Quantifying drilling efficiency and disruption: Field data vs. theoreticalmodel

Mechanical and kinematics data at the bit have been collected during drilli ng operations and organized to identify optimal drilling efficiency. Transi tional conditions towards abnormal behavior have been distinguished Normal drilling is usually characterized by regular linear relationships between g lobal control variables at the bit, which approximately correspond to an up scaling of a local Mohr-Coulomb-type failure mechanism of the rock. Most of our experimental data confirm this classical disposition. However, conside rable departure from these relationships is noted, especially when there is bottomhole assembly (BHA) resonance or when there is partial balling at th e bit. The field tests presented here allow us to verify classical theory a nd to draw up new guidelines for behavior in nonoptimal or dangerous, evolv ing operating conditions, such as those that occur under poor cleaning cond itions. The main features of a mechanical structuring model involving trans ition towards low-efficiency conditions are presented, and it is shown how this model confirms our interpretation of field data. The model couples the drillability of rock and the general dynamics of the bit in an original an d synthetic way. It makes it possible to understand and test the sensitivit y of bit response to variables such as "weight on hook," torque on the dril lstring, and flow rate of the fluid.