The Effect of Bit Type on Reactive Torque and Consequent Toolface Control Anomalies

Abstract

The oil and gas drilling industry has developed a large body of knowledge about methods for drilling directional wells with steerable motors. Experience indicates that more aggressive drill bits are harder to steer. This is commonly attributed to the fact that bits with higher aggressivity produce larger torque changes for a given change in bit weight. The actual mechanics, however, of toolface disorientation during slide events is poorly understood. This paper reports on tests conducted on a full-size drill rig aimed at understanding the mechanics of toolface control. Toolface orientation and other data were measured downhole at 100 Hz. Nine different bits ranging from PDC to hybrid to roller-cone bits were tested on a motor/AKO BHA in slide mode. These tests confirm the common industry notions about the effect of aggressivity on toolface control. They also show that angular motion of the BHA while sliding is overdamped. Toolface orientation consequently follows the average of the torque signal generated by the bit. Furthermore, the toolface orientation is more easily disoriented by a torque signal at a frequency near or less than the natural frequency of the drillstring. PDC bits excite this more readily than bits with rolling cones. We also identify a toolface disorientation anomaly which we call a fast torque anomaly (FTA). FTAs occur because the bent motor/AKO has a preferred angular orientation in the borehole. FTAs have not been previously recognized, probably because they are not identifiable in mud-pulse signals. A BHA suffering FTAs would simply appear as a chaotic and profound loss of toolface control in mud-pulse data. Hybrid and roller-cone bits caused fewer FTAs than PDC bits.