Abstract
A modified model for predicting the friction force between drill-string and borehole wall under in-plane vibrations was developed. It was found that the frictional coefficient in sliding direction decreased significantly after applying in-plane vibration on the bottom specimen. The friction reduction is due to the direction change of friction force, elastic deformation of surface asperities and the change of frictional coefficient. Normal load, surface topography, vibration direction, velocity ratio and interfacial shear factor are the main influence factors of friction force in sliding direction. Lower driving force can be realized for a pair of determinate rubbing surfaces under constant normal load by setting the driving direction along the minimum arithmetic average attack angle direction, and applying intense longitudinal vibration on the rubbing pair. The modified model can significantly improve the accuracy in predicting frictional coefficient under vibrating conditions, especially under the condition of lower velocity ratio. The results provide a theoretical gist for friction reduction technology by vibrating drill-string, and provide a reference for determination of frictional coefficient during petroleum drilling process, which has great significance for realizing digitized and intelligent drilling.
Highlights
The phenomenon that vibration, including normal vibration and in-plane tangential vibration, can reduce the friction between rubbing pairs has been proven by many studies [1,2,3,4], and has been used to obtain lower friction in many engineering fields for many years, such as vibration compactors and plates, metal working, wire drawing, cutting, atomic force microscopes and so on [5]
A modified model is developed for predicting the frictional coefficient under in-plane vibration conditions
The reasons for friction reduction under in-plane vibrations are attributed to the direction change of friction force, tangential deformation of surface asperities and the change of frictional coefficient generated by the asperities of harder surface penetrating the softer surface
Summary
The phenomenon that vibration, including normal vibration and in-plane tangential vibration, can reduce the friction between rubbing pairs has been proven by many studies [1,2,3,4], and has been used to obtain lower friction in many engineering fields for many years, such as vibration compactors and plates, metal working, wire drawing, cutting, atomic force microscopes and so on [5]. In order to reduce the friction force between drill-string and borehole to improve the transfer efficiency of weight on bit, petroleum-drilling engineering developed Agitator [6], Slider [7] and Xciter [8], which can excite drill-string vibrating axially, torsionally and laterally, respectively. Higher rate of penetration (ROP), improved tool-surface control, reduced drag and stick/slip are some of the benefits to reduce friction force along the drill-string through exciting the drill-string mildly while drilling directional wells in the process of petroleum exploitation. The drill-strings are generally thousands of meters, a higher proportion of drill-string can be excited under axial or torsional vibration than lateral vibration. The lateral vibration of drill-string is more likely to cause fatigue rupture. The materials of steel and rocks made by many research achievements cannot be applied directly
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