Abstract
Triaxial hydraulic fracturing experiments were used to study the initiation pressure variation and acoustic emission characteristics of different guide seams sizes during roof hydraulic fracturing. Numerical simulations were used to explore the feasibility of multiple boreholes with prefabricated guide seams. An experiment of hydraulic fracturing on a pillar-free working face was also carried out in a coal mine. The results show that the specimens with guide seams reduced the initiation pressure, with the number of acoustic emission events and initiation pressure being inversely proportional to the size of the guide seams. Specimens without guide seams were deflected by stress and produced multi-level cracks, while the specimens with guide seams did not produce large secondary cracks and deflection. When the stress difference was small, three holes penetrated but not under large stress differences. The hydraulic fracturing technology of prefabricated longitudinal guide seams was tested in the Ningtiaota Coal Mine, and the auxiliary transportation roadway of S1201 working face was successfully retained for reuse in adjacent working faces.
Highlights
In recent years, the number of roof accidents and single fatalities dominated by rockbursts in China coal mines has been on the rise
It can be seen from the figure that the longest fracturing duration of the sandstone sample without guide seams is about 67 s, and the shortest fracturing duration of the sandstone sample with guide seam size of 100 mm  20 mm is about 52 s
A larger guide seam size corresponds to a lower initiation pressure
Summary
The number of roof accidents and single fatalities dominated by rockbursts in China coal mines has been on the rise. The directional energy-accumulating blasting technology, the key layer determination method, and the roadway surrounding rock stress-displacement transmission mechanism have been studied (Hu et al, 2019; Liu et al, 2019; Xue et al, 2019; Yang et al, 2019a, 2019b, 2020, 2021; Zhang et al, 2020). There are hydraulic fracturing methods that use multiple boreholes to achieve directional hydraulic fracturing; for example, dense boreholes have been used to change the stress state and form free surfaces to achieve through fractures for multiple boreholes (Duan et al, 2021; Liu et al, 2019; Song et al, 2019). There are transverse guide seam fracturing methods (Deng et al, 2016; Lin et al, 2016), longitudinal guide seam fracturing methods (Cheng et al, 2018, 2020; Ge et al, 2019; Zhong et al, 2021), guide seams, and directional hole fracturing methods (Lu and He, 2020)
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