Stability and tightness evaluation of bedded rock salt formations for underground gas/oil storage

The presence of interfaces has a critical influence on the stability and tightness of underground gas/oil storages. In China, these energy storages are constructed mainly in bedded salt formations and are widely distributed. Therefore, it is necessary to study the sedimentary rhythm and mechanical characteristics of the interfaces between beds. The petrologic study of core samples from exploratory wells in Yunying Salt Mine, Hubei Provence, China, reveals a sedimentary rhythm and multiple interfaces of bedded salt formations: (1) Mudstone–anhydrite–glauberite–Glauber’s salt–salt rock–mudstone are periodically deposited in sequences. Chemical deposition, the predominant formation mechanism, and mechanical deposition appear alternately and result in the formation of sedimentary multicycles; (2) depending on whether a certain component of the rhythm is lacking or not, interfaces are divided into sequential depositional interfaces and intermittent depositional interfaces. Depending on the deposition frequency, interfaces can also be classified into dominant interfaces and secondary interfaces. Depending on whether the mineral composition changes gradually or abruptly, interfaces can be divided into gradual transition interfaces and discrete discontinuous interfaces. Based on the classification of the interfaces together with scanning electron microscope analyses, the cementation state and strength of the interfaces are discussed. Both field investigations and laboratory tests show that the strengths of the chemical deposition interfaces are higher than those of the mechanical deposition interfaces. Specifically, interfaces of mudstone and rock salt are mostly weak. Taking into account the presence of weak interfaces, the strength of interbedded salt formations around caverns can be represented by a U-shaped lower-bound strength envelope curve, which means that shear failure may occur more easily at the haunches of the cavern. Therefore, when setting the range of the internal operating pressures, more attention should be paid to the shear strength of the weak interlayers and weak interfaces, in particular those at the haunches of the cavern, to ensure the stability and tightness of underground gas/oil stores.