Visual investigation of extra-heavy oil recovery by CO2/N2 foam injection

Compared to light oil, application of CO2 injection in heavy oil reservoirs has received much less attention. CO2 lacks acceptable sweep efficiency due to a large viscosity contrast between CO2 and heavy oil and the displacement process is almost always immiscible in these reservoirs. The use of CO2-foam may represent an opportunity to reduce mobility of CO2 and improve sweep efficiency. Application of foam injection for enhanced heavy oil recovery has not been widely reported in the literature and hence the potential of foam for improving heavy oil recovery is by and large untested. In our previous works, we reported a series of micro-scale visualisation experiments using CO2 for immiscible displacement and recovery of an extra-heavy crude oil. The results of these experiments showed that, despite a very large viscosity difference between CO2 and the heavy oil, injection of CO2 for an extended period of time effectively produced the heavy crude oil due to CO2 dissolution and subsequent viscosity reduction. In this paper, we present the results of a new series of visualisation experiments carried out in high-pressure transparent porous media (micromodels) to study the performance of subcritical CO2 and N2 foam injection in the same crude oil. In the first part of the paper, the pore-scale interactions of CO2-foam/surfactant/extra-heavy oil and the mechanisms of oil displacement and recovery are presented through vivid micromodel images. The results show that if strong CO2-foam forms in the porous media, it can dramatically improve the recovery of extra-heavy oil and accelerate the process of oil recovery by CO2 injection. In the second part of the paper, the results of an experiment conducted using the same extra-heavy crude oil and surfactant but N2-foam instead of CO2 is presented. The results show that despite forming strong foam in the micromodel, the oil displacement process is much less efficient during N2-foam injection compared to the case of CO2-foam. The results clearly demonstrate the potential of CO2-foam for enhancing heavy oil recovery for very viscous crudes as a technically viable non-thermal oil recovery method. The results of these experiments significantly improve our understanding of the processes involved in heavy oil recovery by CO2-foam and the pore scale differences with the case of N2-foam injection.