Friedhelm Makohl

Drilling Performance Improvements Using Downhole Thrusters

Thrusters generate weight-on-bit by using drilling fluid hydraulics. The pump-open behaviour of thrusters decouples the lower part of the bottom hole assembly from the remainder of the drillstring, and in so doing provides a constant, controllable weight-on-bit (WOB) that dampens out axial vibrations and shocks. In more than one hundred and twenty jobs, thrusters have proven their ability significantly to improve drilling performance in holes with vibration- or shock related problems. The technique has been used to increase overall penetration rates and bit live, improve steerability and reduce downhole failures in hole sizes from 12 1/4 to 3 7/8 diameter, in both horizontal and vertical well, and at deopths to 5400m. The system has been used mainly on fixed rigs, but also on floating rigs, substantially reducing cost per foot. As the thruster interacts with other hydraulic components of the drillstring (bit, mud motor, restrictor subs, MWD pulser), the proper tuning of the drilling system is required to obtain optimum performance. This paper shows the function and principle of thrusters. It explains possibilities and limits for their use, and details criteria for the otpimum thruster in certain applications. Detailed field data from 4 wells in 4 geological areas are presented