RIASSUNTO
Abstract
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 lives, improve steerability and reduce downhole failures in hole sizes from 12 1/4"" to 3 7/8"" diameter, in both horizontal and vertical wells, and at depths to -5400 m (1650 ft.). 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 optimum thruster in certain applications. Detailed field data from 4 wells in 4 geological areas are presented.
Background
Drill string vibrations are undesirable as they reduce penetration rates, cause twist offs and consequent fishing time, and contribute to premature downhole failures of MWD systems, motors and bits. They have detrimental influence on borehole stability and reduce the life of tool joints between recuts. Therefore, a reduction in drill string vibrations can contribute to reducing the operator's total well cost.
There are longitudinal, lateral and torsional drill string vibrations, all of them being highly nonlinear (chaotic), so it is difficult to predict when they will occur. Ongoing work in this area is documented in the literature.
All vibrations are coupled (linked), so that a reduction of one form of vibration usually reduces the others. A special case of drill string vibration occurs in oriented drilling with a steerable motor in horizontal or extended reach wells. Here, longitudinal vibrations induced by uncontrolled sliding of the string make it difficult to hold constant weight on bit. This limits the longitudinal displacement where steerability can be maintained. For steering, tricone bits rather than PDC bits are often preferred, as their lower aggressiveness means that they react less sensitively on WOB changes.
When drilling from a floating rig, using a drill string heave compensator can give rise to irregular longitudinal vibrations in the string, making oriented drilling for steering difficult. Again, this leads to the use of tricone, rather than PDC bits when kicking off of horizontal sections. vibrations occur in all wells, but are a particular problem in downsized or slim wells where the drill string is much weaker. These phenomena have been studied in considerable detail, and a new concept of longitudinal vibration dampening has been developed. The method involves using a novel bottom hole assembly component, the thruster. In this device, the differential pressure between the inside of the BHA and the annulus is used to provide weight on bit. (See Fig.1.)
P. 729