In the United States, natural gas has been stored in solution-mined caverns in salt domes since 1961. Although the mechanics of salt are reasonably well understood and there is experience with salt dome storage, there have been recent unexpected failures of the steel casing above such caverns. Casing failures associated with caverns in bedded salt domes have been documented, but less so for caverns in fairly homogeneous domal salt. We review the design and operation of gas storage caverns in domal salt. We then examine the factors that can generate tensile strains and stresses in the casing. Salt is viscoplastic and creep strains can be significant during cavern operations. Low storage pressure can accelerate creep closure of a cavern, transmitting tensile strain to the casing. High storage pressure can cause increased permeability of the salt, by induced microcracks, which can result in migration of the stored gas into the salt.
1.1. Gas Storage in Salt Caverns
It has been reported that one of the first uses of a solution mined salt cavern for the storage of compressed natural gas (CNG) was in 1961, in which Southwestern Michigan Gas Company leased and converted an abandoned cavern from the Morton Salt Company . He also reported that the first cavern specifically solution mined and designed for CNG storage was by the Saskatchewan Power company in 1963. Historical information on the storage of liquid and gaseous hydrocarbons in salt caverns has been published in , in it was reported that such storage dates back to the early 1940s in Canada. Although CNG storage in solution mined salt caverns is attractive for a variety operational and financial reasons, as described in  and , storage of CNG in salt caverns accounts for only about 3%, of the total CNG stored, with porous rock oil and gas reservoirs accounting for the vast majority of the stored CNG . Storage of CNG accelerated rapidly after 1993 after the passage of FERC Order 636, which produced significant changes in the natural gas industry . The safe storage of CNG and its transportation is a concern due to the highly flammable and explosive nature of CNG. Several case histories of the failures of salt caverns in which gas or liquid hydrocarbons were stored have been documented in . In 1995 the Gas Research Institute (GRI) conducted a risk assessment of the conversion of solution mined salt caverns to CNG storage . The GRI assessment was a consequence-based hazards analysis for salt caverns converted to CNG storage, which produced an expert-based estimated ?severity ranking? on a severity scale of greatest at 4.0 to least of 1.0 (i.e., the potential hazards of storage cavern ?failure events? were ranked, based on the estimated probability of occurrence and severity of the consequences). Example hazards are a wellhead failure due to collision with rolling machinery with a severity rank of 4.0; gas leak due to infiltration of gas into a high permeability layer or highly soluble layer with a rank of 3.0; cavern-to-cavern communication due to fracture of a pillar between caverns with a rank or 2.5; and modes of casing leaks ranging from 1.3 to 1.5.