# Preparation of Modified Colloidal Gas Aphrons and Analysis of the Oil Displacement Effect
## 1 Introduction  
Commonly used improved recovery methods in oil fields include chemical flooding, thermal oil recovery, and miscible flooding. Foam flooding, as a type of chemical flooding, is widely used in oil fields. Addition of a certain concentration of foaming agent into the water is supplemented by gas (air, natural gas, or flue gas) to form foam, and its high viscosity helps reduce the mobility ratio and increase the sweep efficiency of the injectant [1; 2]. The mechanism of foam flooding can be divided into microcosmic and macroscopic oil displacement [3; 4]. Under normal conditions, after entering the reservoir, the foaming agent firstly enters the large pore channels where the resistance may be higher than that in the small channels due to the gas blockage effect, which may lead to temporary blockages. Regarding the size difference of foam and its pseudoplastic effects, when the resistance in the macropores increases, resulting in a higher driving gradient pressure than that in the micropores, the water phase in the foam system may permeate the micropores, and the foam disintegrates. Starting from the frontal drive, the whole driving process alternately changes and, finally, an oil-rich zone is built up along the foam front, which is driven to the well [5]. At present, domestic and international researchers conducted a series of foam flooding field tests on the basis of inhouse laboratory investigations and attained certain effects. The displacement method not only greatly improves sweep efficiency, but also reduces the environmental damage commonly caused by chemical flooding, which is a promising recovery technology [6].  
Compared with conventional colloidal liquid aphrons (CLAs) [7; 8; 9; 10; 11], colloidal gas aphrons (CGAs; first proposed in 1971) [2; 5; 12; 13] prepared by high-speed stirring (faster than 8000 rpm) feature small particles (10-100 \(\upmu\)m), a large specific surface area, and a relatively stable performance in surfactant solutions. The CGA consists of a bubble core and a soap film layer that envelops the gas phase.  
The soap film includes inner and outer surfaces, and a surfactant monolayer is adsorbed onto both surfaces [14]. Compared with common foams, CGAs contain microbubbles and have a high gas bearing capacity, large specific surface area, good coalescence, dynamic stability, and viscosity and fluidity similar to water. The CGA consists of a gaseous core and a layer of soap-coated gaseous phase. The soap film is divided into internal and external surfaces. On each surface, a surfactant monolayer is adsorbed. Hydrophobic groups are directed outward, and hydrophilic groups are oriented toward the inside. However, at the interface between the gas and water phases, the hydrophobic groups are oriented toward the soap film, and the hydrophilic groups are oriented toward the bulk water phase, which results in double electric layers. Therefore, there is a large difference between natural and synthetic bubbles [9]. CGAs offer advantages in improving oil recovery, but resin and asphaltene deposition problems still occur in CGA flooding. Based on this issue, a new modified colloidal foam system is developed by introducing a modifier in CGA preparation, which provides an effective solution to the aforementioned problems and is of guiding significance for further improving the oil displacement efficiency of gas foam.