Modelling carbonate and sulphide scales in sour systems

Abstract

Carbonate and sulphide scales are pH-dependent inorganic deposits that often form in oil and gas wells posing a significant threat to production efficiency, system integrity, fluid quality and production safety. Despite the frequent occurrence of these scales, their prediction and mitigation still present numerous challenges, particularly in the case of sulphides. This work is aimed at laying out and addressing these challenges to ultimately improve field scale management programs. Carbonate and sulphide scales are directly coupled together and differ from other inorganic scales because they are intimately linked to the in-situ concentration of carbon dioxide (CO2) and hydrogen sulphide (H2S), which influence the local pH and availability of reactive species. . Pressure-volume-temperature (PVT) and scale prediction calculations must be combined to accurately model three phase component distributions and to determine the scaling risk profile from reservoir to separator conditions. Although some commercial software integrates these two sets of calculations, these packages are usually more targeted to one or the other applications; i.e. either oil/gas PVT or aqueous phase chemistry. This work defines a rigorous step-by-step procedure (workflow) which allows us to obtain carbonate and sulphide scale prediction profiles from commonly available field data using any choice of PVT and scale prediction software. The capability of decoupling PVT and scale prediction software offered by this workflow (as opposed to using an integrated software package) enabled a thorough study of the impact of field measurements and software choice on the final pH-dependent scale prediction results. The newly developed workflow was also applied to two real field case scenarios to show how it can be used to answer key operational questions.