Liquid nitrogen cyclic freeze-thaw permeability-improvement gas drainage method based on horizontal directional borehole

摘要

#$%^&*AU2015383062A120170223.pdf#####ABSTRACT A liquid nitrogen cyclic freeze-thaw permeability-improvement gas drainage method based on horizontal directional boreholes comprises: first constructing a main borehole at an intake roadway or a return roadway, a low-level roadway and a high-level roadway, after 5 a drill bit reaches a pre-set target position of a coal bed, uniformly and directionally constructing a plurality of branch boreholes along a horizontal direction of the coal bed, injecting water to the coal bed, opening a valve, filling the main boreholes with liquid nitrogen, rapidly freezing the water injected into the branch boreholes and the periphery of the coal bed, and stopping injecting the nitrogen when an average temperature of a 10 pre-permeability-improvement area monitored by a temperature measuring hole is lowered to -2 'C or lower. When the coal mass is under the collective action of a water phase change frost heaving force, a liquid nitrogen gasification expansive force and microporous liquid flowing osmotic pressure, macro cracks and micro cracks are forced to be expanded and communicated to form a crack net, thereby improving the permeability of the coal bed. 15 After the liquid nitrogen is completely injected, the gas drainage is carried out. According to the variation of a gas drainage effect, water and liquid nitrogen can be injected into the boreholes for multiple times, thereby realizing a purpose of increasing the permeability of the coal bed surrounding the boreholes and rapidly and efficiently draining the gas. 16