Solution-mined caverns used for storing liquid hydrocarbons must be tested for tightness. The simplest test consists of rapidly increasing cavern pressure and monitoring further pressure evolution. A large leak results in a fast pressure drop rate. However, together with an actual leak, pre-existing and test-triggered phenomena contribute to cavern pressure evolution, making the apparent leak faster or slower than the actual leak. These phenomena can be accurately described and numerically computed.
Solution-mined caverns are used world-wide to store liquid hydrocarbons. These caverns are tested on a regular basis to prove the absence of significant leaks. Various tightness tests are currently used Fig.1). The Nitrogen Leak Test consists of lowering a nitrogen column below the casing shoe in the annular space and tracking the nitrogen-brine interface. In this paper, the simplest tightness test (Liquid-Liquid Test) is discussed: the annular space is filled with a light hydrocarbon and, at the beginning of the test, cavern pressure rapidly is built up by p1, and further pressure evolution as a function of time or p = p(t) is recorded during several days . A significant pressure drop rate is a clear sign of poor tightness. In fact, together with an actual liquid leak, several phenomena may explain the pressure drop observed after a cavern has been rapidly pressurized. The objective of this paper is to identify those phenomena that might contribute to the "apparent" leak and, when properly accounted for, can reduce the gap between the apparent (as-observed) leak and the actual leak.
A first group of phenomena pre-exist the test: they include brine thermal expansion (caverns are created by circulating cold soft water in a deep salt formation where geothermal temperature is warm.) and pre-existing salt creep. A second group consists of test-triggered 1264 phenomena. They include transient brine permeation through the salt formation (pure rock salt permeability is exceedingly small; however salt-beds often contain a fair amount of insoluble rocks whose permeability is larger), additional dissolution (the amount of salt that can be dissolved in a given mass of water is a function of brine pressure; pressure build up in a closed cavern leads to additional dissolution; in the process the volume of cavern brine + dissolved salt decreases and pressure drops), brine cooling (a rapid pressure increase leads to an instantaneous adiabatic warming of cavern brine) and transient salt creep. According to the Le Chatelier-Braun principle, test-triggered phenomena make the apparent leak smaller than the actual leak.
When a certain volume of brine or v inj is injected in a closed cavern, cavern brine pressure incresases.