Storing gas in rock caverns is a proven technique. The cavern may be lined, unlined or solution-mined. The stored gas may be pressurized with high pressure and cooled down to low temperature, even liquefied. This paper discusses the potential rock mechanics problems associated with different types of gas storage rock caverns.
Storing natural gas in rock caverns is a proven technique. The cavern may be excavated or solution-mined; lined or unlined. The stored gas may be pressurized with high pressure and cooled down to low temperature, even liquefied. Various rock mechanics problems may be encountered for different types of gas storage caverns. The steel-plate lined cavern (LRC) can be located at a shallow depth and subjected to extremely high internal gas pressure. The steel liner can be as thin as a few millimeters, while the recorded pressure has reached 52 MPa. The size of this type of storage cavern is moderate. The cavern can be emptied entirely and gas can be withdrawn completely. The gas will not be contaminated in any way, neither the ground water. In this sense, LRC is environment-friendly storage. The basic concept is that the pressure is taken care of by the rock and the steel liner serves for tightness. The typical example is the pilot cavern at Grangesberg, Sweden. For the unlined caverns in hard rock, the rock, together with ground water, takes care or both pressure and leakage, as well as the thermal stress, if applies. For storing liquefied gas, such such as propane, the storage temperature is lower than -40°C and the corresponding vapor pressure is usually a little higher than the atmospheric pressure. The main problems are thermal stress inducedstability during cooling-down as well the leakage during operation. The Karsto and Mongstad caverns are such projects. In the extreme cases of LNG storage, both low temperature (-160°C) and high pressure (up to 10MPa) may be encountered. Creating a cavity in rock salt formation by leaching for gas storage has been widely used for decades in some European countries.The storage volume of existing caverns may reach a half million cubic meters. Research has demonstrated the feasibililty of creating large volume cavern up to 3 Mm3. The major challenge is the storage volume loss resulting from the creep character of the host salt that will reduce the life span or the cavern. It is also attempted to reduce the spacing between the caverns so that the salt formation can be fully used. A new idea of concrete tank resting inside the rock cavern For LNG storage is proposed recently (Klungland 1998). As Shown in Figure 1, the structure consists of three layers: concrete, insulation and tightening membrane. The span of the cavern is determined by the quality of the rock and the height is limited by the compressive strength of the insulation. Due to the insulation the rock will not be exposed to the extreme temperature, such avoiding thermal stresses, which may otherwise cause stability problems.
