More than 60% of the world’s oil reserves are held in carbonate reservoirs. Many unfavorable factors<br>contribute to low oil recovery in these reservoirs. Fractured and oil-wet are two leading factors.<br>Therefore, many research focuses have been put on these factors. Apparently, there is an increasing<br>interest in using chemicals to alter wettability. Injection of chemicals can result in various effects, for<br>example, wettability alteration and reduction in interfacial tension (IFT). The question is how much<br>contribution from each mechanism to the increase in oil recovery. There is lack of such information in<br>the literature. The information is very important because it will guide us to select what chemicals to be used.<br>This paper is to evaluate the effect of wettability alteration on oil recovery in carbonate reservoirs. The<br>main objective is to quantify different mechanisms of wettability alteration in oil recovery related to<br>chemical EOR. Particularly, we compare the effects of wettability alteration and interfacial tension. Both<br>fractured and non-fractured reservoirs are addressed. Analytical models and numerical simulation<br>models are used. Our results show that wettability alteration only plays important roles when IFT is<br>high, and it is effective in the early time. IFT plays very important roles with or without wettability<br>alteration and is effective during the entire process. The implication is that anionics used to reduce IFT<br>is preferred to cationics used to alter wettability. Other observations are that in surfactant-induced<br>wettability alteration with low IFT, gravity drive is a very important mechanism. Molecular diffusion of<br>chemicals affects oil recovery rate in the early time, but not ultimate oil recovery.


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