Scale inhibitors are commonly used for mitigating scale deposition risks in many oil and gas wells worldwide. Of the various chemistries used for scale inhibition, much research has gone into the various conditions in which each chemistry performs best (i.e. temperature, brine solubility, salinity, etc.)4-6 . Furthermore, it is known that dissolved iron (Fe2+ and Fe3+) can hinder the performance of scale inhibitors, some more than others3 . Thus, applying this knowledge we can extrapolate which inhibitor chemistries might perform best under a given set of conditions. This knowledge can then be applied regionally where most production comes from the same or similar reservoirs and production conditions.

However, less research has been conducted on the effects of pre-existing iron sulfide deposits on the performance of scale inhibitors. Iron sulfide solids are becoming increasingly problematic in the oil field. The combination of iron sulfide with more conventional scaling deposits and the fact that scale inhibitors are surface active and tend to adsorb onto surfaces can yield very challenging situations. This paper discusses testing conducted on various scale inhibitor chemistries and evaluates how exposure to pre-existing FeS solids may impact performance. The various scale inhibitors were evaluated for inhibition performance against a set of controls (no FeS exposure) utilizing the NACE Standard TM0137-2007 "Laboratory Screening Tests to Determine the Ability of Scale Inhibitors to Prevent the Precipitation of Calcium Sulfate and Calcium Carbonate from Solution (for Oil and Gas Production Systems)" with an additional pre-test procedure to expose scale inhibitors in stock solution to a set weight of reagent grade ferrous sulfide (FeS).

Scale inhibitor chemistries evaluated include two polymers (scale inhibitor A and B) and five phosphorous based scale inhibitors (scale inhibitors C through F). The various configurations tested included: scale inhibitors alone, scale inhibitor plus FeS solids, scale inhibitor without FeS plus crude oil, scale inhibitor plus FeS and crude oil. The inclusion of the crude oil allowed an interface for potential micelle interactions. The results indicate scale inhibitors A, C and G were least affected by the presence of FeS with no regard to the presence of crude oil. With this study a scale inhibitor that worked best in the presence of FeS solids for the customer's field in the Permian Basin, where FeS has become an increasing issue, was recommended. This also allowed the customer to treat the FeS solids issue via the method that works best for them.

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