Abstract
Optimal control of the drilling process requires that relevant physical models must be used while drilling. The reliability of these models must first be improved by comparing their results with actual downhole and surface data. Measurements of numerous drilling parameters and variables, sampled at high frequency, are then required.
Trafor is an MWD system designed and operated by the Institut Français du Pétrole for research on drill-string dynamics. It measures 15 downhole and 5 surface mechanical parameters. Because Trafor is fitted with an electric link adapted to rotary drilling, it transmits the downhole measurements to the surface in real time with a very high sampling rate of 1800 measurements per second. Furthermore, all the signals are fully synchronized. Hence, Trafor allows cross analysis between any signals, regardless of whether they are from downhole or from the surface.
New data recorded in a 1060 m deep well exhibit stick-slip behavior for various weights-ön-bit and rotary speeds.
Downhole data analysis coupled with numerical simulations shows that, in this field case, stick-slip occurred as an unstable drilling process. The origin of the instability lies in the relationship between the torque at the bit and the rotary speed. From this relationship, we can explain why stick-slip is known to disappear whenever the rotary speed (rpm) is increased or weight-on-bit is decreased. A stability analysis is used to derive operating conditions that prevent stick-slip.
Simulations of drilling with adjusted PID or with a downhole "anti stiçk-slip" tool are described. For both simulations, stick-slip can be cured.
