Analysis of Stuck Pipe in Deviated Boreholes

Abstract

Abstract The complexity of the wells have increased significantly in later years. Reach has more than doubled, and high inclination and fully 3-D well paths are common. However, statistics shows that sidetracking the boreholes due to stuck pipe has also shown a significant increase, and is presently a high cost factor. The margins between success and failure are now much smaller. A larger study was initiated to understand the stuck pipe situation better, and to develop improved procedures. A mechanistic approach was chosen. The following elements were analyzed:the forces developed during differential sticking,pipe strength under combined loads; tension, torque and pressure,effects of buoyancy under various conditions like equal or different mud densities in drillpipe and annulus,wellbore friction as related to torque and drag. This paper presents new equations to determine depth to the stuck point in deviated wellbores, based on pulling tests and torsion tests. In particular it is shown that bends in the wellbore leads to more friction, which with the new equations results in a deeper stuck point in a deviated well compared to a similar vertical well. Knowing all the forces involved in a stuck pipe case, another analysis was performed to determine the action that has largest impact to free the pipe. One of the main conclusions is that the most important element to free the string is to keep the bottomhole pressure as low as possible. The paper will present three methods to free the pipe, which where developed from the analysis:maximum mechanical force method,minimum density method, andmaximum buoyancy method. A detailed field case from the Yme field in the North Sea will demonstrate these methods, and show the effect on the stuck point using each method. In addition to the field case, the paper will in the Appendices present the complete equations for pipe stress and strength under 3-dimensional loading, and define the effects of buoyancy in deviated wellbores. In particular is the buoyancy issue resolved, showing the differences and similarities between "the piston force approach" and the law of Archimedes, as applied to deviated wellbores.