Geometrical versus rheological transient creep closure in a salt cavern

An in-situ test performed in a brine-filled cavern proves that, when brine pressure decreases rapidly, the creep closure rate increases drastically. Conversely, a rapid pressure increase leads to “reverse” creep closure: cavern volume increases, even when, at cavern depth, fluid pressure is lower than geostatic pressure. It is tempting to explain these two phenomena by transient salt creep, a characteristic feature of salt rheological behavior commonly observed during laboratory creep tests. In fact, computations performed on an idealized cylindrical cavern excavated from a Norton-Hoff rock mass (a constitutive law that includes no transient component) prove that these two phenomena are, at least partly, of a structural nature: their origin is in the slow redistribution of stresses following any pressure change.