Experimental study of directional fracturing in sandstones with prefabricated cracks in the axial direction of borehole

To address the potential disaster caused by the un-stability of hard roof in coal mines,the directional hy- draulic fracturing technology using prefabricated cracks along the borehole axial direction is proposed for reducing the length of suspended roof. Sixteen specimens were prepared using purple sandstone samples with the size of 300 mm× 300 mm×300 mm. In each specimen,the authors prefabricated symmetrical cracks with the diameter of 25 mm and the length of 10 mm along the borehole with different orientations. Hydraulic directional fracturing tests were carried out u- sing a true triaxial fracturing test platform and a constant-flow / constant-pressure injection pump. The variation curve of injection pressure against injection time was recorded and the crack propagation path was monitored. The effect of pre- fabricated crack inclination angles, horizontal stress difference coefficients and injection rates on the initiation and propagation of hydraulic fractures was studied. The initiation and propagation mechanism of hydraulic fracture in test results were discussed by combining Hubbert-Willis elastic hydraulic fracturing model and hydraulic fracturing model of mixed crack type I-II. The test results show that a single fracture is formed,and the fracture morphology can be clas- sified into two types:propagating along the maximum horizontal stress direction or deflecting. The crack initiation pres- sure increases with the increase of prefabricated fracture angle or injection rates,and decreases with the increase of horizontal stress difference coefficient. At the same deflection angle,the crack propagation length decreases first and then increases with the increase of prefabricated crack inclination angle (attaining the smallest values at 45 degrees), decreases with the increase of horizontal stress difference coefficient,and increases with the increase of injection rate. Based on the experimental results and theoretical model analysis,it is concluded that the hydraulic fracturing model of mixed crack type I-II can well predict the initiation pressure and crack deflection law when the prefabricated fracture comes into play,and the Hubbert-Willis elastic hydraulic fracturing model can be used to compute the crack initiation pressure when the prefabricated fracture fails to control the crack propagation direction. The research results provide a reference for the parameter design of the technology of directional fracturing with prefabricated cracks in the field ap- plication.