The invention discloses a
coalbed methane horizontal well underbalance
drilling system and a method thereof. The
coalbed methane horizontal well underbalance
drilling system comprises a casing
pipe, adrill rod, a fluid director, a
top drive, degassing valves, a water-
gas mixing supply unit and a three-phase separator. According to stable underbalance drilling, the displacement and the pressure ofcompressed air are adjusted according to the displacement and the pressure of
drilling fluid, a certain proportion of
compressed air is injected into the
drill rod, and the
compressed air is separated from a
coal seam borehole far away from the position of a
drill bit through the set of degassing valves installed on the
drill rod and rapidly expands. Expanded air bubbles ascend and continuously expand, the expanded air bubbles can replace or
discharge drilling fluid with the same volume, and the density of the
compressed air bubbles is far smaller than that of the
drilling fluid, so that thedensity of a fluid in an annulus channel is reduced, and the pressure of an annulus
liquid column acting on the
coal seam borehole is reduced; and in this way, an underbalance or near-balanced drilling state that the pressure of the annulus
liquid column acting on the
coal seam borehole is smaller than or approximately equal to the coal seam gap pressure is formed.