Mechanism of enhanced oil recovery by wettability control-based low salinity water flooding in carbonate reservoir

Low salinity water flooding is a new technology for enhancing oil recovery by adjusting the ion composition or concentration of injected water. However, the applicable reservoir conditions and enhanced oil recovery mechanism of low salinity water flooding have not yet reached a consensus. In this paper, a series of laboratory experiments of wettability control-based low salinity flooding are carried out with plunger rock samples from marine carbonate reservoirs in the Middle East as the research object. Based on the theory of Derjaguin-Landau-Verwey-Overbeek theory (DLVO), an interfacial reaction model of a typical crude oil/brine/rock system is established, and the contact angle and total separation pressure are calculated simultaneously with the augmented Young-Laplace formula. The reliability of the model is verified by the literature experimental data, and the effects of ion concentration and ion type on the separation pressure curve and contact angle are clarified. The results show that in low salinity environments, the pore surface of carbonate rock is more water-wet under the action of fluid flushing, the oil displacement efficiency is higher, and the low salinity water improves the crude oil recovery by 3.2%; under the assumption of constant charge, the mathematical model established based on the DLVO theory for the crude oil/brine/rock system can accurately predict the change of contact angle; compared with the ion concentration, ion type has a greater impact on separation pressure and contact angle. Among divalent ions, Mg²⁺ ions exhibit a more pronounced influence on wettability control compared to Ca²⁺ ions. When the water film thickness is minimal, van der Waals force is the main force affecting the separation pressure. As the thickness of water film increases, the electric double layer force gradually becomes the main force. This study contributes to a deeper understanding of the wettability control mechanism of low salinity water flooding for enhanced oil recovery.