Abstract: A conventional well bore drilling system includes a coiled tubing mounted on a reel and connected to a mud supply for supplying mud through the tubing to a downhole end of the tubing to which is attached the downhole drilling tool. The system is modified by the addition of an inner tubing attached to the reel shaft and extending through the outer tubing to the drilling tool. The drilling mud is supplied in the space between the inner and outer tubes and a gas is supplied through the inner tube, The gas is vented through a coupling at the drilling tool into the well bore so as to enter the well bore and reduce the hydrostatic head of the fluids within the well bore to an under balanced condition below the pressure of the producing zone.