Research On The Mechanism Of Vibration Excitation And The Drillstring Dynamics In Ultra-Deep Wells

Oil and gas are the main energy and resources of human society,and their continuously rising consumption is driving the exploration and development to the deeper reserves.Drilling is an indispensable process for the exploration of oil and gas,and provides the channel for oil and gas production.As one of the important components of the drilling system,drillstring operates under extreme downhole conditions,and suffers from severe vibrations,which may not only cause premature failure of drillstring components,excessive wear of bit,and instability of borehole wall,but also lead to drilling catastrophe that results in the borehole abandonment.Since the 1960 s,researchers have conducted a lot of works on the drillstring dynamics through experiments and numerical analysis,and fruitful accomplishments have been achieved.However,with the increasing drilling depth and the wide application of downhole special tools,some problems relating to drillstring dynamics emerge.First of all,the excitations of downhole special tools are seldom considered,which makes it impossible to correctly simulate the dynamics of drillstring with downhole special tools.Secondly,the transitions among different regimes of the polycrystalline-diamond-compact(PDC)bit-rock interaction are not taken into consideration,which makes it difficult to reveal the intrinsic mechanism of the velocity weakening effect of torque on bit.Thirdly,the fluctuation of rotary speed of downhole drillstring is neglected in the previous drillstring dynamics studies,so that the drillstring whirling behavior is unable to be accurately characterized when stick-slip oscillations occur.Fourthly,the lateral vibration of bottom-hole-assembly(BHA)with downhole special tools needs to be studied with an effective technique.The main contributions of this paper are summarized as follows:(1)The Lagrange equation governing the motion of drillstring is derived from the Hamilton principle.According to the load and motion states of the drillstring,the expressions of kinetic energy,potential energy and work of the external force are formulated and then the Euler-Bernoulli beam elements are used to discretize the drillstring to obtain the finite element equations of the drillstring.In the deviation process,besides taking the geometry nonlinearity of the drillstring,friction contact between the drillstring and wellbore,and the added mass and damping effects of the drilling fluid into consideration,the influence of the fluctuation of rotary speeds is also considered,which makes the axial-torsional-lateral vibrations of drillstring fully coupled.(2)Based on the trajectory of the PDC bit,the bit-rock interaction model is established,and the transition conditions of bit-rock interaction among different regimes are derived.The study on this bit-rock interaction model reveals the intrinsic mechanism of the velocity weakening effect of torque on bit,and a suitable model of the PDC bit excitation for the finite element analysis is proposed.Furthermore,the depth of cut is reformulated with the bit trajectory function,which avoids the introduction of a state-dependent delay into the bit-rock interaction model and thus greatly improves the computational efficiency of the bit-rock interaction model.(3)The weighted residual method in conjunction with the finite element method is used to solve the lateral vibrations of BHA with downhole special tools.The effects of drilling parameters,added mass of drilling fluid,and the configuration of the BHA on the dynamic responses of the lateral vibration are analyzed.In addition,the Newmark method combined with an optimized iterative method is utilized to solve the equations of motion of the drillstring to obtain the transient dynamics of the drillstring.(4)The downhole drillstring accelerations are measured with a tri-axial accelerometer in an ultra-deep well,and the analyses of the measurement data show the characteristics of downhole drillstring vibrations.The proposed finite element model is then used to numerically simulate the dynamics of the experimental drillstring,and the simulation results agree well with the measurement data,which verifies the fidelity of the finite element model.On this basis,the characteristics of drillstring stick-slip oscillations and whirling are simulated and the results show that the stick-slip oscillations of drillstring have a strengthening effect on the whirling behavior.(5)The effects of drilling parameters on the stick-slip oscillations and whirling are studied in order to find the effective measures to mitigate the stick-slip oscillations.The simulation results show that raising the surface rotary speed to suppress the occurrence of stick-slip oscillations will induce more severe BHA whirling.Hence,adjustment of the weight on bit is a reasonable way to eliminate the stick-slip vibrations,which is in accordance with the on-site practices.