Finite Element Analysis Of Dynamic Characteristics Of Drillstring Based On Actual Well Path

During drilling operations, drillstring vibrations occur frequently and can't be avoided completely, which not only reduce the rate of penetration and the longevity of down-hole tool, but also result in drillstring failure. Many research works have been carried out on preventing drillstring failure, but were mainly focused on the structure and material of drillstring. Up to the end of the 20th century, break-through in the field of dynamics had been achieved by some overseas companies, such as Baker Hughes, Schlumberger, et al., to prevent the drillstring vibrations and failure, and a great deal of profit have been obtained. However, limited relevant studies and practical vibration control methods associated with drillstring dynamics are reported in China, so the study of drillstring dynamic characteristics has practical value to ensure the safety of drillstring in deep and ultra-deep oilwell.Although the finite element method has been widely utilized in dynamic analysis of drillstring for decades, it was limited in straight beam element and two-dimensional arc curved beam element. When they are used to analyze dynamic characteristics of drillstring in 3D-curved wellbore, a long time will be taken, and, thus, both?accuracy and efficiency of calculation are decreased. All of these make the dynamic characteristics analysis of drillstring in whole borehole difficult, especially for the drillstring of deep and ultra-deep well. An effective method which uses spatial curved beam element to analyze the drillstring dynamic characteristics in 3D-curved wellbore has not been reported yet. Furthermore, evaluation of drillstring dynamic safety has become a major concern of drilling engineers, but it is rarely studied by scholars. In order to understand the motion of downhole drillstring, minimize the drillstring failure and ensure the safety and reliability of drillstring, the spatial curved beam finite element method has been studied and a simulation program has been developed. At the same time, evaluation method of drillstring dynamic safety was studied in this thesis. The main contributions are summarized as follows:First, based on the spatial curved beam element, a new model of drillstring dynamics was proposed to analyze the dynamic characteristics of drillstring with double nonlinearity based on actual well path.Second, an optimized iterative method has been used to solve the nonlinear dynamics of drillstring effectively, which is compatible with straight beam element and spatial curved beam element. The adaptability and reliability of straight beam element and spatial curved beam element have been studied. Based on the new dynamic model and the optimized computation method, the simulation software DDCS (Drillstring Dynamic Characteristics Simulation System) has been developed, and the simulation results are consistent with the analytical as well as measurement data, which validated the model and the computation method.Third, the dynamic characteristics of drillstring in whole borehole, including whirl, lateral vibration and axial vibration, were analyzed by DDCS. Case studies on both stick-slip motion and dynamic characteristics analysis with and without vibration absorber were presented, and then the model and DDCS were then validated practically.Fourth, an analysis of dynamic characteristics of drillstring with aluminum alloy drill pipes was carried out. The differences between the dynamic characteristics of steel and aluminum alloy drill pipes were also obtained. The reasonable explanation was presented why aluminum alloy drill pipe is easy to scratch and indentation damage on its surface, and an important conclusion was summarized that aluminum alloy drill pipes prefer to be driven by power equipment at bottom hole.Fifth, the dynamic characteristics, the whirl orbit ,whirl speed and acceleration, et al., of drillstring over 7,000 meters long in the ultra-deep well was simulated successfully with DDCS, and, the effects of WOB (weight on bit) and rotary speed on the dynamic characteristics were obtained.Finally, in view of the contact and collision between drillstring and wellbore as well as the static stress of drillstring, a evaluation model of drillstring dynamic safety was firstly established based on drillstring dynamic characteristics and the fourth strength theory. Case study on dynamic safety of drillstring of DB3 well in Tarim Oilfield was made, and the reasonable rotary speed and WOB were presented.