Rapid gas depressurization leads to gas cooling followed by slow gas warming when the cavern is kept idle. Gas temperature drop depends upon withdrawal rate and cavern size. Thermal tensile stresses, resulting from gas cooling, may generate fractures at the wall and roof of a salt cavern. These fractures are perpendicular to the cavern wall; in most cases their depth of penetration is small. The distance between two parallel fractures becomes larger when fractures penetrate deeper in the rock mass, as some fractures stop growing. These conclusions can be supported by numerical computations based on fracture mechanics. Salt slabs are created. These slabs remain strongly bounded to the rock mass and it is believed that in many cases their weight is not large enough to allow them to break off the cavern wall. However, depth of penetration of the fractures must be computed to prove that they cannot be a concern from the point of view of cavern tightness.
Gas storage caverns used to be developed mainly for seasonal storage, with one cycle per year and a moderate pressure rate between the maximum and minimum operation pressure (1MPa/day often was a maximum depressurization rate). However, the needs of energy traders are prompting change toward more aggressive operating modes. Typically, high-deliverability caverns (HFCGSC) can be emptied in 10 days and refilled in 30 days or less. At the same time, Compressed Air Energy Storage (CAES) is experiencing a rise in interest. They are designed to deliver full-power capacity in a very short time period.
Both types of facilities imply high gas-production rates and multiple yearly pressure cycles. This cycled mode of operation raises questions regarding frequently repeated, extreme, mechanical and thermal loading.
