Laboratory Evaluation On EOR By CO₂ Flooding In Ultra-Low Permeability Reservoirs Of Changqing Oilfield

Ultra-low permeability reservoirs in China are widely distributed and abundant in resources.The exploitation reserves of them are an important part of the total amount of oil and gas resources in China.These reservoirs are characterized by low natural energy,and the pressure drop sharply after exploitation.In order to achieve high efficiency development,more technologies for enhancing oil recovery are needed.The block studied in this paper is Xinghe Block of Changqing Ansai Oilfield in Ordos Basin,which belongs to a typical ultra-low permeability sandstone oil reservoir.It is characterized by low porosity,low permeability and strong heterogeneity.The crude oil in the block has good properties such as low viscosity,low density and low content of gum.Compared with conventional water flooding,CO2 flooding,as a common method of enhancing oil recovery by gas flooding,can effectively expand sweep volume,improve oil displacement efficiency and achieve high-efficiency development of ultra-low permeability reservoirs.On the basis of fully investigating the related research of CO2 flooding(including the current technology,oil displacement mechanism,influencing factors and reservoir type adaptability),in this paper,several methods like theoretical analysis,physical simulation(PVT analyzer,slim tube experiments,core flooding experiments)and nuclear magnetic resonance technology are used to carry out laboratory experiments and applicability evaluation of reservoirs.Firstly,the characteristics of crude oil in the study block are analyzed by PVT analyzer.Then the MMP was determined to be 16.8MPa by slim tube experiments.Finally,experiments of CO2 flooding were carried out to explore the variation of recovery of CO2 flooding under different experimental conditions,and the relevant experimental data were quantitatively evaluated by NMR technology.The results show that the recovery of CO2 flooding in the study area is positively correlated with injection pressure,injection volume and injection speed,but the increase of oil displacement efficiency in each stage is different.Meanwhile,NMR data show that near miscible flooding and miscible flooding are the most efficient stages of oil displacement.At this stage,the oil displacement efficiency of both larger and smaller pore is obviously increased.Through comprehensive analysis,it is found that the miscible state is the main factor affecting the displacement effect of CO2 flooding in the study area.The displacement efficiency in miscible state is much higher than that in immiscible state.Based on the basic geological characteristics of the study area,combined with the previous literature research and experimental conclusions,it is considered that the Xinghe block is suitable for CO2 immiscible displacement.