Study On Mechanism Of CO₂ Flooding And Optimization Of Injection-production Parameters In Extra-low Permeability Reservoir

Development effect of water flooding is poor in extra-low permeability reservoir,so it is urgent need of new way of development to improve development effect.The development way of CO₂ flooding is one of research frontiers at present.Because the Daqing Yushulin oilfield belongs to extra-low permeability reservoir,pilot test of CO₂flooding is carried out in a new area named Shu 101 block of which.With the help of field experiment to collect a lot of field data,the paper uses laboratory experiment and numerical simulation to study displacement mechanism of CO₂ flooding for difficult reserves and optimization adjustment way of injection-production parameters,which provide a better technical support to ensure implementation effect of CO₂ flooding.The research achievements also direct the commissioning of the industrial experiment block（Shu 16）where water flooding is translated into CO₂ flooding,and has achieved the desired effect.It can be said that the research achievements provide a new way to develop new area or old area adjustment of similar extra-low permeability reservoirs.It is also of great significance to develop effectively difficult reserves.The paper gets following research achievements:1.It deepens displacement mechanism of CO₂ flooding by using laboratory experiment,combining with the geology and reservoir of Yushulin oilfield.Laboratory results show that CO₂ solubility in the crude oil is about 3 times of water in oilfield,and three fifths of virgin kerosene.When oil and water phase is coexistence,effective mass transfer diffusivity for CO₂ in water phase is only one twenty-fifths of the water phase alone.CO₂ can changes the wettability of rock surface under the condition of certain differential pressure,and makes the wettability change to neutral,this can improve oil displacement efficiency.The study of nuclear magnetic resonance（NMR）shows that the immiscible gas flooding can produce 75%of the crude oil in the large pore.However miscible gas flooding can produce 92.5%of the crude oil in the large pore.The viscosity of residual oil increases after CO₂flooding,which results in colloidal bitumen precipitation,and makes part of the movable fluid in pores turn to unmovable fluid.2.It studies efficiency of CO₂ flooding.Laboratory results show that displacement efficiency of immiscible gas and miscible gas flooding is respectively 48%and 64%,while the water displacement efficiency is only 29%.The time of gas flooding to achieve high recovery is much earlier than water flooding,which shows that rate of oil production for gas flooding is much higher than water flooding.Displacement efficiency of alternating water-gas flooding is about 8%higher than that of continuous CO₂ flooding,because way of water-gas alternating injection can control the velocity of CO₂escape and enlarge maximally sweep volume.3.It gives the quantitative characterization of CO₂ displacement.The components of crude oil and minimum miscible pressure will change during the process of CO₂flooding.The displacement process is divided into miscible,transition and immiscible area,according to the relationship of formation pressure and minimum miscible pressure.4.It studies optimized technology of injection-production parameters by means of numerical simulation while using CO₂flooding.It is designed that well row direction should be consistent in the maximum horizontal principal stress（NE77^o）direction.Injected medium drives oil along the side of the crack while taking rectangular five-spot and linear well pattern,and inject in advance 0.02HCPV,then take periodic gas injection and oil production.Research shows that limit of gas-oil ratio is 3100 m³/t when oil well is shut-in.In order to the goal of"enlarging swept volume,realizing uniform displacement",it raises three stages of adjustment mode of optimization of injection-production parameters,namely early development stage,effective scale stage and gas breakthrough stage.