EVALUATION OF THE EFFECT OF WETTABILITY ALTERATION ON OIL RECOVERY IN CARBONATE RESERVOIRS

More than 60% of the worldC_'s oil reserves are held in carbonate reservoirs. Many unfavorable factors contribute to low oil recovery in these reservoirs. Fractured and oil‑wet are the two main factors. Apparently, there is an increasing interest in using chemicals to alter wettability. Injection of chemicals can result in various effects, for example, wettability alteration and reduction in interfacial tension (IFT). The question is how much contribution is from each mechanism to the increase in oil recovery. There is lack of such information in the literature. The information is very important because it will guide us to select which chemicals to be used, as some chemicals can effectively reduce IFT, while others alter wettability. This paper is to quantify different mechanisms in oil recovery related to chemical EOR. Particularly, we compared the effects of wettability alteration and IFT reduction. Analytical and numerical simulation models were used. Our results show that wettability alteration plays important roles when IFT is high, and it is effective in the early time. IFT plays very important roles with or without wettability alteration and is effective during the entire process. The implication is that anionic surfactants are preferred to cationic surfactants in chemical EOR, as the former are generally used to reduce IFT, while the latter are used to alter wettability. Other observations are that in surfactant‑induced wettability alteration with low IFT, gravity drive is a very important mechanism. Molecular diffusion of chemicals affects oil recovery rate in the early time, but not ultimate oil recovery.