Case Study—Calcium-Carbonate-Scale Inhibitor Performance Degradation Because of H2S-Scavenger Injection in Semoga Field
Semoga and Kaji fields experienced reservoir souring and suffered a history of calcium-carbonate (CaCO3) -scale cases before a proper scale-inhibition program was implemented. Problems with a free water knockout discovered continued scale issues, leading to investigation of the reasons.
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.