Research and application of pressure relief technology for fracturing thick hard roof in super-high mining face

Aiming at the problems of super-large mining space, strong ore pressure and ultra-high coal wall stability control faced by full-mechanized mining face with ultra-large mining height and thick hard roof, this paper takes the 122104 working face of Caojiatan Coal Mine, the world’s first ultra-10 m super-mechanized mining face, as the research background. Systematic research on pressure relief mechanism of heavy hard roof fracturing, true triaxial hydraulic fracturing fracture reconstruction experiment, three-dimensional fracture propagation fluid-structure coupling numerical simulation, high-flow composite fracturing technology and process, hydraulic fracturing effect monitoring and evaluation are carried out. The results show that the pressure relief mechanism of hydraulic fracturing of thick hard roof is mainly embodied in structural regulation, stress transfer, energy release, strength deterioration and so on. Compared with Zhiluo formation, Yan’an Formation has a higher content of brittle minerals and is easy to form complex fracture network. The average fracture radius increases from 21.9 m to 32.5 m with the displacement increasing from 1 m3/min to 5 m3/min, an increase of about 48.40%. The larger the displacement, the larger the hydraulic fracture scale. The hydraulic fracturing pressure relief technology and high-flow fracturing equipment for thick hard roof have been developed. The composite fracturing technology of abrasive jet pre-cut slit and 5 m3/min large flow rate was developed, and a “one game, one policy” zoning design concept of directional hydraulic fracturing for long horizontal drilling was proposed. A set of directional hydraulic fracturing methods for long horizontal drilling with dynamic update of fracturing zones and fracturing plans was formed, and engineering application tests were carried out in Caojiatan Coal Mine. It has realized scientific and effective prevention and control of ore pressure in 10 m super-high working face. The surface microseismic real-time monitoring technology captures the spread track of the fracture network in real time, effectively guides the adjustment of hydraulic fracturing scheme, and obtains the spatial distribution characteristics of roof hydraulic fractures. The multi-factor comprehensive evaluation of hydraulic fracturing effect is carried out, and the working resistance of hydraulic support, the pressing step distance and continuous distance of working face cycle, the dynamic load coefficient, microseismic events and energy are comprehensively analyzed to evaluate the hydraulic fracturing effect. The underground test results show that the underground hydraulic cracks in Caojiatan Coal mine are mainly horizontal cracks, and the average spreading distance along both sides of the borehole is about 80 m, which effectively weakens the overlying thick and complete roof within the working face, and realizes the effective control of regional roof structure. By using zone fracturing technology, the thick hard rock group is pre-fractured in advance, reducing the pressure step and dynamic load coefficient, and ensuring the safe and efficient production of 10 m super-high mining face.