Three-dimensional collaborative hydraulic fracturing control for thick hard roofs: mechanism, simulation, and field application in Yadian Coal Mine

Thick hard roof with high strength, large breaking step and strong mine pressure is one of the major problems in coal mine safety production. Aiming at the problems of large thickness, high strength and difficult collapse of the roof slab of Yadian Coal Mine in Binchang Mining Area, we integrated theoretical analysis, UDEC numerical modeling, and engineering validation to mechanistically analyze the overlying rock structure. We proposed a regionalized control method using high-low three-dimensional synergistic directional hydraulic fracturing, simulated its effect via UDEC, and conducted large-scale industrial tests at the ZF1407 working face (strike length 1602 m; burial depth 634–794 m). Seven directional boreholes achieved 3381 m3 fracturing volume with 141 segments. The average spacing of the fracturing holes was 24 m, and the total number of fracturing segments was 141, which realised the coverage of the entire working face by the fracturing and reforming area. The applicability and effectiveness of the directional hydraulic fracturing technology for thick hard roofs were verified by monitoring means such as microseismic, transient electromagnetic and ground sound monitoring. On-site monitoring results show that: through the implementation of hydraulic fracturing measures on the thick hard roof of the working face, the thick hard roof rock layer has been effectively weakened; the number of microseismic events has been significantly reduced, and the number of microseismic events in the middle and late stages of fracturing has been reduced by 77.0% and 78.5% respectively compared with the number of events in the pre-fracturing stage, and the microseismic activities before and after the fracturing have been developed in the direction of low-energy, low-frequency from high-energy, high-frequency; and the distribution of the electrical resistivity contours in the thick hard rock layer has been changed from a smooth layer distribution to an irregular distribution. The distribution of resistivity contours of thick hard rock formations changed from the smooth layer distribution before fracturing to irregular distribution, and the resistivity of rock formations increased by about 50%, and the local resistivity was as high as 90 due to the large number of cracks generated in the rock formations after fracturing. The ground sound energy in the fracturing process can better reflect the energy release of rock formation rupture and crack extension through the high-pressure water. The results of the study can provide a reference for solving the disaster control of mine pressure caused by the structural instability of multi-layer thick and hard rock formations in the roof of similar longwall workings.