The Influence of Cyclic Loading on Deformation of Rock Salt

Ma, H. L. (The Chinese Academy of Science) | Yang, C. H. (The Chinese Academy of Science) | Liu, J. F. (The Chinese Academy of Science) | Chen, J. W. (China University of Geosciences)

OnePetro 

Abstract

Salt caverns are utilized widely as natural gas storage and compressed air energy storage because of rheology and low-permeability of salt. For both natural gas storage or energy storage, gas injection and gas withdrawal are carried on alternately, two cycles one year for natural gas storage while one cycle every day for compressed air energy storage. For investigation of mechanical characteristics of rock salt under injection and withdrawal cycles, triaxial cyclic loading tests were conducted using a MTS test system. Linear relationships between stress and strain under unloading and reloading were observed. The linear character of the stress-strain response during reloading and unloading is more apparent at an increased confining pressure. Confining pressure reduces the difference of deformation modulus between unloading and reloading. The constitutive relationship for loading and unloading was constructed and proved by fatigue testes.

Introduction

Because of the rheology, low-permeability, and damage recovery of rock salt, salt caverns were widely used as natural gas storage and compressed air energy storage. The creep property of salt rock has been studied in many view points for several decades (Hunsche, 1984, Cristescu, 1996, Hampel, 1996). Several constitutive models have been constructed (Cristescu, 1993, Chan, 1996, Hou, 2003). Permeability values derived by laboratory measurements on undisturbed rock salt samples are generally less than 10-20m2 (Schulze, 2001). Even on a large scale in the field, measured permeability is less than 10-18m2 (Berest, 1996).

Salt caverns have been used for gas storage for several decades in Europe and America. Gas storage in Germany has a history of more than 35 years, and more than 106m3 volume of storage has been designed and constructed(Lux, 2009).

In China, underground gas storage (UGS) in rock salt is still at the primary stage. Jintan UGS,which provides service forWest-East Pipeline Project I, started gas injection in the end of 2007, and about 60 salt caverns (2.5×105 m3 volume per cavern) are planning to constructed in the future to supply about 1.8×109m3 volume of natural gas. Till 2013, there are 10 salt caverns constructed and 20 salt caverns under construction. One salt cavern is leaching out by now at Yunying UGS, which is to provide service for West-East Pipeline Project II, and will have 15 salt caverns (1.0×105 m3 volume per cavern) in the future. The working volume ofYunying UGS is 0.6×109 m3 or so. Qianjiang UGS, which is intended to service for Sichuan-East Pipeline Project, is in the leaching stage. Qianjiang gas storage is also the deepest rock salt underground storage in China, which is about 1 900~2 080m in depth.