Assessing Net and Gross Storage of CO2 in the Subsurface and the Implication for CO2 Credits

This paper examines the definition of a CO₂ credit within the context of geological storage. At present, there is no international standard in place for the calculation of CO₂ emission reduction credits from geological storage. The determination of the gross storage of CO₂ in a geological setting is relatively easy through the use of standard metering systems at the wellhead. This does not account for the many factors that could be considered to affect the net storage of CO₂. Net calculations will require the development of standards that are consistent with other forms of storage (biological storage for example) and will include surface leakage, length of time storage is required, definition of the injection zone, subsurface CO₂ migration, recycling penalties, etc. The CO₂ credit may well be less than the gross amount of CO₂ injected because of any CO₂ penalty that might be imposed. This could be considered as a discount factor that would be applied to each tonne of CO₂ stored in the subsurface. Determining a discount factor will require a complete understanding of the lifecycle of the CO₂ injected into the subsurface, including projections of the fate of this CO₂ over time. Allocation of emissions at various stages of the lifecycle will be achieved by application of a clear set of definitions that will allow these emissions to be inventoried. There will need to be a clear understanding of what is meant by leakage and of how the inventories will be developed. This paper focuses on the assessment of net and gross storage calculations for the CO₂ to be injected into the subsurface, with a brief review of some of the issues requiring definition to assess subsurface net storage calculations.