Abstract
Coil Tubing Drilling substantially lowered the cost for drilling Prudhoe Bay sidetrack wells. The program was nevertheless fraught with MWD (Measurement While Drilling) instrument failures. Downhole tool damage happened so often it was considered routine. Generally there was agreement that excessive vibration played a key role in downhole tool damage. Without causes the only solution was to build more durable tools.
To monitor and record downhole vibration, a DDS collar (Drill String Dynamics Sensor) was added to the BHA. The DDS tool was capable of recording lateral, torsional, and axial accelerations. It also incorporated spectral frequency recordings during periods of highest vibration. Utilizing the DDS sensor in the BHA over multiple runs, engineers were able to pinpoint specific intervals of destructive vibration. These studies allowed the rig team to identify causes and ultimately focus corrective modifications upon equipment and drilling practices. Frequency analysis was instrumental in helping to isolate the destructive excitation mode of vibration as bit generated WHIRL. Detailed information helped bit vendors modify bits, which resulted in the virtual elimination of WHIRL. MWD components were upgraded; improved drilling practices were identified and adopted by the operator.
In conclusion: since the vibration analysis has been compiled and changes made, MWD failures have gone from averaging more than one failure per well to zero failures in over twenty-seven completed wells. This sudden decrease in trouble time has dramatically enhanced the efficiency of Coil Tubing Drilling Operations. Vibration analysis is continuing within the Coil Tubing Drilling Program for Prudhoe Bay using new generation vibration sensors incorporated within the MWD Tools.
