Quantifying Drilling Efficiency and Disruption: Field Data vs. Theoretical Model

Abstract

Mechanical and kinematics data at the bit have been collected during drilling operations and organized to identify optimal drilling efficiency. Transitional conditions towards abnormal behavior have been distinguished. Normal drilling is usually characterized by regular linear relationships between global control variables at the bit, which approximately correspond to an upscaling of a local Mohr-Coulomb-type failure mechanism of the rock. Most of our experimental data confirm this classical disposition. However, considerable departure from these relationships is noted, especially when there is bottomhole assembly ͑BHA͒ resonance or when there is partial balling at the bit. The field tests presented here allow us to verify classical theory and to draw up new guidelines for behavior in nonoptimal or dangerous, evolving operating conditions, such as those that occur under poor cleaning conditions. The main features of a mechanical structuring model involving transition 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 and synthetic way. It makes it possible to understand and test the sensitivity of bit response to variables such as ''weight on hook,'' torque on the drillstring, and flow rate of the fluid. IntroductionCommon Perception of Drilling Efficiency. The idea of combining the specific energy associated with rock crushing and the energy from hydraulics in order to achieve a synthetic view of bit efficiency in soft rock is not new. Previous studies 1,2 have defined semiempirical relationships which take into account the tendency for the rate of penetration to level off when weight on the bit increases beyond normal values. These schemes have been verified through numerous tests but, as they cover ordinary conditions, cannot describe conditions like transients to balling, for example.Meanwhile, some experts think that it is time for a new perception of hydraulics. According to Fear et al., 3 ''Generally, when people talk about bit hydraulics, they are referring to some measure of flow rate and pressure drop, but those factors cannot be considered independent of the particular bit design. The major issue is that the bit should be designed and operated in such a way that drilled cuttings do not impede drilling efficiency. When drilling in soft rocks, preventing bit and cutter balling is a primary concern.''