Research on gas leakage and collapse in the cavern roof of underground natural gas storage in thinly bedded salt rocks

In the thinly bedded salt rocks, the roofs of salt cavern gas storage may have different lithology, some cavern roofs of these caverns may be damaged and lose tightness. Thus it is significant to study the characteristic of the gas leakage through the cavern roof of the gas storage cavern in bedded salt rocks. To approach such a goal, the gas leakage through a cavern roof and the induced collapse are investigated in this study. At first, the salt cavern of ZJ-block of Huai'an salt mine is selected as the potential gas storage site, which has a roof consisting of salt rock, a thin gypsum layer, and thick argillaceous siltstone. And then, the porosity and permeability of rock samples of the roof strata are measured in the laboratory. Thirdly, a numerical simulation model is established based on the geo-conditions of the ZJ-block. The gas seepage and cavern tightness under three different cavern roof conditions are simulated and analyzed. The results show that, if the cavern roof is integral, the main gas seepage channels of the cavern are the interlayers, and the tightness of gas storage salt cavern is satisfactory once the permeability of interlayers is around 10(-17) m(2) or lower. After the salt rock layer of the roof is damaged and a partial gypsum layer is exposed, the gas leakage amount can increase about one order of magnitude. But the seepage rate and pore pressure both change slowly in the roof. Thus the operators have sufficient time to survey the accidents and transfer the gas away. However, after the thin gypsum layer is damaged and results in the exposure of argillaceous siltstone, the gas seepage sharply increases. Including the leakage amount, the seepage range, and the pore pressure are all increased much than that of the above two conditions. Due to the serious consequence of storage safety, this condition should be absolutely avoided. To ensure the roof safety and tightness of gas storage, it is suggested that the reasonable construction of a cavern roof is the prerequisite, and the internal pressure monitoring, as well as sonar measuring, should be engaged regularly during the operation period.