3 Underground storage options

Facilities for the underground storage of gases fall into two main categories:

  • Porous media storage, either in partially depleted oil or gas fields or aquifers, in which the gas occupies the naturally occurring pore space between mineral grains or crystals in sandstones or porous carbonates;
  • Cavern storage, in which the gas is contained in excavated or solution-mined cavities in dense rock.

Both the storage categories have to satisfy two main requirements: providing sufficient storage capacity and containment of the stored gas.

In porous media storage, these requirements are met by a porous reservoir rock and an overlying confining enclosure, whereas in cavern storage, capacity is achieved from the chamber volume with containment provided by the impermeable host rock surrounding the cavern.

Several factors may influence the capacity and containment capability for a given storage mode, in particular storage pressure. As most host rock lithologies are not absolutely impermeable, the lower limiting pressure for some forms of underground storage is related to the hydrostatic pressure gradient, while the upper limiting pressure is related to the ultimate overburden pressure gradient. The overburden pressure is the load of the rock column and, when approached, may result in hydraulic fracturing, or lifting, of the overburden.

Most existing underground storage facilities for natural gas have maximum operating pressures in the range of 70 to 170 bar, although there are facilities operating at both extremes, from a low pressure of 10 bar to a maximum of more than 270 bar.

As the storage pressure increases, a lower volume capacity is required for a given quantity of stored gas. On the other hand, a number of factors limit the maximum depth and pressure desirable for underground storage, including the costs of drilling wells or sinking shafts, the cost of compression, and the geothermal gradient, because high storage temperatures partially offset the volumetric efficiency gained by greater pressure. Except in the case of depleted fields, the higher cost of exploration at greater depth also is a limiting factor, whereas the depth of storage caverns in salt is limited by the rheological properties of salt.

Depending on the mechanism adopted for withdrawing the gas from the reservoir, the storage can be at constant or variable pressure.

If water entries in the previously gas-filled portion of the reservoir, the reservoir operates at essentially constant pressure. If volumetric expansion occurs during the withdrawal cycle, the reservoir pressure drops down.

3.1 Porous media storage

An underground storage in porous-media requires the following features, as shown in Figure 3.1-1:

  • A stratum of porous rock, usually sand or sandstone, at 150-900 m below the surface, sufficiently porous to provide a reasonable storage volume and sufficiently permeable to provide an adequate injection and withdrawal rate;
  • A caprock of adequate thickness, overlying the reservoir;
  • A suitable dome-shaped geological structure such as the anticline, that provides structural closure to limit lateral and vertical upward movement of the gas, together with an underlying gas/water contact, that prevents downward movement of the gas.

Figure 3.1-1: Elements of porous media underground storage

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During operation, a minimum base gas or ‘cushion gas’ has to be maintained in the reservoir. The cushion gas is a volume of gas that remains as permanent inventory in the storage reservoir to maintain adequate pressure and deliverability rates.

In the case of hydrogen storage, a cushion gas of different nature, such as natural gas, can be used to displace a hydrogen-rich gas, but only if gas stratification can be maintained between cushion gas and hydrogen, by avoiding inter-diffusion or “fingering”. Nevertheless, this would also require an efficient gas separator (membrane or PSA) in the gas station at ground level.

Whether this mixing should be encouraged or discouraged depends also on the use of the stored gas. If hydrogen will be used as a chemical feedstock, then high purity is required, thus limiting the amount of mixing that can be tolerated.

3.1.1 Depleted natural gas or oil field storage

The oldest, most widespread and most economical mode of underground gas storage is the re-injection of gas into existing fields, partially depleted by prior production. For natural gas storage, the use of such fields is advantageous, because it virtually eliminates exploratory cost and risk and because these fields normally contain sufficient residual gas to fulfil all or part of the base gas requirement.

Conversion to storage may require only the reworking of wells and the installation of compressor facilities. In the case of hydrogen storage, the presence of residual natural gas may be more of a problem than a benefit, because until it is fully displaced, mixing of the natural gas and hydrogen results in the production of gas characterised by a widely varying heating values.

3.1.2 Aquifer storage

In case no suitable depleted field is located near the market area or the pipelines facilities, it has been possible to develop similar fields, converting natural aquifer to gas storage reservoirs, by injecting gas to displace water from a portion of the aquifer.

A natural aquifer is suitable for gas storage if the water-bearing sedimentary rock formation is overlaid with an impermeable cap rock. While the geology of aquifers is similar to depleted production fields, their use in gas storage usually requires more base (cushion) gas and greater monitoring of withdrawal and injection performance. The base gas may represent from one-third to two-thirds of the total field capacity. Deliverability rates may be enhanced by the presence of an active water drive.

3.2 Cavern storage

Unlike depleted field and aquifer storage systems, cavern storage involves large open, void spaces to be filled with gas.

Underground manmade caverns are mined with access to the surface with wells. The most common type of cavern is the solution-mined cavern in salt domes, often found in form of layers that can be hundreds of meters thick. Alternatively caverns can be drilled in hard-rocks. Furthermore, efforts have been made to use abandoned mines to store compressed gas.

One important advantage of the cavern storage is that it is geologically feasible in many areas where porous-media storage is not. An additional advantage is that there is no limitation on gas deliverability, with respect to the porous-media storage where withdrawal rates are limited by the permeability of the reservoir formation and the number of wells available. Finally, cushion gas requirements are relatively low.

On the other hand, a more complex structural analysis is therefore required to establish feasibility. For example, if the pressure in the cavity is allowed to drop significantly below ambient pressure, a collapsing stress situation is created, which might result in loss of structural integrity of the storage volume. The cavern pressure has to be maintained above a safety limit, providing a proper amount of cushion gas or replacing the drawn off gases with water.

Two approaches can be followed to design a gas storage cavern: constant-pressure and variable-pressure design.

Constant-pressure or pressure-compensated design requires to keep the cavern partially filled with water, providing a connection with a surface water or brine pond, as shown in Figure 3.2-1. The pressure is kept constant by the hydraulic head of water that connects the water in the cavern to a reservoir at the surface, while the working volumes changes. During withdrawal periods, water is allowed to enter the chamber and displace the stored gas. The water level is lowered in the cavern during gas injection, as water is returned to the surface pond through the shaft that connects the cavern with the reservoir.

Reservoirs for compensated cavern storage do not always require surface ponds and can be designed as an underground chamber above the storage cavern, as shown in Figure 3.2-2. This water-compensating pressure system of cavern storage operates with a minimal volume of base gas, as the water maintained the pressure in the cavern providing the driving force to displace the gas during withdrawal operation.

Figure 3.2-1: Pressure-compensate storage caverns

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Figure 3.2-2: Pressure-compensate storage caverns with underground brine reservoir

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The variable-pressure cavern shown in Figure 3.2-3 is a closed system in which the storage pressure is determined by the amount of gas stored in the cavern. Pressure fluctuates as the gas inventory changes. Maximum storage pressure is established by hydrostatic pressure. Minimum storage pressure can be determined by pipeline or compressor input pressures.

Figure 3.2-3: Variable-pressure storage caverns

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3.2.1 Solution-mined salt caverns

Mines-solution cavern in salt domes is the most common type of manmade cavern storage. The cavern is created dissolving the salt layer with fresh water and removing the brine via a single well, which is used both for gas injection and withdrawal.

Salt caverns can be both vertically mined or horizontally mined, depending of the salt layer thickness. If the salt layer is between 60 to 100 metres thick, a horizontal drilling with solution mining techniques is preferred for storing the required volume of hydrogen, with respect to a collection of smaller and inter-connected vertical solution-mined caverns.

Salt caverns provide very high withdrawal and injection rates relative to their working gas capacity. Base gas requirements are relatively low and can be totally recovered with brine injection.