Sulfate Stripping from Injection Seawater in a Sandstone Reservoir and its Implications on Scale Management Options and Decisions

Abstract This paper presents an interesting case study of a Chevron-operated oilfield in offshore West Africa where the oil reservoir has been flooded with full-sulfate seawater (without sulfate removal treatment) since 2010 and extensive produced water analytical data were available from multiple production wells. Detailed analysis of the produced water chemistry data, especially sulfate and scaling cation concentrations, and scale modeling were conducted. This review of the water chemistry data and scale modeling revealed significant sulfate stripping inside the reservoir (>> 50% sulfate depletion from injection seawater) that is not caused by brine mixing and associated scale precipitation inside the reservoir formation but due to rock-water interactions. Likely reactions of rock minerals with the injection seawater and precipitation of calcium sulfate anhydrite and magnesium-containing carbonates are postulated and presented in the paper. The main conclusion developed from this study is that sulfate stripping in this field is so significant that it has moved the mixed formation water and injection seawater (namely the produced water) out of a sulfate scaling regime into a non-scaling regime. Based on this field case study and the findings derived therein, this paper goes on to discuss the implications on scale management options and decisions for both existing offshore fields and development projects. For a brown field with ongoing full-sulfate seawater flooding, it may call for rethinking the need for frequent scale squeeze treatments. For a new project for which seawater water flooding is planned, it may influence the decision on whether to install a sulfate removal treatment facility and consequently impact the project’s Capex and footprint as well as the future Opex.