Abstract: Representative samples are selected from Chang 8 reservoir of Xifeng oil field of Ordos basin to develop water displacing oil of in-situ sandstone microscopic model and water displacing oil of CT scanning for analyzing the impact of microscopic pore structure on flow mechanism of ultra-low permeability reservoir. The experimental results of water displacing oil indicate the water displacement efficiency always presents direct ratio with permeability for the core samples without microcrack. However, the existence of microcrack increases the heterogeneity of microscopic pore structure，then the flow mechanism changes completely, and the water displacement efficiency differs. The laboratory findings of water displacing oil of CT scanning also demonstrate microcrack increases the microscopic heterogeneity of pore structure to some great extent, makes the water displacement effect worse. Casing pressure has major affection on water displacement efficiency of dual porous medium reservoir. It is obvious that the microscopic pore structure heterogeneity is the main reason to cause low sweep efficiency and worse water displacement effects for ultra low permeability reservoirs.

Key words: microscopic pore structure heterogeneity, water displacement efficiency, flow mechanism, X-CT imagery, Xifeng oil field