Summary
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 tool-face 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 tool-face control. Tool-face orientation and other data were measured downhole at 100 Hz. Nine different bits ranging from polycrystalline diamond compact (PDC) to hybrid to roller-cone bits were tested on an adjustable-kick-off (AKO) motor bottomhole assembly (BHA) in slide mode. These tests confirm the common industry notions about the effect of aggressivity on tool-face control. They also show that angular motion of the BHA while sliding is overdamped. Tool-face orientation consequently follows the average of the torque signal generated by the bit. Furthermore, the tool-face orientation is more easily disoriented by a torque signal at a frequency near to or less than the torsional natural frequency of the drillstring. PDC bits excite this more readily than bits with rolling cones. We also identify a tool-face disorientation anomaly that 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 tool-face control in mud-pulse data. Hybrid and roller-cone bits caused fewer FTAs than PDC bits.
