Optimization And Effect Evaluation Of Gas Flooding Development Methods For Low Permeability Reservoirs In JiDong Oilfield

Macroscopically,the unproduced part of the low-permeability and ultra-low-permeability reservoirs in the G fault block in Ji Dong Oilfield has the characteristics of"one deep,two large,two small,and one strong".In specific,it refers to deep burial,large span of oil-bearing well sections,large stress difference between the two phases,small fault blocks,small sand body scale,and strong reservoir heterogeneity.Microscopically,it has the characteristics of"two small,one strong and one complex",that is,the mainstream throat radius is small,the movable fluid saturation is small,the reservoir stress sensitivity is strong,and the pore throat structure is complex.These characteristics result in increased seepage resistance,slow energy transfer,and difficulty in establishing an effective pressure displacement system.From various aspects such as nuclear magnetic resonance,laboratory displacement experiments and numerical simulation,,this thesis carried out research,evaluated the displacement and huff and puff effects of different injection media for different types of reservoirs,and reached the following conclusions and understandings:（1）Fault block G can be divided into classⅠ,Ⅱreservoirs and classⅢreservoirs with poor connectivity according to the physical properties of the reservoir from good to bad.Based onⅡreservoir cores,on-line NMR tests of the 1displacement effects of CO₂,hydrocarbon gas and N₂under various pressure differences were carried out,and the microscopic displacement effects of different media were compared.Experiments showed that:for reservoirⅡ,under the same conditions,the displacement effect and the production degree of small pores are CO₂>hydrocarbon gas>N_2。（2）Through the high temperature and high pressure core flooding device,it carried out indoor core flooding experiments with different injection media,and compared the oil displacement effects of CO₂flooding,N₂flooding and hydrocarbon gas flooding.Experiments showed that theⅡreservoir can be developed by gas injection,and the displacement effect of carbon dioxide is optimal,followed by hydrocarbon gas,and the effect of nitrogen flooding is the worst;Under the mixed-phase condition,the displacement effect of carbon dioxide is better,and the recovery degree of crude oil is 71.5%.（3）Through the laboratory unstable phase permeability experiment,with three gases of CO₂,hydrocarbon gas,and N₂,the law of oil and gas phase permeability under different pressure gradients was obtained.The experiment found that the pressure gradient was increased from 0.15MPa/cm to 0.8MPa/cm,and the CO₂flooding efficiency was increased by 14%,the displacement effect is better than hydrocarbon gas and N₂.The relative permeability of the oil phase and gas phase increased by 0.5 and 1times,respectively.（4）By selecting the better physical propertiesⅢreservoir in the small-scale sand body to conduct the water injection huff and puff core experiment,the recovery degree of active water huff and puff can be about 5%higher than that of water injection huff and puff.Three cycles of throughput were carried out in the experiment.With the increase of the huff and puff cycle,the extraction degree increased,and the increase range decreased.The huff and puff experiments of CO₂injection,hydrocarbon gas injection and N₂injection were carried out on the cores of the poor physical propertiesⅢreservoir,respectively.As the huff and puff cycle increased,the cycle recovery factor decreased.In the process,the recovery degree of the first huff and puff cycle was the highest,about 15%of carbon dioxide,12%of hydrocarbon gas,and 8%of nitrogen gas,and the recovery degree of the second huff and puff cycle was greatly reduced.（5）The reservoir scale numerical simulation of small-scale sand body reservoir is carried out by CMG.Firstly,the huff and puff effect of CO₂,hydrocarbon gas and N₂is numerically simulated.The results show that the huff and puff effect of CO₂is much better than that of hydrocarbon gas and N₂.Then,the injection rounds,injection speed and injection pressure of the three gases are optimized respectively.This study is of great significance for the optimization of different flooding methods forⅡandⅢreservoirs in the fault block G in Ji Dong Oilfield,and provides some reference for CO₂flooding in other low-permeability oil reservoirs.