Visualization of Oil Recovery by CO2-Foam Injection; Effect of Oil Viscosity and Gas Type

Abstract

Abstract CO2 injection is a well-established enhanced oil recovery technique which has been successfully applied in conventional oil reservoirs where it can vaporize light hydrocarbon components and displace the oil miscibly. In heavy oil reservoir, CO2 does not develop miscibility but it can significantly promote recovery by dissolving in the oil and reducing its viscosity. Heavy oil viscosity reduction upon mixing with CO2 can be substantial and reductions up to two orders of magnitude can be achieved. However, displacement of viscous and heavy oil by CO2 is generally perceived as being inefficient and hence, very limited number of heavy oil fields have been considered for CO2 injection. There are two main reasons for this; (1) the large viscosity contrast between heavy oil and CO2 which results in viscous fingering and; (2) the slow rate of CO2 diffusion in heavy oil. The use of CO2 foam represents an opportunity to reduce mobility of CO2 and improve sweep efficiency and at the same time facilitate CO2 dissolution by increasing the contact area between the oil and CO2. The 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 visualization experiments to investigate potentials of sub- critical CO2 and CO2-foam injection for recovery of a medium-heavy crude oil. The results showed that if strong CO2-foam forms in porous medium, it can drastically increase the rate of oil recovery and reduce the residual oil saturation. This paper presents the results of a new series of visualization experiments to investigate the performance of sub-critical CO2-foam for recovery improvement in an extra-heavy crude oil. In the first part of this paper, the effect of oil viscosity on oil recovery and displacement mechanisms has been investigated. The results show that while foam can significantly accelerate and improve oil recovery by CO2 injection, the increase in oil viscosity slows down the process of oil recovery by CO2-foam and may result in weakening of some of the pore scale displacement mechanisms. In the second part of the paper, the results of an experiment conducted using the same extra-heavy crude oil and surfactant with N2-foam (instead of CO2) is presented. The results show that despite forming strong foam in the porous medium, the oil displacement process is much less efficient during N2-foam injection compared to 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 example of N2-foam injection.