An experimental investigation of wettability alteration during CO₂ immiscible flooding

Wettability has been recognized as one of the key parameters controlling the remaining oil-in-place. The knowledge of reservoir wettability is essential to understand the fluid displacement mechanisms, and to develop strategies for achieving higher recovery factors. One of the effective approaches to enhance oil recovery significantly is wettability alteration which has been investigated extensively in the literature. This paper investigates wettability alteration experimentally, on continuous basis, during CO₂ immiscible flooding. Measurements of the contact angle between oil, carbonated brine and a slice of rock cut from a carbonate core plug were conducted. The results indicate that the rock wettability is altered from oil-wet to intermediate-wet when the oil/rock system is exposed to dissolved CO₂. The extent of wettability alteration is controlled by CO₂ exposure time; as such time is increased, alteration of wettability progresses towards an apparent limit. It is also found that as the CO₂ concentration increases in the brine, wettability alteration increases. Based on the experimental finding, an empirical model is developed to describe such continuous wettability alteration. The findings of this study can be applied to the cases where CO₂ is injected in a watered out, oil-wet reservoir at a pressure below the miscibility pressure.