Design method for drillout bi-center bits

A method of designing a bi-center drill bit comprising a bit body, the bit body having a longitudinal axis, a first end adapted to be detachably secured to a drill string, a pilot section on a second, opposite end of the bit body and an eccentric reamer section intermediate the first and second ends, and a plurality of cutting elements on the pilot section, the method comprising,

• defining a first center of rotation of the pilot section about the longitudinal axis,
• defining a radius of rotation R1 of the drill bit about the first center of rotation,
• defining a second center of rotation of the pilot section spaced apart from the first center of rotation by a distance D,
• defining a radius of rotation R2 of the drill bit about the second center of rotation,
• setting the relationship between D, R1 and R2 such that R1 is less than the sum of R2 and D, and fixing a plurality of gauge cutting cutter elements upon the bit body at radius R1.

A method of designing a bi-center drill bit comprising a bit body with a first end adapted to be detachably secured to a drill string, a pilot section on a second, opposite end of the bit body, a reamer section intermediate the first and second ends, and a plurality of cutting elements on the pilot section, the method comprising

• defining a first center of rotation of the pilot section,
• defining a first cutting face surface on the pilot section generated by the plurality of cutting elements as they are rotated about the first center of rotation of the pilot section,
• defining a second center of rotation of the pilot section spaced apart from the first center of rotation by a distance D,
• defining a second cutting face surface on the pilot section generated by the plurality of cutting elements as they are rotated about the second center of rotation of the pilot section,
• defining a first region of the pilot section centered about the first center of rotation having a radius of D,
• defining a second region of the pilot section centered about the second center of rotation having a radius of D,
• and defining a third region of the pilot section formed by the intersection of the first region and the second region,
• wherein no cutting elements lying within the third region of the pilot section contact both the first cutting face surface and the second cutting face surface.