Thousands of cavities have been leached in salt beds or salt domes throughout the world, either to withdraw brine for chemical plants or to store oil and gas. The present paper is specially devoted to deep cavities (say, when the depth is at least five times more than the largest dimension of the cavern). The stability of such cavities has raised up many theoretical publications; this interest can be explained by the remarkable particularities of the 'rock salt rheological behaviour.

Time dependent or delayed part of this behaviour has sometimes brought to inadequate interpretations of experiments carried out in laboratory and then predictions inconsistent with actual behaviour of cavities. Fortunately, a large experience is now available; it is possible and necessary to pay particular attention to "in-situ" datas.

MAIN FACTORS INFLUENCING THE BEHAVIOUR OF A DEEP CAVITY

We can draw up a list of parameters which can be supposed to have a major influence:

  • the depth of the cavity,

  • the mean value and the variations of the internal pressure,

  • the shape of the cavity,

  • the composition of the overlying strata,

  • the vicinity of others cavities, if any.

In this list we have not mentionned the differences between the properties of rock-salt, from one site to another. Microscopical analysis proves large variations in structure and composition; their effect on macrostopical behaviour is likely. But, interpretation of laboratory measurements appearing uncertain, explanations based on quantitative differences in rheological behaviour risk to be rather arbitrary. We prefer at the present stage postulate that the behaviour is independent of the site under study.

The influence of some of the above parameters, as the shape of the cavity for instance, can only be appreciated in a qualitative way. But, the two first parameters allow a quantitative study.

(Figure in full paper)

DEPTH AND INTERNAL PRESSURE

  • Only but few measurements of in situ stresses in rock-salt are available. However advances in salt tectonics knowledge allow to estimate that the state of stresses must not be very different of isotropic pressure equaled to the overburden load where H is the depth (meters) and P the pressure (Mega pascal).

  • The internal pressure in an underground storage is strongly dependent of the nature of the stored products:

    • for liquid or liquefied products, the central tubing is filled with brine up to the surface, for any movement of products must be balanced by an equivalent movement of brine, so that the cavity and the tubings remain fulled up, whatever be the stored quantity of hydrocarbons. The small variations of pressure, due to losses of load during injection or withdrawal, can be neglected, so that the internal pressure is quite close to:

    • For natural gas, the internal pressure can vary in a large extent, only restricted by safety rules. The storage must remain tight and stable; these two restraints lead to select a relevant maximal pressure (for tightness) and minimal pressure (for stability).

This content is only available via PDF.
You can access this article if you purchase or spend a download.