Feasibility of Ion-Modified Water for Low Salinity Water Flooding: A Case Study for Ultra-High-Salinity Carbonate Reservoir in Akanskoe Oilfield Tatarstan, Russia

Abstract

This paper presents the feasibility of the application of ion-modified water for enhanced oil recovery (EOR) in low permeable carbonate reservoir with ultra-high salinity of more than 220000 mg/L. Influence of different ions on wettability alteration, interfacial tension (IFT), scale tendency, recovery factor, and water injectivity was investigated. For choosing the optimized injection-water sequence, different types of water (formation water, distilled water, fresh water, and ion-modified water) were used. First, their effects on wettability alteration by measuring contact angle (oil-water-rock) and IFT were evaluated. Then, core flooding experiments were carried out to investigate how different injection sequence affects the oil recovery and injectivity. Furthermore, the scale tendency of different salts was simulated. The results showed that Mg2+ is the most effective ion. The addition of Mg2+ can fast change the oil-wet (130°) carbonate rock to water-wet (29°). The presence of mono-valent ions has negative effects on the effectiveness of Mg2+ on wettability alteration. Also, the presence of Mg2+ in fresh water and distilled water can reduce oil-water IFT two times lower. Core flooding experiments showed that after fresh water or formation water flooding (until 100% water cut), the sequent diluted formation water (diluted 10 times) yielded incremental oil recovery of about 3-5%, while the Mg2+ modified water obtained incremental oil recovery of about 8-18%. This indicates that Mg2+ modified water has a promising prospect in EOR in carbonate reservoirs. A comprehensive analysis combining contact angle measurements, IFT testing, and core flooding experiments indicates that the high efficiency of Mg2+ modified fresh water for EOR mainly benefits from its strong wettability alteration ability. In addition, it was found that the existence of Mg2+ and SO42− can reduce the tendency of precipitation of salts compared with using only fresh water or diluted formation water. This work proves that ion-modified water by adding Mg2+ to fresh water can be an effective, low cost and environment-friendly EOR method for low-permeability carbonate reservoirs with ultra-high salinity. Simultaneously, this research provides some basic data that can help to enrich the theory for developing low salinity water flooding for EOR.