Abstract

Vibration of drillstring was found responsible for the severe damage and premature failure of drillstrings during drilling the gas-bearing igneous formations in the Songliao Basin, China. The vibration significantly shortened the life of drillstrings and had a great impact on drilling performance in the area. The objective of this study was to identify the key factors affecting drillstring vibration and develop a control-mechanism to reduce the failure rate of drillstring.

This study investigated the combined effect of axial and torsional vibrations on the damage of drillstring. A computerized model was built to simulate loads from axial and torsional vibrations. An analytical method for predicting the fatigue life of drillstring was developed. The method uses the output from the computerized model. Engineering charts were also generated for typical drilling conditions. These charts have been adapted in a Drillstring Damage Control Program implemented in the Songliao Basin, China.

Introduction

Natural gas was found at the beginning of this century in the igneous formations in the Songliao Basin, China. With high contents of quartz and other dense minerals, these formations are very hard and abrasive. They are also heterogeneous with natural fractures and vugs. Types of minerals in the formations vary with depth. When these formations were drilled, severe axial and torsional vibrations of drillstring were observed at surface. Premature failures of drillstring and drill bits were very common in the area. This study was initiated to identify the key factors affecting drillstring vibration and develop a control-mechanism to reduce failure rate of drillstring to an acceptable level.

It has long been recognized that vibration of drillstring is detrimental to the surface and downhole equipment in well drilling process (Li and feng, 1990). The vibrations cause premature wear and fatigue failure of drillstring itself. Combination of axial and torsional vibrations can significantly shorten the life of drillstring and have a great impact on drilling economics in harsh drilling areas. Guo et al. (1994) presented a mathematical model coupling transverse and torsional vibrations while axial vibration was neglected. Quantitative analyses of the effects of axial-torsional-combined vibration on the fatigue life of drillstring have been very limited in the petroleum literature. Existing mathematical models that consider axial vibration only are not adequate for predicting fatigue life of drillstrings in hash drilling conditions.

This study focused on two aspects:

  1. building a computer simulator to predict the maximum axial and torsional stresses; and

  2. developing a method to predict the fatigue life of drillstring using the results from the simulator. After completion of these works, engineering charts were generated for typical drilling conditions, which have been utilized in an implemented Drillstring Damage Control Program in the Songliao Basin, China. A significant improvement of drilling performance in the area is expected.

Mathematical Model

Drillstrings vibrate in three modes: axial, torsional and transversal vibrations. Usually one of them dominates in a given system. Combination of two or three of them can play important roles in harsh conditions. At present, some mathematical models are available for analyzing axial, torsional, and transversal vibrations of drillstring separately, but none of them reflects the actual downhole conditions in drilling operations. It was believed that a more relevant model should be developed to consider the combined effect of at least two vibration modes. The mathematical model described in this paper was formulated by rigorously coupling the axial and torsional vibration components.

Coupling of Axial and Torsional Vibrations

Consider roller bits that are used for drilling hard formations. Because the teeth of drill bit contact hole bottom alternately, an axial vibration in the drillstring is generated by the cyclic force from the hole bottom (Figure 1). It is also expected that the torque on the drill bit will be cyclic and it will generate torsional vibration in the drillstring (Figure 2).

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