Research On Whirl Law Of Drill String Based On The Principle Of Minimum Power Consumption Rate

In oil drilling,whirl is one of the main factors causing the damage of drilling tools,well deviation and poor hole quality.Many researchers have carried out research on the drill string whirl and achieved many results,but some problems have not been solved.This paper mainly focuses on the rotary compression drill string in the drilling fluid,and analyzes the whirl law of the drill string under the dynamic lubrication of drilling fluid.Based on the work related to the two national natural science foundation projects,the main studies are as follows:The present situation and development trend of drilling string whirl are investigated and the main problems in drilling string whirl are analyzed.It is pointed out that the combination of drilling string dynamics and dynamic lubrication is the development direction of drilling string dynamics.The three principles of drill string mechanics are explained: every point and every moment of drill string must meet the mechanical equilibrium equation.If there are multiple solutions,the principle of minimum potential energy is used to judge first.If multiple solutions exist,the principle of minimum dissipative power is used to determine.A mathematical model for the buckling parameters of drill string under sliding friction force and drilling fluid force is established.Analysis of the drill string material resistance and the principle of minimum dissipative power in the conversion of the role of drill string motion state,the dynamic character of the initial bending of the drill string and disturbance initial bending backward whirl stability of the drill string.According to the theory of rotor dynamics,it is assumed that the BHA with stabilizer at both ends is a single disk rotor structure.The calculation of the interaction force and dissipation energy between drill string and fluid based on the lubrication theory of radial sliding bearing is derived.In view of the problems existing in the study of drill string whirl,the paper designs and establishes a drill string whirl simulation device based on the principle of minimum dissipation power and considering the effect of drilling fluid dynamic lubrication.Theinfluencing factors of drill string whirl are studied.The dynamic characteristics of the initial bending drill string,the material lag effect on the drill string trajectory,the deviation angle on the critical axial pressure of the drill string backward whirl,the effects of different fluids on the drill string whirl,the effects of the wellbore inner on the drill string backward whirl,and the effects of the buckling form of the drill string on the backward whirl are studied experimentally,the principle of minimum dissipation power is verified by experiments.It is found that the dynamic lubrication of drilling fluid can reduce the value of positive whirl and prevent the formation of reverse whirl.The dynamic lubricity of drilling fluid is improved,the diameter of drilling is reduced,and the angle of drilling is increased.Two reverse vortices may occur with increasing axial pressure.The movement state transformation of drill string conforms to the principle of minimum dissipative power.In conclusion,under the complex force environment in the wellbore,there are many kinds of whirl forms,and the whirl state can be judged by three principles of drill string mechanics.The dynamic lubrication force of drilling fluid plays an important role in reducing the rotation amplitude and inhibiting the backward whirl of drilling string.Based on the principle of minimum dissipation power,the rule of drill string whirl is studied,which considers the effect of drilling fluid dynamic lubrication force and the internal resistance of drill string material on the criterion of force and deformation of drill string and the transition of motion state,it has important theoretical and practical significance for well trajectory control,wellbore stability,drilling tool design and damage prevention,increasing penetration rate and so on.