Gas seepage around bedded salt cavern gas storage

Abstract

Gas seepage serves as one of the indexes to evaluate the feasibility of salt cavern underground gas storage (UGS) in bedded salt formations because it has a significant effect on the tightness of the cavern. Tests have been carried out to obtain the permeabilities and porosities of the mudstone and interlayer cored from Jintan salt mine. Based on the experimental data and formation characteristics of the Jintan salt mine, a 3D geomechanical model of two adjacent caverns has been established to investigate the effects of operating time, interlayer permeability, pillar width, injection-production mode, and internal gas pressure on the gas seepage in the rock masses. The cavern dimensions and operating parameters are verified based on the numerical simulation results. Experimental results show that the porosity and permeability of the upper and lower formations of the rock salt layer range from about 1% to 20%, and from 1.0 × 10−16 m2 to 6.6 × 10−16 m2, and those of the interlayers range from 3.1% to 15.5%, and from 1.0 × 10−15 m2 to 1.0 × 10−16 m2 respectively. The porosity and permeability of interlayers at different locations show a great variability. This is mainly because of the presence of calcareous clay and brown mudstone in the interlayers, which show a notable characteristic of delamination and randomness. Numerical simulations indicate that the permeability of the interlayer has a greatly influence on the gas seepage pressure in the rock masses around the caverns, and it is also the key factor in the cavern sealing performance. The pillar between two adjacent caverns should satisfy both the requirements of mechanical stability and of gas seepage control. Asynchronous injection–production mode has great detrimental effect on the gas seepage pressure in pillars, particularly for narrow pillars. Synchronized injection–production is recommended for bedded rock salt cavern gas storages when pillars are narrow.