Study on Microscopic Seepage Mechanism During Fracturing Process in Tight Reservoir

Tight reservoirs usually have ultra-low permeability, so fracturing must be carried out during production. Due to different injection velocities and rock-fluid properties, in the fracturing process, oil-water migration law in the pore and throat is complicated. In this paper, to figure out oil-water microscopic seepage mechanism in the fracturing process, we establish two digital core models with and without fractures by using CT scanning method, based on a typical tight reservoir core. Then we study the interfacial position changes and micro-force characteristics during oil and water two-phase movement, using pore scale simulation method. The results show that: ① In the digital core model without fracture, the injected fluid does not advance along a fixed channel, it flows backward when encounters at the parallel channels under imbibition force. This phenomenon leads to the oil remaining in the pore and difficult to use. ② In the digital core model with fractures, a new phenomenon called “reverse imbibition” occurs near the injected fluid entrance. The reverse imbibition is a phenomenon that oil moves from low pressure to high pressure affected by capillary force. It is occurred when the displacement force less than imbibition force. Analysis shows that the reverse imbibition helps oil advance from micro-pore-throat to fracture space during fracturing process. This is the main microscopic mechanism of enhanced oil recovery by fracturing. Under quasi-static conditions, the reverse imbibition only occurs when wetting angle is less than 45°. Therefore, in the hydraulic fracturing process, adding water wetting surfactants is beneficial to increase imbibition capacity, and then can improve oil recovery effectively.