Determination of Optimal Shape for Gas Storage Salt Caverns

Abstract

In this study, the optimal shape for a gas storage cavern was determined by considering the elements that affect its stability and convergence. The considered factors influencing the stability of caverns include the bulk modulus of salt rock, ambient temperature, internal gas pressure, cavern depth, and cavern shape. By varying these parameters and creating various combinations, 45 scenarios were defined. Numerical models were constructed for each scenario to systematically investigate the factors affecting cavern stability. Through a comparison of the results from these numerical models, the most stable cavern shape under different conditions was determined. The study focuses on the pre-salt environments in the Santos Basin, southeast Brazil. The findings of this study may aid in the construction of gas storage salt caverns. The results indicate that the cavern's size and geometry have a greater effect on its volume loss in salt layers with a lower bulk modulus (between 5 and 15 GPa). Additionally, when the bulk modulus is low, the rate of change of the cavern convergence to the bulk modulus is larger. Moreover, the effects of the rock salt characteristics on the cavern convergence are much less pronounced at larger depths, so a depth of 1200 m can be ignored. In comparison to the bulk modulus of salt rock, internal gas pressure has a far greater effect on the convergence of salt caverns. At shallow depths, the salt creep phenomena primarily affect the cavern's roof area, and as the depth of the cavern deepens, it increasingly damages the floor and middle walls. When precise control of the gas pressure in proportion to the cavern depth is not attainable, the optimal form for designing gas storage salt caverns with varying depths based on the minimal convergence criterion is a horizontal ellipsoid. In contrast, the vertical ellipsoidal cavern always results in the greatest volume loss and displacement and is hence regarded as the least acceptable alternative. A downward pear-shaped cavern can be a good alternative to a horizontal ellipsoidal cavern for higher depths. The design and construction of the downward pear-shaped cavern instead of the upward pear-shaped cavern leads to better control and the reduction of the displacements.