Drilling and completion technology
Drilling: When it comes to drilling technology, the means is the end.
Drilling a well using a deep drilling system is one of the most important steps in the excavation of a cavern. The first step is to drill a borehole to explore an unknown salt deposit. If the geological conditions are favourable, the hole is developed further and becomes a cavern well. If the deposit has already been explored, the borehole is drilled in such a way that a cavern can be created by leaching and then used for storage directly.
Oil and gas industry techniques are used for drilling. The entire process takes two to three months. Long-life cutter discs are typically used for drilling the well. This is critical because the entire drill string must be disassembled when the tool needs to be replaced. During drilling, bore samples are taken at regular intervals. The geomechanical and geological properties of the bore samples are examined. This provides important information for the brine process that will be performed later.
By using directional drilling, several boreholes can be drilled from a central location. This reduces the impact on the environment and the changeover times for the drilling rig.
The drilling duration and cost depend on the diameter of the borehole, the geology, and the distance to be covered. The salt layers themselves are actually cut through relatively quickly. Near the future cavern, the borehole is made wider to facilitate leaching.
The size of the bore hole is determined by the planned use of the cavern. Several concentric casings are inserted into the borehole and are held in place with cement to provide mechanical safety and to seal off the casings from the surrounding rock.
Standard casings are:
- 28“-surface casing (approx. 30 metres/110 feet) for fixing additional pipes and protection against unstable formations
- 18 ⅝“-anchor casing for holding the casing string in place and protecting aquifers
- 13 ⅜“-last cemented casing up to the neck of the future cavern for sealing off the surrounding rock.
Later, additional casings and underground equipment for leaching and cavern operation are installed. This is called completion..
Completion: Pumping and storing safely
At the end of the drilling process, casings are inserted into the well and held in place with cement. In addition, cavern boreholes are temporarily equipped with expandable casing strings. In storage operation, this is completed with underground equipment (well completion). During the leaching process, which takes between two and a half and four years, the external and internal brine tubes are installed in the well. The storage medium is injected and pumped through the production casing. To hold it in place, a so-called packer is inserted into the last cemented casing. This seals off the annulus between the last cemented casing and the production casing from the cavern. To protect the last cemented casing and to monitor the pressure, the annulus is filled with an anti-corrosion protective fluid. The production casing can be uninstalled and replaced once it is worn.
The cavern head above ground is primarily used for providing a safe cover for the cavern as well as holding the casings into place and providing access to the ring or circle shaped production spaces. The valves visible above ground that are used to protect the production space access openings are typical of cavern heads. These are designed to be manually and automatically redundant to ensure that
they remain safely closed in an emergency.
Another safety feature is the so-called downhole safety valve (DSV). This is a device that automatically closes the production casing in the event of an emergency or catastrophic failure of surface equipment, e.g. if the cavern head is damaged. Casing wall thicknesses and other completion properties are adapted to the depth of the borehole and the pressures present when using the storage system.
Drilling into the depth
Caverns in salt: Drilling and leaching