COAL SEAM PERMEABILITY INCREASING METHOD UTILIZING CYCLIC DAMAGE OF LIQUID NITROGEN COLD IMPACT AND PHASE CHANGE GAS

摘要

Disclosed is a coal seam permeability increasing method utilizing cyclic damage of liquid nitrogen cold impact and phase change gas. According to the method, a through-layer borehole (8) running through a rock layer to a coal seam (1) is first drilled in a tunnel; a plurality of cracks perpendicular to the through-layer borehole (8) are cut from the coal seam (1) in the through-layer borehole (8); a liquid nitrogen injection pipe (6) and an exhaust pipe (9) are inserted into the through-layer borehole (8); the exhaust pipe (9) is connected to two gas pipelines respectively provided with a safety valve (5) and a ball valve (4); the ball valve (4) as a switch controls the discharge of nitrogen when liquid nitrogen is injected into the through-layer borehole (8); after the ball valve (4) is closed, a threshold of the safety valve (5) is set so that the safety valve (5) controls the pressure in the through-layer borehole (8); when air pressure in the through-layer borehole (8) exceeds the threshold, the safety valve is opened so that the internal nitrogen is discharged for pressure relief, and the safety valve (5) is automatically closed after pressure relief. The method can ensure rapid and continuous injection of liquid nitrogen into the through-layer borehole (8); moreover, the cold impact of liquid nitrogen, the expansion pressure of phase change gas, and the frost heaving pressure of moisture in the cracks are utilized to crack the coal seam, and the permeability increasing effect is effectively ensured.