Optimization and Threshold of Soaking Time in Fracturing-Driven Huff-n-Puff for Tight Reservoirs Based on Dynamic...

In recent years, oil and gas development has gradually shifted toward low-permeability, tight, and difficult-to-produce unconventional reservoirs. Due to the difficulty of energy replenishment via water injection in such reservoirs, fracturing-driven huff-n-puff (FDHP) technology has emerged as a key technique for their efficient development. Tight reservoirs are characterized by fine pore throats and complex pore structures, and the dynamic capillary force effect during the shut-in period can significantly influence fluid seepage behavior. However, most existing studies on shut-in time optimization are based on the static capillary force assumption, which fails to accurately characterize the actual features of unsteady-state spontaneous imbibition processes. In this study, dynamic capillary forces during production in reservoirs with different permeabilities were experimentally measured, and the evolution of the pseudodynamic coefficient with water saturation was revealed. Based on a macroscopic characterization model of the dynamic capillary force effect, combined with the flow characteristics of surfactant solution and crude oil in porous media, a single-well matrix-fracture coupled shut-in production model was constructed. Production simulations were conducted on well T-1, an FDHP well in the Daqing Oilfield. The production performance and remaining oil distribution under different shut-in durations were analyzed using the proposed model, and the effects of bottomhole flowing pressure, crude oil viscosity, reservoir permeability, and oil-water interfacial tension on shut-in time were investigated. The results show that under permeability ranging from 0.81 to 20.31 × 10^-3 μm² and injection rates from 0.01 to 0.2 mL/min, the pseudodynamic coefficient during water flooding ranges from 0.04 to 76.3 MPa s, exhibiting a nonlinear positive correlation with water saturation. Higher displacement rates and lower core permeability lead to a more pronounced dynamic capillary force effect and a larger pseudodynamic coefficient. Shut-in time exhibits a nonlinear relationship with cumulative oil production, with an optimal interval existing. An appropriate shut-in duration can effectively expand the reservoir energy supplementation range, mitigate the oil production decline rate, and enhance oil recovery. The optimal shut-in time for the target well is 15-20 d: short-term shut-in relies on driving pressure for production enhancement, while the effectiveness of long-term shut-in is constrained by fluid flowback efficiency. Sensitivity analysis of shut-in time for FDHP in tight reservoirs demonstrates that the optimal shut-in time shortens with increasing bottomhole flowing pressure, extends with rising crude oil viscosity and injected fluid-crude oil interfacial tension, and exhibits a negative correlation with reservoir permeability. The findings of this study can improve the application of FDHP technology in tight oil reservoirs.