Experimental study on crack propagation control and mechanism analysis of directional hydraulic fracturing

摘要

Hydraulic fracturing is mainly used for increasing large-scale coal seam permeability in coal mines to exploit coalbed methane and prevent coal and gas outbursts. However, conventional hydraulic fracturing cracks tend to propagate along the direction of maximum principal stress, which is inconsistent with reinforcement direction engineering and/or project area needs and makes identification of the orientation or specified location of increased coal seam permeability difficult. To address these problems, we have conducted physical similarity simulation experiments and numerical analysis of directional hydraulic fracturing (DHF) and obtained the crack propagation law of DHF technology. By analyzing the variation law of the maximum principal stress inside the rock mass, the crack propagation control mechanism of DHF technique is revealed. The influence of horizontal stress difference coefficients and angles between the hydraulic slotting direction and maximum principal stress direction (i.e., the hydraulic slotting deviation angle) on the crack propagation deflection is investigated. The results show that the DHF technique can achieve crack-oriented propagation along the desired direction. The maximum principal stress range in the rock mass is redistributed after slotting. A directional fracturing induction region is formed between the slots. In addition, DHF hydraulic pressure curves show a secondary fracturing stage when cracks connect the hydraulic fracturing and hydraulic slotting boreholes. Initiation pressures and values of maximum principle stress in the directional fracturing zone increase with increasing horizontal stress difference coefficients and slotting deviation angles. However, increasing the horizontal stress difference coefficient does not significantly influence the directional fracturing zone range. The results provide a reliable basis for subsequent theoretical research and engineering applications.