Seawater injection is regularly used for pressure maintenance in oilfields during secondary oil recovery. Typically oxygen is removed from the seawater via a process of mechanical deaeration and oxygen scavenger is added to remove final traces. An oxidizing biocide such as hypochlorite is commonly added at the seawater lift pumps, to provide protection to the system from microbially influenced corrosion (MIC) up to the deaerator, after which sulfite based oxygen scavengers will react with residual oxidizing biocides. Therefore, the deaerator itself and the water injection pipelines downstream of the deaerator require protection from microbial contamination, which is normally achieved through dosing a non- oxidizing biocide. Despite potential incompatibility issues, non-oxidizing biocides and oxygen scavengers can be dosed simultaneously, due to unavoidable operational issues, with the operator having to choose between MIC and oxygen corrosion risks.
John Crane has been able to compare the effectiveness, on a consortium of sessile oilfield microorganisms, of two commercially available biocides. One with the active ingredient isothiazolinone and the other with 2,2-dibromo-3-nitrilopropionamide (DBNPA). Tests have been carried out with and without the presence of a sulfite oxygen scavenger.
Sessile microorganisms were grown in biofilms on carbon steel studs and exposed to isothiazolinone at 100, 200, 300 and 400 ppm and DBNPA at 100 ppm and 200 ppm, in the presence and absence of ammonium hydrogen sulfite oxygen scavenger. After exposure times of two and four hours, the microorganisms were enumerated using the most probable number (MPN) technique. Furthermore, the sessile microorganisms were exposed to 400 ppm isothiazolinone and 200 ppm DBNPA for four hours in a dynamic flow loop system to determine the effectiveness of batch dosing over time. Regrowth of the microbial community was measured by MPN and residual microorganisms were identified using next generation sequencing (NGS).
The methods employed showed that 400 ppm isothiazolinone produced a total kill of 84% after two hours without oxygen scavenger, but only 52% with scavenger present. After four hours the effectiveness rates were 99% and 97%. The difference was more dramatic with 200 ppm DBNPA killing 95% of bacteria after two hours with no oxygen scavenger, but only 25% when scavenger was present. After four hours the figures were 99% vs 88%. Therefore isothiazolinone appeared to be less affected by the presence of the oxygen scavenger than DBNPA.