Cement sheath is an important barrier element for maintaining well integrity. Poor quality cement has often been cited as one of the major causes for loss of well control events. This paper presents a useful approach for quick assessment of fitness for service and mechanical integrity of cement sheath.
For this study, three dimensional finite element models consisting of concentric casings and annular cement sheath were created. Different wellbore and annular pressure loads were applied. Performance of cement sheath was assessed by analyzing radial, hoop, axial, and maximum shear stresses for mechanical failure. The FEA model was validated by analytical calculation of the stresses.
Parametric study was performed by individually varying influencing factors such as — cement sheath material properties, its dimensions, and wellbore pressure loads. Sensitivity response curve was developed by normalizing influential parameters and representing them against a mechanical stress on the same plot. The plot can be used to estimate whether cement will structurally fail due to various operational wellbore pressure changes, or variation in material properties caused by aging or temperature changes. The plot can also be utilized to identify the most critical factors influencing cement’s performance.
This paper presents a simple and effective approach of developing sensitivity response curves for a set cement sheath. Industry standards or guidelines can be improved by adding sensitivity response curves for standard cement recipes. This can help design engineers, and regulators alike in quickly evaluating short-term or long-term “fitness for service” of cement sheath from structural integrity perspective.