| At present,CO2 flooding technology has become one of the most effective methods for enhancing oil recovery in low permeability reservoirs.However,CO2-crude oil system is difficult to reach the miscible state due to the high minimum miscible pressure in the process of CO2 flooding,which seriously restricts the development effect of low permeability reservoirs.Therefore,it is a key problem to effectively reduce the minimum miscibility pressure and promote miscibility state.Since the composition of crude oil can be changed by the function of cavitation,vibration and cracking in ultrasonic.In this work,ultrasonic wave is used to reduce the minimum miscible pressure(MMP)of CO2-crude oil system.Based on the theories of reservoir physics,seepage mechanics and reservoir engineering,experimental testing and theoretical derivation were used comprehensively to focus on the change characteristics of flow and solid properties under ultrasonic action and the change characteristics of minimum miscible pressure,and a dynamic mathematical characterization model of ultrasonic assisted CO2 flooding was established.It provides a theoretical basis for the application of ultrasonic assisted CO2 flooding in oil field.Firstly,the characteristics of pore structure and rock mineral composition after ultrasonic treatment were analyzed by nitrogen adsorption,high-pressure mercury injection,NMR and XRD experiments.The results shown that the clay minerals on the pore surface have a dynamic change of abrogation,peristalsis and precipitation under the action of vibration and cavitation with ultrasonic treatment,resulting in the increase of the total number of micro/nano pores in the core.Porosity and permeability increase by3.3%～6.1% and 65.4～167.7%,respectively.The pore-throat connectivity of samples was improved,inaddition,part of large pores and small pores become mesopores,and the microscopic heterogeneity is weakenedand.This study lays a foundation for the establishment of the dynamic mathematical characterization model of ultrasonic assisted CO2 flooding.Then,the change of crude oil properties with ultrasonic is studied and the essential reason of reducing the minimum miscible pressure is revealed by combined viscosity test,four component test,gas chromatography and PVT test.The result show that the C-O,CC and C≡C chemical bonds in crude oil molecules were broken and hydrogenated to form C-H chemical bonds after ultrasonic treatment,and resulting in the asphaltene and colloid contents decreased by 37.2% and 49.2%,respectively,while saturated hydrocarbon and aromatic hydrocarbon contents increased by 18.7% and 45.6%,respectively,which is the primary reason for the decrease of crude oil viscosity and increase of CO2 solubility.Under the acoustic seismic effect,the interfacial tension of CO2-crude oil decreases by 2.2～0.7 m N/m at the pressure of 5～16 MPa,and the minimum miscible pressure decreases by 0.79～1.1 MPa,which promotes the dynamic transformation of CO2 flooding from immiscible to miscible.The calculation model of minimum miscible pressure under ultrasonic wave was established by using line method.The analysis shows that the minimum miscible pressure decreases from 16.7 MPa to 15.7MPa with ultrasonic wave.Finally,the increase mechanism of CO2 flooding recovery under ultrasonic assisted was clarified by physical experiment,the distribution characteristics of residual oil were quantitatively described,and the dynamic mathematical characterization model of ultrasonic assisted CO2 flooding was established based on the research results.As a result,ultrasonic assisted CO2 immiscible flooding increased recovery by 9.2% and miscible flooding increased recovery by 3.0% compared with CO2 flooding.The CO2 flow around and inrush reducing due to ultrasonic reduces the minimum miscible pressure,improves the pore structure and decreases the viscosity of crude oil,thus the oil displacement efficiency is improved.The residual oil saturation of ultrasonic assisted CO2 immiscible flooding is 4.0% and the residual oil size distribution ranges from 0.12μm to 25.9μm,the large residual oil cluster decreases.Based on the physical experiment of ultrasonic attenuation and the dynamic model of heavy organic matter precipitation and stripping by ultrasonic,the mathematical characterization model of ultrasonic assisted CO2 flooding was established,and the errors were within 5% with the experimental results.The research results of this paper are helpful to promote the application of ultrasonic in CO2 flooding in low permeability reservoirs... |
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