A complex of Tvrdonice underground gas storage structures is located in the south-eastern Moravia in cadastral areas of Tvrdonice, Kostice, Hrušky, and Týnec municipalities near Břeclav. It is the first and the oldest underground gas storage in the territory of former Czechoslovakia which stores the natural gas in original partially exploited natural deposits of gaseous hydrocarbons. Its operation has provided valuable experience for construction of other underground gas storages.
The idea of using the underground deposits near Hrušky for storage of surplus gas during the summer time and its delivery into the gas network during increased consumption appeared sometimes before 1968, i.e., at the time of intensive exploitation of both oil and gas deposits. Main reasons included the necessity of balancing the growing seasonal differences between natural gas consumption and supplies due to progressing gas introduction.
Construction of the underground gas storage commenced already in 1971.
Pilot operation commenced in advance already in 1973 when the first compressor was commissioned and gas injection into the storage system commenced. The first phase of construction was completed in 1975. The second phase commenced in 1980 and it was completed three years later. The purpose of the second phase was to increase the daily capacity, reliability of operation, and better storage space utilization. Two additional satellite collection centers were constructed during this phase. These centers enabled connection of other injection-withdrawal wells while maintaining the optimal length of the high pressure connections.
The last phase of construction and modernization commenced in 2004 and it ended in 2005. It focused exclusively on increasing the safety and reliability of operation. A control system was commissioned (data collection, fault reporting, etc.) and selected technological processes were fully automated. At the same time, we performed upgrade of the underground equipment and surface technology of the wells connected during the first phase of construction.
The underground gas storage belongs to early Tertiary Vienna basin spreading over parts of the Czech Republic, Slovak Republic, and Austria. This basin used to belong among the richest oil and gas bearing areas in Central Europe. The underground gas storage is located in the eastern part of central Moravian depression in Hrušky deposit.
From the structural point of view, this is an extended semi-vault along Lanžhot-Hrušky fault. Several of numerous depths (levels) of Hrušky oil-gas deposit are used for natural gas storage.
The most significant storage capacities are located on 8th, 12th through to 14th Sarmatian and 9th Baden horizons, which are located in the depths between 1,050 and 1,600 meters. The collector (rock) with the thickness between 7 and 30 meters saturated by natural gas consists of less consolidated fine grained sands which require heavy production wells with gravel packs in the event of Sarmatian horizons.
Another horizon administered by RWE Gas Storage which is considered for operation is the 9th through to 11th Sarmatian horizon.
When compared to other underground gas storages, Tvrdonice underground gas storage is characterized by greater storage depths and resulting higher values of deposit pressures.
Considering the complicated geological structure and number of storage facilities, another characteristic feature is the relatively high number of both exploitation and monitoring wells including special well for gravity injection of mine water.
Specific is also the fact that oil and gas is still exploited by Moravské naftové doly, a.s. Hodonín from other deposit facilities simultaneously with natural gas storage. Each organization has its own deposit facility with relevant level of protection under mining regulations dedicated for its activities and information about any and all activities and identified anomalies must be mutually exchanged.