Numerical Simulation of Casing Stress during Multi-stage Fracturing Based on Stage Finite Element Method

During multi-stage hydro-fracturing operation, casing deformation issues have widely occurred in the Changning-Weiyuan National Shale Gas Demonstration Area, which dramatically reduce the productions of shale gas wells. It is important to access the influence of multiple factors on casing during fracturing operations. In this paper, a stage-finite-element method is presented to establish three-dimensional physical and finite element casing-cement sheath-formation (CCF) models during volume fracturing processes. It considers the influences of initial stress and displacement state at and near the wellbore. Loads from the initial stress field, fracturing pressure, and transient temperature changes are incorporated in the models. Cement channelling, cement elastic modulus, casing eccentricity, and fracturing pump rate are also the variables taken into account. Sensitivity analyses are conduced to reveal which one has the greatest influence on casing stress. The results indicate that casing stress is highly affected by cement channelling. Low casing stress can be guaranteed by high cement sheath modulus and optimal fracturing pressure. Casing transient temperature is significantly reduced by the large pump rate, then a certain thermal stress generating in the casing. Among all of the factors, casing eccentricity has a minor influence on casing stress.