No abstract

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 CO 2 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 CO 2 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.

The success of CO 2 storage in deep saline aquifers and depleted oil and gas reservoirs is largely controlled by interfacial phenomena among fluid phases and rock pore spaces. Particularly, the wettability of the rock matrix has a strong effect on capillary pressure, relative permeability, and the distribution of phases within the pore space and thus on the entire displacement mechanism and storage capacity. Precise understanding of wettability behavior is therefore fundamental when injecting CO 2 into geological formations to sequestrate CO 2 and/or to enhance gas/oil production. In this study, the contact angles of Bentheimer sandstone/water/CO 2 or flue gas have been evaluated experimentally using the captive-bubble technique in the pressure range from 0.2 to 15 MPa. The experiments were conducted using different compositions of aqueous phase with respect to CO 2 , i.e., unsaturated and fully saturated. It has been shown that a reliable contact-angle determination needs to be conducted using a pre-equilibrated aqueous phase to eliminate dissolution effects. In the fully saturated aqueous phase, the Bentheimer sandstone/water system is (and remains) water-wet even at high pressures against CO 2 and/or flue gas. In these systems, the data of the stable contact angle demonstrate a strong dependence on the bubble size, which can be mainly explained by the gravity (buoyancy) effect on bubble shape. However, the surface nonideality and roughness have significant influence on the reliability of the contact-angle determination. The results of this study prove that in order to avoid the dependency of the contact angle on the bubble size in these systems, the effect of gravity (buoyancy) on bubble shape has to be considered by calculation of the Bond number; for systems characterized by Bond numbers less than 0.9, the influence of the bubble radius on the contact angle becomes insignificant. The experimental results show that, in contrast to quartz, the phase transition of CO 2 from subcritical to supercritical has no effect on the wettability of the Bentheimer sandstone/water system, which originates from differences in the surface charges of quartz and Bentheimer sandstone. In an unsaturated system, two dissolution regimes are observed, which may be explained by density-driven natural convection and molecular diffusion.

No abstract