The development of a methodology for defining safety criteria for CO2 geological storage, realised under the ongoing CRISCO2 project, is depicted in this paper.Generic lists of eleven risk events and nine types of targets are proposed to serve an expert panel for identifying risk scenarios. Assessing the effects of risk events and comparing targets exposure to critical thresholds should lead to setting safety requirements. We focus on simple modelling, such as abacuses, analytical or semi-analytical models. An uncertainty management framework completes this methodology.Further work will concentrate on effectively deducing safety criteria.