Drillstring Dynamic Characteristics Analysis And Dynamic Safety Evaluation For Ultra-deep Well

Since the oil and gas resources in shallow formations have been explorated and developed for a long time, deep petroleum resources become more and more important for the further development of China. The technology of ultra-deep wells has been widely used to develop the deep oil and gas. However, with increasing number of ultradeep wells, the drillstring failure accidents raise, leading to the rapid increase of drilling costs. Experts have recognized that many complex scientific issues and key technical problems for the ultra-deep drilling need to be solved urgently. In particular, the theoretical study and application technology on drillstring dynamics for ultra-deep wells are lack of research in China, compared similar study performed abroad. Therefore, it is necessary to conduct the drillstring dynamics research of ultra-deep wells, combined with the practical engineering. The working state of drillstring in the wellbore and the failure mechanism of drilling tool can be better understood, with the dynamics analysis of the ultra-deep well drillstring, therefore, the risk of failure can also be reduced and the drilling efficiency will be improved.Studies on the drillstring dynamics have been proceeded over decades, and many fruitful results have been achieved. Some of the results have already been applied in practical productions, but certain problems still exist. The mutual coupling among various forms of vibration has been considered in the existing models of drillstring dynamics analysis, but there is no complete unified incentive model, which causes the drillstring dynamics simulation of torsional vibration and whirling motion to adopt different incentive model. The drilling fluid viscosity is only considered when the Rayleigh damping model is used to simulate the drillstring vibration, but the influence of the inside and annular fluid motion towards the drillstring cannot be considered. In the current study, the evaluation of drillstring fatigue failure merely depends on the static analysis, whereas the dynamical stress increase and the impact of vibration frequency were not taken into consideration. Therefore, it is of great significance to conduct an theoretical and application study on the dynamics and dynamic evaluation of drill string for ultra-deep well.In order to reduce the drillstring failure accidents and improve drilling efficiency, the motion feature and stress state of drillstring in the ultra-deep well were obtained through the dynamic characteristics analysis in this paper. The drillstring dynamic safety in the ultra-deep well was evaluated. The main contributions were summarized as follows:1) Considering the couple among axial, lateral and torsional vibration as well as the interaction between the drilling fluid and the drillstring, the drillstring dynamics model was established. Then, the model was solved by the whole matrix, the node iteration and the Newmark method.2) Based on the interaction between the bit and the formation, a unified incentive model, which can reflect the characteristics of both stick-slip vibration and whirling motion, was proposed. This model can explain the incentive mechanism of stick-slip vibration and whirling motion well, and it is closely related to the lithology parameters of formation.3) Based on the proposed model, the drillstring dynamics simulation program for the ultra-deep well(UDCSP) has been developed. The simulation results are consistent with the analytical solution as well as the field measured data, which validates the model and the computation method.4) Considering the field measured data, the formation influence coefficient of the bit-formation interaction model was inversed through UDCSP. The results shew that the stick-slip vibration tends to occur in the formation of high mudstone content and the formation influence coefficient is higher, while the drillstring is prone to whirl in the formation of high sandstone content and the formation influence coefficient is lower.5) The simulation results of stick-slip shew that the stick-slip vibration of drillstring is more obvious for the position close to the bit. The rotary speed at the surface and the weight on bit have greater effects on the amplitude of downhole rotary speed while the well depth has a significant influence on the period of stick-slip vibration.6) Combined with the dynamic stress solved by UDCSP and the dynamic safety coefficient method, the dynamic safety of different drillstring structures were evaluated and related optimization charts of drilling parameters were obtained.7) A method of fatigue frequency ratio was proposed to rapidly evaluate the drillstring fatigue failure, by which the fatigue failure risk for the drillstring of a specific well was assessed. Results shew that the fatigue frequency ratio method can more effectively evaluate the fatigue failure risk of drillstring.