Study On The Formation Mechanism And Migration Law Of Miscible Zone In One-dimensional Long-distance CO₂ Miscible Flooding

Enhanced oil and gas recovery is the focus and core content of oil and gas field development.CO₂ injection development,as a way of tertiary oil recovery to enhance crude oil recovery,especially CO₂ miscible oil flooding technology,has been found to increase the oil recovery efficiency to 90%and above through pilot tests at home and abroad.Therefore,it is necessary to study and explore the miscibility process and miscibility characteristics of CO₂ in contact with formation crude oil,so as to provide a theoretical basis for better CO₂ miscible flooding in the field.In this thesis,the oil sample from Well 30-101 in Jiyuan Oilfield is used as the experimental oil sample,the minimum miscibility pressure for CO₂ and formation crude oil to achieve miscibility is determined by the experimental method and the empirical formula method,the advantages and disadvantages of different methods are analyzed,and the linear fitting method is used to deduce the MMP prediction formula suitable for Jiyuan Oilfield was developed;determination of basic parameters of oil samples by phase experiment,the formation mechanism during the displacement process are theoretically analyzed by using the P-X phase diagram method,the pseudo-ternary phase diagram method,and the multiple contact experiment method;the self-developed long one-dimensional multi-measurement point visualization simulation system is used to carry out one-dimensional long-distance CO₂ miscible oil displacement experiments to divide the leading and trailing edges of the miscible zone,and define the parameters of the miscible zone to characterize the migration law of the miscible zone.Based on the above experiments and theoretical analysis,this thesis draws the following conclusions:（1）The thin tube experiment method in the experimental method is the most reliable method to determine the MMP.According to the reservoir conditions and oil sample characteristics,the MMP prediction formula of the target oil reservoir is fitted and regressed,which is the most suitable theoretical calculation formula.（2）In the process of miscible displacement,CO₂ contacts with crude oil for many times,and the continuous vaporization-condensation effect realizes the formation of miscible zone.The evolution process after the formation of miscible zone mainly depends on hydrodynamic dispersion.（3）After CO₂ contacts with formation crude oil for many times,it forms pure CO₂gas zone,low oil miscible zone,miscible zone,high interfacial tension immiscible zone and pure crude oil zone.（4）The miscible zone was divided by the molar mass content of CO₂ in the oil sample,and the positions where the molar mass concentration of CO₂ in the oil sample was 0.2 and 0.8 were defined as the leading and trailing edges of the miscible zone.（5）Before the leading edge of the miscible zone breaks through,with the increase of the CO₂ injection,the sweep coefficients of the leading and trailing edges increase linearly,and the oil displacement efficiency of the model has a linear relationship with the amount of CO₂ injection;after the leading edge breakthrough,the growth rate of the sweeping coefficient of the leading edge dropped significantly,and the sweeping coefficient of the trailing edge began to slow down and stabilize after maintaining a linear growth for a short time,the oil displacement efficiency basically does not change after the trailing edge breakthrough of the miscible zone.（6）Before the breakthrough of the front edge,the length of the dimensionless miscible zone shows a steady increasing trend,from fast to slow,and the length of the miscible zone reaches the maximum at the time of breakthrough,accounting for about46.7%of the length of the long sand-packing pipe model.After the edge breakthrough,the length of the dimensionless miscible zone becomes shorter than the growth rate of the miscible zone until the miscible zone disappears.