Study Of Gravity Segregation Law And Related Mechanisms Of CO₂ Flooding In Thick Oil Layer

Low permeability thick oil reservoirs have low permeability and good crude oil properties.CO₂ flooding development has good applicability to low permeability oil reservoir,but the thickness of the oil layer is large,and it is easy to form gravity segregation to reduce the longitudinal sweep efficiency of CO₂ flooding.therefore,in order to provide a theoretical basis for improving the development effect of CO₂flooding in low permeability reservoirs,experimental study on gravity segregation law of CO₂ flooding in thick oil layer is necessary.Firstly,a physical model that can simulate gravity segregation of gas flooding was designed independently.The laboratory experiments of long core displacement were used,degree of gravity segregation was evaluated based on the difference between the fluid produced from the upper and lower layers of the core and the distribution of remaining oil in the displacement profile.Factors such as gas injection rate,oil-gas density difference,permeability,core length-to-height ratio that inlfuence gravity segregation law were studied during CO₂ immisicible/miscible flooding.then,mechanisms that how these factors influence gravity segregation were analyzed.The model designed independently can simulate gravity segregation of CO₂ in thick oil layer.Based on the difference of fluid production between top and bottom layers of core,the characterization method of gravity segregation is optimized.Under certain experimental conditions,the results show that inhibition of viscous force on gravity is increased with the injection rate is increased,and the gravity segregation is weakened;The increase in density difference directly enhances the gravity force and intensifies gravity segregation whether in immiscible or miscible flooding;The difference in oil-gas density is large in immiscible flooding,and the change of permeability of permeability obviously increases gravity.The difference in oil-gas density of miscible flooding is small,the increase in permeability does not increase the degree of gravity segregation obviously in the same permeability changing range;The experimental results of core length-to-height ratio simulating the ration of injection-production well spacing to oil layer thickness show that the reduction of core length will limit the space for the formation and development of gravity segregation in immiscible flooding,and gravity segregation is weakend.The small and medium core length reduces the degree of oil and gas miscibility,and the decrease in length is difficult to limit the development of faster gravity segregation,gravity segregation intensifies in miscible flooding.Thus,miscible flooding should be applied as much as possible to reduce difference in oil-gas density,increasing gas injection rate appropriately at the same time to suppress gravity segregation,reasonably determining the injection-production well spacing according to the type of displacement to avoid gravity segregation develped,using permeability to affect the law of gravity segregation to improve the sweep efficiency of heterogeneous reservoirs,to avoide the develpoment of gravity segregation and enhance oil recovery of CO₂ flooding in thick oil layers.