A salt cavern abandonment test

Thousands of caverns have been leached out from deep salt formations. They are used for saturated brine production and/or hydrocarbons storage. They w ill be abandoned some day: the access well will be plugged with cement, iso lating a large bubble of saturated brine. The later evolution of such a bub ble raises serious concerns for environmental protection; salt creep and br ine thermal expansion can lead to brine pressure build-up and rock-mass fra cture, then brine seepage can lead to pollution of overlying water-bearing strata. Taking into account salt formation permeability leads to less pessi mistic scenarios. An 18-month test has been performed on a deep brine-fille d cavern. The objective was to measure the brine equilibrium pressure reach ed 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 leve ls and observing whether the pressure increased (or decreased) with respect to time. Data misinterpretation (i.e., a well leak instead of a cavern-pro per leak) was precluded by a special monitoring system. The observed equili brium pressure was significantly smaller than geostatic pressure, alleviati ng any fracture risk for a sealed and abandoned cavern in this salt formati on. (C) 2001 Elsevier Science Ltd. All rights reserved.