Mechanical property design method of cement sheath in a horizontal shale oil well under fracturing conditions

Based on the elastoplastic model of cement sheath considering the influence of three-dimensional principal stress and the interface stress field model, a mechanical performance design method of cement sheath is established to meet the wellbore sealing requirements during fracturing. This method takes the failure types of the cement sheath, such as tensile failure, plastic yield, interface propagation crack, and zigzag propagation crack into account. Meanwhile, the Young's modulus and Poisson's ratio quantitative design charts of cement sheath are constructed based on this method, and they define the safety and risk areas of wellbores, and quantify the yield strength and tensile strength indexes of cement sheath. The results show that decreasing Young's modulus, increasing yield strength and Poisson's ratio of cement sheath can avoid plastic deformation of cement sheath; increasing the tensile strength of cement sheath can prevent its tensile failure; increasing Young's modulus and Poisson's ratio of cement sheath is good for shortening the length of the interface crack, but will increase the risk of interface cracks zigzagging into cement sheath. The model calculation and case verification has proved that the method in this paper can give accurate calculation results and is convenient for field application.