The operation of a gas storage facility at a harvested natural gas field is handled by a number of wellbores for shut-in and production. After running the storage facility for several years deformations of the casings have been detected in some wells. The suspicious cross-sections have been located in the Upper Permian saliferous layers. The deformations ranged from a slight ovality to the collapse of the casings accompanied by the effect of a reduced flow-rate and minor performance in production. To prevent the slightly oval-ized casings from total collapse and to save the cost of expensive sidetracks numerical models have been developed to back-analyze the deformations and to find optimized solutions that guarantee the economical long-term stability of the wells. The back-analysis of slightly deformed cross-sections can be successfully performed with the material properties determined at our rock-testing laboratory. The calculations have shown that the deformations are time-dependent. It is possible that they increase up to collapse depending on the casing's resistance. With a variety of numerical simulations it is possible to evaluate the risk of collapse for existing casings as well as for projected wellbores. In the presented example it has been possible to determine the critical cross-sections and to recommend a solution for a long-term-stable well.
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