Carbon dioxide flooding is of great significance in emission reduction and enhanced oil recovery, but it is difficult to realize miscible flooding in many reservoirs in China because the pressure is lower than the minimum miscible pressure, resulting in low oil recovery and poor economic benefits. Methods to reduce the minimum miscible pressure have safety and economic limitations, and ultrasound assisted carbon dioxide flooding has the effect of increasing production, but there is a lack of visual experiments and in-depth research on the oil displacement mechanism.
After the carbon dioxide immiscible flooding experiment pretreated and saturated the core, the data were recorded and NMR was measured. The model was treated, saturated, and recorded after oil and carbon dioxide injection. Clean the device before the interfacial tension test measurement, measure after injecting carbon dioxide and crude oil; The experiment of high pressure mercury injection follows the national standard.
NMR and high-pressure mercury injection experiments show that the remaining oil exists in the form of oil film and oil sheet without ultrasound. With ultrasonic assistance, the maximum remaining oil drop radius decreases, the recovery rate of remaining oil in the oil sheet increases, and the thickness of remaining oil in the oil film decreases.
Ultrasound can increase the radius of pore throat and reduce capillary force, so that carbon dioxide can enter the pores that are difficult to enter and displace crude oil. At the same time, ultrasound reduces the interfacial tension of the CO2-oil system, helping to reduce the capillary force, allowing more oil to be expelled.
Ultrasonic-assisted CO2 immiscible flooding can improve oil recovery. The residual oil of CO2 immiscible drive in oil-wet reservoir can be divided into two types: oil film and oil slice. Ultrasound mainly improves the recovery rate of residual oil in the oil sheet and slightly improves the recovery rate of residual oil in the oil film. The mechanism is to increase the radius of the pore throat, reduce the interfacial tension and reduce the thickness of the oil film by high-frequency vibration.
