Experimental Study On Enhanced Oil Recovery By CO₂ Flooding In Chang 7 Tight Reservoir In Southern Hubei

With the rapidly growing demand of oil and gas energy in the world and the consumption of conventional oil and gas resources,tight oil reservoirs with good exploitation potentiality have gradually become the focus of oil and gas exploration and development in recent years.Tight oil reservoirs are characterized by intricate lithology,a large number of micro-nano pores and extremely low permeability.Conventional water flooding is difficult to inject and has low recovery.CO₂ flooding is more suitable for tight oil reservoirs,and CO₂ miscible flooding has the best effect.However,by contrast with traditional reservoirs,the average pressure of most tight reservoirs is lower,and CO₂ and crude oil are difficult to miscible under formation conditions.Therefore,how to reduce the minimum miscible pressure and improve oil recovery is of great significance for the development of tight reservoirs.In this paper,CO₂ displacement experiments were conducted to explore the influence of different factors on the CO₂displacement effect and optimize the CO₂ injection method,which provides some insights for the development of tight oil.As to the Chang 7 tight oil reservoir in southern Hubei,the basic physical properties and oil and gas seepage characteristics of tight reservoir were obtained by the field core porosity and permeability parameter test and unsteady oil and gas phase permeability test.Based on PVT test device and thin pipe test device,the high pressure physical properties and minimum miscibility pressure of the field crude oil were tested,so as to clarify the mechanism of CO₂displacement,and at the same time to screen the alcohol additives that can reduce the minimum miscibility pressure.By building an experimental device for CO₂ displacement in tight cores,the effects of core permeability,injection pressure,gas oil and other factors on the recovery degree were studied to verify the application effect of alcohol additives in cores and further optimize the CO₂ injection method.The results show that the core porosity and permeability of the tight reservoir are low,the reservoir permeability decreases rapidly,and the gas seepage ability is weak.CO₂ can effectively improve the physical properties and enhance the fluidity of crude oil.The minimum miscibility pressure between crude oil and CO₂ is 22.8 MPa.The addition of alcohol additives can improve the degree of oil recovery,of which ethanol is the best,and the method of CO₂carried ethanol displacement can significantly reduce MMP.The CO₂ flooding rule of tight core is as follows:the higher the core permeability,the higher the recovery degree;the higher the CO₂ injection pressure,the higher the recovery degree,and the increament of the recovery degree decreases with the increase of the pressure.With the use of simulated gas oil,the oil production rate is accelerated,and the recovery degree decreases with the increase of injection-production pressure difference;Periodic injection combining CO₂ carried ethanol displacement with shut-in well can delay the breakthrough of CO₂,so that ethanol and CO₂ can be dissolved and diffused in the crude oil more fully,thus further improving the recovery degree.