Cases, causes and classifications of craters above salt caverns

Abstract

Based on the description of a dozen of cases, this paper suggests a categorization of craters formed above brine-production caverns for “piston” and “hourglass” types. The deliberate creation of three craters in Lorraine (France) above the Keuper salt formation is described first. After the cavern roof reaches the top of the salt formation, stoping takes place. A rigid cylinder of rock (a piston) drops abruptly in the cavern, experiencing no deformation and creating vertical crater edges. In the same mining district, a room-and-pillar panel collapsed abruptly in 1873. Multiple pieces of evidence have proven that, here again, no deformation occurred in the cylinder that dropped into the mine. These examples prove that cavern drop is more abrupt when the cavern is filled partly with air, as less brine must be evacuated from the cavern during collapse. The Haoud Berkaoui (Algeria) and Bereznikovsky (Russia) craters also belong to the piston type; less information is available. The main features of the piston mechanism can be captured by a simple model: failure takes place when vertical shear forces along the cylinder edge, plus cavern internal pressure, are not able to balance cylinder weight. This model suggests that the contour of the crater must be a circle and that collapse is easier when the ratio between cavern radius and cavern depth is larger. This may explain why no example of a collapse above a hydrocarbon storage cavern is known: in most cases, this ratio is very small. Three sinkholes formed above salt caverns leached out from the Hutchinson salt formation in Kansas (USA) epitomize the hourglass type. Here, again, stoping occurs until the cavern roof reaches loose sediments at shallow depth. A sinkhole grows when sediments flow to a central hole to fill the cavern underneath, generating an upward flow of brine to the sinkhole. Such a phenomenon also can occur in a salt dome. At Bayou Choctaw (Louisiana, USA) a brine cavern rose through the caprock, allowing loose shallow sediments to flow to the cavern. At Bayou Corne (near Napoleonville in the same state), a cavern was within a short distance from the flank of a dome; a 1500-m-deep breach was created, and loose sediments in the dome sheath, accumulated during geological times, filled the cavern—a process that lasted more than one year and created a sinkhole at ground level. In both cases (piston and hourglass) lakes form in the crater and gravity-driven waves are observed. Sinkhole creation can be prevented when the distance between the cavern roof and the salt top (or dome flanks) is large enough and when the ratio between cavern radius and cavern depth is small enough.