Study Of Longitudinal Vibration Of Horizontal Well Drilling String

The vibration of drilling string in horizontal well is closely related with the overall length of the drill string and structural shape. In this paper ,a mechanical model of longitudinal vibration of drill strings is established by studying the overall drilling string,seeing a hole as arbitrary bending curve in three-dimensional space. Three-dimensional dynamic finite element methods and theories , improved the Ritz vector method and vibration shapes superposition method are applied to analyze and calculate the dynamic response of drilling string.When analyze the forced vibration of drill string ,it is taken into consideration that the friction between drill string and wellbore impact on the vibration of the drill string.So static analysis of the drill string is included in this paper. By using static gap element theory, calculating the drill string friction through nonlinear mechanics iterative, the size and distribution rule of friction are found . By taking the stress at pin end as the calculated stress, the drillstring fatigue limit stress formula is raised, thus for the quantitative assessment and prediction of drillstring fatigue has a theoretical basis. While based on theory and calculation the effective measures to reduce vibrations are provided. In this paper, using the formula and model to analyze and validate the field data from some well,the following are get:The mechanical model of longitudinal vibration of drill strings and the finite element analysis method is effective and can be applied to the program. The use of Improved Ritz vector method greatly improve the calculation speed , reduce the computational workload and create favorable conditions to practice in project. the drillstring fatigue limit stress formula is correct, providing the quantitative assessment and prediction of drillstring fatigue for a theoretical basis. Applying this model ,the drillstring fatigue strength and safety evaluation can not only be assessed or predicted ( at drilling project design), but also be analyzed(after drilling). It is useful to analyze the cause of drilling string fracture accident in drilling horizontal well and make improvements.