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.