Carbonated (CO 2 -enriched) water injection has been shown to improve waterflood performance over conventional water flood. Carbonated water can be purposely injected in an oil reservoir but it also forms spontaneously during conventional CO 2 floods or CO 2 WAG injection. It is therefore important to understand the rock-fluid and fluid-fluid interactions that take place in an oil reservoir when carbonated water contacts the oil and the reservoir rock. Due to dissolution of CO 2 in brine, the pH of injection water is reduced during carbonated water injection. This reduction in brine pH may affect the electric charges on water-rock interfaces and hence, may alter the wetting characteristics of the surface. This wettability alteration will have a direct effect on oil recovery and the amount of oil remaining after waterflood.In order to assess and quantify the extent of possible wettability alteration due to carbonation of water a series of contact angle measurements have been performed in this study. Three different minerals namely; Quarts, Mica, and Calcite were exposed to plain and then carbonated water under a wide range of pressures between 100 and 3500 psi. The temperature of the measurements was kept constant at 100 ⁰F. For each mineral, two situations were considered; an un-aged (clean) rock system and an aged rock system. The captive bubble method was used for measuring the contact angles.The results for the un-aged measurements show that carbonated water can change the wettability of clean minerals in varying degrees. The observed change in the measured contact angles was a function of pressure and it increased as the pressures increased. For the un-aged substrates, the change in wettability by carbonated water was moderate with the maximum change of around 6 degrees taking place for Quartz.The results of the aged minerals revealed a much higher change in wettability by carbonated water compared to the un-aged substrates. For the aged quartz sample, at the pressure of 2500 psi, when CO 2 was introduced to the top of plain brine and CW was formed, the contact angle changed from 76 to 61, and for the aged mica at the same pressure the contact angle changed from 89 to 63. For the aged Calcite, carbonated water brought about a larger change in wettability with the contact angle changing from 144 to 97.The results of the study show that under real reservoir conditions where the rock is usually mixed-wet or oil-wet, the dissolved CO 2 content of water can have a major impact on the wettability of the reservoir, which in turn would significantly affect the oil displacement efficiency and the recovery factor.

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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.

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