Carbon dioxide (CO2) flooding projects have shown a large potential for recovering additional oil beyond waterflooding, but in many cases this process is handicapped, especially in thick reservoirs, by CO2 gravity override. Different methods have been used to overcome the CO2 gravity override by either increasing CO2 density, viscosity, or reducing its relative permeability. Reducing the relative permeability of CO2 by the water alternate with gas process has received much experimental and theoretical attention. Although this method helps to control fingering, it also introduces gravity segregation issues. An alternative approach to reduce the mobility is to disperse CO2 as a foam or as an emulsion in a continuous aqueous liquid phase. This paper provides some data and results for a new emulsion system that behaves as a conformance control agent, and also to enhance the recovery of oil following water and CO2 floods.
The CO2 emulsion system used in this study consists of 70 vol% supercritical CO2 (sc-CO2), 30 vol% of water-based polysaccharide linear polymer and a foaming agent (surfactant). This new emulsion system has the ability to be created in situ to provide better mobility control of the injected CO2. Special parallel dual coreflooding experiments were conducted using live oil at reservoir conditions to investigate the effectiveness of the CO2 emulsion system in conformance control and enhancing oil recovery. Several experiments were conducted to explore the effect of the injection rate, injection mode and slug volume on incremental oil recovery using dual core carbonate composite stacks with permeability contrasts up to 60 based on brine permeability.
Results based on this study show the CO2 emulsion's ability to significantly reduce permeability of the higher permeable cores resulting in enhancement of incremental oil recovery from the lower permeable cores. The majority of oil was recovered from the high permeability core during the initial water and CO2 flooding when both cores were placed in parallel. The oil recovery from the low permeable cores by waterflooding was similar to that of the high permeability core, but significantly less during the first CO2 flood. After a slug of CO2 emulsion was injected that blocked the high permeability core, it resulted in 21% incremental oil recovery from the low permeability core during a second CO2 injection. The results demonstrate the improvement in both areal and vertical sweep efficiencies by CO2 emulsions by stabilizing viscous fingering and gravity override issues. The results bode well for scale up for a field trial.