The Research On Self-Excited Stick-Slip Vibration Characteristics Of Deep-Well Drillstring

With the rapid development of society and the progress of science and technology,human demand for oil and gas resources increased and oil drilling gradually developed into the deep stratigraphy.When the drilling depth increases,the drillstring equivalent torsional stiffness decreases.Under the influence of the complicated underground geological conditions and the harsh working environment,the drillstring system is prone to stick-slip vibration,which seriously restricts the completion cycle and greatly increases the drilling cost.Therefore,it is very significant to study the stick-slip vibration characteristics of deep well drillstring to improve the drilling efficiency and reduce the oil drilling cost.In this paper,a two degree of freedom lumped mass torsion pendulum model is established by rationally simplifying the drillstring system in vertical deep well.Based on the rotary speed control and the friction between bit and rock,the Lagrange theory is used to derive the stick-slip vibration equation.It is found that the nonlinear friction torque between drill bit and rock is the root cause of stick-slip vibration.The effects of drillstring length,weight on bit,rotary speed,the friction coefficient between drill bit and rock and the system viscous damping on the stick-slip vibration of drillstring system are analyzed in detail by MATLAB program.The following conclusions are obtained:(1)with the increase of the drillstring length,the possibility of drillstring system stick-slip vibration increases;(2)improve rotary speed and reduce weight on bit can effectively inhibit or even eliminate stick-slip vibration;(3)the difference of static and dynamic friction coefficient between drill bit and rock formation is one of the important reasons for stick-slip vibration,with the increase of static and dynamic friction coefficient difference,the system stick-slip vibration intensifies;(4)increasing the system's viscous damping ratio can inhibit stick-slip vibration but at the same time will increase the energy dissipation.By rationally simplifying and equivalent the drillstring system,the drillstring stick-slip vibration experiment rig is designed and machined.The experiment rig is mainly composed of frame,input module(servo motor and electromagnetic exciter),output module(discs and drillstring),data acquisition module(camera)and so on.The moving image of lower disc is processed by digital image processing technology,the trajectory and instantaneous radial velocity of the lower disc center are obtained.According to the results of the rotary speeds and friction coefficient experiment,it can be seen that the improvement of the rotary speed can eliminate the stick-slip vibration,and the stick-slip phenomenon occurs in the case of large friction coefficient,which is consistent with the theoretical analysis.