An operator was facing significant challenges because of iron sulfide (FeS) scale deposits in the tubing and ESP in a Black Sea well. The scale was severely hindering oil production, resulting in >50% decrease in output, and leading to high pressure and temperature at the ESP intake. Acid treatments would liberate hazardous H2S gas and consequently, the operator was seeking a safer and more controlled solution.
PYRISOL iron sulfide scale dissolver was selected for the task; it is alkaline in nature and does not release H2S during scale dissolution. Laboratory tests with a 50% solution resulted in dissolving 62% of the scale in 24 h. Following this test, a treatment was designed based on circulating PYRISOL dissolver down the tubing annulus.
After a preflush with 2 m3 of an aromatic mutual solvent PI-7430 to remove hydrocarbon deposits, the scale dissolver was pumped in two batches. Downhole equipment was allowed to soak for an hour in the first batch, and then the remaining treatment was pumped down; the entire 8 m3 of dissolver was circulated for 24 h at a flow rate of 40 L/min. During that period, 12 samples of the circulating fluid were analyzed and showed a gradual increase in the amount of dissolved iron.
The treatment fluid was passed through a sieve assembly at surface, and 350 kg of finely dispersed scale was collected; an estimated 450–500 kg of scale was removed in total. Following the treatment, ESP intake temperature and pressure normalized and production was fully restored, increasing from 50 m3/d to 110 m3/d.
