LAUSR

Abstract Compared with the conventional tubing with threaded connection, the coiled tubing (CT) has the advantages of no screwing on and off and it has been widely used in drilling, work over and fracturing operations. But the difficulties of exerting bottom weight-on-bit and horizontal extended lengths are the key problems that restrict the CT drilling technology. In this work, based on energy method, the critical loads of sinusoidal and helical buckling are presented for the CT under bottom weight-on-bit in the curved borehole. According to the geometrical shape of sinusoidal or helical buckling in the curved borehole, introducing the sliding displacement boundary conditions of CT in the Lagrangian multiplier method, the gravitational potential energy and friction resistance dissipated energy are considered in the energy method, the contact force, friction resistance between CT and wellbore are then proposed with factors of well trajectory curvature and well deviation. Consider a sidetrack horizontal well with straight section length of 200 m and bending section length of 174 m, the buckling behavior and friction resistances are conducted for 2 3 8 ' ' CT with different horizontal section lengths. The limit extended lengths are finally obtained for CT under different weight-on-bit and friction coefficients in the sidetrack horizontal well. This finding is important for CT applying in drilling, work over and fracturing operations.