Direct observation of CO2 transport and oil displacement mechanisms in CO2/water/oil systems

As the concern over the greenhouse gas effects of carbon dioxide (CO2) increases, its injection into oil and gas reservoirs for enhancing hydrocarbon recovery and in saline aquifers for storage is on the rise. Variation of reservoir pressure and temperature affects the properties of CO2 and its interaction with reservoirs resident phases. Although in most cases CO2 would be in supercritical state at reservoir conditions, however, it is necessary to understand the flow and displacement mechanisms of CO2/water/oil systems in porous media under various reservoir conditions. In this paper we present the results of a series of direct flow visualisation experiments using CO2/water/oil systems in high-pressure transparent porous media. We demonstrate mechanisms of CO2 transport and oil displacement and recovery at both sub-critical and supercritical conditions and the implications of these conditions on oil recovery. We have also investigated the effect of CO2-enriched water on the flow of CO2 and oil recovery. The results show that both swelling and evaporation of oil as a result of contact with CO2 play an important role in mechanisms and quantity of oil recovery by CO2 injection but the extent of the impact of each mechanism is highly dependent on the state of CO2 at reservoir conditions. High water saturation was found to be detrimental to the performance of CO2 injection however, oil swelling and subsequent elimination of water separating CO2 and oil could to some extent alleviate this problem.