Experimental Study On Improvement Of Oil Displacement Efficiency By CO₂ Flooding In Conglomerate Reservoir

The 530 well field of the conglomerate reservoir in Xinjiang Oilfield has low permeability and strong heterogeneity.It adopts waterflood development mode,with serious water breakthrough,low recovery ratio and high water content.CO2 flooding is a method to effectively enhance oil recovery for low permeability reservoir.It is widely used at home and abroad,and the oil-increasing effect is remarkable.In this paper,aiming at the reservoir geology and development efficiency of the 530 well field,the long core test,nuclear magnetic resonance technology,real rock plate model and CO2 foam flooding technology are used to explore the feasibility of CO2 flooding and subsequent CO2 foam flooding technology to improve waterflood efficiency and enhance oil recovery.The research has achieved the following results and understanding:(1)The minimum miscibility pressure of CO2/crude oil was tested by tubule experimenta to be 24.1 MPa.Core microscopic analysis show that pores mainly exist in the form of dissolved pores,intergranular pores and micro-cracks.The core wettability test result show hydrophilicity.(2)The high temperature and high pressure core displacement device was used to study the influence of different pressure and gas injection timing on CO2 flooding.The CO2 flooding experiments after water flooding were carried out under different pressure of 18MPa,24MPa and 30MPa.The result show that with the increase of pressure,CO2 and crude oil can reach the miscible phase more easily and the recovery degree is higher in the gas flooding stage.Under 30MPa pressure,CO2 flooding enhanced oil recovery by 30.75%.Three experiments of different gas injection timing were carried out at water content of 65%(current w content at the mine),80%and 100%.The result show that the earlier the gas injection timing,the later the gas breakthrough time and the higher the cumulative degree of reserve recovery.when the water content is 65%,the cumulative degree of reserve recovery is the highest with 75.05%.(3)Using nuclear magnetic resonance technology,two groups of nuclear magnetic online flooding experiments(natural core with poor physical properties and good physical properties)were carried out.The result of the core with poor physical properties show that oil in the small and medium pores is mainly driven by water flooding.Oil in the macropores is not much reduced.After CO2 flooding,the remaining oil in the small pores is driven the most,the middle pores is the second,and the large pores is the worst.The water displacement efficiency is 35.52%,and the CO2 displacement efficiency is 14.17%.For the core with better physical properties.After water flooding,the amount of oil in the large pores is driven away the most.The oil in the small and medium pores is partially driven.After CO2 flooding,the large and medium pores are driven better,and the small pores is also effectively driven.The water displacement efficiency is 42.46%,and the C02 displacement efficiency is 17.98%.(4)A two-dimensional visualization plane model was fabricated by real conglomerate rock plates for displacement experiments.We can visually observe the migration of oil,gas and water.we can also observe the formation process of the dominant channel of the fluid during water flood and gas flood.The remaining oil after water flood mainly exists in the form of clusters,membranes,columns and blind ends.After C02 flooding,some of the remaining oil still exists in the form of islands,clusters,and membranes,and there is obvious gravity overlap.The CO2 flooding after waterflood has significantly improved the sweep efficiency and displacement efficiency obviously.(5)The CO2 foam system was selected under simulated reservoir conditions:0.3%NP-1+0.03%DY-1.The foam system has good temperature,salt tolerance and stability.The foaming volume was 712 mL and the foam half-life was 206 min.The accumulated recovery ratio of the long core foam displacement experiment is 80.42%,and the foam flooding further improves the recovery factor by 8.63%.The research results of this paper provide a reference for the field application that CO2 flooding enhanced oil recovery technology in the 530 well block in Xinjiang.It also have a guiding role in the research of CO2 flooding technology in other low permeability reservoirs.