Research Of Near Miscible Carbon Dioxide Flooding For Enhanced Oil Recovery Technology In Ultra-low Permeability Reservoirs

It is an important criterion that the oil can miscible with carbon dioxide or not during the screening process of reservoir that suitable for carbon dioxide flooding,but the pressure of carbon dioxide flooding is difficult to reach,and the effect of immiscible flooding is not very good,therefore,the research of near miscible flooding was promoted.Carbon dioxide near miscible phase region is a pressure region below the minimum miscibility pressure,in this region,oil displacement efficiency is slightly lower than miscible flooding but much higher than immiscible flooding,however,it is not strict for the injection pressure,therefore it is more suitable for the real requirements of oil field production.As to the low permeability district of Yanchang Yougou oil field,based on reservoir physical stimulation,we established a method that can measure near miscible pressure.Firstly,research on the influence factors of carbon dioxide flooding,find the effect of four different factors on oil recovery,including core permeability,core length,displace speed and core heterogeneity;Then,ensure the near miscible flooding region by the change of displacement efficiency under different pressure,research the differences between the three aspects of the near miscible phase,the miscible phase and the immiscible phase by oil recovery,gas oil ratio and displacement pressure;finally,optimize different miscible solvents,and make a preliminary study on the technology of reducing miscible pressure flooding by using miscible solvent.The results show that higher or lower permeability will reduce the oil efficiency;With the increase of the length of the core,the phenomenon of fingering will decrease oil recovery;The lower the displacement speed,the longer the contact time between carbon dioxide and crude oil,and the higher the oil displacement efficiency;The stronger the heterogeneity of the core,the harder to active the remaining oil in the low permeability core matrix,the lower the oil recovery;The minimum miscible pressure ensured by the results of core experiment is 18.5MPa,higher than the result of slim tube experiment;The near miscible pressure region ensured by the oil recovery,gas oil ratio and displacement pressure is 16.5~18.5MPa;The usage of miscible solvent can reduce the miscible pressure,and in all solvents,the effect of light hydrocarbon is better than that of surfactant.The results of the experiment research further deepen the realization of the mechanism of carbon dioxide flooding,and provided a basic reference for the design of carbon dioxide flooding scheme.