# A Review of Evaporite Beds Potential for Storage Caverns: Uncovering New Opportunities
## 3 Properties and Geomechanics of Bedded Evaporites
### Main Geomechanical Considerations in Bedded Evaporite Caverns
#### 3.3.3 Cyclic Loading  
Cyclic loading, characterized by periodic injection and withdrawal of gas, imposes significant stress on salt formations, often resulting in cavern instability. The main forms of damage caused by cyclic loading are fatigue, creep deformation, crack formation, and wellbore instability. Fatigue, defined as the deterioration of material strength due to repeated stress, occurs in two forms: cyclic fatigue from repeated loading cycles and static fatigue (or creep rupture) from prolonged stress [241]. Both fatigue modes can happen in salt cavern gas storage because the rock salt is subjected to both cyclic gas storage and an extended storage period. Cyclic loading experiments conducted in [242] demonstrated a logarithmic relationship between fatigue life and gas pressure. Fatigue models in [243,244] identified two phases of damage: rapid initial accumulation followed by a stable phase. A microstructural study was performed in [245] to record the differences in microstructures during cyclic loading, using fatigue and NMR tests. The damage evolution of rock salt showed three stages of damage, including minor damage in the first two stages and rapid damage in the third stage when the rock salt is near failure. Fatigue tests were also conducted in [246] to determine the effect of high-stress intervals during cyclic loading on the rock around the salt cavern. A mixture of shear and tensile cracks was observed in the rock samples in addition to intra-grain and inter-grain cracks in salt crystals. The main factors affecting rock fatigue behavior are maximum applied stress, amplitude, loading frequency and waveform, and temperature, based on experimental works in [247,248]. The creep behavior of salt results in cavern volume shrinkage under constant gas pressures. A study in [249] investigated the cavern creep shrinkage under constant gas pressures for a sphere- or cylindrical-shaped cavern. The authors derived equations for small and large deformations of cavern shrinkage. The behavior of salt caverns under both cyclic loading and average cycle pressure was studied in [250]. The results indicated a faster volume loss under cyclic pressures than under average cycle pressure. The authors of [173] investigated the effect of cyclic loading on rock deformations in bedded salts. The results showed a brittle deformation of gypsum and an elastoplastic deformation of rock salt. The study in [251] investigated the deformations of mudstone interbeds in salt caverns, demonstrating significant differences in the creep behavior of salt and mudstone under highly effective stress conditions. Using a geomechanical model, ref. [252] studied the deformation around a cavern. Large displacements around cavern walls were simulated in [253] using FLAC3D software (Version 5.0), showing significant deformations after 20 years of cavern creep. Another numerical simulation study of horizontal salt caverns in two mining layers showed larger displacement and volume shrinkage compared to caverns in single mining layers [155]. Tensile failure is the primary cause of fractures in bedded salt caverns. According to [254], tensile fractures can occur if cavern pressure exceeds the operating limit. Another study of rock salt with mudstone interbeds in [251] concluded that tensile failure in mudstone interlayers could compromise cavern integrity. According to the study in [255], the risk of tensile failure in bedded salt rock caverns increases with higher extraction rates and repeated hydrogen storage cycles. Wellbore instability is another challenge associated with cyclic loading. Gas production rates play a crucial role in cavern and wellbore stability. The study in [256] proposed optimal gas production rate methods to avoid issues such as cavern volume shrinkage, tubing erosion and corrosion, hydrate and condensate formation at the wellhead, and stability concerns. The effect of cyclic internal pressure and cavern volume shrinkage on cement sheath integrity was also studied through experimental work and numerical simulations in [257].