Tommy M. Warren

Bit Whirl: A New Theory of PDC Bit Failure

Results of a study showing that the most harmful PDC bit vibrations can be attributed to a phenomenon called bit whirl. Bit whirl causes cutters to move sideways, backwards, and much faster than those on a true rotating bit, with the result that holes become overgauge. the impact loads associated with the motion cause PDC cutters to chip, which accelerates wear.Laboratory and fields results show the detrimental effects of whirl on PDC bit rate of penetration (ROP) and life. Factors affecting bit whirl are discussed.

Simultaneous drilling and reaming with fixed blade reamers

Deep well drilling programs often require the use of multiple casings or liners. Designing these wells require compromises between using the smallest surface hole possible to minimize tubular costs and maintaining the largest hole possible to allow for contingencies and for the optimum completion design. The conflict between these two considerations can be resolved somewhat by nesting the casing strings with the minimum clearance between them. Unfortunately this often requires that the section drilled below a casing string be reamed to a larger diameter than the casing ID. The reaming process is often fraught with many inefficiencies. This paper describes a technique for simultaneously drilling and reaming with a bi-center reamer run on a stabilized assembly above a pilot bit. The successful use of the technique in three field wells is reported.

Development of a whirl-resistant bit

Bit whirl is a major cause of early failure and reduced performance of polycrystalline-diamond-compact (PDC) bits. Attempts to control bit whirl by stabilizing the drill string have been unsuccessful, but a low-friction bit design has been discovered that substantially eliminates whirl. The low-friction design is based on placing the cutters so that the net imbalance force from the cutters is directed toward a smooth pad that slides along the wellbore wall.

Field Application of Diamond-bit Hydraulic-Lift Principles (includes associated papers 17470 and 17553 )

Hydraulic lift can have a major effect on the operation of diamond drill bits. The actual weight on bit (WOB) is less than the measured weight by the amount of hydraulic lift, or pumpoff force, acting beneath the bit. Pumpoff forces in excess of 15,000 lbf (67 kN) have been recorded. Historically, the hydraulic-lift effect has been recognized but not understood well enough to be accounted for reliably. Failure to account for hydraulic lift can adversely affect actual bit performance and interpretation of the results. It is now possible to determine hydraulic lift reliably with the practical techniques presented here. Field tests show that pumpoff force tends to double as the bit dulls. Appropriate WOB adjustments, determined through pumpoff tests run on the rig, are required to maintain bit performance. The applications to diamond bits run on mud motors are particularly advantageous.

Diamond-Bit Hydraulics Model

This paper describes an analytical technique that calculates the fluid velocities and pressures under a surface-set diamond bit. The technique is based on solving the continuity and energy equations for a series of flow cells that covers the bit face. Each cell is described with geometrical dimensions and an integrated property model to account for flow around the cutters. Experimental data from simulated diamond-bit segments provide friction-factor data and model validation. The results of analyzing a commercial diamond bit are also presented. The model is being developed to improve the hydraulic design of diamond bits.