John D. Macpherson

Field Verification of Model-Derived Natural Frequencies of a Drill String

Vibrations generated in a drill string while drilling generally lead to a reduction in drilling efficiency and often cause premature failure of drill string components and bit damage. It is also known that lateral vibrations, in particular, are responsible for most measurement-while-drilling (MWD) tool failures while drilling. One way to increase drilling efficiency and avoid tool damage is to monitor and analyze drilling vibrations so that drilling parameters can be adjusted while drilling to reduce such vibrations. An alternative method is to analyze and determine the natural frequencies of the bottom-hole assembly (BHA) so that resonant conditions caused by various excitation mechanisms in the drill string can be avoided. Even though models have been developed in the past in the drilling industry to determine the natural frequencies of a BHA, few attempts have been made to demonstrate that such models do actually help reduce vibrations or failures. This paper deals with the process of field validation of model-derived frequencies for axial, torsional and lateral vibrations. The results presented in this paper are based on the analysis of drilling data from afield test using downhole vibration measurement sensors. The downhole measurements included X and Y bending moments, axial acceleration, dynamic weight-on-bit, dynamic torque, and X and Y-axis magnetometers mounted in an MWD sub. The data analysis demonstrates that the natural frequencies predicted by the models match well with actual field (measured) values at the locations of interest, particularly for lateral vibrations. This analysis therefore shows that model derived results can be used with a degree of confidence to help avoid resonant conditions in a BHA while drilling and to help reduce failures.

Analysis Of The Stick-Slip Phenomenon Using Downhole Drillstring Rotation Data

The stick-slip phenomenon has been identified in the industry as an inefficient and often damaging drilling vibrational condition. Numerous studies, predominantly focusing on measurements of drillstring torque. have detailed the nature of this phenomenon, and have shown its detrimental effects on drill-string components and especially on PDC bits. Using surface rotary motor current, drilling contractors and surface logging companies have attempted to monitor drillstring torque so as to recognize and correct for this torsional instability. Recent MWD tool developments using downhole drilling rotational speed have established a superior method to identify stick-slip and establish its severity. Measuring actual rotational behavior of the lower BHA gives a better insight to the torsional movement of the drill bit during the drilling process, without the problems involved when inferring this behavior from the surface torque readings, obtained at the opposite end of the drillstring. This paper describes the method of determining the stick-slip behavior downhole from magnetometer readings. The sensors are sampled at a high enough frequency to compute near-instantaneous values of downhole drillstring rotation, and variations in the RPM are then used to establish the incidence and severity of stick-slip. This information is telemetered to the surface and provided to the driller in order to allow for real-time changes in drilling parameters to correct for this mefficiency. Also presented in the paper are several examples using actual field data that detail stick-slip behavior and demonstrate how real-time information about downhole RPM variations can help to reduce the problems associated with this torsional drilling dysfunction.

Drilling trajectory optimization

This is an excellent introduction to contemporary drilling technology. A well is not just a hole in the ground; putting the hole where you want it is no small matter.