Development Pattern And Parameter Optimization Of CO₂ Flooding After Water Flooding In Low Permeability Reservoir

CO₂ flooding is a widely used tertiary enhance oil recovery technology（EOR）,and its effect is affected by many aspects,including reservoir parameters,fluid properties and gas injection methods.In this paper,the indoor driving method and numerical simulation method are used to study the oil displacement characteristics and influencing factors of CO₂continuous flooding and CO₂ water–alternating-gas（WAG）flooding after water flooding,which provides reference for timely adjustment of development scheme.Firstly,through a series of low-permeability long core CO₂ continuous displacement experiments after water flooding,the oil displacement characteristics and related parameters are systematically studied.The experimental results show that CO₂ flooding can effectively increase oil recovery,which can increase 33.55%～45.59%based on water flooding,and can effectively reduce the water content to 10%～20%and then slowly increase to 40%～50%.The CO₂ flooding efficiency is mainly affected by the pore pressure,and the effect of near-miscible flooding and miscible flooding is obviously better than that of immiscible flooding.Secondly,the long core flooding experiments are carried out to study the oil displacement characteristics and related parameters of WAG flooding after water flooding（WAGa.WF）in low permeability reservoirs,and the control parameters were optimized.The experimental results show that the formation pressure should be maintained as high as possible in the WAG flooding stage.The optimum slug size,gas-water ratio and injection speed are 0.1 PV,2:1 and0.1 ml/min,respectively.The oil recovery increased by 40.24%based on water flooding.Finally,the method of CO₂ flooding control in low permeability reservoirs under well pattern conditions is studied by streamline numerical simulation.After the water flooding,the CO₂ flooding in the anti-five-spot,the anti-seven-spot and the anti-nine-spot pattern theoretical model are obtained,and the streamline distribution maps of different development stages are obtained.The results show that the area of the streamlined area of WAG flooding after water flooding is significantly larger than that of CO₂ continuous flooding after water flooding.The oil recovery of WAG flooding is higher than that of CO₂ continuous flooding under different well network pattern.Among them,the anti-five-spot well network has the highest oil recovery.The research results deepen the understanding of CO₂ enhanced oil recovery after water flooding in low permeability reservoirs,provide theoretical basis for CO₂ development scheme design and production practice,and have important significance for rational and effective development of low permeability reservoirs.