Directional hydraulic fracturing technology serves as an effective means to overcome the stress constraints in deep coal mines and achieve targeted depressurization and permeability enhancement in specific areas. This study conducted directional hydraulic fracturing tests on samples with pre-cast slots under true triaxial conditions. Considering the azimuth angle (i.e., the angle between the long axis of the pre-cast slot and the maximum principal stress) and long axis length, the stress distribution along the fracture edge was determined using the method of complex variable function, and the evolution mechanism of the stress distribution along the fracture edge was investigated. The main conclusions are as follows: Experimental results indicate that the initiation pressure and directional extension effectiveness show a relative growth pattern of “slow-fast-slow” in the azimuth angle range of 30° to 90°, with the most significant decrease in initiation pressure occurring around 45°, and the enhancement trend of directional extension is most pronounced in the range of 45° to 60°. When the ratio of the long axis to the short axis of the pre-cast slot changes from 2:1 to 1:1 and 3:1, the initiation pressure increases by −30.44% and 28.08% respectively, while the amplification of directional extension effect is −74.6% and 166.8%. Considering the azimuth angle, an expression for the circumferential stress at the edge of the pre-cast slot was derived, and a criterion for determining the initiation of the pre-cast slot edge was proposed. As the azimuth angle increases from 0° to 90°, the circumferential stress at the edge of the pre-cast slot fluctuates, which is consistent with the trend of initiation pressure observed in the experimental results. As the long axis length increases, the initiation pressure decreases, and the initiation position moves closer to the tip.
Read full abstract