Research On Stick-slip Vibration Characteristics Of Drillstring And Mechanism Of Torsional Impacts Mitigate Stick-slip Vibration

The works in this paper are on the basis of mechanical vibration,structural mechanics and differential equations,and laboratory experiments and finite element analysis are also used to conduct the basic research.The main points of this paper are listed as following.(1)The research status of stick-slip vibration in oilwell drillstring is investigated,including the theoretical studies and suppression approaches of this phenomenon.In this study,the theoretical studies are divided into mechanical model research and experimental research,and the suppression approaches are divided into active control and passive control.(2)Combining with operating features of the drilling system,a lumped pendulum model of the drilling system is established,differential equation of motion of the drill bit is derived and piecewise solutions of the equation are obtained.By referring to the actual drilling conditions,a drilling system with the drillstring length 3000 m is studied,vibration characteristics of the drill bit is analyzed and stick-slip vibration of the drill bit is found.In order to obtain the conditions of occurrence of stick-slip phenomenon,the cases of non-friction,static friction coefficient equals to kinetic friction coefficient,and the weight on bit(WOB)vibrates harmonically are studied.(3)For the operating drilling system,the friction between the bit and formation and the damping of drilling fluid constitute the damper unit,the rotation of rotary table becomes the energy input unit,and the drillstring connects the rotary table and drill bit becomes the regulator.As a result,the drilling system has the conditions of forming self-excited vibration.In order to investigate the self-excited characteristics of the stick-slip bit,dynamic responses of the drill bit under different initial conditions are analyzed.Results show that the stick-slip vibration of drillstring is only related to the own property of the drilling system,and has nothing to do with the initial conditions,which means the stick-slip vibration is a type of self-excited vibration.For the stick-slip vibration of drilling system,the phase trajectory of'the drill bit on the phase plane converges into a limit cycle.(4)In order to obtain the influences of system parameters on stick-slip vibration,different groups of static friction coefficients and kinetic friction coefficients,WOB,drillstring length,table velocity,and damping coefficient are analyzed.According to the vibration characteristics for different parameters,the reason of stick-slip phenomenon is easily occurred in deep well is interpreted.In addition,the results obtained in this part can be the theoretical basis of increasing the table velocity or lowering the WOB to reduce stick-slip vibration.(5)Based on the Drucker-Prager criterion and the equivalent plastic strain criterion,numerical simulation models of the drill bit crushes the formation are established.The models are run to simulate the dynamic rock crushing process and to obtain the influences of high-frequency torsional impact on rock damage.The bit dynamics under high-frequency torsional impacts are analyzed and the mechanism of high-frequency torsional impact mitigates stick-slip vibrations is obtained.Also,the mechanism of high-frequency torsional impacts improve the drilling efficiency is obtained.(6)In order to study the effects of high-frequency torsional impacts on drillstring vibration,a continuous system model of torsional vibration of the drillstring is established.Under the actions of high-frequency torsional impacts,the dynamic responses of the drillstring in stick phase and slip phase are analyzed,and then the influences of torsional impacts on drillstring vibration are obtained.Results show that the torsional impacts mitigate the drillstring vibration.Combining with the results of drillstring vibration and efficiency in breaking the rock with the use of high-frequency torsional impacts,the influences of impact frequency and placement of the torsional impact generator on the drilling process are investigated.The basic theoretical researches in this paper systematically illustrate the mechanisms of the occurrence of stick-slip vibration,vibration characteristics,and influences of system parameters on stick-slip vibration,and reveal the mechanism of torsional impact mitigates stick-slip vibration and improves drilling efficiency.The achievement presented in this paper will be helpful to the researchers in this area and field engineers.The results help them to understand the stick-slip dysfunction and thus design approaches to mitigate these vibrations.In addition,the studies of high-frequency torsional impacts play role in aggravating the rock damage and mitigating the stick-slip vibration give the mechanism of torsional impact improves the drilling efficiency,which is substantial important for the researchers and drilling operators to understand and grasp the technology,and finally promote the development of this technology in our country.