Permeability and Fissuring Studies to Ascertain the Imperviousness of Salt Caverns

摘要

An important criterion for the construction of storage sites or for the utilization of existing subterranean cavities as storage sites is the impermeability of the rock surrounding the cavity. The maximum allowable working pressures in the cavity from this point of view must be selected in such a manner that no fissuring of the rock should be expected, thus precluding leaks. Results are reported from studies related to the rock surrounding saltierra cavities of interest for gas storage. Results led to the conclusion that porosities and permeabilities do not represent simply rock constants, but petrophysical characteristics which depend, among others, essentially on external load and on the deformation properties of the rock in question, and that permeability measurements on saliferous rocks in the laboratory cannot be applied to those in situ. The recovery of samples from a rock association is connected with more or less rapid relief according to the stress condition predominating there. This process leads to a structural loosening. Moreover stresses which arise when the samples in question are being bored out cannot be precluded. Both processes are irreversible since they represent failure to a certain degree, and result in the genesis or enlargement of pore volumes and thus a permeability not present in bedrock. As the result of a creep process, simulation of overlying pressure on the inelastically behaving saliferous rock in laboratory studies results in the pore volumes being partially closed, partially separated so far from one another that such a saliferous rock under pressure for a sufficient length of time loses almost all effective porosity and thus is impermeable.