GAS EXTRACTION METHOD BASED ON LIQUID NITROGEN CYCLIC FREEZE-THAW ANTI-REFLECTION BY DRILL HOLES IN HORIZONTAL ORIENTATION

摘要

A horizontal directional drilling liquid nitrogen freeze-thaw cycle permeability improvement method for gas extraction, comprising the following steps: firstly, constructing a main drill hole (3) in an intake airway or a return airway (6), a low position roadway (11) and a high position roadway (12), when a drill bit reaches a predetermined target position in a coal seam (7), a plurality of branch drill holes (1) are arranged uniformly and constructed directionally along the horizontal direction of the coal seam (7); water injection is carried out in the coal seam (7), and then a valve is opened to inject liquid nitrogen into the main drill hole (3); water injected into branch drill holes (1) in the coal seam and surroundings thereof is cooled quickly; the injection of liquid nitrogen is stopped when the average temperature of an area pending permeability improvement drops below -2 °C is monitored through a temperature measuring hole (9). Under combined effect of frozen-heave force of water, gasification bulging force of liquid nitrogen, and osmotic pressure of micro-pore liquid flow, macroscopic and microscopic cracks expand and connect to form a crack network in the coal body and improve the air permeability of coal seam. Gas extraction is executed upon completion of liquid nitrogen injection. Water injections and liquid nitrogen injections can be repeated for a plurality of times for the drill holes, depending on the gas extraction results, so as to achieve improved air permeability of the coal seam surrounding the drill holes, and realize gas extraction.