Investigation of Microscopic Oil Flow Characteristics During Fracturing Fluid Invasion and Flowback in Shale Oil Reservoirs

After hydraulic fracturing of C shale formation, it is difficult to get stable production, and the flowback efficiency of fracturing fluid is low. In order to reveal the law of fracturing fluid invasion and flowback behavior and to investigate the microscopic characteristics of its effect in improving the oil recovery factor, the experiments were innovatively carried out by using online Nuclear Magnetic Resonance (NMR) technology combined with huff-n-puff experiments using formulated fracturing fluid on two shale rocks, with the core inlet end representing the fractured surface in the reservoir. The dominant pores, Minimum Pore-Producing Radius (MPPR), and invasion depth for upper sweet-spot core (with smaller K and Φ) and lower sweet-spot core (with larger K and Φ) during fracturing fluid invasion and flowback were compared. The results show that small pores in upper sweet-spot core are the dominant pores, while mesopores are dominant in the lower sweet-spot core. The MPPR is 0.0087 μm and 0.024 μm, respectively, for the upper and lower sweet spot. As invasion pressure difference increases from 10 MPa to 20 MPa, the invasion depth of fracturing fluid into the upper sweet-spot core increases from 0.15 cm to 0.29 cm, and for the lower sweet-spot core it increases from 0.23 cm to 0.36 cm, about 1.2 times that of the upper core. Based on the similarity criterion, the formula for calculating on-site effective invasion depth of fracturing fluid Lf is derived, and it is approximately 0.46 m and 0.70 m in the upper and lower sweet spot after a 30-day well soaking. Based on experiments, the flowback efficiency of fracturing fluid is obtained, which is 23.6% in the upper sweet spot and 17.66% in the lower sweet spot. Imbibition tests were also performed for two shale core samples, and it is found that the imbibition recovery degree of the upper sweet-spot core is higher than that of the lower sweet-spot core. Dimensionless time calculated by using Ma’s model yields good fitting results for imbibition, and the soaking time is upscaled to reservoir conditions. The research results provide important reference for hydraulic fracturing practice and thus to improve the oil recovery factor in shale oil reservoirs.