It has long been recognized that an alkaline-surfactant (AS) flood is an effective enhanced oil recovery (EOR) process. When an AS solution is injected into an acidic oil reservoir, it causes the interfacial tension (IFT) reduction and wettability alteration, which subsequently affect the flood performance to a large extent. Quantitative descriptions of this dynamic IFT phenomenon and wettability alteration are of great interest to the petroleum industry. In this paper, the axisymmetric drop shape analysis (ADSA) technique for the sessile drop case is employed to measure the dynamic IFT and the contact angle of a crude oil drop on a reservoir rock in several aqueous solutions. Based on the modern digital image acquisition, analysis and processing techniques, the ADSA technique obtains the accurate digital image of the sessile oil drop and accomplishes the relevant measurements. The aqueous solutions include alkaline solutions and AS solutions at different alkali, cationic and anionic surfactant concentrations. It has been found that the measured IFT varies with time. This dynamic IFT phenomenon is then interpreted in terms of the surfactant generation and adsorption at the interface and its subsequent dissolution into the adjacent bulk phases as well. Meanwhile, it has also been observed that the measured contact angle changes with time in a peculiar manner. When the crude oil is made in contact with an alkaline solution, a water-wet reservoir rock quickly becomes more water-wet to different degrees at different alkali concentrations. At the beginning, the contact angle increases but eventually decreases close to its initial value. This wettability alteration corresponds well to the measured dynamic IFTs. In addition, the synergistic effects of AS solutions on wettability alteration are studied. It has been found that the wettability alteration in a cationic surfactant-alkaline solution has a rather different trend from that in an anionic surfactant-alkaline solution. More specifically, a cationic surfactant can make a water-wet reservoir rock become oil-wet, whereas an anionic surfactant can only cause minor change of its wettability. It is speculated that adsorption of the cationic surfactant onto the rock surface is strong and stable so that the wettability alteration is irreversible. This laboratory study not only provides an improved understanding of the interfacial interactions in the crude oil-reservoir rock-AS solution systems but also helps to fully develop the oil recovery potential of AS flood processes in field applications.

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