Mechanical analysis and design of casing in directional well under in-situ stresses

• The current casing design isn't suitable for the casing in directional well under the action of in-situ stresses.
• The influence factors on casing stress in directional well under in-situ stresses are found.
• The analytical result proposed in this paper is verified by the numerical simulation.
• The casing mechanical analysis in directional well under in-situ stresses remedies the limitation of current standards.

Casing deformation and collapse hamper the regular drilling, production and downhole operation. The casing in directional well under the action of inelastic surrounding rock displays a complex mechanical state. Taking the in-situ stresses and well trajectory into account, the mechanical model of casing in directional well under in-situ stresses is established. By coordinate rotation, the three principal in-situ stresses are converted to a stress tensor in the wellbore coordinate. The mechanical interaction of casing and surrounding rock is simplified to a generalized plane strain problem. Based on the theory of pressurized hole in plate, the stress of the infinite surrounding rock is calculated. To analyze the casing behavior, the complicated solution is divided into three simpler problems: the elastic mechanics analyses under mean stress, deviator stress and shear stress respectively. The analytical expressions of casing stress and load in directional well under in-situ stresses and inside hydrostatic pressure are deduced. This analytical solution is verified by numerical simulation. Furthermore, casing design for the directional well through complex formation in an oilfield has been conducted. The results can remedy the current standards of casing design and guide the well integrity design in directional well under in-situ stresses.