Natural gas is generally stored in underground storage facilities such as subsurface geological formations that once held oil or gas deposits. The stored natural gas can be injected into the pores in the reservoir rock after hydrocarbon deposits have been partially or fully produced from these formations.

The Uhřice UGS Facility consists of two main underground storage structures.

Originally a gas deposit discovered in 1984 by the Uhřice 8 well, the Uhřice storage facility has been used for gas storage purposes since 2001. In terms of its geological structure, it is situated on the north-eastern slope of the Nesvačilka Trench, featuring a fairly complex composition on the south-eastern slopes of the Bohemian Massif. The Nesvačilka Trench area is represented by autochtonous sediments ranging from the Lower Paleozoic to the Upper Tertiary. The deposit itself is associated with basal Jurassic rocks – Gresten Formation arenites represented by a 100 – 135 m deep sandy complex. The overlying cap rock is made up of dark clay. The semi-domed reservoir trap, located at the interval of 1,700 – 1,850 m, is elongated from west-northwest to east-southeast, sloping towards the southeast. It is tectonically bound in the north, while an erosional edge is presumed in its eastern part. In the lower structural positions in the southwest there is an aquifer. The reservoir rock is predominantly represented by medium to coarse grained quartzose sandstone and, in the pores wherein the gas is stored, by conglomerates.

A seismic section of the area with deposits in the Gresten Formation.

The gas is being injected into and withdrawn from the facility using a set of eight large-diameter wells which link the Uhřice storage formation to the surface UGS technology. Four observation wells (one in the gas section of the horizon, one at the gas-water contact, one in the aquifer and one in the overburden) are designed to monitor the storage formation and its vicinity.

Uhřice storage facility wellheads.

The oil-gas deposit – the second storage structure, Uhřice-Jih was discovered via the Uhřice 57 well in 2001. It is located in the immediate vicinity of the Uhřice structure at a depth interval of 1,800 – 1,900 m. The oil and natural gas accumulate in the sandstones and conglomerates of the basal section of the Jurassic (the Gresten Formation – arenite beds), i.e. in the same rocks as the gas within the Uhřice structure. These deposits are separated from each other by an erosion furrow filled with Paleogene pelite sediments which ensure their hydrodynamic separation. To the north lies the Dambořice oil reservoir which is separated from the Uhřice-Jih deposit tectonically (by means of an impermeable fault). The reservoir formation is underlain by Early Carboniferous Kulmian sandstone and shale, with Paleogene and Outter Carpathian Ždánice Unit sediments in the overburden. The deposit is elliptical in shape with the ellipse axes up to 400–650 m long. In the southwest direction, the deposit is lined with erosion talus sandy sediments which form a portion of the deposit’s aquifer and are lithologically bound.

The Nesvačilka Trench oil and gas deposits (3D seismic survey).

Conversion of the Uhřice-Jih deposit was initiated by trial injection in 2010.
The year-on-year volume of stored gas had been steadily increasing until the initial phase was completed in 2017 when the structure reached its projected maximum storage capacity. Gas storage at the deposit is conducted in parallel with oil production. Once production operations are terminated here, the oil wells will be re-completed and utilized for gas injection/withdrawal purposes. In the end, the facility will have a total of 8 operational and 5 observation wells.

The wellhead layout and trajectories of the existing wells at the Uhřice-Jih structure (top view).

3D seismic survey data were used to develop three-dimensional geologic and simulation reservoir models for both storage facilities, making it possible to predict the behavior of the storage formations under different operating conditions. The models are periodically updated with new geological and operational-production data.

Uhřice-Jih reservoir model 3D network with a well layout.

The exceptional reservoir properties of the storage formation, favorable parameters of both storage facilities and high-performance wells allow for high daily withdrawal/injection rates despite the relatively small storage capacity. This ranks the Uhřice UGS among leading storage facilities and, with the flexibility of an automated operation of state-of-the-art surface technology, making it a unique and indispensable component of the gas system capable of meeting immediate customer requirements.