Sealing a solution-mined cavern in a salt formation that has been used for mineral production, hydrocarbon storage, or waste disposal has been a contemporary topic for many years. As a result of knowledge gained from a series of succinct sealed well and cavern field tests performed in the late 1980s and early 1990s by Gaz de France, issues related to the abandonment of salt caverns became more focused. The main difficulty of the abandonment problematic is linked to fluid pressure increase in the cavern after sealing. This phenomenon is the result of different factors that interact with each other: creep, heat transfer, dissolution, percolation, for instance. All these factors must be taken into account for designing the abandonment process. This paper will discuss these factors and will highlight their practical impacts on a deep salt cavern abandonment experiment (1500 meters deep). First measurements will be also presented.

As a result of knowledge gained initially from a series of succinct sealed well and cavern field tests performed in the late 1980s and early 1990s by Gaz de France (EZ58; EZ53), issues related to the long term abandonment of salt caverns became more focused. The tests were performed in cooperation with École Polytechnique (France) and were partially funded by the Solution Mining Research Institute (USA). The long-term abandonment (sealing or plugging) of a solution-mined cavern in a salt formation that has been used for mineral production, hydrocarbon storage, or waste disposal has been a contemporary topic for many years. Sealing and abandonment of caverns in salt formations differs from sealing of an oil and gas well principally for two reasons: - cavern wells are generally completed with much larger casings than oil and gas exploration and production wells; - the cavern fluid pressure rises after sealing. The first difference is not a significant deterrent to effective cavern sealing and abandonment. Essentially all of the uncertainties and complexities associated with cavern sealing and abandonment are a direct result of the second difference – the fact that the cavern fluid pressure rises through time. After a short introduction to solution mining of salt caverns and their use as underground gas storages, the paper will first discuss the factors affecting cavern fluid pressure increase after sealing. The second part of the paper will highlight the practical impacts of these factors on a deep salt cavern abandonment experiment (Cavern TE02; 1500 meters deep), currently being performed on the natural gas storage of Tersanne operated for over 35 years by Gaz de France.

Thousands of caverns have been leached out from deep salt formations. They are used for saturated brine production and/or hydrocarbons storage. They will be abandoned some day: the access well will be plugged with cement, isolating a large bubble of saturated brine. The later evolution of such a bubble raises serious concerns for environmental protection; salt creep and brine thermal expansion can lead to brine pressure build-up and rock-mass fracture, then brine seepage can lead to pollution of overlying water-bearing strata. Taking into account salt formation permeability leads to less pessimistic scenarios. An 18-month test has been performed on a deep brine-filled cavern. The objective was to measure the brine equilibrium pressure reached when the cavern is closed. Such an equilibrium is reached when salt mass creep, which leads to cavern shrinkage, balances brine permeation through the cavern wall. This objective was met by imposing different pressure levels and observing whether the pressure increased (or decreased) with respect to time. Data misinterpretation (i.e., a well leak instead of a cavern-proper leak) was precluded by a special monitoring system. The observed equilibrium pressure was significantly smaller than geostatic pressure, alleviating any fracture risk for a sealed and abandoned cavern in this salt formation.

Thousands of caverns have been leached out from salt formations. They are used for storing a variety of fluid products ranging from compressed air and hydrogen to LPG, natural gas and crude oil, which requires that the caverns be tight. The main factors in the onset of well leakage and its prevention are discussed: fluid pressure distribution, geological environment, cementing workmanship and well architecture. The Mont Belvieu accident is described to illustrate the importance of periodic cavern testing. Test methods are discussed; apparent, corrected and actual leaks arc distinguished. Factors contributing to apparent leaks are described, as are two actual in situ tests that use fuel oil and nitrogen as test fluids. It is proven that a thorough test analysis allows good estimations of actual leaks.

A test has been performed on a deep brine-filled cavern, whose objective was to measure the equilibrium pressure reached when the cavern is closed. Such an equilibrium is reached when salt mass creep, which leads to cavern shrinkage, balances the permeation of brine through the cavern wall. This pressure is significantly smaller than the lithostatic pressure at cavern depth, alleviating any natural fracturation risk.

In an underground cavern leached out from a salt formation, cavern brine pressure builds up after the cavern is sealed, due to salt mass creep, brine warning and brine seepage through the rock mass. These effects have been quantified to predict the equilibrium pressure reached at the end of the process. An 18-month-long test allowed to validate the prediction. The observed discrepancy may be attributed to large-scale permeability assessment which is especially difficult.

Thousands of caverns have been leached out from deep salt formations. They are used for saturated brine production and/or hydrocarbons storage. They will be abandoned some day: the access well will be plugged with cement, isolating a large bubble of saturated brine. The later evolution of such a bubble raises serious concerns for environmental pro- tection; salt creep and brine thermal expansion can lead to brine pressure build-up and rock-mass fracture, then brine seepage can lead to pollution of overlying water-bearing strata. Taking into account salt formation permeability leads to less pessimistic scenarios. An 18-month test has been performed on a deep brine-lled cavern. The objective was to measure the brine equilibrium pressure reached when the cavern is closed. Such an equilibrium is reached when salt mass creep, which leads to cavern shrinkage, balances brine permeation through the cavern wall. This objective was met by imposing different pressure levels and observing whether the pressure increased (or decreased) with respect to time. Data misinterpretation (i.e., a well leak instead of a cavern-proper leak) was precluded by a special monitoring system. The observed equilibrium pressure was signicantly smaller than geostatic pressure, alleviating any fracture risk for a sealed and abandoned cavern in this salt formation. This result allows to optimize the period during which the cavern must be left opened before nal abandonment.