Study on the Fracture Extension Law of Hydraulic Fracturing in Shale Reservoirs

This study aims to deeply investigate the fracture extension law of hydraulic fracturing in shale reservoirs, as well as the effects of rock mechanical anisotropy and engineering operation parameters on the fracture extension behavior. Matlab programming combined with the theoretical model of fracture extension was used to establish a computational model of the stress on the hole wall of a shale isotropic matrix shot hole, and the effects of injection displacement, horizontal stress difference, elastic modulus and Poisson’s ratio on fracture extension were systematically analyzed through a series of true triaxial hydraulic fracturing physical simulation experiments. The results show that the injection completion can significantly reduce the fracture initiation pressure compared with the barehole completion, especially when the injection azimuth angle is 90°, the fracture initiation pressure reaches the lowest value of 52.3 MPa. The anisotropy of rock mechanics has a significant effect on the fracture initiation pressure, especially when the horizontal stress difference coefficient is more than 0.13, the larger the ratio of Young’s modulus is, the lower the fracture initiation pressure is, and the larger the value of Poisson’s ratio is when the value of Poisson’s ratio is more than 0.25, the larger the value of Poisson’s ratio is, the higher the value of Poisson’s ratio is. The larger the value of Poisson’s ratio is above 0.25, the higher the crack initiation pressure is. The increase of injection displacement from 0.5 mL/min to 5 mL/min increased the crack length from 15.2 cm to 66.3 cm, and the crack extension rate from 0.51 cm/min to 2.21 cm/min, which significantly increased the crack extension rate and length. While the fracture extension rate decreased from 1.8 cm/min to 0.8 cm/min when the elastic modulus increased from 20 GPa to 60 GPa, the rock with high elastic modulus formed a smaller plastic zone near the fracture tip, which limited the fracture extension rate and morphology. This study provides a new theoretical basis for the fracture expansion mechanism of hydraulic fracturing in shale reservoirs, which is of great significance for optimizing hydraulic fracturing design and improving the efficiency of shale gas development.