Real-Time Evaluation of Structural Integrity of Tubulars in Directional Wellbores

Abstract

The actual forces acting on the drill string in directional drilling is relatively complex than vertical drilling. In this work, the different forces acting on the drill string during directional drilling are analyzed using actual drilling data. The calculation of such forces can help driller to predict downhole complications that are caused due to drill string failures. The estimation of effective tension force at the top of the drill string requires both true tension forces and buckling stability forces acting on the drill string. True tension is a function of weight component of the drill string, the forces acting on BHA due to change in cross-sectional area and bottom pressure force acting on the drill bit and the drag forces acting on the string. The buckling stability force is defined as the difference between the internal and external force acting on the drill string. The effective tension is used to calculate the hookload and normal forces acting on the drill string. The calculation of the hookload at the deadline can help the driller to compare with actual hookload and take corrective action before the complication occurs. Further, that requires the relationship between the effective tension force at the top of the drill string and the hookload measured at the deadline. Such a relationship can be established by knowing the efficiency of the rig components such as sheave, block and tackle system, hydraulic lines and weight parameter for remaining components. Considering the unavailability of the efficiency of these components, the following model parameters are introduced: sheave efficiency, correction factor for efficiency of block and tackle system, hydraulic lines and weight parameter for the remaining components. All the three parameters are estimated by tuning the model with actual directional drilling data. In another aspect, the true tension is used to locate the position of neutral point by calculating the axial stress along the drill string. The proposed model is capable of predicting the hookload at the deadline, position of neutral point and normal forces acting along the drill string. The abnormal behavior of the normal forces along the drill string is used to locate the key-seating zones. Further, the model is validated with actual directional drilling data and successfully implemented in real-time monitoring platform and the model is found to be capable of predicting downhole complications such as drill string parting and improper hole cleaning. This study is expected to provide theoretical bases for understanding the stability regions of directional well.