Mechanism of strata deformation under protective seam and its application for relieved methane control

In this paper, an equation correlated with normal stress and permeability was developed and FLAC3D software was used to investigate the rock mass stress evolution and distribution to understand the methane flow characteristics. Research results show that the rock mass under the protective coal seam can be divided into three belts (zones) in the vertical direction, including total de-stressed belt, vertical de-stressed belt and original stress belt. Methane in the total de-stressed belt can flow into the working face of protective coal seam by its own pressure gradient. Methane in the vertical de-stressed belt can only be extracted by boreholes. In the horizontal direction, the rock mass was also divided into five zones, including original zone, compression zone, expansion zone, recovering zone and re-compacted zone, which have been proved correct by the field experiments. The rock mass permeability in the de-stressed belts doesn't increase until stepping into the expansion zone, and from then on higher concentration of methane can be extracted. The methane in original regions is difficult to extract because rock stresses stay the same. The division of “three belts and five zones” reveals the rock mass stress and permeability distribution and evolution and supplies theoretical guidance for relieved methane control.

Research Highlights
► We studied the stress distribution of the rock mass below the protective coal seam.
► The underlying rock mass was divided into three belts and five zones.
► We analyzed the methane flow characteristics in different belts and zones.
► Methane in de-stressed belts can't freely flow until getting into the expansion zone.
► The findings provide a theoretical guidance for methane control in coalmines.