Steerable drilling system and method

摘要

A bottom hole assembly 10 for drilling a deviated borehole includes a positive displacement motor 12 having a substantially uniform diameter motor housing outer surface without stabilizers extending radially therefrom. The motor housing 14 has a fixed bend therein between an upper power section 16 and a lower bearing section 18. The long gauge bit 20 powered by the motor 10 has a bit face 22 with cutters 28 thereon and a gauge section 24 having a uniform diameter cylindrical surface 26. The gauge section 24 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 is less than ten times the bit diameter. According to the method of the present invention, fluid is pumped through the downhole motor to rotate the bit at a speed of less than 350 rpm. A substantial portion of the curved borehole section may be drilled while sliding rather than rotating the motor housing.