Study On The Characteristics Of Stick-Slip Effect Under Axial

With the sustainable development of petroleum exploration and development,exploration and development turn to deep oil and gas reservoirs,resulting in the increasing difficulty of drilling.Compared with other bits,PDC bits can effectively improve drilling efficiency by shearing and breaking rocks.Therefore,it has been widely used in the petroleum industry.However,in deep wells and hard formations,stick-slip effect is often accompanied by the use of PDC bits.This effect is one of the main reasons for premature failure of drill string and low drilling efficiency.At present,the main measures to weaken the stick-slip effect are to increase the rotating speed of the turntable and reduce weight on the bit(WOB).However,when the rotational speed increases to a certain value,the axial and transverse vibration will intensify.And reducing WOB will affect the rate of penetration(ROP).In order to shorten drilling cycle and reduce drilling cost,it is urgent to thoroughly understand the root causes of stick-slip effect and find effective measures to weaken it.Based on the rock breaking mechanism of PDC bits,this paper reveals the root cause of stick-slip effect and studies the effect of axial vibration on stick-slip effect.The main research contents and achievements of this paper are as follows:1.Based on the stress analysis of rock at the bore hole bottom,a PDC single-tooth cutting model based on in-situ stress is established,and the effects of horizontal in-situ stress,rock strength,effective vertical in-situ stress and cutting angle on cutting force are analyzed.The accuracy of the model is verified by laboratory and field experiments in the literature,and the suitable conditions of the model are found.2.On the basis of PDC single-tooth cutting model,combined with the movement pattern of single-tooth in actual drilling,the axial velocity of single-tooth in the cutting process was considered,and the friction force of single-tooth was modified.Using analogy method,the interaction model between PDC bits and the rock is established,and it is proved by analysis that the interaction between rock and bit is the fundamental reason for the coupling of the axial motion and the torsional motion of the bit.3.Focusing on the stick-slip effect,the drill string system is simplified,and a simplified two-degree-of-freedom drill string model is established.By solving the mathematical model,the motion equation of the drill bit in the drilling process is obtained.Furthermore,combined with the interaction model between drill bits and the rock,the process of stick-slip effect in conventional drilling process is elaborated,and the weakening effect of axial impact vibration on the stick-slip effect is confirmed.4.Based on the field data,LS-DYNA software is used to simulate the whole drilling process.In order to facilitate the analysis,the burrs in the data of bit weight and torque caused by the unsteady movement of the bit are processed by using the fast Fourier algorithm and excellent noise reduction model.Moreover,the absolute angular displacement,the frequency and amplitude of the torque are taken as the important basis for describing stick-slip effect.Based on the results of finite element software analysis,the motion characteristics of the bit during the stick-slip effect are described,and the effects of axial velocity and rotational speed on the stick-slip effect are analyzed.Moreover,the frequency and amplitude of stick-slip vibration are weakened by the axial vibration impact is confirmed.