The Salt Dome Whirlpool

Hydrostatic Dissolution and the Draining of Lake Peigneur

Beneath the shallow waters of Lake Peigneur in Iberia Parish, Louisiana, rose the Jefferson Island salt dome. Since 1920, the Diamond Crystal Salt Company had excavated that subterranean pillar of pure halite ($\text{NaCl}$) into a five-level labyrinth of dry caverns supported by eighty-foot salt pillars. On the surface above, Lake Peigneur was a tranquil ten-foot-deep freshwater lake spanning 1,300 acres. On November 20, 1980, a contract drilling crew aboard the floating pontoon barge Wilson Brothers Rig No. 35 was boring an exploratory oil well for Texaco into the lakebed. At a depth of 1,245 feet, the 14-inch steel drill bit seized abruptly. As the crew worked to free the drill string, loud subterranean thuds vibrated up the pipe. Within minutes, the floating derrick tilted sharply toward the water, prompting the toolpusher to evacuate the barge.

Directly below on the third level of the Diamond Crystal mine—1,300 feet beneath the surface—the 14-inch oil drill bit had punched straight through the ceiling of an excavated salt gallery instead of the surrounding shale flank of the dome. When muddy water began pouring around a salt pillar and shorting out electrical lines, mine electrician Randy LaSalle flashed the corridor lamps three times to trigger the emergency evacuation protocol. All fifty-five miners working across the third and fourth levels rode the slow hoist cage up the main shaft to daylight before the lower galleries flooded completely.

Post-accident engineering discussions quickly focused on a suspected surveying coordinate mistake, frequently cited in petroleum engineering literature as a confusion between Universal Transverse Mercator (UTM) coordinates and the modified Lambert grid projection used on subterranean mine maps. However, because the ensuing vortex scoured away both the surface wellhead platform and the upper mine shafts, federal Mine Safety and Health Administration (MSHA) investigators concluded in their official August 1981 report that the exact cause of the drill mislocation could never be definitively proven. Texaco and Wilson Brothers subsequently settled out of court with Diamond Crystal for $45 million without judicial assignment of a specific mapping error.

Whatever placed the drill bit inside the cavern, the puncture triggered an irreversible coupled cascade between orifice hydraulics and chemical dissolution. By Torricelli's hydraulic flow law, the volumetric discharge $Q$ through a submerged orifice of cross-sectional area $A$ under a vertical hydrostatic head $\Delta h$ is:

With $\Delta h \approx 1,300\text{ feet}$ ($396\text{ meters}$), fresh lake water jetted through the initial 14-inch puncture at more than 80 meters per second under a pressure differential exceeding 550 pounds per square inch. Had the cavern roof been insoluble granite, a 14-inch hole would have drained the lake slowly over weeks. Halite, however, dissolves aggressively in unsaturated freshwater up to a saturation limit of 360 grams of sodium chloride per liter.

High-velocity turbulent water scoured and dissolved the salt walls of the borehole, rapidly expanding the orifice area $A$. As $A$ widened from fourteen inches to thirty feet across, the volumetric flow rate $Q$ multiplied hundreds of times over and dissolved the supporting salt pillars around the shaft. Within three hours, a quarter-mile-wide whirlpool swallowed the $5 million drilling rig, eleven commercial canal barges, a tugboat, and sixty-five acres of the adjacent Rip Van Winkle botanical gardens.

As 3.5 billion gallons of water drained into the caverns, displaced subterranean air erupted up the ventilation shafts in 400-foot geysers. Once the lakebed emptied, the twelve-mile Delcambre Canal connecting the lake to Vermilion Bay reversed direction. For forty-eight hours, the canal pulled Gulf of Mexico saltwater backward over a 150-foot temporary waterfall, permanently converting a ten-foot freshwater lake into a 200-foot saltwater bay.