Study On Microscopic Remaining Oil Distribution After Displacement Of Different Media In Low Permeability Reservoirs In Jidong Oilfield

In recent years,the research on oil and gas production mechanism of low permeability tight reservoir is becoming more and more important.To further understand the microscopic distribution of remaining oil in low permeability reservoirs after different media flooding in Jidong Oilfield,nuclear magnetic resonance imaging,T₂ spectra and pore radius distribution maps of cores representing different types of reservoirs（type I,II and III）in the process of water,surfactant,CO₂,N₂ and CH₄ displacement were obtained by on-line nuclear magnetic resonance（NMR）displacement experiments.Based on the pore distribution characteristics,the pores are divided into large,medium and small types based on the limits of 1μm and 0.2μm,and the oil displacement effects of different pore intervals are analyzed.Based on indoor experiments,numerical simulation is carried out to study the distribution pattern of remaining oil.The electron microscope scanning images of cores were obtained by electron microscope scanning experiment,and the images were processed by software MATLAB and Image J,and a clear and quality binary image of pore structure was obtained.At the same time,based on the binary diagram,the COMSOL multi-physical field simulation software is used to model the two-dimensional pore model and simulate the remaining oil distribution.The results show that:（1）Type I cores have the best physical properties and can be flooded with water and surfactants.The required pressure difference is less than 4MPa,and the final oil displacement efficiency exceeds 70%.The remaining oil accounts for more than 55%in small pores and medium pores.accounted for more than 32%.The permeability of type II cores is lower than that of type I,and the pressure difference required to achieve the same recovery degree by water flooding and surfactant flooding is 4-5MPa higher than that of type I,and the proportion of remaining oil in medium and small pores exceeds40%.The oil displacement efficiency of type I and type II cores using surfactants is about4%higher than that of water flooding,and the proportion of remaining oil in small pores is lower than that in water flooding.Analysis shows that surfactants improve the imbibition capacity of small pores.Under indoor conditions,immiscible gas flooding was carried out on Type II cores,which could reduce the displacement pressure difference of4 MPa,and the final oil displacement effect was CO₂>CH₄>N₂.（2）The physical property of type III core is the worst,and the permeability is only about 0.1m D.When the water flooding pressure difference is 8MPa,the oil displacement efficiency is only 36.32%,and the remaining oil accounts for more than 60%of the small holes.Similarly,CO₂ flooding can reduce the displacement pressure difference of type III cores,but the oil displacement efficiency is only about 30%,and the proportion of remaining oil in small pores is more than 80%,which is much higher than that of water flooding.In this study,the reason for the low oil displacement efficiency of gas is that the experimental conditions do not reach miscible flooding,and gas channeling is easy to occur in the process of immiscible displacement.The influence of gas channeling on typeⅢcores is stronger.（3）The morphology of remaining oil mainly includes flake,strip,network,ball,cluster and film.Gas flooding is better than water flooding,and CO₂ flooding is the best,followed by CH₄,both of which are better than nitrogen flooding.The shape of remaining oil in each medium is different,but it is mainly flake and strip.On this basis,it is speculated that the flow of N₂ can drive the flow of flake and strip remaining oil to form a network of remaining oil,while the displacement ability of CO₂ and CH₄ is better,and the displacement is more complete to form independent ball remaining oil.