Study On CO₂ Flooding Technology In Medium-high Permeability Reservoir

Nanhua 2 block is a typical medium-high permeability reservoir.After natural energy and water flooding,the water production of oil wells increases,the oil production decreases,and the stable production is difficult.It is urgent to explore new technologies to improve the utilization of remaining oil and further improve the oil recovery rate of the block.Compared with water flooding,CO₂ flooding can improve the swept volume and the ability to use remaining oil.It can realize the effective utilization and geological storage of CO₂,and has both economic and environmental benefits,which has a good application prospect.However,compared with low permeability and ultra-low permeability reservoirs,the phenomenon of gas viscous fingering in medium and high permeability reservoirs is serious.Whether the channeling can be effectively plugged is the key to improve the recovery of CO₂ flooding in medium-high permeability reservoirs.Therefore,this paper studies the CO₂ flooding technology in medium-high permeability reservoirs by carrying out various indoor experiments.Among which the research on the plugging limits and combinations of different channeling systems for CO₂ flooding in medium-high permeability reservoirs after water flooding is the focus and innovation of this paper.The research content involves the high-pressure physical property test of crude oil in the study area,the determination of the minimum miscible pressure of CO₂,the long core flooding experiment,the analysis of the influencing factors of gas channeling,the research and development of the plugging system and the combination optimization of the plugging system.The evaluation system of CO₂-EOR flooding effect in medium-high permeability reservoirs was established,and the plugging limits of three different sealing systems of water gas alternation,CO₂ foam and gel were quantified,and a set of optimal plugging system combinations suitable for CO₂-EOR in medium-high permeability reservoirs was determined.This study can provide the necessary theoretical basis and technical support for the implementation of CO₂-EOR in medium-high permeability reservoirs.Firstly,the experimental results of high pressure physical property analysis and CO₂minimum miscibility pressure test show that the crude oil in Nanhua 2 block belongs to typical black oil,and the elastic expansion energy is insufficient.The implementation of CO₂ flooding can play a role in increasing energy.The minimum miscible pressure of CO₂ is 23.7 MPa,and the crude oil cannot reach miscible under the condition of original formation pressure.Through long core flooding experiments,it is determined that it is feasible to implement CO₂flooding in the study area.CO₂ flooding still has good oil flooding effect under high water cut conditions.The best injection time is 60%water cut,the best injection volume is 0.5 PV,and the best injection method is water gas alternation.Secondly,based on the analysis of the influencing factors of gas channeling in the study area,a CO₂ foam sealing system and a high-strength gel sealing system suitable for gas channeling plugging in the study area were developed.The flooding pressure difference is the main factors affecting gas channeling.Water gas alternation can effectively prevent gas channeling,but when the permeability is high,the effect becomes worse,and other high-strength channeling sealing systems must be combined to control gas channeling.Finally,on the basis of using high temperature and high pressure flooding+nuclear magnetic resonance method to determine the plugging limits of three different channeling systems of water gas alternation,CO₂ foam and gel,different plugging systems are combined for two reservoirs without and with large pores.The ability of different plugging system combinations to produce remaining oil is evaluated from both micro and macro aspects.The experimental results show that the plugging limit of water gas alternating system is 1.51～21.35μm;the plugging limit of CO₂ foam system is 0.07～28.23μm.The gel system can effectively block the large pores above 7μm,and it is difficult to enter the small pore throat.When developing reservoirs without large pores,high-strength CO₂foam plugging is first used,and then water gas alternating flooding is carried out.The smaller pore throat has a higher degree of utilization and a better oil flooding effect.The combination of water flooding+foam slug flooding+water gas alternating flooding+foam flooding+water flooding is the best.In the development of reservoirs with large pores,water gas alternation is carried out first,and medium and small pore throats are used.After low strength leads to gas channeling,high-strength gel is used for plugging.The degree of pore throat utilization is high and the oil displacement effect is good.The combination of water flooding+water gas alternating flooding+gel flooding+water gas alternating flooding+water flooding is the best.