Chemical injection using HR-2746 removed large quantities of H₂S from the production stream while also reducing chemical usage and discharges, schmoo deposition, oil content in produced water, and the frequency of cleaning surface equipment. At the same time, it eliminated the need for major modifications to the offshore infrastructure.

Seawater injection for reservoir pressure support began in Draugen Field* in 1994. Bacteria in the reservoir reduce sulfate in the water to H₂S. When the seawater reached production wells, OKEA—the operator—started encountering significant challenges related to production of H₂S and precipitation of schmoo, which consists mainly of iron sulfide (FeS) together with organic compounds such as hydrocarbons, paraffin, asphaltene, and biomass.

The FeS was often wrapped in an oil film, leading to more oil in produced water. The result was fouling of the water reinjection system and greater environmental impact from the discharged water. Frequent cleanouts of the hydrocyclones were required, costing time and money. The precipitation mostly occurred in the Garn West separator, where oil streams from all subsea wells and one platform well were routed. An array of treatments was used, including flocculants, a scale inhibitor, and topside injection of MMA triazine, which increased opex but did not solve the challenges.

SLB suggested subsea injection of its multiphase H₂S scavenger HR-2746. Unlike standard nitrogen-based scavengers, which increase pH and consequently the risk of scale formation, HR-2746 is pH neutral. It was also the only H₂S scavenger available in the region that was qualified for subsea conditions. OKEA agreed to a field trial involving injection of the scavenger into one well via a spare methanol line. As a result,

• H₂S levels in the well stream reduced from 170 ppm to 60 ppm, halving the amount of H₂S scavenger used topside
• less H₂S led to a reduction in precipitated FeS, confirmed by an increase in the levels of dissolved iron in produced water
• oil in produced water decreased significantly
• hydrocyclone cleanouts that were once required monthly have been eliminated
• flocculants at surface are no longer used
• reduced chemical consumption and less oil in discharged water mean a reduction in environmental impact.

In addition, the topside scavenger MMA triazine has been replaced by the more water-soluble MEA triazine, reducing issues with particle formation in the condensate system and resulting in significant cost savings.

* OKEA ASA, Petoro AS, and M Vest Energy AS are license partners for the Draugen Field.