An operator within the North Sea had historically been using seawater injection for pressure support, with produced water being discharged to sea. Water injection was via subsea injection wells, with a subsea flowline transporting water from the host platform to these wells. For the last 6 years, this strategy has changed – seawater injection has been halted and the objective has been to reinject all produced water. During this period of time, the field had experienced rising H2S levels within the produced water and increasing oil in produced water, due to problems with separation that were controlled by application of flocculants and monthly cleaning of separation equipment. The solids found within the hydrocyclones were determined to be iron sulphide, reservoir silicates, barium sulphate scale, and biomass. Over time it became clear that the subsea water injection pipeline was also facing challenges with increased pressure drop resulting in lower injection capacity.

Injection line fouling

Cleaning the subsea water injection line was evaluated initially via batch application of chemicals such as hydrocarbon solvents, mutual solvents (hydrocarbon removal and water wetting), dilute acid solutions and THPS solutions (sulphide dissolver), chelants (sulphate dissolvers) and oxidising agents (biomass dissolvers). Evaluation of the volumes of chemical required to treat the flowline, in addition to the aggressive nature of these materials and the logistic issues associated with the application of such large chemical volumes resulted in another cleaning program option being investigated. The laboratory testing of a novel dissolver/dispersant not based on THPS (tetrakis (hydroxymethyl) phosphonium sulphate) was carried out leading to a field application of the cleaning chemical at treatment rates between 5 and 40 ppm into the produced water for a trial period of 3 months.

The application of the continual injection cleaning chemical program to remove the fouling in the produced water system using the novel dissolver/dispersant and control of additional biofilm formation with the biocide program applied to the produced water resulted in the following changes to the produced water injection system.

  • Produced water injection rate increased from 729 m3/h to 777 m3/hour (7% increase).

  • Injection pressure reduction from ∼50 bar to 40 bar (a 20% decrease despite injection rate increasing as above).

  • Increased production as the injection line fouling was reducing reservoir voidage replacement.

This paper outlines the development/deployment of this novel non THPS based iron sulphide cleaning chemical package which is suitable for application within the North Sea.

You can access this article if you purchase or spend a download.