Mechanisms of Imbibition Diffusion and Recovery Enhancing of Fracturing Fluids in Tight Reservoirs

Spontaneous imbibition is an essential method to enhance recovery during fracturing shut-in of tight reservoirs. However, most of the investigations are primarily focused on the mechanism of water imbibition to enhance recovery; the mechanism of fracturing fluid to enhance recovery and the lower limit of imbibition flow are remained insufficiently investigated. This paper identifies the pore structure characteristic of tight reservoirs, the fracturing fluid imbibition, and the lower limit of imbibition flow by combining nuclear magnetic resonance and high-pressure mercury intrusion methods, clarifying the mechanism of fracturing fluid recovery enhancement. The core-scale super diffusion model of fracturing fluid was established, and the sensitivity analysis of imbibition-influencing factors was conducted. The experimental results indicated that high interfacial tension increased the driving force and decreased the lower limit of imbibition flow, while low interfacial tension decreased the crude oil flow resistance and increased the lower limit of imbibition flow. The imbibition recovery improved with the increase of matrix permeability and decreased with the increase of crude oil viscosity. The degree of mobilization of micropores decreases and the degree of mobilization of mesopores with macropores increases with decreasing interfacial tension. The increase in matrix permeability elevated the degree of mobilization of each pore, while the increase of crude oil viscosity decreased the reduction of each pore. The simulation results fitted well with the experimental data, verifying the reasonableness of the model. The results indicated that the imbibition recovery was positively correlated with the characteristic parameters of the capillary force and negatively correlated with the core length and the viscosity of the fracturing fluid. The investigation in this paper provides a new theoretical basis for the improvement of recovery in tight reservoirs.