Pierre Berest

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 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.

Review of static and dynamic compressibility issues relating to deep underground salt caverns

Abstract Compressibility of deep fluids-filled cavern is discussed. Compressibility is measured both through statical and dynamical tests. Statical compressibility is influenced by cavern shape and the nature of cavern fluids. This parameter plays an important role for such applications as the determination of stored hydrocarbons volume, of volume lost during a blow-out and of pressure build-up rate in a closed cavern. Dynamical compressibility is measured through the periods of waves triggered by pressure changes. Both tube waves and longer period waves associated to the existence of an interface between a liquid and a gas can be observed. They allow checking of the results of the statical tests and can provide additional information, for instance the existence of trapped gas in the well-head.

Static and Dynamic Compressibility of Deep Underground Caverns

Compressibility of deep fluids-filled cavern is discussed. Compressibility is measured both through statical and dynamical tests. Statical compressibility is influenced by cavern shape and cavern fluids nature. This parameter plays an important role for such applications as the determination of stored hydrocarbons volume, of volume lost during a blow-out, and of pressure build-up rate in a closed cavern. Dynamical compressibility is measured through the periods of waves triggered by pressure changes. Both tube waves and longer period waves associated to the existence of an interface between a liquid and a gas can be observed. They can provide additional information, for instance the existence of trapped gas in the well-head.

Mechanical Behavior of Salt Caverns: Closed-Form Solutions vs Numerical Computations

Creep closure and structural stability of a cylindrical elongated cavern leached out from a salt formation are discussed. The Norton-Hoff creep law, or “power law”, is used to capture the main features of salt rheological behavior. Two failure criteria are considered: (1) shear stresses must not be larger than a certain fraction of the mean stress (dilation criterion); and (2) the effective stress at the cavern wall (actual stress plus cavern fluid pressure) must not be tensile. The case of a brine-filled cavern whose pressure is kept constant is discussed first. It is proved that creep closure reaches a steady state such that stresses in the rock mass remain constant. However, decades are needed to reach such a state. During the transient phase that results from the slow redistribution of stresses in the rock mass, deviatoric stresses decrease at the vicinity of the cavern wall, and onset of dilation is less and less likely. At this point, the case of a rapid brine pressure increase, typical of a tightness test, is considered. It is proved that during such a swift pressure increase, cavern behavior is almost perfectly elastic; there is no risk of dilation onset. However, even when cavern pressure remains significantly smaller than geostatic, the effective stress at cavern wall can become tensile. These results, obtained through numerical computations, are confirmed by closed-form solutions obtained in the case of an idealized perfectly cylindrical cavern; these solutions provide a better insight into the main structural features of the behavior of the cavern.

Influence of the leaching phase on the mechanical behavior of salt caverns

Abstract Big caverns can be leached out from salt formations. During the leaching process, cold water is injected in the cavern through tubing; warmer brine is withdrawn through an annular space. Thermal balance is relatively intricate, because salt dissolution is an endothermic process, whereas brine and water exchange heat through the steel tubing inside the well. As a whole, the rock mass is cooler around the cavern after leaching than before, resulting in a small reduction of cavern creep.

Rock Mechanics for Underground Nuclear Waste Disposal in France

Publisher Summary This chapter discusses rock mechanics for underground nuclear waste disposal in France. Disposal of nuclear waste raises a series of scientific and philosophical questions, which require a truly pluridisciplinary approach. The behavior of the medium, where a highly active nuclear waste-disposal site is placed, is complex and depends on the type of material chosen. The mechanical consequences of thermal expansion are particularly pronounced in salt because the coefficient of thermal expansion is high. The risks of signs of failure in the far field in the salt formation, and the overburden, must be examined with all the more care since salt is very vulnerable to water circulation. The coefficient of thermal conduction of salt is high, which allows quicker dissipation of the heat released by the radioactive waste deposited. The phenomena of thermal conduction can bring about problems of heating the medium in the far field and especially near the surface.

Une mesure de la perméabilité et du fluage d'une caverne dans le sel

Abstract Rock mass properties differ significantly from those measured on samples in the laboratory. A test has been performed on a deep brine-filled cavern, with the objective of measuring the equilibrium pressure reached when the cavern was closed. Such an equilibrium is reached when salt mass creep, which leads to cavern shrinkage, balances brine permeation through the cavern wall. A K = 2·10 −19 m 2 value of the average in situ intrinsic permeability has been deduced from the test; it is significantly higher than the intrinsic permeability measured in a well or in the laboratory. This result supports cavern abandonment scenarios in which the risk of natural fracturation due to high brine pressures is alleviated.