No abstract

Iron sulfide scale is prevalent in the industry and its chemical prevention is an area of recent focus by several research groups. There are very few examples where true inhibitor chemistry has been effective at controlling the scale, rather stoichiometric amounts of chelating agents are required. Partial inhibition has been achieved using classic scale inhibitor species such as phosphonates or maleic acid or sulfonated copolymers. Efficacy of these classic inhibitors against sulfide scale is poor and often uneconomically high concentrations are required. This paper summarizes the work performed to deliver to the industry a true, high-performance sulfide scale inhibitor. This chemistry offers a true step-change in performance from existing technologies. An industry best-in-class rapid screening technique was used to systematically evaluate all current technologies in the market place and from this develop a detailed understanding on the structure-performance relationships of functional groups. By ranking the correlations, a range of novel polymeric chemistries were synthesized which provided significantly superior inhibition. Test methods are presented that mimic formation of these scales in the field and order of magnitude increases in performance over standard species are reported, thus representing a true step-change in the efficacy of sulfide scale inhibition. The paper is largely dedicated to a field trial of this novel chemistry in USA. A severe iron sulfide fouling issue was being experienced in the water handling system. Intense sampling was performed when trialing classic chemistries such as THPS and phosphonate scale inhibitors as well as the novel polymeric product. The results speak for themselves and show that threshold concentrations of the new chemistry controlled sulfide deposition whereas order of magnitude more of standard chemistries are required to achieve the same efficacy.

Prevention of sulfide scale through the use of chemistry is a developing area of focus within the oil industry. There are few examples of a single chemical approach working, where scale inhibitor species function at threshold concentrations. Partial inhibition may be provided by ‘standard’ scale inhibitors using established chemistries such as phosphonates or polymeric species. However, the efficacy of these inhibitors against sulfide scale is generally poor and high concentrations are typically required. Based on an industry need for a true, high-performance sulfide scale inhibitor, work was undertaken to develop a novel chemistry that would offer a step-change in performance from existing technologies. Using a new advanced rapid screening technique, a wide range of ‘standard’ scale inhibitors were assessed, which proved that the majority of these chemistries display no efficacy against sulfide scale. A select few of the standard scale inhibitors displayed limited efficacy and from this data common molecular features which contributed to sulfide scale inhibition were identified. Utilizing this knowledge, a range of novel polymeric chemistries were synthesized which provide significantly superior inhibition than any other postulated for this application. It has been possible to identify specific moieties within these complex polymers which are required for sulfide scale inhibition and to theorize on likely molecular structure-performance relationships for this new class of scale inhibitor. Additionally, hypotheses on the specific mechanisms by which these inhibitors function have been provided, showing why they are so successful at sulfide crystal growth retardation. Static and dynamic test methods are described that accurately mimic formation of these scales in the field, in comparing the novel polymeric chemistry with the best-performing ‘standard’ scale inhibitors. Order of magnitude increases in performance over standard species are reported which represents a true step-change in the efficacy of sulfide scale inhibition.

An operator in West Africa required a scale inhibitor for an application with several technical challenges. The scale inhibitor was required to meet the Angolan decree on environmental acceptability, had to be compatible with F22 alloy present in the wellhead, and had to be suitable for application via umbilical. The scale in question was a reasonably harsh barium sulphate condition. A number of chemical formulations were suitable for use with F22, however, these chemicals did not meet the performance requirements set for barium sulphate inhibition. Testing was conducted starting with corrosion tendency screening, whilst several potential raw materials were checked for their environmental credentials. It was possible to meet the corrosion screening conditions set by the operator with one raw material; however, this did not meet the scale inhibition criteria. Several chemicals were examined in dynamic scale loop testing, with none achieving the required performance. It was decided to evaluate blends of generic scale inhibitors, to assess whether a synergy between two different chemicals could offer improved inhibition performance, whilst still meeting the corrosion rates. The chemicals selected were known to have lower corrosivity – one a polymer which had been used successfully with F22 alloy, plus another polymer widely used in the North Sea where low corrosivity generally is required along with good environmental properties. This blend was successful in meeting the scale inhibition performance target and was found to offer good compatibility with F22 alloy under application conditions. Furthermore, both chemicals were suitable to meet Angolan decree on environmental acceptability. The paper describes in detail the development test work carried out for corrosion assessment, inhibitor performance, formulation stability, brine compatibility to select this formulation and highlight the importance of synergistic blends for meeting particularly challenging scaling environments.