Styrbart boresystem og fremgangsmåde

A bottom hole assembly (10) for drilling a deviated borehole includes a positive displacement motor (PDM) (12) or a rotary steerable device (RSD) (110) having a substantially uniform diameter motor housing outer surface without stabilisers extending radially therefrom. In a PDM Application, the motor housing (14) may have a fixed bend therein between an upper power section (16) and a lower bearing section (18). The long gauge bi9t (20) powered by the motor (10) may have a bit face (22) with cutters (28) thereon and a gauge section (24) having a uniform diameter cylindrical surface (26). The gauge section (24) preferably has an axial length at least 75% of the bit diameter. The axial spacing between the bit face and the bend of the motor housing preferably is less than twelve times the bit diameter. According to the method of the present invention, the bit may be rotated at a speed of less than 350 rpm by the PDM and/or rotation of the RSD from the surface.