Effect of crude oil on CO₂–Foam stability

The use of foams is a promising technique to overcome gas mobility challenges in petroleum reservoirs. Foam reduces the gas mobility by increasing the gas apparent viscosity and reducing its relative permeability. A major challenge facing foam application in reservoirs is its long-term stability. Foam effectiveness and stability depends on a number of factors and will typically diminish over time due to degradation as well as the foam-rock-oil interactions. In this study, the effect of crude oil on CO₂-foam stability and mobility will be addressed. Two-phase flow emulsification test (oil-surfactant solutions) and dynamic foam tests (in the absence and presence of crude oil) were conducted to perform a comparative assessment for different surfactant solutions. A microfluidics device was used to evaluate the foam strength in the presence and absence of crude oil. The assessment was conducted using five surfactant formulations, and using different oil fractions. The role of foam quality (volume of gas/total volume) on foam stability was also addressed in this study. The mobility reduction factor (MRF) for CO₂-foam was measured in the absence and presence of crude oil using high salinity water and at elevated temperatures. The results indicated that foam stability has an inverse relationship with the amount of crude oil. Crude oil has a detrimental effect on foams, and foam stability decreased as the amount of crude oil was increased. There is good agreement between the results obtained from the two-phase emulsification test with those obtained from the dynamic foam tests in presence of crude oil. Depending on the surfactant type, the existence of crude oil in porous media, even at very low concentrations 5%, can significaly impact the foam stability and strength. The oil can act as an antifoaming agent; it enters into the thin aqueous films and destabilizes the film. This, ultimately, resulting in lower foam viscosity and less stable foams. Thus, the CO₂ MRF dropped significantly with higher oil fractions.