Mechanisms and experimental research of ion-matched waterflooding to enhance oil recovery in carbonate reservoirs: A case of Cretaceous limestone reservoirs in Halfaya Oilfield, Middle East

Based on systematically summarizing the achievements of previous ion-matched waterflooding researches, the diversity and synergy of oil recovery enhancement mechanisms and the interaction between mechanisms are examined according to two classification standards, and the influence of behaviors of different ions on different mechanisms and oil displacement efficiency are investigated. Ionic strength is proposed to characterize the behavior differences of univalent and divalent ions, the relationships between ionic strength, effective concentration, and mechanisms are established to characterize the ion behavior behind various mechanisms, and evaluate the performance of ion-matched injection water. The mechanisms of enhancing oil recovery by ion-matched waterflooding include:(1) The ion-matched water can reduce the ion strength and match the ion composition of formation water, thereby reducing the difference between the effective concentration of univalent ions and divalent ions on the surface of carbonate rocks, and improving the effective concentration of potential determining ions(especially SO42-).(2) It can improve wettability, oil-water interface properties, pore structure and physical properties of the reservoir, and finally enable the establishment of a new ionic equilibrium conducive to waterflooding while breaking the original equilibrium. In this study, experiments such as relative permeability curve, interfacial tension, and core-flooding were carried out on carbonate core samples from the Cretaceous Mishrif Formation reservoirs in Halfaya Oilfield, Middle East, a method for injection water evaluation was established and the injection water suitable for these reservoirs was selected: 6 times diluted seawater. Compared with ordinary seawater, oil displacement efficiency can be increased by more than 4.60% and compared with the optimum dilution of formation water, oil displacement efficiency can be increased by 3.14%.