Steerable drilling system and method

A bottom hole assembly for drilling a deviated borehole, the bottom hole assembly comprising:

• a positive displacement motor driven by pumping downhole fluid through a motor stator to rotate an axially curved motor rotor, the motor having an upper power section, a bend section having a bend therein, and a lower bearing section, a lower bearing section central axis offset at a selected bend angle from the power section central axis by the bend in the bend section;
• a long gauge bit powered by the positive displacement motor, the long gauge bit having a bit face defining a bit diameter and a gauge section having a uniform diameter cylindrical surface spaced above the bit face, the gauge section having an axial length of at least 75% of the bit diameter;
• and
• an axial spacing along the lower bearing section central axis between the bend and the bit face being less than ten times the bit diameter.

A method of drilling a deviated borehole utilizing a bottom hole assembly including a positive displacement motor having an upper power section with a power section central axis and a lower bearing section with a lower bearing section central axis offset at an selected bend angle from the power section central axis by a bend in a motor housing, the bottom hole assembly further including a bit rotated by the motor and having a bit face defining a bit diameter, the bit being rotated by the motor relative to the motor housing, the method comprising:

• (a) providing the motor housing having a substantially uniform diameter outer surface extending axially from an uppermost end of the upper power section to a lowermost end of the lower bearing section;
• (b) providing a gauge section on the bit, the gauge section having a uniform diameter cylindrical surface thereon along an axial length of at least 75% of the bit diameter;
• and
• (c) pumping fluid through the downhole motor to rotate the bit at a speed of less than 350 rpm while the non-rotating motor housing slidably engages a wall of the borehole to form a cured section of the deviated borehole.

A method of drilling a deviated borehole utilizing a bottom hole assembly including a positive displacement motor having an upper power section with a power section central axis and a lower bearing section with a lower bearing section central axis offset at an selected bend angle from the power section central axis by a bend in a motor housing, the bottom hole assembly further including a bit rotated by the motor and having a bit face defining a bit diameter, the method comprising:

• (a) providing a motor housing having a substantially uniform diameter outer surface extending axially from an uppermost end of the upper power section to a lowermost end of the lower bearing section;
• (b) providing a gauge section on the bit, the gauge section having a uniform diameter cylindrical surface thereon along an axial length of at least 75% of the bit diameter;
• (c) axially spacing the bend from the bit face less than ten times the bit diameter;
• and
• (d) pumping fluid through the downhole motor to rotate the bit at a speed of less than 350 rpm while the non-rotating motor housing slidably engages a wall of the borehole to form a curved section of the deviated borehole.