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.

With wet tree developments now outweighing dry tree developments in the North Sea, the challenge of accurate surveillance of scaling in wells is increasing. Often, multiple wells feed into manifolds which themselves feed into comingled systems. The need to differentiate multiple scale inhibitor molecules in the same sample of produced water is now essential in order to monitor the scale health of any given well in that system. Accurate surveillance can save potentially millions of dollars of deferred oil production, intervention vessel hire and chemical and operational costs. This paper details a case history from the North Sea where the subsea field was located 21 km from the host platform. The field comprised three wet tree wells that each required scale inhibitor squeezing, and the flowline itself also had a scale inhibitor injected at the manifold. Commingling occurred at a manifold and with no test line all fluids were produced to the same dedicated separator on the host platform, which was the further point upstream where sampling could occur. Details are given on the selection methodologies employed to determine the most appropriate chemistry for each squeeze application, as well as for subsea injection to the manifold. The primary aim was differentiation of all the chemistries from one another. The paper shows how squeeze treatment volume and frequency were optimized, leading to increased production up-time and deferment of chemical costs. It also shows the accurate detection of scale inhibitor to very low levels, providing confidence that the wells remained protected with no scaling or loss of production. All chemistries selected were also done so with the environmental classification in mind, which led to a significant improvement in environmental profile and impact footprint.

In scale inhibitor squeeze treatments, precipitation of the inhibitor within the formation can lead to extended squeeze lifetimes. However, such processes also have the potential to cause formation damage unless they are carefully designed and controlled. The formation of a partially soluble inhibitor/metal complex within a reservoir is the objective for almost all precipitation squeeze packages. However, historically there are numerous ways this is achieved almost all of which require a limited operational window to be deployed successfully. In this paper, we describe the development of a novel dual chelant system which provides a method for controlling both the "wanted" and "unwanted" precipitation of the scale inhibitor package within the formation. The highly tunable nature of the system allows for ease of pumping at more extreme conditions (higher and low temperatures, calcium levels etc.) than have previously been possible. By using the dual chelant mechanism described in this paper, a package can be tuned to precipitate within a certain time frame both at low and high temperatures in brines with varying degrees of salinity and hardness. The scale inhibitor (SI) itself is a chelant or ligand for divalent ions present (mainly Ca2+) and this is denoted L2 and the second chelant, L1, is added to the system at certain design concentrations, as explained in the paper. In many situations, the high divalent metal ion content of a produced brine, or formation water can limit the successful pumping of a scale inhibitor due to high levels of calcium, for example. Under these conditions the dual chelant mechanism can also be deployed to prevent scale inhibitor phase separation. This paper discloses the theory of how the dual chelant mechanism works using computer modeling and the subsequent confirmation of the simulations by laboratory testing. The importance of the pKa of the SI (L2) and the added chelant, L1, and the relative metal binding constant interactions between L1/ L2 and Ca2+ are explained and investigated. The comparison of the dual chelant mechanism versus conventional packages is demonstrated by core flood experiments. The dual chelant mechanism gives a clear improvement in squeeze lifetime and controllability and provides a platform for the development of many types of controlled solubility scale inhibitor treatment.

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.