This paper details the inception, modeling, laboratory, and field testing of a concept polycrystalline diamond compact (PDC) drill bit when removing its vestigial features. The extent to which length, gauge geometry, and bit design can affect directional response on push-the-bit rotary steerable systems (RSS) and a bent housing motor is examined. Results from laboratory testing provided feedback into analytical models for validation and are then field-tested in several different directional applications.
A comprehensive study was conducted to determine the benefits of removing the bit shank, which can be considered a redundant structure, alike the human appendix. Following this, both a steering-sensitive drill string model and computer-aided design (CAD) analyses were conducted to predict directional response and steering force values of both conventional and short-shank configurations. Drill bits were then manufactured and repeatedly tested on a push-the-bit RSS tool in a horizontal drill test. The testing results were used to validate and optimize the drill string model. This validated model was then used to design a drill bit for a field application where directional work was historically challenging.
The results from this paper focus on validating the short-shank concept known as the ultra-short make-up (USMU) bit via the drill string model and rigorous testing, as well as confirming the potential drilling benefits in challenging directional applications. The initial assumptions and approach from the drill string model and CAD analyses are presented. Validation by several high and low side tests on push the-bit RSS using drill bits with and without the short-shank modification were conducted. The dependent variables for testing were the directional capability, bit stability, borehole quality, and gauge pad-to-borehole interaction while biasing. The drilling parameters were kept constant to allow for a fair comparison of this concept design. The results of the post-horizontal testing were examined against the drill string model and adaptations made to both the model and its initial assumptions. This concept was then field-tested in a challenging directional application in Oman where steerable roller cone bits were predominately used. This led to back-to-back field rate-of-penetration (ROP) records, stable tool face, and excellent steering response from two hole sizes.
The vestigial concept generally applies to genetically determined structures or attributes that have apparently lost most or all their ancestral functions, such as a human's appendix or coccyx. This approach has rarely been used to drive technological developments within oil and gas tools. It also gives a refreshing new framework for concept, testing, and validation of changes to this unique directional drill bit design. A future development phase of this concept is to test it on the various rotary steerable systems and beyond.
