Modelling the effect of ice swelling in the rock mass around an LNG underground storage cavern using FRACOD

Abstract

Underground storage of liquefied natural gas (LNG) has many advantages over the conventional above-ground LNG storage due to its safety and energy efficiency. One of the key technical challenges for the underground storage of LNG is to understand the behaviour of the rock mass in the vicinity of the LNG cavern and hence to take measures to prevent leakage caused by possible rock fracturing in response to the cooling of the rock mass. With the extremely low temperature of the LNG ( − 162°C) in the storage cavern, sub-zero temperatures will be induced in the surrounding rock mass, which on one hand may cause tensile stress due to thermo-mechanical effect and on the other will cause the formation of ice in pores and cracks of the saturated rock mass. Ice formation is likely to cause compressive stress in the rock mass due to its expansion in volume. The combined effect of rock cooling and ice swelling is complicated as cooling tends to create open fractures whereas ice swelling tends to cause compression and the closure of these discontinuities. Understanding this effect is crucially important as it is related to the integrity of the surrounding rock mass for leakage prevention. This study presents recent developments of the thermal-mechanical coupling and ice swelling functions in FRACOD, a numerical code designed to predict rock fracturing processes in fractured rock masses. The new functions enable us to investigate the complicated response of an in situ rock mass to the excavation of LNG cavern and the storage of low temperature LNG in a most realistic way. The pilot LNG storage cavern experiment at Daejeon Korea is used as a case study site, and the measured temperature in the surrounding rock mass are used to validate the numerical model and the modelling method. The ice swelling effect was modelled by introducing a large number of random cracks in the rock mass, which have certain hydraulic apertures to hold water. When the local rock temperature is below zero, the aperture water will become ice and cause crack expansion. It has been found in the study that ice swelling has a major effect on the displacement and stress distribution in the rock mass. It causes a compression zone around the LNG cavern, and effectively prevents tensile fracturing and fluid leakage.