Scaled Tortuosity Index: Quantification of Borehole Undulations in terms of Hole Curvature, Clearance and Pipe Stiffness

Abstract

Abstract Wellbore tortuosity is a potential source of friction, which can lead to problems while running casing and completions. Describing a smooth trajectory qualitatively as one with the least amount of bends is easy. However, quantitative tortuosity indices based on survey trajectory data alone have shortcomings that make comparisons and interpretation very difficult: Their computational results depend on the survey interval (averaging effect). While 90-ft surveys hide important wellpath features, short spacing surveys might yield alarmingly high tortuosity, which does not reflect the curvature of the pipe in the borehole. Varying stiffness of the pipe and clearance between the pipe and borehole can have different levels of negative consequences with the same survey data. Stiffness and clearance have a direct impact on physical bending moments and sideforces, which drive frictional losses when moving pipe in an out of a borehole. The scale of the wellbore undulations relative to the clearances is relevant to be able to distinguish relatively benign undulations from tortuosity that limits wellbore construction. In formulating a scaled tortuosity index we have taken into account that actual hole curvature is not the curvature of the pipe in that hole, which is governed by stiffness and clearance. The main driver for the index is the bending moment profile along the pipe as it is positioned in the wellbore. The proposed index describes the total amount of required elastic energy (originating from these bending moments) to bend all the components in the string in all the required configurations to move it to total depth. The index is linked to nominal hole and casing sizes, (i.e. 7in casing in 8.5in hole) to make it applicable and comparable. Hence this concept is different from the existing dimensionless wellbore energy index.