Several remarkable features of the mechanical behavior of salt caverns are presented. How these features can be explained by laboratory experiments and micro-mechanical investigations is discussed. Other aspects of the mechanical behavior of salt (slow strain rates, humidity, temperature, stress triaxiality, transient creep and salt failure) are analyzed.
The mechanical behavior of salt can be studied at various scales: the scale of crystals, at which dislocations climb, glide and pile up; the scale of grains and the interface between grains, where micro-cracks can form; and the scale of the salt formation, in which the salt, clay or anhydrite layers generate specific interactions. These scales are provided by Nature. Two other scales are man-made. The first of these is the laboratory scale, which is quite helpful but not fully relevant—laboratory tests alone cannot provide a full picture of salt mechanical behavior. The most significant scale is the scale of the body under study: mine, cavern, or repository. To start the discussion, the next Sections are devoted to a brief overview of salt-cavern behavior.
The elastic behavior of a brine-filled cavern can be observed easily when an additional amount of brine (Vinj) is injected in the cavern: the resulting cavernpressure increase, or pc, is proportional to the injected volume of brine, β Vpc =Vinj, where V is the cavern volume. The slope of the curve, or βV, is called the cavern compressibility (in m3/MPa or bbls/psi). An example of such a test is described in Thiel (1993) and illustrated in Figure 1. The cavern compressibility coefficient, β, is the sum of the brine-compressibility coefficient (which is βb =2.7 10−4/MPa) and of the "hole" compressibility coefficient, βc, which can be written βc =3(1+ ν) f (Ω)/2E, depending on the elastic properties (E, ν) of the salt formation and on cavern shape (Ω) (Bérest et al. 1999).When the cavern is a perfect sphere, f (Ω)=1; for a perfect cylinder, f (Ω)=4/3; when the cavern is "flat", f (Ω) is much larger than 1.A typical value is βc =1.3×10−4/MPa, which proves that the in situ elastic modulus of salt is E =15 GPa or so, a figure that is consistent with the results of laboratory experiments. For instance, Hansen et al. (1984) measured the elastic moduli of natural rock-salt samples from ten different locations in the U.S.; mean values were in the range E =24–32 GPa.
Cavern compressibility plays a significant role in various phenomena: the mechanical-integrity test, pressure build-up in a closed cavern, or blow-out scenarios (Bérest et al. 1999).