Wellbore Tortuosity Analysed by a Novel Method May Help to Improve Drilling, Completion, and Production Operations
Small scale wellbore tortuosity – variations in attitude on a length scale smaller than standard survey intervals of 30 m (100 feet) – is generally neglected due to its small effect on the final position of the well and its unclear relation to traditional dogleg severity. However, it is well known that such tortuosity may have significant influence on the drilling process and the drilling efficiency. Furthermore, it is a crucial factor for the design and installation of completions and production equipment, since a highly tortuous wellbore section may exert strong bending forces on such equipment, or high friction on moving parts.
This paper describes a novel methodology for analysing the wellbore tortuosity, and shows examples of application to field data. The methods utilise high resolution continuous gyroscopic survey data (inclination and azimuth), and an analytical definition of tortuosity. Based on a user-defined length as single external parameter, the methods have the robustness and flexibility to analyse tortuosity at any length scale greater than the input survey data interval. In one method, a tortuosity parameter is defined over a pre-defined wellbore section as the ratio between the along-hole length and the length of the straight line between the end points. The tortuosity parameter can be displayed as a graph versus measured depth. In another method, the transverse deviations of a set of survey points from an established reference line are calculated. These transverse deviations describe how the wellbore bends on a small scale over the analysed section, causing restrictions to the maximum diameter of a device that shall pass through the section, or be permanently placed at this depth. The results can be displayed as a graph of maximum allowed diameter as a function of measured depth, or as a 3D view of the actual wellbore shape.
Results from various field case examples are included, in which the tortuosity analysis has been applied to field survey data. In all the field cases, the novel methods revealed sections of considerable tortuosity that were either unnoticed, or located with unacceptably low accuracy, by the conventional methods. Although verification of the methods is still needed, some of these field results have already 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 identification of low tortuosity sections where downhole pumps or other equipment will not be subject to excess bending. In addition, the tortuosity results may help in evaluating the drilling equipment and the drilling process.
The tortuosity analysis has the potential to contribute to technical and procedural improvements and cost savings in areas ranging from drilling operations to the completion phase and initial and long term production.