Pushkar Nath Jogi

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.

Using Dynamics Measurements While Drilling to Detect Lithology Changes and to Model Drilling Dynamics

As a result of bit-rock interaction, downhole weight-on-bit, downhole torque, instantaneous downhole rotational speed and bit motion (acceleration and rate of penetration) are directly affected by the formations being drilled. Since these measurements react differently to different lithologies, and assuming that drilling problems do not effect these measurements, any changes in the measurements in some way will reflect changes in the properties of the lithology. If, based on these measurements, the lithology is assumed to have certain properties, then it is possible to derive models for the interaction between bit, formation and drillstring. With these models it is possible to simulate the dynamic behavior of the system including phenomena like stick-slip. Rate of penetration has long been used as a lithology indicator, and drilling models have been developed using surface measured drilling parameters to infer changes in lithology. With the advent of MWD measurements, significant improvements were made in the mathematical models by involving downhole torque. The model derived parameters were shown to be related to rock strength (drilling and shear strength) and proved to be good indicators of formation changes. Similar expressions in the form of simple bit models can be used in combination with a finite element model of the drillstring to simulate the dynamic behavior of the complete system. A significant improvement in this analysis can be affected by introducing measurements from the dynamics tool, such as instantaneous torque, weight and rotation rate, as well as the bit acceleration. These measurements provide not only static but also dynamic data which can be used to validate simulations and the underlying models. The present analysis explores the use of the dynamic measurements and the application of some drilling models in analyzing formation changes while drilling, and the use of these data and models in simulating drilling dynamics.Copyright