale are pro-
duced in an open environment, special handling permits may be required
from regulatory authorities to dispose of it, and it needs to be properly
packed and shipped to registered recipient for proper disposal.