Visualization observation of formation of a new oil phase during immiscible dense CO₂ injection in porous media

Miscibility is not a target nor achievable for CO₂ injection in heavy oil reservoirs but immiscible CO₂ injection has shown significant potential in enhancing heavy oil recovery mainly through a substantial reduction in the oil viscosity. Another major benefit of CO₂ for enhanced heavy oil recovery is a favorable improvement in the composition of the oil which results in the production of higher quality oil compared to the original oil. To identify the actual mechanisms that affect the physical properties of heavy oil during various CO₂ injection strategies, the behavior of CO₂ and heavy oil was visually investigated under reservoir conditions. For this purpose, a setup was designed to visually observe the contact between injected CO₂ and heavy oil under reservoir conditions. In addition, several flow experiments were performed using a transparent micromodel setup. It was observed that as soon as CO₂ contacts the oil, the extraction of hydrocarbons, mostly methane at the beginning, from the oil starts. The extraction of hydrocarbons by CO₂ initially led to a sudden swelling of the oil mainly because of the liberation of dissolved methane from oil. Because of the extraction strength of CO₂, an accumulation of light and intermediate compounds of the oil takes place in the oil phase, near the interface of oil and CO₂. The results of direct visualization of the flow of CO₂ and heavy oil reveal that the contact of CO₂ and heavy oil results in the formation of a light and less viscous oil-rich phase. This oil-rich phase has a relatively lighter color than the oil in contact with CO₂ and it is very mobile in pore spaces.