Casing Failure in Salt Rock: Numerical Investigation of its Causes

Numerous cases of casing failure have been reported worldwide. Depletion-induced compaction is one of the main factors that may cause casing failure. Stress perturbation in salt layers may accommodate rock flow which endangers the stability of cased wells. Besides, poor cementing jobs are recognized as one of the wellbore instability causes. These factors need to be considered to determine the mechanism behind the casing failure. In this study, the creep behavior of the caprock salt layer in the southwest Iranian oil fields is experimentally studied through a number of creep tests under conditions of elevated temperature and pressure. Then, a three-dimensional finite element analysis is utilized to examine the validity of various casing failure scenarios after calibrating the model with well logs and experimental data. The results confirmed that the combined effects of creep behavior of rock salt and cementing imperfection can be the main cause of failure in the wellbore under study, because the failure time and depth predicted by the numerical simulation were found to be in accordance with the real field data. By history matching, the geomechanical parameter of the salt layer has been modified for study of the casing failure in other wells in the same formation.