Mined caverns for the storage of hydrocarbons and carbon-free products
The first mined rock caverns: Chambers and pillars
Mined hydrocarbon storage caverns were first designed and developed in the United States in the 1950s. At the time, the technology was purely mining-oriented and caverns were of the “room and pillar” type conventional in the mining industry. These underground caverns were designed to store liquid or low-pressure hydrocarbons such as LPG, using only the intrinsic sealing of the host rock to contain the product. Accordingly, geologists and engineers researched and constructed caverns in very low permeability rocky soil masses (typically with a hydraulic conductivity of less than ~ 10-8 m/s on the scale of the rock mass), such as shale or low permeability carbonated rocks, including limestone, dolomite and compact chalk, ideal for achieving this objective. The caverns were also sufficiently deep for the pore-water pressures in the rock (pressures exerted by the water naturally present in the rock) to be greater than the pressure of the stored product. This was a positive addition to the intrinsic sealing of the rock but did not necessarily fully meet the hydrodynamic containment criterion. This hydrodynamic containment criterion (see below) was not fulfilled until a few decades later.
The first LPG storage cavern built in France was developed on the basis of this concept imported from the United States in the mid-1960s by Fenix & Scisson. However, the storage capacity of this type of cavern remained modest, at around a few tens of thousands of square metres. Furthermore, owing to the use of the intrinsic sealing principle, this technique could not be developed in geological environments with substantial rock mass permeability, which is often the case.
Large longitudinal galleries
Thanks to improvements in excavation equipment and the development of rock mechanics, a new concept of large caverns for the storage of hydrocarbons in unlined mined caverns emerged, first in Northern Europe (thanks to good geological conditions) and then in France. Storage in caverns in rooms and pillars therefore mainly gave way to the concept of storage in large longitudinal galleries with cross-sections of up to 600 m2 over several hundred metres.
The hydrodynamic containment concept marked a turning point in the design of unlined mined caverns for hydrocarbon storage, making them much more adaptable to a wide variety of geological conditions, including fractured rocks, which was not the case in the previous concept. Many geological environments (sedimentary, volcanic, plutonic, metamorphic rocks) became favourable candidates for the creation of storage caverns, boosting interest in this underground storage technology.
Storage cavern stability is ensured by conventional methods of support by shotcrete (preferably reinforced with steel fibre) and reinforcement through bolting (steel or fibreglass bolts) installed as the caverns excavated. The thickness of the shotcrete, the number and length of the bolts and the characteristics of the materials to be used are defined according to the geotechnical quality of the rock mass. This quality is assessed in real time as the work progresses by geological mapping and is based on proven empirical methods (for example, Q-system and RMR) for assessment purposes. In some cases, a heavier support, or even a pre-support, may be necessary (steel arches, reinforced shotcrete ribs, forepoling) to ensure the long-term stability of the galleries, particularly in fault areas or heavily fractured areas.
To limit the amount of water to be managed in underground caverns, rock grouting is carried out during excavation work in the most permeable areas.
Geostock has been involved in the vast majority of mined caverns around the world since the 1980s.
Lined Rock Cavern
Recently, the development of lined rock cavern storage concepts has proven suitable for the storage of cryogenic products such as liquefied natural gas and for the storage of gaseous products at high pressure (natural gas or hydrogen).
For cryogenic storage in lined mined caverns (liquefied natural gas storage), a lining consisting of a stainless-steel corrugated membrane ensures the containment of the product and a layer of polyurethane foam placed on a concrete wall is used to insulate the rock against thermal shocks. A drainage system consisting of a network of boreholes placed approximately 10 to 15 m under the storage galleries desaturates the rock mass before the lining is put in place such as avoid water arrival, as well as overpressure generated by the formation of ice lenses during the preliminary phase of the storage operation. The depth of storage is such that the frozen zone above the storage remains sufficiently far from the surface, the crown of the storage galleries being placed at a depth of approximately 50 m. A (liquid nitrogen) pilot cavern has been built and tested in South Korea as part of a partnership between Geostock, Saipem and SKEC, proving the feasibility of the concept and the operation of this type of storage. The concept allows for the storage of large volumes of natural gas and a low boil-off rate compared with the storage of LNG in surface tanks.
An alternative concept was developed to store gaseous products at high pressure (initially natural gas). The maximum pressure envisaged is around 20 MPa. The concept of hydrodynamic containment would require prohibitive depths for this level of cavern pressure. A steel membrane is therefore necessary to seal the product while maintaining a standard storage depth for mined caverns, at about one hundred to one hundred-fifty metres below ground surface. For a given maximum operating pressure, the depth of the underground cavern is sized to prevent cavern uplift pressure. The architecture of the underground storage facility depends on the mass capacity to be stored and may consist of vertical storage (in wells or silos) for small or moderate volumes or horizontal tunnels for larger volumes. The steel membrane must be sized to accommodate the deformation of the rock mass resulting from the relatively large pressure variations in the cavern and to resist the effects of fatigue owing to operating cycles. Geostock is currently working to develop the most optimised technical and economic solutions for the storage of gaseous hydrogen at high pressure in lined mined caverns.
Regardless of the lined mined cavern technology, vertical pipes are used to circulate stored products between the cavern and the surface. Pumps are installed at the bottom of the cavern for this purpose. If the cavern is not lined, seepage water (or mine water) needs to be regularly drained using special bottom pumps and vertical pipes. These lines, along with the instrumentation lines, are usually grouped together in a large diameter shaft.
Unlined mined caverns also require surface facilities for product treatment (dryer) before shipment and for treating the seepage water (stripper). The surface equipment also includes metering benches.