It is expected in Japan that the import of natural gas will increase more and more in the future. To handle the large amount of natural gas; the underground storage method has been discussed. Rock mechanics investigations on rock specimen at cryogenic temperature down to -180°C have been carried out. And finite element analysis and laboratory tests have been conducted to clarify the behavior of rock when cryogenic liquified gas is stored in unlined cavern.
The amount of LNG imported reached to 11.7 million tons in 1978 from 1.0 million tons in 1970, and it is expected that its import will expand progressively in the future. On handling such a large amount of LNG, underground storage method has become a major interest lately. Under these circumstances. the authors have conducted theoretical and experimental investigations to clarify the behavior of rock when cryogenic gas is stored in rock cavern. First, the physical properties of rock at low temperature are studied. Then, temperature distribution and thermal stress drived around cylindrical cavern are analysed with FEM by supplying data on physical properties of rock previously obtained. Laboratory tests on two dimensional specimen are also carried out to observe the behavior of rock when cryogenic gas is stored in rock cavern.
Granite is thought to be the most suitable rock in Japan for oil and liquified gas storage. so that grani te cores in water saturated, "wet specimen" and in dry state. "dry specimen" are used at cryogenic temperature to know the properties such as uniaxial compressive strength, tensile strength, Young's modulus, Poisson's ratio, the coefficient of thermal expansion, thermal conductivity and porosity are measured. The hydraulic loading machine with cryogenic chamber (Fig.l) capable of keeping constant temperature down to -180°C is used. Rock cores used are cylindrical ones with 30rnm in diameter. Cotes are cooled down to -10, -40, -70, -100, -130, -150 and -180°C at the rate of -20°C/hr. At each temperature level, tests were carried out. The uniaxial compressive strength is plotted versus temperature for both dry and wet specimen in Fig. 2. At 20°C, the strength of dry specimen is 1.5 times higher than of wet specimen. Cooling cores results in increased strength. but this increase is more remarkable in wet specimen than in dry one. Fig. 3 illustrates tensile strength of specimen as a function of temperature obtained by Brazilian test. The strength of wet specimen is lower than that of dry specimen at 20°C. As can be seen, it tends to increase when temperature is lowered to -100°C and decreases slightly following further 1owering of temperature. On the contrary, the strength of dry specimen will not vary much according to the temperature change. Stress-strain relations obtained by axial loading are shown in Fig. 4 for wet specimen and in Fig. 5 for dry specimen. As evident from these figures, axial strain is almost linearly related to stress up to the failure point.