Despite the development of models describing Polycrystalline Diamond Compact (PDC) bit performance, bit selection is often made on the basis of visible, geometric features.
Bits can now be modelled routinely and described in terms of their technical interaction with the drilling assembly and the formation. Selection on the basis of modelling and this interaction is novel and valuable, and will make for more consistent selection and improved performance.
A new set of performance indices is presented for PDC bits. These are derived from a sophisticated mathematical model and describe performance in terms of:
ROP - Rate of Penetration, or how fast the bit will drill for a given Weight on Bit (WOB)
Durability - how resistant the bit is to abrasive wear
Stability - how resistant the bit is to lateral vibration
Steerability - how the bit responds to side forces and therefore how steerable it is on Rotary Steerable Systems.
Once the relative importance of each index is established, the optimal bit for the specific application can be selected.
A specialist interface is presented for Rotary Steerable Systems. Using this, the user can input system type (from the variety of systems available) and well profile, and the software will create a list of bits ranked by decreasing suitability, based on a variety of factors including some aspects of bit geometry but primarily mathematically modelled response.
The paper presents results from pilot studies and demonstrates a scientific and rational approach to bit selection - giving not only improved but also more consistent and reliable results.
All bit designs can be described in terms of their performance. Historically, this was done by recording actual performance in bit records and using examples of good performance, in close offsets, to justify repeat runs of specific designs. To a limited extent, this technique can be effective, but the danger is that because not all possible options are considered, it is by no means assured that the bit selected will be optimum for the application.
As the art of roller cone bit design, and subsequently of PDC bit design, matured, it became possible to select bits based on basic geometric features. For example, the IADC coding systems were developed for roller-cone and PDC bit design classification1, 2, and PDC bits have increasingly been characterised in terms of their blade count and cutter size.
So, for example, a PDC bit with many blades would be selected for a hard or abrasive formation - or a bit with few blades and large cutters where penetration rate was most important.
As easy as they are to understand, these simplistic classification systems miss the point. As will be seen, there are subtle differences between bit designs that can lead to significant changes in bit performance.
Others have recently shown how bits can be ranked in terms of performance in specific case histories, as an aid to bit selection and optimisation3. There have also been publications of methods of classification of bits according to geometric features, for example cutting profile, number of blades and number of cutters4, 5.
Over the last decade, sophisticated PDC bit models have been developed that allow the loads on each bit design feature, as a result of interaction with the formation, whether it be cutting or rubbing, to be calculated6, 7.