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Natural gas storage in salt caverns requires fast injection / withdrawal cycles due to the increasing dynamics of the energy market. High rates induce rapid changes in the internal pressure of the stored gas causing important temperature changes susceptible to damage the rock salt mass. To experimentally observe this, the Starfish project aimed to characterize the damage caused by purely thermal stresses at the surface of a large bloc of rock in the salt mine of Varangeville (France). The objective was to determine the type of failure mechanism involved with repeated cooling stages. Since the salt is favourable to the generation of Acoustic Emissions (AE) and the propagation of stress waves, acoustic monitoring was chosen as one of the methods to follow the impact of the salt cooling. In addition to thermal and mechanical sensors , an acoustic monitoring device consisting of 16 ultrasonic sensors was installed on the free surface and in boreholes . It enabled to record and locate a large number of AE (58,426) located with good accuracy (2.5 cm). Those AE can be correlated to the evolution of salt fracturing. Acoustic activity is very intense at the start of each cooling cycle, then decreases with time to reach a very low level (background) after about 15 days. The average localisation depth reached by the AE is about 90 cm during the first cooling period. For subsequent cooling cycles, this depth is limited to 74 cm. These results show that the first cooling period is decisive, as it contains the strongest and deepest acoustic emissions.

Author(s): Blanco-Martin, L; Rouabhi, A; Billiotte, J; Hadj-Hassen, F; Tessier, B; Hevin, G; Balland, C; Hertz, E | Abstract: High frequency cycling of salt caverns is becoming common practice to meet the needs of energy markets and to foster underground energy storage. In the case of rapid cooling, tensile stresses and thermally-induced fractures can appear in the surrounding rock, with potential detrimental consequences to the integrity of the storage project. To further investigate the effects of rapid cycles on the integrity of rock salt, a thermo-mechanical test was performed in a salt mine. It consisted in cooling rapidly several times a salt surface of 10 m2 (ΔT=−20 °C in about 8 h). Extensive monitoring allowed tracking the thermo-mechanical response of the rock, including possible fracture creation and propagation. Although more research is needed, the test demonstrated that tensile fracturing due to rapid cooling is possible. Thermo-mechanical modeling allowed reproducing fairly well the location, orientation and timing of the first fracture; indeed, fractures should be avoided to ensure cavern integrity, and therefore knowledge about the critical zones where fractures could appear is sufficient at the design stage.