Underground Natural Gas Storage

Underground natural-gas storage uses a subsurface geological formation to hold gas until it is needed. In a depleted hydrocarbon reservoir, pipeline gas is injected into the pore spaces of a formerly producing oil or gas field and later withdrawn through wells; it is not stored in a large open cavern.

Reservoir conversion and containment

A suitable field contains porous, permeable reservoir rock beneath an impermeable cap rock that previously trapped hydrocarbons. Production records provide evidence about its geology, pressure behaviour and containment, while existing wells and pipelines may be reused after integrity testing and modification.

• Injection and withdrawal wells, compressors, gathering lines, meters and gas-treatment facilities connect the reservoir to the pipeline network.
• Operating pressure is controlled to protect the reservoir, wells and sealing formation.

Injection, storage and withdrawal cycle

During periods of lower demand, compressors raise the pressure of pipeline gas and inject it through wells into reservoir pores. During higher demand, the pressure difference drives gas back toward production wells; compression may assist withdrawal as reservoir pressure declines.

• Cushion gas, also called base gas, remains underground to maintain adequate pressure and deliverability.
• Working gas is the portion repeatedly injected and withdrawn during operating cycles.
• Withdrawn gas is measured and, where necessary, dehydrated or otherwise treated before re-entering the pipeline.

Performance and suitability

Usable capacity depends on pore volume and the permitted pressure range, while deliverability depends mainly on pressure, permeability and well capacity. Deliverability generally falls as working-gas inventory and reservoir pressure decline.

• Depleted fields can offer large storage capacity and may benefit from known geology and existing hydrocarbon infrastructure.
• They are commonly suited to seasonal supply balancing, but require continuing surveillance of wells, pressure, water movement and possible leakage pathways.