Analysis and Quantification of Wellbore Tortuosity

Abstract

Summary Small-scale wellbore tortuosity—variations in attitude on a length scale smaller than standard survey intervals of 30 m (100 ft)—is generally neglected because of its small effect on the final position of the well and its unclear relation to traditional dogleg severity (DLS). However, it is well-known that such tortuosity may have significant influence on the drilling process and on drilling efficiency. Furthermore, it is a crucial factor for the design and installation of completions and production equipment, because a highly tortuous wellbore section, depending on borehole diameter and tortuosity amplitude and along-hole distribution, may exert strong bending forces on such equipment, or high friction on moving parts. This paper describes a novel methodology for analyzing the tortuosity of openhole wellbores, casing, drillstring, and production tubing. Several related tortuosity parameters are described, and examples of application to field data are included. The methods use high-resolution survey data [measured depth (MD), inclination, and azimuth], which may, in principle, originate from any surveying tool or service capable of providing such data. The methodology requires input data from a single survey only. On the basis of a user-defined length as single external parameter, tortuosity can be analyzed on any length scale greater than approximately 10 times the input survey-data interval, and with a maximum resolution equaling twice the data interval. The processed parameters include relative elongation of a tortuous section compared with a straight-line section, transverse displacements from the straight line, and maximum available diameter for a downhole device caused by the small-scale bendings of the section. The results can be displayed as graphs vs. MD, or as 3D-rendered views of the actual wellbore or tubing shape. Results from various field cases are included, in which the tortuosity analysis was applied to high-resolution continuous gyro survey data collected in cased wellbores. In all the field cases, the novel methods revealed sections of considerable tortuosity that were either unnoticed, or located with unacceptably low accuracy, by conventional methods. These results led to re-evaluation of the planned locations for completion and production equipment. The characterization of the wellbore in terms of tortuosity on various length scales may be of crucial importance for the functionality and lifetime of permanently installed equipment. For example, identification of highly tortuous sections will aid the placement of rod-guide wear sleeves, increasing the rod and casing life and reducing the workover frequency. Another application is the identification of low-tortuosity sections in which downhole pumps or other equipment will not be subject to excess bending. In addition, the tortuosity results may help evaluate the drilling equipment and the drilling process.