Formation And Migration Mechanism Of CO₂ And Crude Oil Miscible Zone

CO₂ flooding is an effective technology to improve oil recovery and reduce greenhouse gas emissions.CO₂ flooding can be divided into miscible flooding and immiscible flooding.The effect of miscible flooding to enhance oil recovery is better than that of immiscible flooding.Miscible zone is an important difference between miscible flooding and immiscible flooding.Therefore,it is necessary to study the formation and migration mechanism of miscible zone and its influence on oil recovery.Firstly,PVT experiment,CO₂ injection experiment and slim tube experiment of crude oil in Shengli Oilfield are carried out,and Winprop is used to fit the experimental data and establish the fluid model.Then,the formation and evolution of miscible zones in displacement process are theoretically analyzed based on pseudo-ternary phase diagram.Subsequently,the oil-gas interfacial tension and CO₂ content in oil phase are taken as the index to divide the miscible zone,and the quantitative characterization method of the characteristics of the miscible zone(including the sweep coefficient and fingering coefficient of the front and rear edges of the miscible zone and the area and width of the miscible zone)and its variation law are studied by CMG,and the relationship between the characteristics of the miscible zone and the recovery factor is discussed.Finally,the influence of reservoir characteristics and development methods on the formation and migration of miscible zones is analyzed.The results show that:(1)The formation of miscible zone is dominated by vaporization-condensation between CO₂ and crude oil,and the evolution of miscible zone is mainly influenced by hydrodynamic dispersion.(2)With the increase of cumulative gas injection,the sweep coefficient and area of the front and rear edges of the miscible zone increase linearly before gas breakthrough.The maximum area of the miscible zone can reach about 25%of the reservoir area.The trend of increasing the width of the miscible zone tends to be gentle,while the index coefficient of the front and rear edges of the miscible zone increases exponentially,and rises sharply when the cumulative gas injection reaches 0.2-0.3PV.(3)In gas breakthrough before and after,the displacement efficiency and expansion coefficient of the miscible zone are constant,but before gas breakthrough,the displacement efficiency is higher and the expansion coefficient is lower.(4)The higher the permeability is,the bigger the reservoir dip angle is,the higher the reservoir pressure is,the higher the CO₂ purity is,the lower the gas injection rate is,the lower the index of front edge and rear edge of the miscible zone is,and the higher the sweep coefficient is.At the same time,the larger the width and area of the miscible zone is,the greater the contribution of expansive displacement is,and the higher the recovery is.(5)CO₂ can aggregate the residual oil of water flooding after water flooding into a miscible zone during the process of miscible flooding and water-gas alternating flooding.Compared with continuous gas injection,the area and width of the miscible zone are larger and the fingering coefficient is smaller.(6)For miscible flooding,recovery comes from CO₂ displacement and crude oil expansion.Before gas breakthrough,the contribution value of expansion to oil recovery is about 7%-17%,and after gas breakthrough,the contribution value of expansion to oil recovery is about 8%-20%.