Investigation of Rock Wettability Alteration by Carbonated Water through Contact Angle Measurements

Carbonated (CO2-enriched) water injection has been shown to improve waterflood performance over conventional water flooding. Carbonated water can be purposely injected in an oil reservoir, but it also forms spontaneously during conventional CO2 floods or CO2 water alternating gas (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. Because of dissolution of CO2 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. To assess and quantify the extent of possible wettability alteration as a result of carbonation of water, a series of contact angle measurements have been performed in this study. Three different minerals, namely, quartz, 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 °F. For each mineral, two situations were considered: an unaged (clean) rock system and an aged rock system. The captive bubble method was used for measuring the contact angles. The results for the unaged 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 the pressure, and it increased as the pressures increased. For the unaged substrates, the change in wettability by carbonated water was moderate, with the maximum change of around 6° taking place for quartz. The results of the aged minerals revealed a much higher change in wettability by carbonated water compared to the unaged substrates. For the aged quartz sample, at the pressure of 2500 psi, when CO2 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 CO2 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.