After clarifying the terminology of man-made caverns and the advantages of mined caverns compared to other storage types, the concept of mined cavern is presented and detailed within a historical frame. The major evolutions of the concept are developed along with the main technical steps, from mining to tunnelling technology. Emphasize is given to the hydrodynamic containment allowing liquid hydrocarbons and LPGs to be stored in unlined caverns for a large range of geological conditions. More recently stability issues of megawedges have been considered in parallel with the general increase of cavern size in the last decades. The conclusion focusses on the contribution of structural geology in this domain.
Underground storages of hydrocarbons are conventionally classified into three major types corresponding to the three geological contexts in which they are developed: storages in porous media, storages in leached salt caverns and storages in mined cavities (Gomez-Montalvo, 1997; Londe, 2017)
Storages in porous environments include aquifers (porous geological structures, sealed by a rock cap) and depleted fields (old oil or gas fields, production phase being terminated). The latter type, for which the tightness of the geological structure has not to be demonstrated, is the oldest underground storage of hydrocarbons, with the 1st facility put in service in 1916 in Zoar, New York (USA).
The second type, storage in salt caverns, represents an adaptation of the cavities leached by fresh water in order to dissolve salt arranged in layers or domes and create a stable volume. This type of storage is the cheapest one by working volume (barrel or cubic-metre).
The third type, mined caverns, have been first designed and developed in the USA and 73 caverns were created between the years 1950 and 1984 in this country. At that time, the technology was a pure mining one, adapted to and modified for the purpose of underground storage. The room-and-pillar design (Fig. 1, left), classical in mining industry, represents this obvious mining heritage. The excavation used compressed air hammers and then explosive (drill-and-blast method) and even the name "mined caverns" refers to mining industry. Caverns were small even very small and the total volume of the 73 American caverns reached a bit less than 3 300 000 cubic-meters (as compared to present caverns which frequently reach 1 000 000 cubic-metre per cavern). Along with the technological developments, the room height increased as well as the room width, giving higher extraction ratios. The extraction ratio which was originally 0.3 to 0.5 in the 50s reached much higher values, 0.60 to even 0.75 at the end of the 70s. Despite a very interesting development, these values represented a technological limit which led to the abandon of room-and-pillar design for underground storages at the beginning of the 80s. A few room-and-pillar mined caverns have been excavated using road-headers, a technological development which increased productivity but with limited sections.
