UGS are an essential component of the gas delivery chain. They are used worldwide to optimally balance fluctuations in a relatively steady supply, irregular daily and seasonal consumption and price. They mitigate the risk of service disruption due to political events, technical problems or accidents (to the transportation system) and emergencies (extreme weather) so they have become critical to safeguard customers but also to better negotiate supplies.

The first UGS became operational as early as 1915. Since then hundreds of facilities have been developed with some 150 in Europe and Central Asia, over 400 across the United States and 59 in Canada.

Underground storage of natural gas has become a large and essential part of the natural gas delivery system and continues to grow. This increasing demand for natural gas calls for an enhancement of underground storage capacity and deliverability both by creating new facilities and by upgrading existing ones.

Base, or cushion, gas may include both native and injected gas and it is the volume that must be left in the underground storage to provide the required pressurization so that the remaining gas can be delivered at reasonable rates.

The amount of gas injected and withdrawn during the normal cycle of storage is called working, or top, gas.

Deliverability is the measure of the amount of gas that can be delivered, or withdrawn, from storage on a daily basis.

By increasing the pressure of the UGS above the original reservoir pressure there is the potential to increase the volume of stored gas and the well deliverability. This is called Delta Pressuring. If the original formation pressure is exceeded both the mechanical integrity and hydraulic sealing capacity of the caprock must be investigated. Well performance monitoring may also be appropriate.

Other considerations such as rock and discontinuity stress-strain behavior, position of spill point(s), and wellbore integrity are important in UGS evaluation. More information can be found in the technical considerations.

Historically, depleted gas reservoirs have been the most sizeable and commonly used formations for natural gas storage. A depleted field is typically the most suitable option as it has proven characteristics to store gas and there is often a considerable level of knowledge already available about the geological, structural and petrophysical characteristics from primary production. Commercially, depleted reservoirs can provide very good storage efficiency both in terms of seasonal moveable, or working gas, volume and peak performance in terms of injection and or withdrawal gas rates.

Other potential UGS candidates include:

• Aquifers: a common alternative to depleted gas reservoirs. They are expensive to develop due to large cushion gas requirements and the need to verify the reservoir’s capability to contain and retain gas. Here the original formation pressure is always exceeded to displace the water initially saturating the porous rock.
• Salt caverns: typically developed where reservoirs are not available. They contain much smaller volumes than depleted reservoirs and aquifers but can provide high delivery rates.