Optimization of shrinkage agents and study of their shrinking mechanism for mudstones in the salt cavern gas storage

Abstract

Salt cavern gas storage takes up a high proportion of gas storage, which allows a large amount of gas production in one day and flexible market response. It has become one of the world's important strategic natural gas reserves and emergency peak-shaving facilities. However, salt cavern gas storage in China is mainly built in the layered salt formation, which is characteristic of thin salt layers with high contents of insoluble mudstone interlayers. During the construction of this type of gas storage, these insoluble interlayers can be left at the bottom of the storage and reduce the storage by 30%–70%. It is urgent to study how to effectively control the volumetric expansion of insoluble mudstones and enlarge the storage capacity. In view of this, we first characterize and quantify the rock salt, interlayer mudstones, and clay minerals by XRD. Then, 12 kinds of organic and inorganic shrinkage agents were compared in the shrinkage experiments. Five factors affecting the shrinkage are considered, including the concentration and diffusion distance of shrinkage agents, the mass ratio and particle sizes of mudstones, and the temperature. The optimal recipe and concentration of chemical agents suitable for mudstone shrinkage in salt cavern reservoir are obtained. Finally, based on the shrinking performance of mudstones, SEM scanning, EDS analysis and Zeta potential test, the shrinking mechanism of insoluble mudstones at the bottom of the cavern was proposed. The results show that: The content of swelling clay in the target gas storage is low and the clay swelling is not the main reason for the volumetric expansion of mudstones. High valence cationic salts or polymers can shrink. Among which, the order of shrinking effectiveness is: inorganic polymers > Inorganic salts > Organic polymers. When the dosage of PAC-30 is 0.7 g/L, the volumetric shrinkage effect of mudstones is more than 40%. It has a great potential to expand the storage capacity of the salt cavern. The study shows that there is an optimum dosage of the shrinkage agent. The increasing temperature and the small particle sizes of mudstones are beneficial to shrinkage. The shrinking process is a diffusion control process, and the shrinkage agents are easy to form a dense layer on the mudstone surface, which hinders the further diffusion of the shrinkage agents. Mechanism studies show that: After the addition of cationic polymer, the Zeta potential on the mudstone grain surface decreased significantly. The electrostatic repulsion between rock particles was decreased, and flocculation, agglomeration, and compaction occurred, which is the main mechanism of mudstone shrinkage in the gas storage cavern. This study provides a theoretical and practical basis for the volumetric shrinking of mudstones at bottom of bedded salt cavern gas storage.