Abstract:Based on the theory of Backly-Leverett water flooding, considering gravity, capillary force, viscous force and threshold pressure gradient, and considering the different characteristics of the same direction imbibition resistance between before and after the leading edge of wetting phase breakthrough the outlet, a one-dimensional low permeability imbibition mathematical model in the same direction is established. Using the mathematical model, by adjusting the coefficient and the index of oil-water phase in the threshold pressure gradient equation, when the calculated curve result of the imbibition rate and time is consistent with the experimental curve result of the imbibition in the same direction,The values and the changes characteristic of the threshold pressure gradient in the imbibition process can be obtained. The results of the five groups experiments and calculations show that the mathematical model can effectively calculate the threshold pressure gradient of spontaneous imbibition. It can be seen that the establishment of the mathematical method is of great significance to further improve the theory of imbibition and rich the calculation method of starting pressure gradient.