Study On Gas Channeling Pattern And Optimization Method Of CO₂ Injection In C4+5 Oil Reservoir In W Oil Region Of Northern Shaanxi

It has been half a century since CO₂-EOR technology was applied to actual reservoirs and achieved good results.Because CO₂ has the low critical pressure,can dissolve in crude oil,inflate crude oil,increase the liquid mobility,mix with the crude oil,and appear both gas and liquid two phase characteristics under the geological conditions,so many scholars focus on it,while achieve significant economic results in most oil fields both at home and abroad.With the development of oil fields,ultra-low permeability reservoirs have come into the view of scholars due to their rich reserves,and the application of CO₂ flooding in ultra-low permeability reservoirs has been increasingly intensified in recent decades.Ordos basin is a typical low permeability sandstone reservoir at home,it has abundant reserves,strong heterogeneity,fracture-developed,it need geological evaluation and production projections before CO₂-flooding in the study area,to analyze the influence factors of gas channeling,find a reasonable way of gas injection,so as to guide the on-site production.The study object is the C4+5 reservoir in the W oil region of northern shaanxi,which is located in the central shaanxi slope of the ordos basin.In this paper,a 3d model of this area is established by using 3d geological modeling software through reservoir feature description and stratigraphic division,and a small model of the target layer is selected.The physical properties of crude oil at high pressure were matched by numerical simulation software and the minimum miscible pressure was studied.The influencing factors of gas channeling in ultra-low permeability reservoirs are analyzed by means of the conceptual fracture model.Finally,the numerical simulation software is used to match the production history by adding compression cracks into the actual geological model.On the base of good matching,analyzed the gas channeling rules in different gas injection and optimized the best injection parameters in the practical model.Through research and analysis,the paper mainly obtained the following understanding:（1）The reservoir in the study area is mainly divided into three layers:C4+5₁,C4+5₂¹ and C4+5₂² through logging curve interpretation,stratigraphic correlation and devision.Among them,C4+5₂² sedimentary microfacies are underwater distributary channel deposition,the rest are abandoned channel deposition,C4+5₂² is the target layer of this study,with large reservoir thickness,continuous development and good physical property.（2）Matched crude oil bubble point experiment,constant component expansion experiment,differential liberation experiment,and reservoir viscosity measurement experiment,it is found that the error of parameter matching value and experimental value is small,and the data is reliable.Through the results of the gas injection expansion experiment,it was found that the system saturation pressure and the viscosity of crude oil match well and met the requirements.Through the numerical simulation,the actual formation fluid phase characteristic can satisfy the later history matching and the production prediction fluid critical parameter.（3）On the basis of previous matching,it was found through the study of minimum miscibility pressure that the minimum miscibility pressure measured by the numerical simulation and the experiment was 17.8MPa,the results were true and reliable,and the miscibility pressure could be reduced by adding intermediate hydrocarbon components to the injected gas to make the oil and gas miscibility（4）The distribution of pressure fractures in the production process has a significant impact on the gas channeling time:the longer the distance between gas injection fractures and production fractures,the later the gas channeling;The bigger of the angle between production fracture and gas flooding direction,the later the gas channeling;The larger the sweep area from the gas injection well to the production well,the later the gas channeling.The main controlling factor of gas channeling in ultra-low permeability reservoir is viscosity fingering.Oil and gas miscibility can reduce the occurrence of viscous fingering and delay gas channeling.（5）After added fractures in the geological model,the results of single well history matching,well group history matching and oil area history matching are all good,which indicates that the fracture adding is reasonable and accord with the real geological model.（6）It was found in the gas channeling patterns study of practical fracture model that:in the initial stage of continuous CO₂ injection flood,gas was oval diffusion with the focal length of fracture half-lengh,while in the later stage of production,the gas channeling was irregular.Pulse gas injection delays the occurrence of gas channeling to a certain extent.The initial stage gas was oval diffusion with the focal length of fracture half-lengh,Finally form irregular radial with the gas arrived to the more production wells.Gas and water alternately flooding promote oil and gas miscibility.The presence of water slug also makes the gas more uniform to drive,effectively delayed the degree of gas channeling.The leading edge of gas drive is square or diamond.（7）Continuous CO₂ injection flooding has a daily gas injection volume of 7,500m³/d in a single well,and a daily liquid production volume of 5m³/d in a single well has the best effect.It predict 20 years that the cumulative increase oil is 358,100 tons.Under the mode of pulsed injection of gas injection 6 months then production 6 months,the best production effect is to increase the daily fluid volume per well as much as possible.It predict 20 years that the cumulative increase oil is 384,800 tons.Under the condition of 3:1 gas-water slug ratio,the daily injection volume of single well is 10000m³/d,the daily injection volume of single well is 20m³/d,and the production effect is best when the daily output volume of single well is 5m³/d,and It predict 20 years that the cumulative increase oil is 390,500 tons.It can be seen that it is best to adopt water alternate gas flooding in this area.