Vibration-based Drillstring Mechanical Performance Analysis And Safety Assessment

The oil and gas reserves are decreasing in the shallow layers of the surface.The oil and gas resources in the deep surface have gradually become the main target for exploitation at present.It is difficult to exploit deep resources,and the length of drilling tools is increasing,which put forward higher requirements for the safety design of drilling tools.The working environment of downhole drilling tools is very complex.Drilling process of drilling tool,the drilling string and rock mass collide and vibrate continuously.Vibrations generated by the deformation of rock formation in response to forces acting on the rock formation.The vibration of drilling tools mainly includes axial vibration,transverse vibration and torsional vibration.Vibration can cause alternating stress on the drill tool.The rapid change of alternating stress results in fatigue effect and strength reduction of drilling tools.Eventually,the drilling tool will be damaged due to fatigue.At present,the safety analysis method of drilling tools is static strength check method.The method takes the axial force and torque of the drilling tool into consideration,which analyzes whether the axial force and torque exceeds the safety allowable range of the drilling tool under the influence of gravity,friction,drilling fluid and other factors.The static check method is to evaluate the safety of drilling tools from a force perspective,which just considers a single stress factor and ignores the impact of vibration on drilling tools.As the length of the drillstring increases,the impact of vibration on the drilling tool increases.This method gradually fails to meet the actual engineering needs.In this paper,we first study the static strength calibration method of drilling tools and analyzes the shortcomings of this method.Then the basic characteristics of drilling tool vibration are studied,and it is realized that the whirling motion of drilling tool is heavily effect on the safety of drilling tool.In the paper,researches on the characteristics of whirl and vibration intensity of whirl have been done.The influence of whirl on the failure of drilling tools is analyzed.The bending stress,axial force,shear stress and other stresses produced by drilling tools are analyzed,and the dynamic stress analysis model of drilling tools is constructed.This method refers to the static check method,which considers the bending stress and fatigue of the drilling tool when the drilling tool whirls backward.We use the fourth strength criterion to comprehensively analyze the safety of the drill tool during drilling.It is found that the rotational speed and weight on bit will have an impact on the safety of drilling tools,and the speed has a greater impact on the safety.There is a certain degree of damage risk in the wellhead of drilling tools,the bottom of weighted drill pipe and the drill collar during the drilling process.Based on the static and dynamic stress analysis models,the program of drilling tool stress analysis is designed to provide analysis tools for drilling tool stress analysis.Finally,the conventional drilling tool design method is used to calculate the drilling capacity under the same conditions.The dynamic stress analysis model of drilling tool is used to analyze the stress of the drilling tool assembly of steel + aluminum alloy drill pipe and steel drill pipe.The analysis found that under the same conditions,the drilling depth capability of the same or similar type of aluminum alloy drill pipe is higher than that of steel pipe drill pipe.Compared with the same length of steel + aluminum alloy drill pipe,the safety of steel + aluminum alloy drill pipe is higher.