The potential scale of carbon dioxide capture and storage (CCS) under long-term decarbonisation scenarios means that analysis on the contribution of large international CO2 storage reservoirs is critical. This paper compares the potentially key role of CCS within cost-optimizing energy systems modelling at the national level (ensuring country-specific technical, economic and policy detail), and the regional level (ensuring transboundary electricity and CO2 trade). Analysis at alternate model scales investigates the full range of drivers on the feasibility and trade-offs in using the Utsira formation as a common North Sea CO2 storage resource. A robust finding is that low carbon electricity is a primary decarbonisation pathway and that CCS plays a key role (32-40%) within this portfolio. This paper confirms that the overall driver of the amount of CCS utilized is the climate policy, with by 2050 a total of 475-570 MtCO(2) captured and stored (of which 110-120 MtCO(2) is stored in Utsira) under an 80% CO2 reduction target. Modelled country differences are much larger due to specific national policies and to regional (EU) commodity trading. From 2030 onwards, Utsira plays a key role within the CO2 storage cost curve, with the Netherlands and the UK being the largest contributors, followed by transboundary flows of CO2 from other countries. However, overall regional CCS flows may be larger (for example under low fossil fuel prices) than the estimated (and uncertain) maximum annual injection rates into Utsira which could potentially represent a significant constraint. (C) 2011 Elsevier Ltd. All rights reserved.