The Feasibility Study Of Impure CO₂ Near Miscible Flooding In Q Oilfield

The crude oil in Q Oilfield has high wax content,low reservoir permeability and strong heterogeneity.The gas source with high CO₂ content is found at the top of the oilfield,and there are a lot of CH₄ and N₂ in the gas source.If injecting high CO₂content gas into this reservoir,the minimum miscible pressure of crude oil is high,and it is difficult to achieve miscibility.If the injection gas is separated and purified,the cost will be greatly increased.Research indicates that there is a transition region near the minimum miscible pressure and it is defined near miscible pressure region.When the injection gas and crude oil reach near miscible region,high displacement efficiency can also be obtained.Compared with miscible flooding,near miscible flooding requires less formation pressure,injection gas purity and crude oil properties,and it is easier to realize in the oilfield.Therefore,considering injecting impure CO₂ into Q reservoir to explore the feasibility of improving oil recovery by achieving CO₂ near miscible flooding.On the basis of the fully investigating of the research status of near miscible flooding,the pressure intervals to achieve near miscibility when injecting CO₂ with different purity are obtained by slim tube simulation and oil displacement efficiency calculation method.The purity interval of injection gas required for achieving CO₂ near miscible flooding under formation pressure is obtained by plotting and calculating.Under formation pressure,the purity of injected gas for CO₂ near miscible flooding in Area E ranges from 57.6%to 99.0%.Aimed at the injection gas whose purity is lower than the limit,the feasibility of adding light hydrocarbon to reduce the difficulty of near miscibility was explored.For every 10%decrease in CO₂ content,it is necessary to add at least 3%C₄ to achieve near miscible flooding.Combined with the geological conceptual model,the injection parameters such as gas injection purity,gas injection speed,gas injection location and gas injection timing are optimized,which can provide reference for the development of CO₂ near miscible flooding in Q Oilfield.With the optimized gas injection parameters,the recovery degree of near miscible flooding simulation development in E area of Q oilfield in 10 years has reached 93.2%of that of miscible flooding,11.44%higher than that of water flooding and 21.80%higher than that of depleted flooding.Under the current gas source conditions,CO₂ near miscible flooding can be achieved in the target area through reasonable control of injection gas composition and optimization of injection parameters.This paper provides technical support for the economic and effective development of Q Oilfield,and it also has certain significance for improving the potential of CO₂ flooding in China.