Study On Mechanism And Adaptability Of CO₂ Flooding In Low Permeability Reservoirs

In recent years,CO₂ flooding has been applied to major oil fields as an important measure to increase oil recovery,and it can also achieve the resource utilization of greenhouse gases while enhancing the oil displacement effect.Therefore,this paper has studied the different CO₂injection modes and oil displacement mechanism of Chang 6³reservoir in An 83 District of Hujianshan Oilfield.In this paper,the expansion experiments and capillary experiments of crude oil after CO₂injection were carried out.The results show that the minimum miscibility pressure of CO₂ and crude oil measured by the thin tube experiment is 12.2MPa,which is lower than the actual reservoir formation pressure of 16MPa.Therefore,the miscibility can be achieved under the formation conditions.The injection of CO₂ makes the formation oil saturated pressure,expansion coefficient and.the ratio of gas and oil increases,meanwhile makes viscosity and density of oil decrease.On this basis,the four groups of natural cores and humans were subjected to flooding efficiency,flooding pressure,and water ratio of the CO₂ miscible flooding,CO₂ immiscible flooding,CO₂ miscible flooding after water flooding,and oil flooding CO₂ immiscible flooding experiment.The results show that:For the four groups of fractures without development of natural cores,CO₂ miscible flooding effect is the best,the displacement efficiency is an average of 81.8%,after the water drive CO₂ miscible drive,the displacement efficiency is an average of71.5%,again after the water drive CO₂ for the immiscible flooding,the average displacement efficiency is 68%,and the most unfavorable displacement effect is the CO₂ immiscible flooding,the average flooding efficiency is 66.4%.The higher the permeability,the higher the displacement efficiency,the lower the displacement pressure,and the lower the water content.The optimal CO₂ injection method for the relatively well-developed cores is the CO₂ miscible flooding after flooding.The displacement efficiency is 57.5%.After the water flooding,the CO₂immiscible phase displaces,the displacement efficiency is 55.1%,and after again is CO₂ the miscible flooding,the displacement efficiency is 52.3%.The worst-displacement effect is the CO₂ immiscible flooding.,the displacement efficiency is 37.2%.At the same time,a series of natural cores and a set of man-made fracture cores were subjected to gas-water alternating flooding under three different gas-water ratios and volume slugs.The results show that the highest core flooding effect is miscible flooding.The optimal combination is a CO₂ slug volume of 0.05 PV,a gas to water ratio of 1:1,and a displacement efficiency of 83.2%.The best effect of man-made fracture core flooding is miscible flooding.The optimal CO₂ slug size is 0.2 PV,the CO₂ to water ratio is 1:2,and the oil displacement efficiency is 62.23%.In a comprehensive comparison,the CO₂ injection method with the best development of relatively low-fracture natural cores and fractures with relatively high degree of development of core flooding is CO₂-water alternate injection,and the optimal air-water ratio and slug combination is 1:1,0.05.PV and 1:2,0.2 PV.