Abstract: Gas drainage from low permeability coal seams has been confronted with many problems, such as low gas drainage efficiency, great drainage difficulty, and heavy workload of borehole construction. This paper was aimed at using hydraulic fracturing to increase coal seam permeability and enhance gas drainage. A damage-stress-seepage coupling model was built based on multiphysics and elastic damage theory, and numerical simulations of hydraulic fracturing and gas drainage in the 15108 panel of Mabao mine were carried out. Changes in the parameters during hydraulic fracturing and gas extraction including elastic modulus, permeability and gas pressure were analyzed. The key parameters were determined and applied in engineering tests. The results show that fracturing water with high energy will rupture the coal seam and cause a steep increase of permeability in the fractured zone which can speed up the migration of gas from coal seam to boreholes, that with the increase of hydraulic fracturing time, the damage zone will be gradually enlarged, that when hydraulic fracturing 10 000 s, the damage zone between two fracture holes will be connected at the middle drainage hole with a fracture influence range of 14 m, that the average and maximum gas drainage fluxes in hydraulic fractured zone are 63.79 m³/d and 126.57 m³/d, 9.65 times and 7.23 times as much as that in non fractured zone respectively, and that the permeability of coal seam is increased by 67 times.

Key words: low permeability coal seam, hydraulic fracturing, gas drainage, increasing penetration and enhancing drainage, damage-stress-seepage coupling model, numerical simulation