The Impact of Reservoir Properties on the Sulphate Content of Produced Brine in Carbonate Reservoirs Under Seawater-Alternating-Gas Injection

Abstract This paper uses reactive transport modelling to investigate how mineralogy and reservoir properties can affect geochemical reactions and the sulphate concentration of brine in carbonate reservoirs under seawater-alternating-gas injection. Geochemical parameters, such as ion concentrations and mineral dissolution or precipitation, are analysed at the propagating injection front. The study is carried out in a 3D reservoir model assuming two zones with distinct mineralogies. A compositional reservoir simulator coupled with a geochemical model is used with the WOLERY database. Pressure, temperature, formation water and injected water compositions are based on the Brazilian pre-salt scenarios. The upper reservoir zone consists of calcite, dolomite, anhydrite, barite, and gypsum, and the lower reservoir zone is only formed by calcite and dolomite. Two different initial oil compositions are tested, the first with 1% CO2 and the second with 18% CO2. For the scenarios proposed, the results show that carbonate reservoirs with 1% or 18% CO2 in the initial oil phase and formed by calcite and dolomite can remove sulphate from the injected seawater as the seawater front flows through the reservoir, which means that sulphate is removed from brine during CO2 WAG EOR and this process is not dependent on the CO2 concentration in the initial oil phase. Calcite dissolution releases Ca2+ ions into the brine, which precipitate with SO42− ions from the injected seawater. The brine reaches the region of the producer well with depleted sulphate concentrations. Hence, injecting desulphated seawater for this scenario is expendable, as it involves the high cost of treating all water using a Sulphate Removal Unit (SRU) before injection, and the reservoir will perform the same function naturally. For carbonate reservoirs with anhydrite in their mineralogy, depending on reservoir properties, the produced brine may be less depleted in sulphate concentration due to anhydrite dissolution. For the models proposed, sulphate is almost fully removed from the injected brine in the 18% CO2 reservoir, and only partially removed from the injected brine in the 1% CO2 reservoir. This work suggests that geochemical reactions in carbonate reservoirs can naturally alter the sulphate content of injected seawater, demonstrating that sulphate can be naturally removed from the injected brine, leading to a produced brine depleted in sulphate. It was also demonstrated that the CO2 content in the original oil composition and mineralogy may have a significant role in the reservoir reactivity. This is a novel finding not reported in previous research.