Investigation on Interfacial Interactions among Crude Oil-Brine-Sandstone Rock-CO2 by Contact Angle Measurements

Wettability plays a crucial role on the performance of enhancing oil recovery techniques because of its effect on fluid saturations and flow behavior in porous medium. This study is directed toward determining contact angles (i.e., wettability) in systems with carbon dioxide, brine, and an oil-saturated rock system. Two situations are considered: Rock system I is partially water-wet, whereas rock system II is effectively oil-wet. Contact angles have been determined experimentally as a function of brine salinity and pressure using the pendant-drop shape analysis. The experiments were carried out at a constant temperature of 318 K and pressures varying between 0.1 up to 16.0 MPa in a pendant-drop cell. For rock system I, the partially water-wet substrate, brine, and CO2 system, the dependence on the pressure at constant salinity is very small. For this system, at a constant pressure, the contact angle decreases for increasing brine salinity. The results show that the carbon dioxide is the nonwetting phase in the pressure and salinity range studied. This behavior can be quantitatively understood in terms of the expected dependencies of the three interfacial tensions (IFTs) in Young's equation on pressure and brine salinity. For rock system II, the effectively oil-wet substrate, brine, and CO2 system, the dependency of contact angle on pressure is considerable. This study proves that carbon dioxide becomes the wetting phase at pressures higher than 10.0 MPa. Beyond 10.0 MPa (i.e., in the supercritical region), the contact angle remains practically constant. The effect of salinity on the contact angle of the oil-wet rock system II is small. The behavior can again be quantitatively understood based on expected trends of the three IFTs that determine the contact angle. It is also shown that use of the equation of state method makes it possible to approach the experimental data quantitatively. We conclude that contact angle measurements form an essential ingredient to determine the efficiency of carbon dioxide flooding and storage.