Improved Calculation of Casing Strains and Stresses in a Steam Injector Well Simulator

Abstract

ABSTRACT This paper describes the modifications implemented in a steam-injection well simulator to model casing strains and stresses produced by the injection/production of hot fluids. These modifications allow the calculation of the axial, radial and hoop stresses, and the axial and radial displacements induced by temperature and pressure changes. With these calculations, it is possible to predict tension and compression failures and casing bursts or collapses, produced by excessive pressures and temperatures. The calculations take into account the thermal and mechanical effects simultaneously. The recommended API calculation is only valid for inner diameter-thickness ratios larger than 20, but since casings with an inner diameter-thickness ratio of less than 20 are not uncommon, an expression was developed to calculate the maximum burst resistance for any casing thickness. The energy distortion theory was used in the analysis of the combined stresses. All of these calculations can be carried out at any point along the casing and with any boundary conditions. Comparisons with other models reveal several differences. The calculated stress values differ because other models do not consider the effects of the reactions at the fixed points, and do not report the radial and hoop stresses, which are important for the cementation. A good prediction of the casing strains and stresses provide valuable information to avoid possible well failures which are very costly and time consuming.