Abstract: In view of low permeability in ultra-low permeability reservoirs, complex pore throat structure, high heterogeneity, difficulty in realizing effective displacement with conventional water injection, strong imbibition between fractures and matrixes, and other problems, dynamic imbibition characteristics and main influencing factors were worked out by indoor imbibition experiments and similar theories on the basis of the mechanism analysis of dynamic imbibition and oil displacement in ultra-low permeability reservoirs, and the laboratory data were applied to the oilfield site. As indicated by experimental results, the main factors affecting dynamic imbibition in ultra-low permeability reservoirs included wettability, injection rate, fracture density, and stimulation turns. Among them, wettability was the primary factor affecting the imbibition. The stronger the hydrophilicity of the rock, the higher the permeability, the more fractures, and the better the dynamic imbibition effect; the water injection rate suitable for the development of ultra-low permeability reservoir was 0.1 mL/min; the reasonable stimulation turn was 2 to 3 cycles, and the optimal shut-in duration was about 30 days. The significant effect of dynamic imbibition was proven by the field tests where the average water cut was reduced by 20 percentage points, and the cumulative oil production was increased by 2 145 t in 5 horizontal wells after fracturing by stimulated reservoir volume and dynamic imbibition in Oilfield A, Ordos Basin. The study results play a guiding role in the formulation of production scheme of ultra-low permeability oilfields.

Key words: ultra-low permeability reservoirs, dynamic imbibition, imbibition recovery efficiency, huff and puff, shut-in duration, Changqing Oilfield