This paper combines the knowledge and experience accumulated at the International Research Institute of Stavanger (IRIS) over a number of years, based on several case studies involving the Tjelbergodden CO 2 value chain, modelling of CO 2 injection at the Ekofisk field (fractured chalk reservoir) and Gullfaks CO 2 injection compositional study.The paper discusses the importance of correctly representing the physical effects when modelling miscible or immiscible CO 2 injection in sandstone and carbonate reservoirs. The following aspects were evaluated:• Transfer of CO 2 from fractures to matrix involving diffusive, gravitational and capillary forces;• Viscous effects;• Formation of low and high concentration liquid hydrocarbon phases;• Compositional effects such as miscible displacement and oil swelling;• Chemical interaction between CO 2 , formation or injection water and reservoir rock. The sensitivity and importance of these effects were studied on several simplified mechanistic models and later applied to actual field cases.An overview of viscous fingering effects is presented. Results from mechanistic studies of the CO 2 matrix-fracture transfer are shown and physical effects are discussed. The paper also presenets critical aspects of Equation-Of-State (EOS) modelling and PVT experiments necessary for a CO 2 flood simulation study. From our experience we suggest a "cooking recipe" for building reservoir simulation models for CO 2 based IOR process evaluations. Case studies of the CO 2 and CO 2 -WAG IOR methods for reservoirs on the Norwegian Continental Shelf (NCS) are presented and oil recovery potentials of these IOR processes are discussed. Physical effects during CO 2 floodingThis chapter presents a short review of important physical effects acting during CO 2 injection process and the simulation aspects associated with them. Viscous FingeringDuring miscible CO 2 flooding, displacement instabilities are functions of rock-fluid properties, fluid saturation distribution, viscous forces, and other parameters such as rock wettability, interfacial tension and miscibility. In cases where a miscible slug is injected, followed by chase gas, fluid cross flow and mixing of the slug with the chase gas may result in front instabilities which reduce the displacement efficiency.Fingering of an interface is a hydrodynamic instability that occurs when a more mobile fluid displaces less mobile one. Mobility variations are usually related to differences in viscosity or density of the two fluids considered. The onset of viscous instability and the subsequent fingering has mostly been studied using a Hele-Shaw cell under conditions of a sharp mobility contrast for both miscible and immiscible conditions (Homsy et al, 1987). Such experiments do not account for the CO 2 mass transfer mechanisms. Nevertheless, the Hele-Shaw cell experiments allow identifying and visualizing the fingering pattern mechanisms, see Figure 1.For both miscible and immiscible conditions, propagation of viscous fingering is governed by different mechan...