Vibration Analysis of a Drillstring in Horizontal Well

The complicated geological conditions will bring great challenges to the drillstring of horizontal wells for the reason that the increase of the well depth in Sichuan-Chongqing region.Since drillstring failure and friction during drilling are generally caused by drillstring vibration,great importance must be attached to computer simulation methods for the prediction of drillstring vibration.A finite element model considering axial,lateral and torsional vibration is established.In order to verify the established numerical model,an indoor experimental device based on the similarity principle was established.The vibration characteristics of three shale gas horizontal wells drillstrings are described.The reasons for the large difference in vibration characteristics between the three wells were discussed by changing the well structure,BHA(bottom hole assembly)and drilling parameters.The simulation results show that the depth of ultra-deep well 3 is only 1.27 times of that of medium-deep well 2,but the fluctuation of WOB(weight on bit)is 2.8 times.The primary factor for the great difference of WOB fluctuation is the change of vertical section depth,the secondary factor is the change of the horizontal section length.Additionally,the reasonable range of speeds has been proposed and applied in the field.The advantage of this numerical analysis method is that it can judge the vibration and friction torque of drillstring in horizontal wells with arbitrary structure,drilling parameters and BHA.This method has certain guiding significance to the field practice.

Mechanical behavior of drillstring with drag reduction oscillators and its effects on sliding drilling limits

Practice has proved that drag reduction oscillators can decrease the axial friction and increase wellbore extension effectively in sliding drilling operations.However,the complicated mechanical behavior of drillstring with drag reduction oscillators has not been revealed sufficiently.In this paper,the mechanical model of drillstring with drag reduction oscillators is established by considering the friction nonlinearity.Further introducing the initial conditions,boundary conditions and continuity conditions,the finite differential equation of drillstring vibration is obtained and solved.The new model has been applied to a case study,in which the drag reduction effects of drillstring with and without oscillators are compared and the effects of relevant factors on drag reduction are analyzed.The results show that the hook loads increase obviously by reducing downhole average friction coefficient for drillstring with oscillators.Increasing vibration amplitude of the drag reduction oscillator can decrease axial friction,but the vibration frequency is nearly irrelevant to drag reduction.Increasing number of drag reduction oscillators can decrease axial friction,but may lead to large hydraulic power loss and high risk of drillstring fatigue.Therefore,there is an optimal number of drag reduction oscillators.The re search re sults are of significant guiding significance for optimal design and safety control in sliding drilling operations.

On the formation of limit cycle of the friction-induced stick-slip vibration in oilwell drillstring

In this paper,the stick-slip vibration in oilwell drillstring is studied.The drilling system is modelled as a lumped-parameter torsional pendulum and the interaction between the drill bit and the rock is treated as Coulomb friction.Equation of motion of the drill bit is established and the dynamic responses of the drill bit are obtained.A drilling system with the drillstring length 3000 m is analysed,in which the system parameters are selected by reference to the actual drilling situation.After the slip vibrations in the initial stage,final state of the drill bit is a stable stick-slip vibration of which the limit cycle is a closed loop.In order to find the stability of the limit cycle corresponds to the periodically stick-slip vibration,different initial conditions for the drill bit are studied.Results show that the drill bit will lead to a periodic motion and the phase trajectories ultimately converge to the same limit cycle corresponds to stable stick-slip vibration.

Study of the influences of rotary table speed on stick-slip vibration of the drilling system

Stick-slip vibration presents one of the major causes of drilling problems,such as premature tool failures,low drilling efficiency and poor wellbore quality.The objective of this work is to investigate the influences of rotary table speed(RTS)on stick-slip phenomenon of the drilling system.In this study,the drilling system is treated as a lumped torsional pendulum model of which the bit/rock interaction is regarded as Coulomb friction.By analyzing cases with different RTS,two types of vibrations on the bit are found:stick-slip vibration and uniform motion.With an increase in the RTS,the stick-slip vibration on the drill bit disappears once the RTS arrives at its critical value.For the cases that stick-slip vibrations occur,the phase trajectories converge toward a limit cycle.For the cases that stick-slip vibration does not appear,the drill bit tends to stabilize at a uniform motion and the phase trajectories correspond to contracting spirals observed in the phase plane.

Investigation into the effect of friction decay factor on the modelling and attenuation of stick-slip vibrations of oilwell drilling systems

The self-excited stick-slip oscillations of oilwell drillstrings are attributed to the nonlinear interaction between the drill-bit and the rock formation.Development of more accurate models will lead to improved predictions allowing more potential for successful suppression of the drillstring vibrations,thus reducing damage to the drilling system,prevention of expensive failures and increased output from the oilwell.In this paper,the effect of the transition from static friction to Coulomb friction on modelling of stick-slip phenomenon of oil well drill string is investigated through an analysis of the so called‘decay factor’.Based on a distributed-lumped parameter model(DLPM)of the drilling system,the governing equations of motion for the system are obtained.By using different values of decay factor(low,high and medium),the stick-slip vibrations of the drill string are validated against published data from full-scale drill strings.The results from the simulation show that lowering the decay factor increases the critical speed and thus reduces the propensity for stick slip motion.However,a reduction in the decay factor also has the effect of inducing worse stick-slip motion once the critical speed has been reached.The results indicate the wider impact of both correct modelling of the decay factor,but also the importance of correct characterisation of the mud viscosity and drill/well contact for more accurate selection of drilling parameters in the field.