Case Study—Calcium-Carbonate-Scale Inhibitor Performance Degradation Because of H2S-Scavenger Injection in Semoga Field
Semoga field is an oil field in the Rimau block, which is located in South Sumatra, Indonesia. There is also a nearby oil field, Kaji field, in this block. These fields experienced reservoir souring and suffered a history of calcium-carbonate (CaCO3) -scale cases before a proper scale-inhibition program was implemented. At the end of January 2011, there was a separation-system problem at free water knock-out (FWKO) #2 Semoga Station. FWKO outlet lines were dismantled for inspection purposes, and it was found that the oil-outlet line at the downstream of the level control valve (LCV) was clogged by deposits, leaving an inner diameter (ID) of only 0.5 in.
X-ray defraction (XRD) and X-ray fluorescence (XRF) analyses confirm that the deposit is CaCO3 scale, even though routine scale-coupon monitoring shows very low scale growth. There is a hydrogen sulfide (H2S) scavenger Brand X injection at the downstream of the LCV on the FWKO #2, with a concentration of 11,050 ppm. The scale-inhibitor dosage in the water line on Semoga Station was only 9–10 ppm. Laboratory simulation shows that H2S scavenger Brand X injection reduced the scale-inhibition percentage from 97.2 to 35.3%, with 9 ppm of scale inhibitor. Meanwhile, no deposits were found in the oil-outlet line at the downstream of the LCV at Kaji Station, which has the same H2S-scavenger-injection point and dosage. The scale-inhibitor dosage in the water line at Kaji Station was 17.5–22 ppm. Laboratory simulation shows that 20 ppm of scale inhibitor resulted in a scale-inhibition percentage of 58%. Laboratory analysis shows that the scale-inhibition percentage increases linearly as the scale inhibitor increases under H2S scavenger Brand X influence.
However, another laboratory test conducted using H2S scavenger Brand Y shows that the scale-inhibition percentage will decrease with the increase of scale-inhibitor concentration, until it reaches the lowest value before increasing again with the increase of scale-inhibitor concentration. Thus, the scale-inhibitor performance under H2S scavenger Brand Y influence behaves parabolically.
It is concluded that H2S-scavenger injection increases system pH, resulting in the increase of scaling tendency. However, adding scale-inhibitor concentration is not always the answer to overcoming scale growth resulting from this effect. It is important to understand scale-inhibitor performance behavior on certain H2S-scavenger brands before taking preventive measures. Scale-inhibitor performance might have a linear correlation between increasing concentration and scale-inhibition percentage, as shown by the behavior with H2S scavenger Brand X, but it might also have a parabolic correlation, as shown by the behavior with H2S scavenger Brand Y or even other correlation types. A proper understanding of scale-inhibitor behavior can prevent a production loss caused by the deposition of 8,190 barrels of oil.
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