Experimental and mechanistic study of mudstone volumetric swelling at the bottom of salt cavern gas storage

Abstract

Salt cavern gas storage is one of the vital strategic natural gas reserves and emergency peak shaving facilities all over the world. However, rock salt in China is primarily bedded salt, usually composed of many thin salt layers and interlayers (e.g., anhydrite, mudstone, and glauberite). During the water solution mining process of the cavern, the insoluble mudstones fall to the bottom and account for 1/3 up to 2/3 of the storage capacity. The bulk volume of the insoluble mudstones is almost twice its in-suit volume. It is of great urgency to investigate the swelling mechanisms of the bottom insoluble mudstones. Given this, we first analyzed the mineral composition of salt rock and insoluble mudstones by using XRD and SEM methods. Then, experimental studies were carried out considering both clay swelling and physical packing. At last, the zeta potential tests were conducted to reveal the swelling mechanisms of the bottom mudstones. Results show that the volumetric expansion of mudstones is made up of three parts: clay swelling, particle surface bound water volume, and pore space free water volume increase. Because the content of expansive clay in the bottom mudstones is less than 2%, and the high salinity brine in the cavern has excellent clay stability performance, clay swelling is not the main contributor to the volumetric expansion of the bottom mudstones. Measurement results show that the surface of the mudstones is negatively charged after hydration. Electrostatic repulsion can increase the spacing between small rock particles and creates approximately 47.6% of the pore space, which is the main factor in the volumetric expansion of mudstones. This study provides a theoretical basis for the mining solution and capacity enlargement during the construction of bedded salt cavern gas storage in China.