Abstract

Injection of Low Sulphate Seawater (LSSW) instead of untreated Full Sulphate Seawater (FSSW) is widely used to mitigate barium sulphate (BaSO4) scaling risk at production wells. LSSW injection may no longer be required when the barium (Ba2+) concentrations in the produced water drop below a certain threshold. Such a trigger value could be estimated from the BaSO4 precipitation tendency. Relaxation of requirements for the Sulphate Reduction Plant (SRP) can significantly reduce operational costs. This study investigates the impact of several parameters on the timing and degree of relaxation of the output sulphate (SO42-) concentration by the SRP. Finally, the optimal switching strategy is proposed for a field case.

The strategy for switching from LSSW to FSSW, e.g. time and method (direct or gradual increase in the SO42- concentration) were initially investigated using generic 2D areal and vertical models. The sensitivity study included the impact of reservoir heterogeneity and initial Ba2+ and SO42- ion concentrations. Findings were later applied on a full field reservoir simulation model followed by a mineral scale prediction software to investigate the specific switching strategy for a field that has multiple wells and significantly more complex heterogeneity.

Results show that Ba2+ concentrations in the formation brine impact the choice of switching time more than the output SO42- concentration produced by the SRP. The degree of heterogeneity around the producers also has a significant impact on the switching time. Another parameter is the contrast in the permeability between layers; higher contrast allows longer period of co-production of the scaling ions and thus delays the switching time. In the field case, switching to FSSW at early times allows higher consumption of Ba2+ ions due to its in situ precipitation. Ba2+ is no longer a limiting ion, and so a higher degree of deep reservoir precipitation reduces the requirement for prolonged LSSW injection. Another strategy is a gradual relaxation of LSSW output, which allows even earlier build-up of the injected SO42- concentration compared to the direct FSSW switch.

The study investigates the reservoir parameters that impact SO42- relaxation of LSSW injection for a field. Following the proposed workflow, the optimal relaxation strategy can be designed for other field cases.

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