Change of joint aperture during formation of ice-ring around underground LNG storage cavern

摘要

The new system for storing LNG in rock caverns had been developed and verified through the design, construction and operation of a LNG pilot plant in Daejeon, Korea. Among the key technologies of the LNG storage system, drainage of groundwater and formation of an ice-ring are very important ones. These are strongly related to the complex mechanism between thermal and hydraulic characteristics of rock mass and groundwater. Through the previous researches, the core of ice-ring design is summarized as follows: propagation distance of 0C isotherms after the injection of LNG; groundwater penetration distance within 0C isotherm; and change of joint aperture during/after ice-ring formation. It was confirmed that the propagation distance of 0C isotherms after the injection of LNG affects the operation period of the drainage system, and can be predicted from the thermal properties of rock mass, and initial temperatures of LNG and surrounding rock mass. In this paper, the generalized Clapeyon equation defined in porous media was used to quantify the phenomena of cryo-suction during ice-ring formation. Also the effect of cryo-suction on joint apertures was investigated by thermo-hydraulic-mechanical coupled analysis with UDEC code. The numerical analyses were carried out with the following two models: simple three joint sets and LNG pilot cavern models. The effect of cryo-suction on aperture of rock joints was verified by the numerical analysis results of the pilot plant model. It was revealed that as temperature dropped, the joint aperture was gradually increased by shrinkage of rock mass as like the results of monitoring. And the cryo-suction occurs as groundwater is recovered and changed to ice for forming an ice-ring, and can affect the joint aperture instantly. Furthermore, the joint apertures around the cavern were decreased up to 0.01…0.035 mm, and the change of the joint aperture had little effect on the stability of the cavern.