A novel active prevention technology for borehole instability under the influence of mining activities

Analysis of the shear failure of surface methane capture boreholes for improving the drainage period efficiency: A lithological perspective

“…Coal measure strata are usually composed of numerous bedded sedimentary formations of varying thickness, strength and stiffness. Therefore, apart from the topographical condition, the strata sequences, the Lithology-controlled performance and the presence of weak interfaces between alternating soft and stiff layers (Ghazvinian et al., 2010; Liang et al., 2017b; Liang et al., 2014; Lu et al., 2016; Majdi et al., 2012; Palchik, 2003; Whittles et al., 2006; Xu et al., 2011b), which have not aroused adequate attention, also significantly affect the deformation and movement of the overburden and the resulting failure and performance of SMCBs, which traverse the overlying strata (Karacan, 2009; Liang et al., 2017b; Liu et al., 2017b; Xu et al., 2011a; Zhai et al., 2015).…”

The impact of time-dependent deformation on coal permeability and gas drainage efficiency​​​​​​​

“…Pressure vs. distance at approximately t=27.8 hours for: a) σx=5 MPa and σz=7 MPa, b) σx=5 MPa and σz=9 MPa, c) σx=7 MPa and σz=5 MPa, and d) σx=9 MPa and σz=5 MPa .............. 111 Figure 6.1. Stress distribution in front of the mining face (modified after Zhai et al, 2015) ......... 114 Figure 6.2. Roadway and the coal around the boreholes (Zhai et al, 2015) ................................... 115 Figure 6.3.…”

“…In-situ stress zone (initial stress zone). (Zhai et al, 2015) As aforementioned, in some researches, it is assumed that the original stress is equal in all directions and hence hydrostatic (Li, Li and Yifeng, 2015;; and the time-dependent deformation does not occur around drainage boreholes for the sake of model simplicity (Zhai et al, 2015). The corresponding failure and deformation zones to the redistributed stresses around drainage boreholes under hydrostatic and deviatoric in-situ stresses are shown in Figure 6.3.…”

“…Their study does not reveal the impact of post-peak and time-dependent deformation on permeability during continuous pore pressure depletion and the effects of matrix shrinkage on permeability evolution are neglected. Xue et al, 2017) In some studies, the stress profile and permeability evolution around drainage boreholes has been considered analogous to that around roadways in a smaller scale (Zhai et al, 2015), where the drainage borehole wall is analogous to the mining face in underground coal mine. Like the mining face, the zones around the borehole wall present a similar stress profile and permeability distribution (Wang, Elsworth and Liu, 2013).…”

“…However, after borehole drilling and disturbance to coal reservoir, the deformation process may accelerate due to the increase of effective stress during the extraction of fluids in the reservoir (Schatz and Carroll, 1981). Instability of gas drainage boreholes due to repeated disturbances exerted by external stress, such as mining activities (Zhai et al, 2015) and roadway and branch borehole excavations (Hu et al, 2015) has been widely studied. In such a case, the collapse of borehole walls is likely to occur, which results in blockage of gas-flow channels and therefore a hindrance to CSG drainage.…”