Study On Enhanced Oil Recovery By Gas Flooding In Tight Reservoirs

China is rich in tight oil reserves.Due to the successful application of volumetric fracturing technology,the tight oil has entered the development stage one after another,but it still faces the problems of low recovery factor and fast decline of production capacity.Therefore,it is extremely important to select an appropriate gas injection medium to supplement energy for formation and improve tight oil recovery.In this study,minimum miscible pressure test experiment,gas injection expansion experiment,mass transfer experiment,long core displacement experiment,and full diameter core displacement experiment before and after fracture were carried out respectively.The results show that the minimum miscible pressures of crude oil with hydrocarbon gas or CO₂ are 37.36MPa and 27.76MPa,respectively.Injected gas dissolved in crude oil can improve the properties of crude oil,reduce the viscosity of crude oil,improve the flow capacity of crude oil,and also expand the volume of crude oil,effectively increase the elastic drive energy of crude oil.Compared with oxygen-reduced air,hydrocarbon gas and CO₂ have better compatibility with formation oil in the study block,and miscible or near miscible flooding can be realized under reservoir conditions.The mass transfer mechanism of hydrocarbon gas with crude oil is different from that of CO₂ with crude oil.Hydrocarbon gas is mainly dissolved and condensate miscible occurs,while CO₂ is mainly extracted and evaporative miscible occurs.In the long core displacement experiment,the overall efficiency of CO₂ displacement is 70～82%,and that of hydrocarbon gas displacement is 55%～70%.The oil displacement efficiency of oxygen-reduced air is 40%～54%.Hydrocarbon gas injection is the best solution in the study block.On the one hand,the gas source is wide and the cost is low.On the other hand,compared with CO₂ flooding,the oil displacement efficiency is only about 15%lower.The larger the size of the core,the more complex the internal situation,the more likely to occur gas channeling,and the greater the influence of sweep efficiency on the ultimate recovery.The injection capacity of core is significantly improved after fracture construction,but it will lead to the premature occurrence of gas channeling phenomenon.Therefore,reasonable planning should be made in the field construction process of tight reservoir to prevent premature gas channeling on the premise of improving injection capacity.