Abstract

Seasonal gas storage in rock salt caverns is a proven technology since several decades. However during the last years new operation modes with high pressure rates are aimed at. The corresponding loading of the rock salt mass differs because the change of gas temperatures during operation and the consequences of these temperature changes on the stressing of the salt rock mass must be taken into account. Within the rock mechanical design a loading history, i.e. a robust pressure versus time dependency, has to be defined which covers the possible withdrawal and injection scenarios. Alternatively the gas flow from or into the individual caverns can be taken as basic requirement. In each case, the thermodynamic influence should be considered in addition to the influence of creep. The paper discusses possible load cases which are defined pressure controlled or volume controlled respectively. It shows the differences of the time dependent state variable as well as the stress regime with respect to the assessment of the safety and economic operation.

1 Introduction

Many different aspects have to be taken into account within the dimensioning and optimization of gas storage caverns in rock salt mass for high-frequency storage. Apart of the mechanical loading the thermal loading of the surrounding rock mass is very important. Besides the determination of the operating pressures for the high-frequency gas storage there is a focus on the velocity of withdrawing the gas from the cavern. To ensure a safe operation of the cavern, next to the useable pressure regime also maximum velocities must be determined defined by pressure change rates or gas flowrates during the dimensioning or optimization. By a fast gas withdrawal from the cavern the stress condition of the surrounding rock mass is extremely influenced by the thermal load so that tensile stresses can appear in the area of the cavern wall. Due to the low tensile strength of rock salt the tensile stresses at the cavern wall must not appear in order to avoid macroscopic fracturing (Staudtmeister, May 2012, Zapf et al. 2012).

For the dimensioning of a gas storage cavern from engineering point of view a design load case has to be defined which covers all possible load scenarios. Such design load case is represented in Figure 1. The design load case contains several withdrawal and injection phases as well as phases without withdrawal or injection at maximum pressure and the pressure at lower limit of the free operating range. That procedure to determine a design load case in detail is described in Zapf et al. (2011) and Leuger et al. (2012).

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