Geomechanical Effects of Cyclic Loading on Rocksalt

Abstract

ABSTRACT: Hydrogen-based solutions are gaining importance as a renewable clean energy source. A geomechanical challenge when hydrogen storage in salt caverns is considered is the cyclic loading and unloading of the host rock (rocksalt). As hydrogen is stored and removed based on fluctuating demand throughout the year the cavern wall will be exposed to pressure cycles, where in times of high demands the hydrogen is supposed to be readily available and in times of low demand, the hydrogen will be stored in the cavern. Ideally these pressure cycles do not influence the integrity of the cavern as a storage site. To investigate the effect of cyclic loading on rocksalt, several cyclic loading experiments have been performed in the lab using two different very pure rocksalt samples from Germany (Asse Speizesalz and Z3 Leine Rocksalt (both > 98% NaCl). Loading stresses, number of cyclic loading event and length of high versus low differential stress have been varied to investigate the effect of cyclic loading. Daily loading cycles versus weekly loading cycles showed very similar strains on the sample. When reaching stresses above the dilatancy boundary of the rocksalt though the samples kept deforming. 1. INTRODUCTION Hydrogen storage is one of the key necessary steps in the energy transition, where hydrogen-based energy solutions are gaining importance as a renewable clean energy (e.g.: Abdalla et al., 2018; Gulcin Caglayan et al., 2020; Lankof et al., 2020; Tarkowski, 2019; Zhang et al., 2016; Zivar et al., 2020). With energy consumption varying daily as well as seasonally, and the hydrogen supply-demand chain persistently being out-of-phase, flexible hydrogen storage solutions are critical. Specifically, in times of low energy demand excess hydrogen must be temporarily stored (Zivar et al., 2020), while in times of high demand the hydrogen must be easily accessible and readily available. A well-known hydrogen storage option is geologic sequestration in salt caverns (e.g.: Tarkowski, 2019; Heinemann et al., 2021). Salt is known as a natural barrier with general low permeability, frequently acting as a seal for hydrocarbons and widely considered as possible host rock for the storage of radioactive waste (e.g.: Arson et al., 2012; Gulcin Caglayan et al., 2020). In the past 30 years, research has been performed on the mechanical behavior of rocksalt (A selection of references: Schutjens, 1991; Peach and Spiers, 1996; van den Bogert et al., 1998; Hunsche and Hampel, 1999; ter Heege et al., 2005; Alkan et al., 2007), but less work has been performed on the cyclic loading of rocksalt.