Cavern storages of natural gas in gaseous state have been constructed only in salt caverns and in abandoned coal mines so far. Háje cavern storage is the first commercial storage constructed in crystalline structures.
Search for a way of balancing peaks in natural gas consumption in Prague and the industrial centres around it begun at the end of 1970s. As the underground storages located in Moravia have proven to be too far away, it was necessary to look for a new location where underground gas storage could be developed.
Among the locations under consideration, a plutonian geological structure in Central Bohemia located approximately 70 kilometers southward from Prague appeared to be the most suitable. Pilot tests in the Bohutín cavern of Příbram ore mines commenced in 1980 and the testing continued in the 68-Milín shaft in 1984.
The lower part of the underground gas storage is located in the granodiorite massif of the Central Bohemian central granite. Granodiorite of Blatná type, which is characterized by medium grain size with facies from fine grain biotitic up to hornblende-biotitic granodiorite, penetrates to the surface.
The natural gas storage space was formed by driving and excavation of rock from the underground. One horizontal tunnel with slope of 5‰ was driven in the depth from 961 meters at the junction of five production wells up to 955 meters at the other end of the storage located approximately 1,350 meters away. The system of tunnels with cross sections between 12 and 15 square meters and with the total length of 45 066 m creates space for natural gas storage. The driven tunnels were left without any surface treatment, permanent bracing was constructed only in places with considerable rock loosening where the risk of tunnel collapse was considerable. Places with water infiltration through cracks in the rock massif were sealed by grouting.
Pressure seals of two transport cross cuts providing access into the underground gas storage area represent important parts of the underground structure. Before commencing with construction of final plugs, a test plug was constructed in the access corridor to the borehole located between the plugs. Mathematic model of the plug was verified on this formation. Investigation comprised verification of the mathematic model of the plug, investigation of volume changes of the plug body in relation to the hydrating heat development, impermeability of the surrounding rock, and the quality and technology of the steel-fibre-reinforced shotcrete. Behavior of the rock massif and the plug at pressures higher than the hydrostatic pressure was verified by pressurizing the area behind the plug towards the borehole between the plugs after the test plug and surrounding rock grouting. The obtained results were used to define the technology of steel-fibre-reinforced shotcrete application, grouting works, and the criteria for water pressure tests were determined. Manual shotcrete application was replaced by application with a handling device. After the end of the tests, an opening was constructed in the test plug in order to provide access to the borehole between the plugs. Changes in mixing technology, transport, and steel-fibre-reinforced shotcrete application was subsequently tested on test plug No. 2.
Each pressure seal consists of four pressure plugs, two on each cross cut. The pressure seals have been constructed using the steel-fibre-reinforced shotcrete technology. Each pressure seal is a concrete block with the total length of 10 meters. The medium part of the seal is conically embedded to the depth of 1.2 meters into the surrounding rock all around its perimeter. Outside sections of the plug fill the current profile of the cross cuts. The concrete formations are provided with steel armoring closing the entire cross section of the cross cuts on both sides. The rock massif in the area of the plugs is reinforced with steel struts. In order to increase the rock impermeability, grouting was performed in the area of the concrete plug. Grouting of joint between the concrete formation and the rock took was performed during the same time as the concrete application. The zones in front of the faces of the concrete formation were lined with concrete and a damp proof foil covering the entire perimeter of the cross cut was applied after lining adaptation. The cross cuts between the plugs are interconnected by an inset which is connected to the access corridor to the borehole between the plugs.
The principle of the rock massif sealing consists in establishing a pressure water curtain around the plugs on the gas side. In order to establish the pressure water curtain preventing gas leakage along the plugs, two borehole rings have been established in front of the plugs on the water side of these plugs. These boreholes are perpendicular to the line of the cross cuts. The pressurized water from the area between the plugs and water supply boreholes penetrates into cracks and fissures in the surrounding rock massif and acts against the pressure of the gas. An overpressure of 0.5 MPa of the water curtain in the area between the plugs is maintained by nitrogen overpressure above the water level. Water is automatically replenished in the event of water level decrease in the borehole between the plugs below 250 meters.
The second technological borehole is a drainage borehole which leads into the lowest point of the underground gas storage. A sump for collection of water from the entire storage is located at this place. The borehole is equipped with a casing, pumping pipes, and ejector pump used for water removal from the underground.
The underground natural gas storage space is connected to the technology on the surface by five production wells which are used for natural gas withdrawal at the same time. The boreholes have been designed with respect to the assignment and also with respect to the fact that they must remain in failure-free operation for the entire service life of the storage. The casing string has the diameter of 9 5/8“ and it can withstand the internal overpressure of 28.3 MPa.
A local seismic network is maintained in the underground gas storage area. Seven monitoring stations enable registration of both local and regional seismic effects. Gas measurement grid monitoring methane content in the ground has the structure identical to that of the seismic station.
The overground part of the underground gas storage is equipped with plants necessary for gas treatment and transport.
Gas is supplied to the underground gas storage area by DN 500 PN 63 Zvěstov - Háje pipeline which is connected to DN 700 PN 63 pipeline from Veselí nad Lužnicí to Prague. The gas first passes through separators in which mechanical and liquid impurities are removed. Cleaned gas can be supplied through pressure regulation station into the distribution pipelines of Středočeská plynárenská leading to Příbram and Kasejovice.
In the injection mode, the natural gas is first preheated and its pressure is subsequently reduced to that of the gas in the cavern. Gas quantity is measured before it enters the cavern. Once the natural gas injection into the cavern by the pressure in the pipeline is no longer possible, the gas passes into the compressor station after quantity measurement and it continues to the cavern only after the compression and possible cooling.
During exploitation, the gas leaving the cavern is first cleaned in separators and subsequently cleaned. Furthermore, the gas is preheated as necessary, its pressure is reduced to that in the pipeline. Quantity of the gas is measured before it enters the pipeline. Withdrawal of the gas from the cavern gradually reduces the pressure in the cavern. Once it is no longer possible to exploit gas by its own pressure, its flow is switched to the compressors. Filtered gas passes to the compressor station and the compressed gas is subsequently transferred for drying. Natural gas is transferred to the gas distribution system after quantity measurement.
Underground gas storage operation is possible on two levels, the third level is planned for the future. The first level is the local level when individual actuators are controlled from local instrument panels or control panels. The second level is available in the control room. Control (operation) from this level is possible by two means. Individual control of individual actuators by commands from the operator’s workplace when commands are executed only if the conditions for safe operation and automatic control of the plant are met and when the system ensures required sequences for transfers between individual operational conditions based on the instructions and other specified inputs.
Construction of the overground part of the underground gas storage commenced in January, 1996. In January, 1998 first natural gas was introduced into the underground storage and both individual and complex tests of gas equipment and pressure seal subsequently took place. Natural gas was injected into the underground storage space on July 14, 1998. Ever since, there was a gradual increase in bearing pressure to the current 12MPa from a maximum of 12.5MPa.