Foaming characteristics of crude oil-CO2 mixture by depressurization: Influence of crude oil viscosity and wax precipitation

With the gradual application of CO2 flooding technology, a large amount of CO2 gas is dissolved in produced fluid. After the produced fluid is lifted to the surface gathering system, the dissolved CO2 will escape from within the crude oil due to the continuous pressure decline, resulting in foaming of the crude oil. In this paper, the effects of crude oil viscosity and wax precipitation on foaming characteristics were investigated by using the depressurization method. In order to explore the mechanism in depth, the solubility of CO2 in crude oil and the interfacial characteristics of crude oil-CO2 system were taken into consideration. The results indicate that the foamability of crude oil tends to decrease with the increase of temperature when the temperature stays below the WAT (wax appearance temperature). Above the WAT, the foamability weakens drastically compared with the situation below the WAT, and it becomes to change barely with temperature. Further experiments show that the CO2 solubility increases with the increase of temperature below the WAT while decreases with the increase of temperature above the WAT. This is not in agreement with the foaming trend, indicating that solubility is not the determining factor of foamability. In addition, when the temperature increases from 26 degrees C to 40 degrees C at 0.5 MPa, the equilibrium interfacial tension between the crude oil and CO2 only decreases by 6.67%, while the oil viscosity decreased by 42.08%, manifesting that the temperature-induced change in oil phase viscosity plays a dominant role in foamability. Next, the foam stability, represented by the foam height decay with time, was measured immediately after the foam height reached the maximum. It is found that the foam stability decreases signifi-cantly with the increase of temperature. Further experimental results of interfacial viscoelasticity exhibit that the interfacial dilational modulus decreases by about 40 mN/m from 26 degrees C to 40 degrees C, proving that the enhancement of film strength caused by the adsorption of asphaltenes and wax crystals at the interface has an important effect on the foam stability.

Automated summary

In this study, the effects of crude oil viscosity and wax precipitation on foaming characteristics were investigated using the depressurization method. The results showed that the foamability of crude oil tends to decrease with increasing temperature below the wax appearance temperature (WAT). However, above the WAT, the foamability weakens drastically and remains relatively constant with temperature. The dominant factor affecting foamability was found to be the temperature-induced change in oil phase viscosity. Additionally, the foam stability decreased significantly with increasing temperature, which was attributed to the decrease in interfacial dilational modulus and the weakened film strength at the crude oil-CO2 interface.