Investigation on the permeability characteristics of bedded salt rocks and the tightness of natural gas caverns in such formations

•Permeability of bedded salt rocks ranges between 10−17 m2 and 10−21 m2 by lab-testing.•The interface did not dilate and show uprush of κ under high deviatoric stress.•Pure rock salt shows the lowest permeability and plastic behaviours when compressed.•Natural gas has a small seepage distance around the cavern in bedded salt rocks.•Tightness of the gas cavern in such bedded salt rocks is satisfactory for storage.

In China, the salt formations close to the lower reaches of the large natural gas routes and the main gas consuming regions are all bedded structures. The presences of non-halite or low-halite mudstone interlayers and interfaces (salt-interlayer) in these bedded salt formations increase the concern on the tightness of the natural gas caverns constructed in such formations. Therefore, different lithotypes of samples of bedded salt rocks have been prepared to determine their gas permeability evolution by steady-state and transient pulse-decay methods. The results show that the permeability of the samples is very low, within the range of 10−17 to 10−21 m2 after well compacted. The permeability of the interface is comparatively the highest, steadily around the order of 10−17 m2. A similar tendency of reducing permeability as deviatoric stress increases is found for all the samples; and unexpectedly there is no dilatancy and consequently steep increase of the permeability for all samples, even under very high differential stress. SEM (scanning electron microscopic) testing, as a supplement of tightness assessment, shows tight microstructure of all the bedded salt rocks cores. The experimental results indicate favourable performances for the tightness of natural gas caverns in such formations. Gas seepage around a gas cavern in such bedded salt formation was simulated by the FLAC3D software over a lifespan of 30 years. The results present that the interfaces act as the main channels for the gas to seep through. But the seepage distances in the vicinity of the cavern are so short that it causes a little influence on the tightness. Several factors, e.g., the internal gas pressure, the locations of interface, as well as the permeabilities of the interlayers were analyzed to investigate their effects on the tightness of a gas cavern. On the whole, the bedded salt rocks have enough low permeability and satisfactory tightness for the natural gas caverns constructed in.