Analytical Model of the Interaction Between a Centered Casing String and Wellbore Keyseats

In directional and horizontal wells, the running of casing strings is complicated by local geometric defects of the wellbore, in particular keyseats, which alter the contact configuration, the normal reactions, and the frictional resistance to casing running. The aim of this work is to develop an analytical model of the contact interaction between a casing string and a keyseat on the wellbore wall that makes it possible to account for the possible use of centralizers on the string and to estimate the resistance forces arising during such contact. The casing string is represented as a slender elastic member whose local side loading in a given cross-section is related to the offset of the pipe axis relative to the wellbore axis. The wellbore and keyseat walls are assumed rigid, and the frictional resistance to casing running is determined through the normal contact forces. The main interaction configurations are considered: contact of the pipe with the keyseat mouth, entry of the string into the keyseat, and the limiting case of contact with its bottom. For a wedge-shaped keyseat profile, relations are obtained for determining the contact reactions and the resistance force. It is shown that the resistance force to casing running depends non-monotonically on the casing radius and on the keyseat mouth width, because a change in these parameters may switch the system from one contact configuration to another. When the string is seated in the keyseat, increasing its effective diameter raises the normal contact reactions and impairs the running ability; at the same time, once the string leaves the zone of geometric confinement, a further increase in diameter may cause a sharp decrease in the resistance force. The originality of the study lies in treating the keyseat as an independent geometric contact object, whereas in traditional approaches, its effect is mostly considered within the general complication of the wellbore. This formulation makes it possible to analytically describe the transition between different interaction configurations and to show that a centralizer may either improve or reduce the running ability of the casing string, depending on its position relative to the keyseat and on the nature of the contact. The proposed model makes it possible to compute the resistance forces during casing running in the presence of keyseats, to justify centralizer parameters, and to increase the reliability of bringing the casing string to its design depth.