The importance of subsurface gas storage as part of the energy transition mix
24 September 2021 | 9 min read
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Natural gas storage is used as a strategic reserve to level out peaks in demand. Based on seasonal variations, gas is injected when demand and price are low, then withdrawn again as needed Gas storage also provides short-term protection against unforeseen supply disruption by providing an immediate and reliable energy source. Unlike spot markets, storage offers the advantage of a physical asset located close to demand. Natural gas storage allows large natural gas consumers to hedge supply and price risk and, should price suddenly spike, capture the upside opportunity.
A natural gas storage system needs a working gas volume, injection capacity, withdrawal capacity and a volume of cushion gas. The working gas volume is the volume that can be injected and withdrawn during normal operations. The cushion gas volume is the gas that is required to remain in store to maintain reservoir pressure.
The injection and withdrawal capacity, which are largely dependent on the storage pressure, control the rate at which gas can be injected or withdrawn. If storage pressure is low, gas can be injected at high rates but withdrawn at low rates. Conversely, if storage pressure is high due to considerable working gas in store, injection rates will be low, but withdrawal rates will be high. There is a well-documented hysteresis in the field's ability to be repeatedly repressurised and depleted which has to be modelled.
The most common types of natural gas storage are in depleted gas reservoirs and salt caverns and, to a lesser extent, in aquifers. Where aquifers and depleted aquifers do have a much more useful storage capacity is in the area of CO2 long-term capture and storage.
Depleted gas reservoirs have advantages over other types of gas storage as any unproduced gas can be used as cushion gas, and the fields may also have very large working gas capacity. Depleted reservoirs are also likely to be linked to existing gas infrastructure, which reduces initial investment costs.
Salt caverns are located in thick halite formations. They are made by pumping fresh water down a borehole into the salt layer to dissolve the salt and circulate the saline solution to the surface. This process continues until the required size of cavern is reached. Hydrocarbon liquid or gas storage in salt caverns is well understood. For example, the US Strategic Petroleum Reserve (SPR), which is the world's largest supply of emergency crude oil, is stored in salt caverns at four sites along the Gulf of Mexico coastline.
Converting an aquifer to a natural gas reservoir requires long, slow injection cycles to displace the water.
Cushion gas in depleted fields can be 50% of the total storage volume, whereas in salt caverns, it may only be 25% of the total storage volume.
Aquifer storage of natural gas requires a very large percentage of cushion gas making this the least efficient option for storage and withdrawal of natural gas. However, since the intent for CO2 storage is that it stay stored, this is not an issue in that case.