SPONTANEOUS CAPILLARY IMBIBITION MODEL IN TIGHT OIL RESERVOIRS ¹⁾

The fracturing construction shows that the lower flowback efficiency is often accompanied with a high production in tight oil reservoirs, which may be due to the spontaneous imbibition of the fracturing fluid. However, the tight oil reservoir has poor physical properties with a complex permeable mechanism. A spontaneous imbibition model for tight oil reservoirs is desirable. Based on the oil-water flow theory, a mathematical model of the spontaneous imbibition is established. The numerical difference method is used to solve the equations and analyze the influencing factors. The results show that the imbibition volume and the imbibition front position of the tight oil reservoir are linear functions of the square root of time, as is consistent with the prediction of the classical Handy imbibition model. It indicates that the imbibition model is reliable. The numerical results show that the phase diffusion coefficient is the main control parameter of the imbibition rate and has a positive correlation with the imbibition rate. The phase diffusion coefficient increases first and then decreases with the water saturation; it increases with the increase of the irreducible water saturation, and the relative endpoint permeability of the oil and the water; it decreases with the increase of the permeability characteristic index, the oil-water viscosity ratio and the residual oil saturation. The results of this paper can help to understand the fracturing fluid imbibition mechanism in the tight oil reservoirs, and can be used for optimizing the flowback regime and improving the production of the tight oil.