Study Of Gas Channeling Law And Injection Method For CO₂ Flooding In Ultra-low Permeability Reservoirs

Huang-3 Block is an ultra-low permeability reservoir in Changqing Oilfield,China which had been waterflooded and was switched to CO₂ flooding due to excess water production.However,the nature fractures have resulted in early CO₂ breakthrough and poor production performance.Therefore the investigation of CO₂ production performance and the method to control CO₂ production becomes a key to continue CO₂-EOR.Outcrop cores are used to perform a series of CO₂ flooding experiments at reservoir conditions of pressure,temperature and formation water salinity.Permeability heterogeneity and injection pressure are considered as two variables to affect gas channeling characteristics.Based on the research of gas channeling characteristics,the limits and the technology parameters of foam system,PLS gel system and two-level gas channeling control system are evaluated.Three production stages are clearly stated based on production performance and experimental observation,including gas-free production stage,oil/gas co-production stage,and gas channeling stage.A significant new finding is that oil/gas co-production stage contributes the most to oil recovery.The injection pressure of CO2 and heterogeneity of reservoir have an certain impact on the performances of all stages of CO2 flooding.The effective ways to improve CO2 flooding in an ultra-low permeability reservoir are to maintain the pressure of the formation,to improve reservoir heterogeneity and to control gas channeling,which can help extend the oil/gas co-production stage as much as possible.By optimizing each gas channeling control system,the adaptability limits,optimal injection volume,injection speed and injection timing of each system were determined.The three dimensional physical simulation experiments have been carried out to verify that three kinds of two level gas channeling control systems have good adaptability for serious heterogeneity and fractured reservoir,and provide experimental basis for selecting reasonable CO₂ channeling control technology in the oil field.