Mechanism and application of transient cyclic water flooding in extra-low permeability oil reservoirs

The Chang 6 oil reservoir in the Block X of Yanchang was taken as the research object in order to give full play to the imbibition between reservoir fractures and matrix, increase the waterflooding swept volume and improve the waterflooding recovery factor. After the influence factors of counter-current imbibition were analyzed by conducting laboratory static imbibition experiments, the parameters of cyclic water flooding in the Block X were optimized be means of numerical simulation technology considering imbibition. It is indicated that counter-current imbibition is the main imbibition mode in the Block X. The imbibition displacement efficiency increases with the increase of reservoir permeability and porosity and decreases with the increase of water saturation. In a certain range, interfacial tension and imbibition displacement efficiency are in an inverse relation. Transient cyclic water flooding is selected in the Block X and its optimized working system is injection for 20 d and stop for 30 d with injection/production ratio of 1.0−1.2 and water injection rate of 8−12 m³/d. Field application results show that oil production rate increases slightly, water cut decreases significantly and recovery factor is 2.5−3.5% higher than that of continuous waterflooding development. The research results provide the effective theoretical foundation and field application basis for the transient cyclic waterflooding development of extra-low permeability oil reservoirs.