Failure analysis of thick interlayer from leaching of bedded salt caverns

• Model calculating the critical collapse span of a thick interlayer is proposed.
• Formulas for the proposed model are obtained based on an energy method.
• This approach can greatly improve the calculation accuracy.
• Different factors influencing the critical collapse span are discussed.

A mathematical model is proposed to calculate the critical collapse span of a thick interlayer during the leaching of a gas storage salt cavern in bedded rock salt. In the proposed model the ratio of the height of the damaged zone to the interlayer thickness is introduced. This approach overcomes the negative effects on the calculation accuracy of the fact that the damaged rock does not fall off from the thick interlayer but is linked to it by contact forces. Brine immersion tests have been carried out to obtain the solution characteristics and the influence of brine immersion on tensile strength and on elastic modulus of interlayer samples of a salt formation from Yingcheng city, Hubei province, China. Experimental results show that these mechanical parameters decrease with immersion in brine and more so with decreasing brine density. To validate the proposed model, a 3D geomechanical model has been built of a salt cavern under construction in a bedded rock salt formation of Yingcheng city. The stresses and deformations of the thick interlayer obtained by the analytical solution and by the numerical simulation are compared, and show good agreement. The critical collapse span of the thick interlayer decreases greatly with a decrease of the ratio of damaged zone height to interlayer thickness and interlayer depth, increases with increasing tensile strength and thickness of interlayer, and is independent of the elastic modulus of the interlayer. The proposed model has been used in the planning of the leaching of the underground salt cavern gas storage of Yingcheng city, Hubei province, China.