The Effect of Bit Type on Reactive Torque and Consequent Tool-Face-Control Anomalies

Abstract

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.

Closed-loop tool face control with the bit off-bottom

“…Compute (∆φ) 2 . 4: If (∆φ) 2 − (∆φ) 1 is too large, repeat steps (1) to (4) reducing d A . 5: Find a suitable bump amplitude A i = A 1 + n i d A using (27).…”

“…Both of which are low bandwidth and high latency. Automated tool face control has been attempted, however, the long latency (1 to 20 seconds) of downhole telemetry has made automatic feedback control significantly slower than manual control [4,5]. Target zones for current wells are often thin (5 to 30 meters thick) layers of reservoir or source rock for hydrocarbons and precise placement of wells is critical for economic production of the insitu hydrocarbon.…”