Gas Seepage Behaviors Revealed by Two Kinds of Typical Soft and Hard Raw Coals Under High Pressure Water

To study the hydraulic fracturing effect of different types coal body, two typical raw samples namely primary structure and tectonic soft coals from the same coal mine were successfully prepared with the “twice moulding” method and their permeability’s were studied with self-designed “gas seepage experiment device loaded by high pressure water” before and after the high pressure water loaded. A big difference on the gas seepage change rule of these two kinds of coal samples was revealed. The study results showed that in the case of the primary structure coal, brittle deformation was found that the internal cracks extend and derive forming extended fracture network, by which the effective porosity was increased and the permeability was significantly improved, whereas, internally plastic deformation appeared in the tectonic soft coal that the coal was compacted itself by which the cracks cannot be fully penetrating and extending and the water blocks the initial fissure of intestine coal body, as a consequence, the flow of gas was weakened and the permeability descends before the high pressure water loaded. The experimental results gave a good explanation why the hydraulic fracturing measure is not suitable to all the coal seam, meanwhile, the research conclusion can provide reference for the optimization of the "gas pressure relief and permeability increase" to the coal body, and further improve the theory and method system of gas drainage in low permeability coal seam.