Steerable drilling system and method

A method of drilling a deviated borehole utilizing a bottom hole assembly including a rotary shaft having a lower central axis offset at a selected bend angle from an upper central axis by a bend, the bottom hole assembly further including a bit rotated by the rotary shaft and having a bit face defining a bit diameter, the method comprising:

• (a) providing a housing having a substantially uniform diameter outer surface surrounding the rotary shaft upper axis, the substantially uniform of diameter outer surface extending from above a bend defining the bend angle to a lowermost end of the housing and without a stabilizer thereon;
• (b) providing a gauge section rotatable fixed to the bit and having a substantially uniform diameter cylindrical surface spaced above the bit face which is substantially equal to the bit diameter, the gauge section having a total gauge length of at least 75% of the bit diameter, and a portion of the total gauge length which has the substantially uniform diameter cylindrical surface is at least 50% of the total gauge length;
• and
• (c) rotating the bit at a speed of less than 350 rpm to form a curved section of the deviated borehole.

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

• a rotary shaft having a lower central axis offset at a selected bend angle from an upper central axis by a bend;
• a housing having a substantially uniform diameter housing outer surface, the housing containing at least a portion of the upper axis of the rotary shaft;
• a bit powered by the rotating shaft, the bit having a bit face defining a bit diameter;
• a piggyback stabilizer positioned above the bit and having a stabilizer gauge section;
• the stabilizer gauge section having a substantially uniform diameter cylindrical surface spaced above the bit face, the bit and stabilizer gauge section together having a total gauge length of at least 75% of the bit diameter and a portion of the total gauge length which has the substantially uniform diameter cylindrical surface is at least 50% of the total gauge length;
• and
• one or more sensors spaced substantially along the stabilizer gauge section for sensing selected parameters while drilling.

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

• a shaft rotated from the surface and having a lower central axis offset at a selected bend angle from an upper central axis by a bend;
• a housing containing at least a portion of the upper central axis of the rotary shaft;
• a gauge section powered by the rotating shaft, a bit having a bit face defining a bit diameter;
• the gauge section rotatably fixed to the bit and having a substantially uniform diameter cylindrical surface spaced above the bit face which is substantially equal to the bit diameter, the gauge section having a total gauge length of at least 75% of the bit diameter and a portion of the total gauge length which has the substantially uniform diameter cylindrical surface is at least 50% of the total gauge length;
• the shaft having a pin connection at its lowermost end;
• and
• the gauge section having a box connection at its upper end for mating interconnection with the pin connection.

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

• a positive displacement motor driven by pumping fluid through a motor stator to rotate a motor rotor, the motor including a rotary motor output shaft having a lower central axis offset at a selected bend angle from a motor rotor central axis by a bend;
• a rotary bit powered by the rotary motor output shaft, the bit having a bit face defining a bit diameter;
• a gauge section rotatably fixed to the rotary bit, the gauge section having a substantially uniform diameter cylindrical surface spaced above the bit face which is substantially equal to the bit diameter, the gauge section having a gauge length of at least 75% of the bit diameter;
• and
• an axial spacing between the bend and the bit face is less than twelve times the bit diameter.

A method of drilling a deviated borehole utilizing a bottom hole assembly including a positive displacement motor driven by pumping fluid through a motor stator to rotate a motor rotor, the motor including a rotary motor output shaft having a lower central axis offset at a selected bend angle from a motor rotor central axis by a bend, the bottom hole assembly further including a bit rotated by the rotary motor output shaft and having a bit face defining a bit diameter, the method comprising:

• (a) providing a gauge section rotatably affixed to the bit, the gauge section having a substantially uniform diameter cylindrical surface spaced above the bit face which is substantially equal to the bit diameter, the gauge section having a gauge length of at least 75% of the bit diameter;
• (b) axially spacing the bend from the bit face less than twelve times the bit diameter;
• and
• (c) powering the positive displacement motor to drill the borehole.

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

• a housing for a rotary output shaft having a lower central axis offset at a selected bend angle from a rotor central axis by a bend;
• a rotary bit powered by the rotary output shaft, the bit having a bit face defining a bit diameter;
• a gauge section rotatably fixed to the rotary bit, the gauge section having a substantially uniform diameter cylindrical surface spaced above the bit face which is substantially equal to the bit diameter, the gauge section having a gauge length of at least 75% of the bit diameter;
• an axial spacing between the bend and the bit face is less than twelve times the bit diameter;
• the rotary output shaft having a pin connection at its lowermost end;
• and
• the gauge section having a box connection at its upper end for mating engagement with the pin connection.

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

• a rotary shaft having a lower central axis offset at a selected bend angle from an upper central axis by a bend;
• a housing extending from above the bend to below the bend and enclosing at least a portion of the rotary shaft;
• a rotary bit powered by the rotary shaft, the bit having a bit face defining a bit diameter;
• a gauge section rotatably fixed to the rotary bit, the gauge section having a substantially uniform diameter cylindrical surface spaced above the bit face which is substantially equal to the bit diameter, the gauge section having a gauge length of at least 75% of the bit diameter;
• and
• an axial spacing between the bend and the bit face is less than twelve times the bit diameter.

A method of drilling a deviated borehole utilizing a bottom hole assembly including a rotary shaft having a lower central axis offset at a selected bend angle from an upper central axis by a bend, the bottom hole assembly further including a bit rotated by the rotary shaft and having a bit face defining a bit diameter, the method comprising:

• providing a housing for the rotary shaft;
• providing an axial spacing between the bend and the bit face of the less than twelve times the bit diameter;
• providing a gauge section rotatable fixed to the bit and having a substantially uniform diameter cylindrical surface spaced above the bit face which is substantially equal to the bit diameter, the gauge section having a total gauge length of at least 75% of the bit diameter;
• and
• rotating the bit at a speed of less than 350 rpm to form a curved section of the deviated borehole.