ase study of a thermo-mechanical field test simulatng
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high frequency gas storage cycles in salt caverns
n experiment was undertaken in an salt mine to assess the influence of high temperature changes on
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the stability of salt cavities as a consequence of high frequency gas storage operation cycles. A salt
rock block was exposed to repetitive rapid cooling cycles while its thermo-mechanical and acoustic re-
sponse was recorded by a multi-parameter monitoring system. Data processing and analysis yield a pre-
cise time lapse image of the damaging effects in the rock salt.
ontext. Underground natural gas storage in salt caverns provides high deliverability as they
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allow high rates of injection and withdrawal. Due to today's highly variable energy market and con-
sumption, injection and withdrawal cycles tends to increase significantly raising questions about the integ-
rity of the salt storage caverns and potential loss of deliverability. These high frequency cycling operations
indeed tend to generate high thermo-mechanical loading suspected to induce damage at the walls and
roof of the cavern. To improve the understanding of the geomechanical behavior of salt caverns under
such thermal loading, the SMRI (Solution Mining Research Institute) agreed to fund the Starfish experi-
mental project led by Storengy in partnership with INERIS, Mines-ParisTech, and CSME (la Compagnie
des Salins du Midi et de l’Est).
olution. A field test was designed and
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performed in the Varangéville salt mine
(France) operated by CSME, where salt content is
~94% and the ambient temperature
~15°C. A
~3mx3mx1.3m sized footwall salt block, equipped
with several monitoring equipments, was thermally
stimulated on its floor surface by a cooling device
allowing repetitive, rapid, temperature drops of 25°C.
Geotechnical monitoring was covered by a high-
resolution multi-parameter monitoring system includ-
ing 16 passive and active acoustic sensors, thermal
gauges as well as strain cells, at different depths.
Time series of the temperature cycles applied to
esults. Besides continuous velocity
the free surface of the salt block during 300 days
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measurements between active and passive
sensors, acoustic monitoring sensors recorded a
total of 58 426 events, with more than 21 600 during
the 1st cooling cycle; number of acoustic
emissions decreased asymptotically dur-
ing the following cooling cycles.
Advanced automated processing of the
waveforms and the acoustic swarm was
optimized based on SYTMIS software
suite. During each cycle, the acoustic
events are initially located at the surface
of the block and then deepen as the salt
cools deeper. Also, the average depth
reached by the acoustic emissions varied
significantly between each cycles, reach-
ing 90 cm during the 1st cooling period;
and only 74 cm for all following cooling
cycles.
Installation of the ultrasonic passive and active sensors inside
3D locations of acoustic events show
the salt rock mass under the experimental cooling chamber
crack initiations and coalescences that
strongly correlate in space and time with
the overall impact, as it is characterized by the strongest
the spreading of the temperature front
and deepest acoustic emissions.
downwards. This corresponds also with a
progressive change from predominantly
Lesson learned. This unique field experiment
tensile fracture mechanisms to shear
allowed to reveal and quantify the thermo-mechanic re-
fracture mechanisms. The magnitude of
sponse of salt rocks hosting cavities to high temperature
the acoustic events was found around -5
drops of 25°C. Ultrasonic monitoring gives an accurate
which is coherent with observed dimen-
3D picture of the progressive development of cracks in-
sions of salt crystals.
side the rock during the thermal cycles among which te
Results support the theoretical mechani-
first has a prominent role. Current research works are
cal concept (Sicsic et Bérest 2014) where
being made to develop exploitation strategies by under-
only the major cracks (figure below) are
ground storage operators to minimize potential damage
selected. The first cooling phase governs
of salt caverns.
Comparison between the macroscopic surface cracks and the cumulated acoustic energy density versus time.
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