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. As the gas is supplied through the inner tube, it is maintained separate from the liquid outside the inner tube and accordingly can be by passed into the well bore without entering the downhole MWD or motor drive systems and in addition the mud pulses generated by the downhole MWD system can be communicated through the drilling mud in conventional manner without losses due to gas within the liquid.