Optimization of Shut-In Time for Hydraulic Fracturing-Assisted Oil Displacement in Offshore Low-Permeability Reservoirs.

To optimize the shut-in time for hydraulic fracturing assisted oil displacement in offshore low-permeability reservoirs, a coupled mathematical model encompassing injection, shut-in well, and production is established. The accuracy of the model is validated using production data. The law of pressure diffusion and oil-water two-phase flow during the shut-in well process is clarified, and the mechanism of hydraulic fracturing assisted oil displacement to enhance oil recovery is summarized. The sensitivity analysis is conducted, and then the main controlling factors affecting the optimal shut-in time for hydraulic fracturing assisted oil displacement are identified. The research results indicate that the primary function of the shut-in well is to enhance the effect of formation energy supplementation after injection, thereby further increasing the swept volume, controlling greater geological reserves, and ultimately enhancing oil recovery. During the initial stage of shut-in well, a larger portion of geological reserves are controlled mainly by fluid pressure diffusion, while in the later stage, oil-water exchange is mainly achieved by imbibition effect. The main controlling factors affecting the shut-in time are cumulative injection volume, formation pressure coefficient before hydraulic fracturing assisted oil displacement, and permeability, which should be given special attention on site. The research results provide a theoretical basis for the design of shut-in time for hydraulic fracturing assisted oil displacement operations in offshore low-permeability reservoirs.