Produced water from a North Sea well showed high quantities of iron and H2S entering the well. Over a span of 2 years, the H2S increased; at the same time, iron trended down, indicating that iron ions were being consumed by the formation of iron sulfide scale, which hinders production efficiency.
Squeeze treatments with scale inhibitors using phosphonate chemistry are longer lasting, but they are less effective in preventing iron-based scale. In contrast, sulfonated polymer, another commonly used chemical, better inhibits formation of these scales but delivers a relatively poor treatment lifetime because it primarily relies on adsorption onto the reservoir rock surface.
In addition, the well has five producing intervals, and the heterogeneous zones challenge even placement of scale inhibitor. This nonuniform distribution combined with the high water production rate of >3,000 m3/d [>790,000 galUS/d]—which would rapidly deplete scale inhibitor concentration in the reservoir—complicated designing an effective squeeze treatment with a satisfactory lifespan.
SLB rose to the challenge and created a chemical treatment plan to address these issues. The first step was dissolving the existing iron scale with acid. This process was followed by squeezing the formation with the sulfonated polymer scale inhibitor SI-4154 to prevent further scale formation.
ResFiks 200 preflush additive was injected before the SI-4154; it acts like glue, improving adsorption of the scale inhibitor onto the reservoir rock. When designing the job, selecting appropriate volumes of both chemicals is key to ensuring effective placement of the scale inhibitor and achieving a satisfactory treatment lifetime that justifies the cost of the operation.
After the squeeze operation, the iron concentration in produced water more than tripled on average, indicating a dramatic reduction in the amount of iron lost to iron sulfide. The treatment was designed to handle 175,000 m3 [about 46 million galUS] of produced water, which corresponds to a treatment lifetime of approximately 58 days. Measurements of residual scale inhibitor concentration over time showed that the treatment would last about three times longer than predicted by design simulations.
