Modelling of geochemical reactions during smart water injection in carbonate reservoirs

Extensive studies have confirmed that altering ionic composition of injection water has a big impact on the ultimate oil recovery. Different mechanisms have been proposed to explain the positive effects of Smart Water injection, but no single one is universally accepted as the dominant mechanism. Therefore, in the paper, we conduct a three-dimensional reactive transport modelling study to investigate the geochemical processes during seawater flooding altering seawater with different sulphate concentrations injected into a carbonate reservoir which can help the understanding of the possible mechanism behind Smart Water injection. A series of calcite and anhydrite mineral reactions are key in situ chemical reactions in the study. At the early stage, CO2 partitioning from the hydrocarbon phase into the brine causes significant calcite dissolution. This process can be enhanced by increasing sulphate concentration in the injection water. Sulphate concentration in the injection has a significant impact on whether the calcite is continuously dissolved or not after the CO2 front passes. In the modelling cases that include thermal transport, reservoir temperature is cooled by injection water, and thus anhydrite precipitation and calcite dissolution are decreased.