An Analytical Model for Axial Force Transfer and the Maximum Compression Point of Work Strings in Extend Reach Drilling

Abstract

Complex work string dynamics are often observed when one is investigating the limit of Extended Reach Drilling (ERD), yet the underlying physical causes of anomalous problems are often not fully understood and is thus a topic of ongoing research interest. Theoretical models capturing tubular dynamics have been previously proposed to analyze force transfer in work strings, yet there is significant confusion regarding these models because their published versions are not entirely consistent and, in many cases, do not meet engineering requirement. Further confusion is introduced through variations in pin-pointing locations where axial compression in the work strings should be checked for mechanical integrity. A simple and yet rigorous mathematical model is essential for adequate prediction of axial compression profiles in work strings for ERD. This article presents a simplified tubular mechanics model for describing axial force transfer under ERD conditions. We discuss the model in finding the point of peak compression in casing strings. This point is predicted to be in the arc section near the heel of a horizontal wellbore, under borehole drilling, and casing running conditions. The exact location of the peak axial compression changes with pipe-wall friction coefficient. For the friction coefficient in the range of 0.15 to 0.35, this point occurs in the range of inclination angle between 70.7 o and 81.5 o , averaging 76.1 o . The model can also be used for other purposes, including prediction of depth limit, bottom hole assembly (BHA) design, and locking up analysis of coiled tubing (CT) strings.