Analysis of mechanical and permeability properties of mudstone interlayers around a strategic petroleum reserve cavern in bedded rock salt

Using salt caverns for underground strategic petroleum reserve (SPR) is the most popular storage method in the world due to its high security and economy. The rock salt resources in China are mainly bedded salt, containing many mudstone interlayers. In this paper, pH test and total acid number (TAN) tests are carried out to determine the different degradation mechanisms caused by brine-mudstone and oil-mudstone interactions. Uniaxial and triaxial compression tests and permeability tests were carried out to determine the mechanical and permeability parameters of the mudstone interlayers under brine and crude oil immersions. Results show that the thermochemical sulfate reduction (TSR) will not occur during the crude oil storage. The reaction between the anhydrite and the naphthenic acid is the major reason that causes damage to the mudstone interlayers. Both the crude oil and brine can cause damage to the mudstone interlayers, but the damage caused by brine to the mudstone interlayers is much greater than that caused by crude oil because of their different reaction mechanisms. Due to the plugging effect by the asphaltene and resin present in crude oil, the permeability of the samples decreases 67.5% after they have been immersed in crude oil for 30 days, even though the rock damage is still accumulating. This property is beneficial for the SPR salt caverns, to keep good tightness.