Microscopic water flooding characteristics and influencing factors in extra-ultra low permeability reservoirs: a case study of Chang 6 and Chang 8 reservoirs in Heshui area of Ordos Basin

Chang 6 and Chang 8 reservoirs in Heshui area of Ordos Basin have some similarities and differences in reservoir characteristics. Controlled by the homogeneity degree, there are three types of oil displacement in Chang 6 reservoir: uniform-reticular, reticular and fingerlike oil displacement, while no finger-like displacement is observed in Chang 8 reservoir.Wettability affects the state of residual oil. The higher the effective permeability, the larger the average throat radius, the better the pore-throat connectivity, and the higher the oil displacement efficiency. In the samples of Chang 8 reservoir, the more developed the dissolution hole is, the higher the oil displacement efficiency will be. The injection pressure increased by 50% based on the start pressure of water injection, when the injection speed is0.05～0.08 mL/min, and the injection volume is 3 PV, the best displacement effect will be reached. The oil displacement efficiency can be predicted by the comprehensive comparison of reservoir parameters, and the oil displacement efficiency can be improved obviously by selecting the appropriate water injection method.