Oil wells (wellbores) are usually drilled with a drill string. The drill string includes a tubular member having a drilling assembly that includes a single drill bit at its bottom end. However, sometimes the drill string includes two spaced-apart drill bits: the first at the bottom of the drilling assembly (referred to as the “pilot drill bit” or “pilot bit”) to drill the wellbore of a first smaller wellbore diameter; and the second drill bit located above, or uphole of, the pilot bit (referred to as the “reamer bit” or “reamer”) to enlarge the wellbore drilled by the pilot bit.
Pilot bits typically include several regions, such as a nose, cone, lower shoulder or lower region and an upper shoulder or upper region, each region having thereon cutting elements (also referred to as “cutters”) that cut into the formation to drill the wellbore of the first smaller diameter. The reamer bit typically includes a lower shoulder or lower region and an upper shoulder or upper region, each such region having a number of cutting elements, which cut into the formation to enlarge the wellbore of the first smaller wellbore. The orientation of a front cutting face of a cutting element may be characterized by a back rake angle and side rake angle, which, in combination with the profile angle of the cutting element, define an effective back rake (or aggressiveness) of the cutting element. The load on a region of a bit during drilling of the wellbore depends upon the effective back rake of the cutting elements in that region. Uneven load distribution between the reamer and the pilot bit often causes problems, especially when the pilot bit is in a soft formation while the reamer bit is in a relatively hard formation. Under such drilling conditions, the reamer bit lower region is typically under a greater load compared to the load on the pilot bit, which can damage the reamer bit or wear it out quickly, while the pilot bit is still in an acceptable condition. The reason generally is that the effective back rake of the lower region of commonly used reamer bits is relatively low (i.e., the aggressiveness is relatively high).
Therefore, there is a need for an improved reamer bit which may be used to selectively distribute (e.g., even) the load between the reamer bit and an associated pilot bit for use in drilling wellbores.