Three main CO2 geological storage scenarios may currently be considered as technologically-advanced – deep saline formations, depleted oil fields as part of enhanced oil recovery (EOR) schemes, and depleted natural gas fields. Much attention is currently focussed on deep saline formations due to these providing the largest theoretical global storage resource (10,000Gt according to the 2004 IPPC Special Report), and on CO2-EOR schemes due to the potential economic benefits. However, depleted gas fields offer significant advantages for CO2 storage: proven capacity and sealing structures to give confidence in storage security; and the presence of existing infrastructure that may be suitable for re-use in storage operations. Whilst some technical challenges remain – for example, controlling the flow of injected CO2 into de-pressurised formations where aquifer ingress is low or absent – storage in depleted gas fields could be regarded in some locations as an ‘early’ opportunity for large scale commercial storage. The southern North Sea provides an example of such a location, where a number of large fields are rapidly approaching exhaustion of recoverable natural gas reserves.
A key objective of this work is to advance the analysis undertaken in the IEA GHG report Barriers to overcome in implementation of CO2 capture and storage: Storage in disused oil and gas fields, which identified a total potential worldwide capacity of 923 Gt CO2, in depleted gas fields, of which 797 Gt is in depleted gas fields.