Patent ID: 7287609

Claim:
A method of drilling a borehole into a subsurface earth formation using a tubular drill string which includes at its lower end a bottom hole assembly comprising a drill bit, a drill steering system, and a surveying system, wherein the drill steering system is provided in the form of a mud motor comprising a tubular stator and a rotor arranged in the tubular stator, and a bit shaft which is arranged to be driven by the rotor and suitable for transmittal of torque to a drill bit, wherein the rotor is releasably connected to the bit shaft so that the rotor can be longitudinally removed from the stator after disconnection from the bit shaft, wherein the drill string includes a passageway for an auxiliary tool from a first position interior of the drill string above the bottom hole assembly to a second position wherein at least part of the auxiliary tool is exterior of the drill string below the bottom hole assembly, which passageway can be selectively closed, part of the passageway being formed by the interior of the stator, and which method comprises the steps of: drilling so as to progress the drill string into the earth formation, until a tool operating condition is met; opening the passageway; passing an auxiliary tool from the first position through the passageway to the second position, and operating the auxiliary tool at the second position.