Selection of Drill Pipes Based on Critical Loading Conditions for Slim Hole Drilling
Authors: Akgun F. , Estrella D. , Rahman S. , Mitchell B. , Eustes A. 
Published: December 2007

Abstract
Slim bole drilling has significant potential to reduce well costs. This cost savings are especially important with increased demand for reduced capital budget under current economic conditions in the oil industry. The savings can be achieved by use of smaller drilling rigs and/or workover rigs, reduced casing size, reducing requirement for drilling consumables and other costs associated with hole size. Cost savings achieved from slim hole drilling, however, can be offset by inability to effectively transmit the weight to the bit, increased mechanical failures of drill pipes and tools, in particular, in drilling at greater depths (depths greater then 1,500 m), reduced horizontal section of drilled hole, and lack of directional control. This paper investigates the effects of borehole parameters on drill pipe stability in an inclined and curved hole by means of non-linear analysis of Finite Elements Methods (FEM). Results are presented in the form of a set of tables and diagrams to predict Critical Buckling Loads (CBL), pipe/hole contact forces, contact points, contact lengths, and buckled pipe bending stresses. Drilling parameters considered are: hole inclination and curvature, pipe size, hole size, pipe thickness, mechanical properties of drill pipes and weight on bit.

Introduction
This paper presents critical buckling loads for drill pipe in vertical, inclined, horizontal, and curved oilfield type of boreholes. These findings are different in that no past work has encompassed both inclined and curved boreholes (Figure 1) in buckling research nor did it use non-linear mechanics which is permitted with the FEM.
Buckling Problems. Buckling of oilfield drill pipe which is confined in a drill hole usually does not cause immediate failure but leads to premature failure through fatigue and erosion. In horizontal wells "lockup" of drill strings terminates the controlled drilling of the reach section. Lockup occurs when the compression at any location within the drill string is equal to or is greater than the drill string's resistance to buckling at that location. In other words, the weight on the bit plus the drag on the drill string is greater than or equal to the CBL of the string. Lockup is analogous to the act of attempting to push a rope through a horizontal section of pipe. The rope (the drill string) coils (buckles) just inside the entrance of the pipe (the well bore) and the far end of rope stops moving within the pipe. The rope is locked up. In a real bore hole, at lockup, the buckling of the drill string prevents the transfer of the weight of the drill string in the vertical section of the hole through the reach section to the drill bit. This is because the drill string no longer lies flat along the bottom of the horizontal section of the hole but is wound around the inside of the bore hole, probably in the shape of a helix. Without the weight on the drill bit, drilling ceases and the drilling of the reach is terminated.
Current buckling models are based on Arthur Lubinski's pioneering work in the 1950's. The Bogy and Paslay paper on buckling of drill pipe in inclined holes was published in 1964. Other buckling work is based on Leonhard Euler's equations for determining buckling of long slender columns published in 1757. Most of the buckling models are based on critical assumptions. The most critical assumption is that the wellbore and drill string together act in a linear manner. For straight and vertical holes, this is an adequate assumption; but, for horizontal and long reach wells, this assumption can and does cause significant errors with respect to actual drilling operations. Adding non-straight conditions to the long reach hole can cause even higher errors. These errors are caused by the non-linear nature of drill string buckling. Current models can not take into account these non-linearities. However, FEM can take these non-linearities into account.
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