Characterization of the permeability and acoustic properties of an outburst-prone sandstone

Two underground coal mines in the Sydney Coalfield, Nova Scotia, Canada have encountered gas outbursts from sandstone formations overlying the coal seams. These have consistently occurred while driving mine roadways into virgin ground at and below mining depths of 700 m. In this investigation, triaxial compression tests were conducted on samples of the outburst-prone sandstone from one of these mines, the Phalen Colliery, while simultaneously measuring gas permeability, acoustic emissions and ultrasonic P-wave velocity and attenuation. Experimental results characterized these sandstone properties over the full range of axial stresses up to compressive failure. These data were intended to assist with (i) evaluating the potential for degassing the sandstone and (ii) evaluating the results of in-seam seismic surveys, which were conducted to map the sandstone ahead of mining. At the estimated in situ conditions in virgin rock at 700 m depth, the sandstone permeability is expected to be in the 0.005 to 0.04 mD range, the P-wave velocity is approximately 4000 m/s and the attenuation quality factor in the 20 to 25 range. Near mine openings at 700 m depth, where the lateral confining pressures are reduced, the permeability can be several orders of magnitude higher in the 4 to 15 mD range, the P-wave velocity is 3800 m/s and the attenuation quality factor is in the 10 to 15 range. Experimental data suggests that microcracking in the sandstone prior to compressive failure does not significantly enhance permeability but there may be local pockets of higher permeability within the sandstone.