Geomechanical investigation of roof failure of China's first gas storage salt cavern

Abstract

JK-A cavern is the first operating cavern of Jintan underground gas storage, which is also the first operating salt cavern gas storage in China. In 2015, the sonar survey carried out under working conditions indicated that a massive roof collapse of the JK-A cavern had taken place. To prevent similar accidents from happening again, finding the causes and the collapse time of JK-A cavern would be valuable. By using the target salt formation information, rock properties, and monitored gas pressure, a 3D geomechanical model of JK-A cavern has been built. The responses of the rock mass around JK-A cavern under leaching, sealing tests, debrining, and gas injection/delivery have been investigated. A safety evaluation system consisting of cavern volume shrinkage, dilatancy safety factor, displacement, vertical stress, and equivalent strain is proposed to find the likely collapse time and the reason for the JK-A cavern roof collapse. Results show that there are two main causes for the cavern roof failure. (1) A large-span flat roof, detrimental for bearing loads. Once local damage takes place, massive collapse may be triggered by the self-weight of the loosened rock. (2) The decrease speed of the internal gas pressure is too fast. The loads applied to the cavern roof cannot be transferred in a timely fashion, which causes a stress concentration zone to form, and local damage develops. The roof collapse of JK-A cavern took place 1.3 years after its gas injection/delivery started. Numerical analysis results show that dilatancy safety factor and vertical stress have high accuracy and sensitivity in the prediction of the cavern roof collapse. There is a high potential for a roof collapse of JK-A cavern to happen again. We propose that the gas pressure and pressure decrease rate are to be controlled strictly in the later operating period to prevent any collapses in the future.