True tri-axial directional hydraulic fracturing test on sandstone with transverse grooves in coal mine

To reveal the initiation and propagation law of directional hydraulic fractures of main roof fine sandstone, the true tri-axial directional hydraulic fracturing and high-energy CT scanning tests were carried out on the 300 mm×300 mm×300 mm large-scale sandstone specimens obtained in the underground mine, a 26 mm diameter fracturing hole is drilled in the center of each sample, and a special grooved drill bit is used to prefabricate a 12 mm three-dimensional wedge-shaped transverse groove in the axial direction. The effects of bedding direction and horizontal stress differences on the hydraulic crack initiation pressure, extension pattern, hydraulic pressure-time curve and fracturing volume are studied, and the concept of directional deflection distance, over which the crack propagates in the direction of the groove without deflection, is introduced to characterize the effect of directional fracturing. The test results show that the premade transverse grooves can drive the crack to initiate and extend along the groove. The crack patterns of specimens with transverse grooves are divided into two types: "一" and "H". The hydraulic pressure-time curve can be divided into gentle wave type and cliff drop type according to the crack propagation stage. The horizontal stress difference affects the directional deflection distance more than the bedding direction. In the case of high horizontal stress difference, the stress concentration at the tip of the groove is high, which promotes the ability of cutting through the bedding planes. After the crack initiation at the tip of the groove, the crack does not deflect when intersecting with the bedding planes, and extends to the boundary of the specimen. In the case of low horizontal stress difference, the crack is prone to deflect when it comes across the bedding planes, resulting in the poor directional propagation performance of the groove. The average directional deflection distance for the high horizontal stress difference is 10 times of that for the low horizontal stress difference. The former can extend the directional crack to the specimen boundary, and the latter can only control the local crack propagation, then the crack will gradually deflect to the direction of the largest principal stress. The average initiation pressure and fracturing volume for specimens with bedding planes parallel to the grooves is 1.7 times of that perpendicular to the grooves, and the average crack initiation pressure and fracturing volume for specimens with high horizontal stress difference is 1.3 times of that with low horizontal stress difference. The bedding effect is obvious at the low horizontal stress difference, especially when the groove is parallel to the bedding plane, and the bedding planes near the groove are most likely activated and extended along the groove direction. Otherwise, it is hard to be activated. However, the bedding planes can be activated under horizontal stress difference, which complex cracks net are formed. © 2020, Editorial Office of Journal of China Coal Society. All right reserved.