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.