Modelling CO2 Injection: IOR Potential after Waterflooding

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 CO2 value chain, modelling of CO2 injection at the Ekofisk field (fractured chalk reservoir) and Gullfaks CO2 injection compositional study. The paper discusses the importance of correctly representing the physical effects when modelling miscible or immiscible CO2 injection in sandstone and carbonate reservoirs. The following aspects were evaluated: Transfer of CO2 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 CO2, 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 CO2 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 CO2 flood simulation study. From our experience we suggest a "cooking recipe" for building reservoir simulation models for CO2 based IOR process evaluations. Case studies of the CO2 and CO2-WAG IOR methods for reservoirs on the Norwegian Continental Shelf (NCS) are presented and oil recovery potentials of these IOR processes are discussed.