A generic model for predicting long-term behavior of storage salt caverns after their abandonment as an aid to risk assessment
Résumé
The most accepted strategy for abandoning solution-mined salt storage caverns involves filling the cavern with brine and sealing the well permanently. The sealing can be delayed waiting for the brine to reach thermal equilibrium with the surrounding salt. This concept is based on the principle that the cavern, once closed, will reach an equilibrium pressure that will assure the cavern׳s long-term mechanical stability. This article provides quantitative information about the evolution of abandoned salt caverns (the value of the equilibrium pressure, the time before thermal equilibrium is achieved, the rate of brine expulsion from the cavern, the time before the cavern closes up) from numerical simulations based on the state of the art related to the phenomena that affect the cavern after it is abandoned. The model reveals the conditions under which an excessive increase in the brine pressure in the cavern may appear, which can lead to the walls of the cavern being damaged. It also provides a basis for recommendations that could help to control these risks. A model has been developed to study the short-, medium- and long-term implications of abandoning storage caverns in underground salt masses. Despite the simplicity of our approach, the results remains close to those obtained with the well-known LOCAS software program and some measurements done during a trial-and-error test. The behavior of the cavern after abandonment varies very widely depending on its volume, shape and depth, as well as the characteristics of the salt that surrounds it. The model reveals the conditions under which an excessive increase in the brine pressure in the cavern may appear, which can lead to the walls of the cavern being damaged. Solutions have been proposed and evaluated to overcome this problem, including heating the brine before injecting it into the cavern or incorporating a small proportion of gas into the brine to make it more compressible.