Interactions between Torque and Helical Buckling in Drilling

Abstract

Abstract The effects of torque on helical buckling are a concern in coiled tubing drilling. These effects are also important in conventional drilling of long horizontal wells. This paper presents theoretical models for the effects of torque on helical buckling, normal contact force, and pitch of helix in the curved well bore. The effects of helical buckling and the post-buckling contact forces on torque and drag in a drilling operation are also modelled. The models are used for realistic simulations for both conventional drilling and coiled tubing applications. Introduction Torque and drag are important factors in extended reach drilling and can represent the limiting factor in how far out a well can be drilled. Buckling is a phenomenon that increases both torque and drag. Buckling occurs when the compression in a tubular exceeds a critical value. The tubular then deforms into a more or less spiral form. This results in increased wall contact forces and frictional effects. Buckling is a major concern in coiled tubing and many theoretical and experimental works have been reported. These papers deal with the buckling of tubulars of various sizes and uses in the petroleum industry. including casing. drill string and coiled tubing. Increased use of coiled tubing. particularly in extended reach wells and horizontal wells has lead to renewed interest in buckling phenomena. Only Miska has previously reported on the relationships between torque and buckling. Buckling is not normally important in drilling operations. With shallow or nearly vertical wells where the drill string is mostly in tension, it is impossible for buckling to occur. However, as wells become longer and more deviated, it is often the case that large parts of the drill string are in compression and buckling effects must be considered. It is necessary to include the interactions between torque and buckling if drag and torque are to be correctly computed for these extreme wells. Interaction Between Drag, Torque and Buckling Buckling is known to increase the contact forces between the drill string and the well bore walls. This will increase both rotating torque and drag. The first motivation for this study was that buckling could give a significant increase in rotating torque. This has been verified in this work. In an attempt to calculate these effects for real wells, it was necessary to also consider the question of how torque can influence buckling. The simultaneous computation of torque. drag and buckling could be complex. The situation is depicted schematically in Figure 1. The link between buckling and the contact force is strong and well established both theoretically and experimentally. The increased contact force due to buckling will increase both torque and drag. Increased drag will increase the compression in the drill string and this will lead to even more buckling and even higher contact forces. Thus the coupling between drag and buckling is strong and this makes it more complicated to compute drag. If torque also strongly affects buckling. either by decreasing the compressional forces necessary to buckle the pipe or by increasing the contact forces in the buckled state. then it will be necessary to develop complex algorithms to compute torque. drag and buckling simultaneously. The theory of torque effects on buckling is outlined below. Later, these effects are visualised through several examples. P. 725