Enhancing Water Imbibition Rate and Oil Recovery by Carbonated Water in Carbonate and Sandstone Rocks

Spontaneous imbibition is regarded as an important mechanism of oil recovery by waterflood, particularly in heterogeneous or fractured reservoirs where direct displacement of oil by water is usually poor. It has been shown that carbonated water injection (CWI) can improve recovery from oil reservoirs. It has also been reported that the dissolution of CO₂ in water can alter water/oil contact angle, which is a direct indication of the wettability of a crude oil/water/rock/system. In this work, our main objective was to experimentally investigate the potential of carbonated water for improving the rate of spontaneous imbibition of water and, hence, the oil recovery from systems where spontaneous imbibition of water would be important. To achieve this objective, a special high-pressure imbibition cell has been designed, and six spontaneous imbibition experiments have been performed on two different types of rocks (carbonate rock and sandstone) at a pressure of 2500 psi and room temperature. Crude oil was used for these experiments together with seawater and CO₂-enriched (carbonated) seawater. The results of the experiments performed on both sandstone and carbonate rocks reveal high potential for carbonated water to enhance water imbibition rate and oil recovery. This positive impact of carbonated water is attributed to wettability alteration toward more water-wet as well as oil swelling, oil viscosity reduction, reduction of interfacial tension between water and oil, and mineral dissolution. Our experimental results show that the extent of the impact of carbonated water on the observed improvement of spontaneous imbibition of water and the consequent oil recovery enhancement is a strong function of the initial wettability of the rock as well as the rock type and mineralogy. Crude oil type and its physical and chemical properties are also expected to impact the results.