The objective of the CCS UK Storage Appraisal Project was to provide a fully auditable and defensible overall estimate of UK CO2 storage capacity in offshore geological formations. Emitters committing to major capital investment as major CCS roll out occurs in the 2020s will require confidence in the availability of sufficient, suitable storage sites to accept the emissions over the project lifetime. Total UK capacity requirement is expected to be ~15 Gt (including industrial sources) to store 100 years of emissions.The project started in September 2009, and the project team (led by Senergy Alternative Energy) has populated almost 600 potential storage units in the North Sea, Western Channel and Eastern Irish Sea with geological data, security of storage and economic assessments.These units cover both depleted oil and gas reservoirs and saline aquifers. Comprehensive and consistent methodologies have been developed and peer reviewed within the team for calculating storage capacity for each unit.These take into account both ‘static’ effects (e.g. how much CO2 can be stored before a maximum pressure is reached) and ‘dynamic effects’ (e.g. how much CO2 can be injected into a large open aquifer before there is a risk of migration beyond the bounds of the storage unit; how fast can CO2 be injected into a ‘closed’ unit through one well).