关键词: 真三轴试验, 水力压裂, 网状裂缝, 层理, 裂缝扩展

Abstract: In this study, hydraulic fracturing experiments were carried out on the large raw coal to investigate the influence of bedding planes on fracture propagation by using a true triaxial hydraulic fracturing experiment system. The extension rule of hydraulic fractures in coal was analysed to reveal the formation mechanism of net-cracking based on the spatial distribution. The results show that the hydraulic fracture tends to bifurcate and swerve at the weak bedding. Then the developed bedding and fracture system provide the premise conditions to establish a fracture network. From the pump pressure curve, its frequent fluctuation indicates a remarkable feature of the net-cracking in coal rock. There are four basic modes on the initiation and extension of hydraulic fractures, and the fracture network is the combination of the four modes. In-situ stress difference coefficient and injection displacement both have great influences on the hydraulic fracture geometry. The small in-situ stress difference coefficient is more likely to form the fracture network. However, a relatively simple fracture geometry is easily formed when the fracturing fluid displacement becomes high, which conversely results in a poor fracturing effect. The experimental methods and corresponding results can provide reference and basis for the design and optimisation of hydraulic fracturing parameters.

Key words: true triaxial test, hydraulic fracturing, net-cracking, bedding, fracture propagation