Novel Large-Scale Three-Dimensional Fracturing Experimental Apparatus and Application to Vertical-Well Hydraulic Fracturing of Hard Roof

Effective control of high-position hard roofs (HHRs) is important to meet the requirements of safe and efficient mining. Vertical-well hydraulic fracturing (VWHF) is an innovative control method developed for releasing strong ground pressure behavior induced by HHRs. Understanding the hydraulic fracture extension law and its spatial form is critical for VWHF applications on HHRs. To address this problem, we have developed a large-scale three-dimensional fracturing simulation apparatus. The effective sample size is 2,060 by 1,200 by 1,200 mm. The primary loading subsystem maintains full closed-loop control of stress and displacement and can perform one-way, two-way, three-way, and step-wise independent loading and unloading with a maximum three-dimensional loading capacity of 10 MPa. The apparatus can achieve gas sealing of 3 MPa. The time-space distribution law of multiple parameters in the apparatus can be monitored using 240 simultaneous test channels, and experimental phenomena can be observed in real time using the visual monitoring system. The fracturing subsystem can achieve flow infinite transformation between 0 to 107 mL/min and water pressure up to 51.7 MPa. The apparatus simulates hydraulic (supercritical carbon dioxide or liquid nitrogen) fracturing of coal-rock mass under complex stress and a gas-containing environment. To the best of our knowledge, these are the first large-scale three-dimensional fracturing simulation experiments performed on hard roof sandstone samples. We analyze the distribution and change of acoustic emission signal and stress before and after fracturing and determine the hydraulic fracture propagation law, which reflects characteristics of hydraulic fracturing hard roof processes in a large-scale fracture network. These experiments validate the capability and reliability of the newly developed apparatus. The results provide strong technical support for VWHF to weaken HHRs.