The purpose of this paper is to provide a case history of methods used to work on an underground storage cavern containing butane. This refined product had become trapped behind the casing set point. The product was recovered using a mechanical casing cutter and a snubbing unit.
In 1991, when a portion of the roof fell, it formed a trap for an estimated 100,000 bbls. of liquid butane. With extensive planning and project meetings, a thorough procedure was prepared. The workover consisted of first, pulling the tubing up +80' and rehanging it. Then, going in with a mechanical casing cutting tool through tubing, to cut the production casing at the newly formed roof. This would allow circulation of the trapped product. Three cuts were done, one at a time, in an attempt to insure the pipe fell into the cavern. A snubbing unit was used to control the pressure while working on the well. After the final cut was made, a tubing extension was snubbed in and hung off as a tail pipe to the tubing string. A blanket of nitrogen was maintained in the snubbing stack during snubbing operations. The work was performed without incident and saved a substantial amount of money by recovering 100,000 bbls. of butane and returning the cavern to operational status.
Because there are differences between cavern operations and oil and gas production operations, a few basics will be covered…
How Caverns Work:
Storage caverns are for storing hydrocarbons to be used by downstream facilities as required. Caverns are created by drilling into a salt formation then dissolving it with fresh water, creating a void in the formation. Once in service, brine is used instead of fresh water to prevent cavity enlargement.
These caverns are developed in a flat, sedimentary laminated salt formation, not in a salt dome.