Research on the Compatibility Between Washover String and Casing in Directional Casing Damage Wells

Nowadays extracting or changing casing is a mature technique used in vertical wells, which can solve the problems of casing breaking and corrosion. But for directional casing damage wells, the phenomena of milling head cutting casing often happen because of the stiffness difference between washover string and casing. In order to evaluate the compatibility between washover string and casing, some studies have been made in this paper: (1) an axial force calculation model is established, which is used for mechanical analysis of washover string in the condition of well wall outside and casing inside. (2) a turning force calculation model is obtained through mechanical analysis of the milling head-ball-casing assembly. The calculation model can reflect the compatibility between washover string and casing. And on this basis the contact stress of the assembly can be simulated with Comsol Multiphysics. When the contact stress exceeds casing's yield stress, yield failure happens. A poor compatibility will result in casing to be cut off and the interruption of extracting operation. The research results provide a method for evaluating the compatibility between washover string and casing.