The Design and Testing of Anti-Whirl Bits

Abstract

SPE Members Abstract Laboratory drilling tests have been used successfully to determine the tendency of a PDC drag bit to whirl. Drill bit whirl has been demonstrated in all four drilling simulators where whirl tests were performed. This paper describes the design and testing of anti-whirl PDC drag bits. The verification process of the anti-whirl concept, which includes both laboratory and field tests, will be emphasized. Results from the laboratory drilling experiments that measure a bit's propensity to vibrate are presented. Additionally, results from more than 60 field runs are summarized and discussed, with three successful field runs discussed in detail. Introduction Backwards whirling motion has been shown to be detrimental to the life of polycrystalline diamond compact (PDC) drag bits. The emergence of technology to suppress this whirling tendency has resulted in the development of anti-whirl drilling bits. Anti-whirl technology is well-documented and consists of positioning the individual cutting elements on the bit so as to create a net imbalance force. This imbalance force pushes the bit against the side of the borehole, which in turn creates a stable rotating condition that resists backwards whirling. The following capabilities must be considered when fabricating drill bits according to anti-whirl technology:*Drilling model capability*Experimental verification of predicted bit stability*Manufacturing process control and verification of accurate final cutter placement; and*A field test and dull bit evaluation program Several authors have discussed drilling models in detail, so for this discussion, these will be addressed only briefly. The emphasis of this discussion will be on the laboratory and field verification of the anti-whirl bit technology. Furthermore, manufacturing process control, which addresses in part the accuracy required for cutter placement to achieve the desired imbalance force and the imbalance force direction, will not be discussed in this paper. Design Rules for Anti-Whirl Bits The inventors of the anti-whirl technology are responsible for the creation of design rules, which dictate, among other things, the magnitude of the imbalance force and the direction of that force with respect to the cutting structure. The design rules also specify the size and location of the cutter devoid area, the style of the gage, the cutter back rake, etc. In accordance with the design rules, for a bit to drill without whirl it is necessary to place the cutters in such as way as to produce the cutting force vector of the proper magnitude and direction. P. 403^