Abstract: Salt caverns have good stability and air-tightness, and thus are ideal places for storage of natural gas, oil, hydrogen, etc. The oil blanket used in the conventional salt caverns created by solution mining has problems such as poor dissolution inhibition effect and heavy oil contamination, which cannot meet the requirements of green mine construction. As a substitute, gas blanket has attracted much attention. However, its application is hindered due to great difficulty in control, and the complex geological conditions of salt rock strata in China. To this end, the characteristics of gas blanket control at different cavern construction stages were analyzed first, and then the methods for predicting the thickness of gas blanket and calculating the gas injection amount at different stages were proposed based on the analysis of the pressure balance at the gas-liquid interface. Combined with an example of a well in the groove construction period, the recommended thickness of gas blanket and its fluctuation range were given, and the variation law of the gas blanket thickness and the gas injection amount with the cavern construction progress was proved. Finally, the key problems related to the application of gas blanket were discussed. The results showed that the gas blanket thickness is mainly controlled during the groove construction and roof formation periods, and the gas-liquid interface position is mainly controlled during the cavern construction and cavern repair periods. It is suggested that the average gas blanket thickness during the groove construction period should not be less than 0.3 m. The thickness of gas blanket fluctuates rapidly at the beginning of the groove construction period, and thus a larger gas blanket thickness should be set up and the nitrogen should be replenished in time. Then the gas blanket thickness tends to be stable, and the gas replenishment interval can be gradually extended. With the increase of cavern construction time, the single gas replenishment volume becomes larger and larger, and the cumulative gas replenishment volume increases linearly as a whole. During cavern construction, the wellhead gas pressure and the gas pressure at the gas-liquid interface increase first and then decrease linearly, and the wellhead gas pressure change shall be monitored in real time. The cost of gas blanket is close to that of oil blanket, but the environmental benefits are significant, and the discharged nitrogen and wellhead gas injection facilities can be reused to save costs. The position of gas-liquid interface should be monitored on site in real time by combining optical fiber monitoring and neutron logging.

Key words: salt cavern storage, nitrogen dissolution inhibition, pressure balance, gas blanket design, gas-liquid interface