Abstract

Vibration of drillstring has detrimental effects on drilling performance. This type of vibration is much more severe in air and gas drilling than in liquid drilling due to the low damping effect of air and gas. It is highly desirable for drilling engineers to have a reliable mathematical model to simulate the vibration and identify resonance conditions before and during the drilling operations. This paper fills the gap.

Traditionally drillstring vibrations have been analyzed using bit-force as a boundary condition. Accuracy of such analyses is often questionable as the discrepancies have been observed between model-predictions and field data. Mathematical models of longitudinal vibration of drill strings excited by both bit-force and bit-displacement have been established in this study. Equations are solved using the method of separation of variables. Calculations with the solutions indicate that the resonance rotary speeds of drillstring predicted by the bit-force model are non-resonance rotary speeds predicted by the bit-displacement model, and vice versa. A field case study is presented in this paper. Data from a field operation with drillstring failure seems to support the bit-displacement model, not the bit-force model. The models are presented to drilling engineers to further verify their accuracies. Once fully validated, the mathematical models will provide drilling engineers an effective tool for designing drill string for air and gas drilling operations against drillstring failure.

Introduction

Use of air and gas as circulating media for drilling oil and gas recovery wells has become an attractive practice in the petroleum industry. The drilling technique is utilized mainly for increasing rate of penetration into hard formations to improve drilling performance.1,2

Vibration of drillstring is much more severe in air and gas drilling than in liquid drilling due to the low damping effect of the light drilling fluids. The string vibration has detrimental effects on drilling operations such as premature failure of drillstring and other equipment, as well as health and environmental problems to the drilling crew due to the vibration-induced noise. It is highly desirable to develop techniques that can be used for reducing drillstring vibration.

Field observations have indicated that drillstrings are generally subjected to severe vibrations induced primarily from two excitation sources: bit/rock iteration and string/borehole interaction. Drillstring vibrate in several ways: axially, torsionally, and laterally, or combinations of these three basic modes.3 While some literatures have shown that these basic modes are interactive,4,5 other studies indicated that the basic modes can be simulated independently.6 Some researchers demonstrated that the lateral vibration is more detrimental,5,7 others emphasized the importance of axial and torsional vibrations.4,8 A practical approach is to investigate particular vibration mechanisms individually and identify and attack the major vibration mode case by case. In this study, we focused on longitudinal (axial) vibration mode only because this mode was identified as the major vibration mode that caused damage of drillstrings in several oil and gas fields in China.9–13

Traditionally drillstring vibrations were analyzed using bit-force as a boundary condition. Accuracy of such analyses has not been verified with field data. Mathematical models of longitudinal vibration of drillstrings excited by both bit-force and bit-displacement have been developed in this study. Equations are solved using the separation of variables method. Calculations with the solutions indicate that the resonance rotary speeds of drillstring given by the bit-force model are non-resonance rotary speeds of drillstring given by the bit-displacement model, and vice versa. A field case study is presented in this paper. Data from a field operation with drillstring failure seems to support the bit-displacement model more than the bit-force model. Both of the models are presented to drilling engineers to further verify their accuracies. Once fully validated, the mathematical models will provide drilling engineers an effective tool for optimizing design of drillstrings and drilling parameters in air and gas drilling operations.

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