The number of extended lateral reach and horizontal wells have been increasing due to resulting economic benefits. The steerable motor is accepted as the standard method of directional drilling but is not ideal for these sections. As a result, rotary steerable systems have been developed and benefit over motors due to continuous pipe rotation whilst deviating. This results in a smoother, cleaner hole and less drag, leading to improved transfer of weight and increased penetration rates.
This technological development has prompted the design of a range of PDC bits specifically for use on rotary steerable systems. Namely, that they possess sufficient lateral cutting ability to successfully perform in highly deviated sections at the reduced rotational speed, as compared to that of conventional motors. The designs must also be stable in rotation to minimise downhole vibrations that could damage the bit or cause premature tool failure.
Through progressive design and continuous field performance analysis, specific bit features have consistently displayed good steerability and very low levels of vibration, without compromising drilling performance.
Detailed analysis has been conducted using vibrational logs and data downloaded from a rotary steerable tool over a multitude of field runs. Overall bit performance was compared between similar designs with differing cutter placement.
Combined results demonstrate that a specific cutter placement methodology is essential for successful bit performance on a rotary steerable tool in terms of both stability and directional capabilities, and has resulted in extended bit life. Design considerations are laid out with regard to lateral cutting ability in terms of profile and cutter placement.
Case studies on field performance illustrate the design progress in producing a steerable, stable bit for use with a rotary steerable system.
Introduction
The increased use of rotary steerable tools has necessitated further consideration into the design of PDC drill bits in order to capitalise on the potential of improved drilling performance that these systems offer. In order to be successful the bit must possess three qualities:SteerabilityStabilityDurability
An existing 8½" diameter, steel bodied PDC bit design was modified and used in the initial runs with the tool with good success, particularly in terms of steerability. Further runs illustrated several weak points affecting both the durability and the stability of this design and thus modifications were made to the body and cutting structure. Through continued development and field monitoring, additional changes to the cutting structure have been made to provide a design that fulfils the requirements above. The design features developed can be transferred to produce a range of drill bits to match specific size and geological application.
The developments of the drill bits are illustrated in several case studies that clearly display the benefit of the modifications. Improved performance from the resultant cutting structure is also demonstrated quantitatively via calculations of anisotropic indices.
