A Case Study of the Influence of Sulfate Ion Concentration Change on Oil Displacement Efficiency of Carbonate Reservoirs in Low Salinity Water Flooding

Measures to enhance the recovery of carbonate reservoirs in the Middle East primarily involve water injection development. The practice of water flooding in various countries has demonstrated that using low salinity water as an alternative medium for water flooding of carbonate reservoirs can yield superior results. However, the impact of sulfate ion concentration in the replacement medium on the development effectiveness during the low salinity water flooding process remains unclear. We collected carbonate reservoir cores from the C layer of the Middle East X oilfield and conducted displacement experiments to alter the sulfate ion concentration in the surface river water at the site and the salinity of the injected water. We investigated how the nature of the injected water affects the efficiency of the oil displacement. The experimental results show that in the process of low salinity water flooding, the change in sulfate ion concentration affects the reservoir pore structure. When the sulfate ion concentration is low, the reservoir becomes blocked, reducing permeability. Conversely, when the sulfate ion concentration increases, the reservoir becomes unblocked, leading to increased permeability. Additionally, the sulfate ion concentration change influences the relative permeability curve. Increasing the sulfate ion concentration shifts the isotonic point, resulting in changes to the relative permeability curve. On one hand, it will shift the isotonic point to the right, while on the other hand, it will widen the co-osmosis interval. There is a positive correlation between sulfate ion concentration and oil displacement efficiency. As the sulfate ion concentration in the injected water increases, the oil displacement efficiency also increases. However, the increase in sulfate ion concentration also leads to increased salinity, which can negatively impact oil displacement efficiency. The optimal concentration of sulfate ion in the current experiment is 3162 mg/L, which results in the highest relative oil displacement efficiency. According to the surface water data at the site, increasing the sulfate ion concentration in the injection water to around 3000 mg/L is beneficial for enhancing the oil recovery efficiency. In this study, the influence of sulfate ion concentration on the water flooding effect in carbonate reservoirs is elucidated. Simultaneously, the optimal sulfate ion concentration limit for carbonate rocks in the Middle East X oilfield is determined. This finding is crucial for devising a rational water flooding development plan for this reservoir. Moreover, the results hold significant implications for the development of carbonate reservoirs with low salinity water flooding in other regions.