Abstract
Abstract
In this paper, an experimental technique is developed to study the interfacial interactions of the crude oil-CO2 systems under reservoir conditions. By using the axisymmetric drop shape analysis (ADSA) for the pendant drop case, this new technique makes it possible to measure the interfacial tension (IFT) and to visualize the interfacial interactions between crude oil and CO2 at high pressures and elevated temperatures. The major component of this experimental set-up is a see-through windowed high-pressure cell. A number of important physical phenomena have been observed when the crude oil is made in contact with CO2. They include oil swelling, light-ends extraction, initial turbulent mixing, skin layer, oil drop movement, wettability alteration, asphaltene precipitation, and bubbling at the crude oil-CO2 interface. In particular, the light-ends extraction, initial turbulent mixing and wettability alteration are the major characteristics of the CO2 flooding processes. It is also found that there always exists a constant low equilibrium IFT as long as the pressure is higher than a threshold value. No ultra low or zero IFT between crude oil and CO2 is found, regardless of the operating pressures and temperatures. Therefore, the measured constant low IFT and the observed interfacial interactions show that only partial miscibility between crude oil and CO2 can be achieved for most reservoirs. In addition, it is anticipated that wettability alteration may have significant effects on the ultimate oil recovery and CO2 sequestration.
Introduction
CO2 flooding is considered as one of the most promising enhanced oil recovery (EOR) techniques because it not only efficiently enhances oil recovery but also considerably reduces greenhouse gas emissions. In the past five decades, there have been extensive laboratory studies and field applications of CO2 EOR processes. It has been found that these processes can enhance oil recovery normally by up to 8–16% of the original oil in place1,2. In the CO2 flooding processes, saturation distribution and flow behavior of crude oil, gas and brine are controlled mainly by the interfacial interactions among crude oil, reservoir brine, CO2 and reservoir rocks. These interfacial interactions include the interfacial tension (IFT), wettability, capillarity and dispersion. Wettability, capillary pressure and dispersion can be closely related to the IFT under certain conditions of pressure, temperature and compositions3–5. Therefore, it is essential that the interfacial interactions of the crude oil-CO2 systems be accurately described under reservoir conditions.
In general, it is found that oil viscosity reduction, oil swelling effect and miscibility between crude oil and CO2 mainly contribute to the ultimate oil recovery in the CO2 flooding processes6,7. Both the oil viscosity reduction and the oil swelling effect lead to mobility improvement, while the miscibility between crude oil and CO2 improves the sweep efficiency. At present, the oil viscosity reduction and the oil swelling effect can be accurately quantified under reservoir conditions8,9. Usually, CO2 is not miscible at first contact with crude oils but may reach the dynamic miscibility through multiple contacts7,10. Miscibility between crude oil and CO2 can be determined by conducting the slim tube test6,7,10 and the rising bubble apparatus test11 as well as using some empirical correlations12. The minimum miscible pressure (MMP) is used to indicate whether there exists miscibility between crude oil and CO2 at a given pressure. In addition, zero IFT can be used to characterize the so-called complete miscibility because there is no interface between the two phases of interest under the miscible condition13–15.