BS/EN 1918-3-2016 pdf download.Gas infrastructure Underground gas storage Part 3: Functional recommendations for storage in solution-mined salt caverns.
4.2.3 General characterization of UGS
UGS are naturally or artificially developed reservoirs respectively artificially developed caverns in subsurface geological formations used for the storage of natural gas (or LPG). An UGS consists of all subsurface and surface facilities required for the storage and for the withdrawal and injection of natural gas (or storage of LPG). Several subsurface storage reservoirs or caverns may be connected to one or several common surface facilities.
The suitability of subsurface geological formations shall be investigated individually for each location, in order to operate the storage facilities in an efficient, safe and environmentally compatible manner.
In order to construct a storage facility, wells are used to establish a controlled connection between the reservoir or cavern and the surface facilities at the wellhead. The wells used for cycling the storage gas are called operating wells. In addition to the operating wells, specially assigned observation wells may be used to monitor the storage performance with respect to pressures and saturations and the quality of reservoir water as well as to monitor any interference in adjacent formations.
For the handling of gas withdrawal and gas injection the surface facilities are the link between the subsurface facilities and the transport system, comprising facilities for gas dehydration/treatment, compression, process control, measurement.
Gas is injected via the operating wells into the pores of a reservoir or into a cavern, thus building up a reservoir of compressed natural gas (or LPG).
Gas is withdrawn using the operating wells. With progressing gas withdrawal the reservoir or cavern pressure declines according to the storage characteristic. For withdrawal re-compression may be needed.
The working gas volume can be withdrawn and injected within the pressure range between the maximum and minimum operating pressure. In order to maintain the minimum operating pressure it is inevitable that a significant quantity of gas, known as cushion gas volume, remains in the reservoir or cavern.
The storage facility comprises the following storage capacities:
— injection rates.
The technical storage performance is given by withdrawal and injection rate profiles versus working gas volume.
Recommendations for the design, construction, operation and abandonment of underground storage facilities are described in Clauses 5. 6, 7, 8 and 9.
Construction of a storage facility begins after the design and exploration phase and should be carried out in accordance with the storage design. It is based on proven experience from the oil and gas industry.
For specific elements of an underground gas storage facility. e.g. wells and surface installations, existing standards should be applied.
4.2.4 Storage In salt caverns
Underground storage of compressed natural gas (CNG) and liquified petroleum gas (LPG) in solution-mined salt caverns is a proven technology for providing storage capacities on a short-term and seasonal basis.
Storages of CNG in salt caverns are artificially developed containments in salt rock usually to provide high withdrawal capacities but may as well be used for the storage of large gas volumes in case numerous caverns are tied into one storage facility.
Salt caverns (see Figure 2) are constructed in suitable salt layers or salt domes by drilling a well into a salt deposit with adequate protection for the underlaying, overlaying and lateral surrounding strata, i.e. mainly by thickness of the salt and completion of the well.
NOTE Some salt caverns may have more than one well, so in this standard the term weII” can also mean “wells”.
It is known that suitable salt layers and salt domes are impermeable to gas up to certain pressures. In addition, cracks and faults in the salt are healed by the viscoplastic behaviour of the salt under the geostatic pressure.
After drilling, salt caverns are leached by the controlled circulation of water or not saturated brine down the weilbore into the salt zone and back as brine to the surface (see Figure 5). Once the geometrical design volume is reached, the brine is displaced from the cavern by the controlled injection of CNG or LPG.
The pressure in a cavern can be cycled between the minimum and the maximum operating pressure of the cavern while considering approved pressure change rates.
Concerning caverns for liquid petroleum gas (LPG), the displaced brine is normally collected in a pond, which has the geometrical volume of the cavern as minimum volume. When it is necessary to withdraw the LPG from the cavern, the brine stored in the pond will be Injected into the cavern. An LPG cavern, in this case, does not require any downhole pumping equipment
This is the most common method for constructing and operating an LPG cavern in salt. With shallow salt caverns, however, the operation may be similar to the operation of a rock cavern for LPG (see EN 1918-4).BS/EN 1918-3-2016 pdf download.