Sulphate Enrichment During Low Sulphate Seawater Injection in Carbonate Reservoirs

This paper uses geochemical modelling coupled with flow simulation to evaluate how mineralogy and reservoir characteristics can influence the sulphate concentration of produced brine in carbonate reservoirs during low sulphate seawater (LSSW) injection. Geochemical parameters, including mineral dissolution or precipitation behaviour, are examined at the advancing injection waterfront.

This study applies reactive transport simulation using the WOLERY database in a 3D reservoir sector with a producer-injector well pair. Pressure, temperature, and formation water and injected water compositions are characteristic of Brazilian pre-salt fields. Reservoir mineralogy comprises calcite, dolomite, anhydrite, gypsum and barite. Two contrasting initial oil compositions are assessed, the first containing 1% CO₂ and the second containing 18% CO₂.

The results demonstrate that LSSW, being undersaturated with respect to sulphate and carbonate minerals, causes anhydrite and calcite dissolution near the injection well in both the 1% and 18% CO₂ cases, enriching the brine with sulphate ions. However, calcite dissolution is significantly enhanced in the 18% CO₂ scenario due to the elevated CO₂ concentration. As the waterfront advances away from the injection well, the brine becomes enriched in calcium and sulphate ions, triggering anhydrite precipitation. Nevertheless, in the 18% CO₂ case, the system exhibits greater ion saturation owing to higher reactivity, resulting in sufficient anhydrite precipitation that the reaction effectively removes sulphate from the brine. Conversely, in the 1% CO₂ case, sulphate removal is only partial. Thus, geochemical processes in carbonate reservoirs can substantially modify the sulphate content during LSSW injection, showing that while initial anhydrite dissolution releases sulphate ions near the injector, subsequent precipitation at higher temperature can remove it from the brine.

The previously undocumented findings in this work reveal that employing a Sulphate Removal Unit (SRU) for 1% CO₂ reservoirs containing anhydrite is not economically justified. Such treatment incurs substantial costs whilst in situ geochemical reactions result in elevated sulphate concentrations in the produced brine and consequent scaling risks.