Mechanistic study of improved heavy oil recovery by CO2-foam injection

CO2 injection is a relatively well-established oil recovery method but it has been mainly applied to conventional (light) oil reservoirs where the displacement of the oil by CO2 is likely to be miscible. Application of CO2 injection in heavy oil reservoirs has received much less attention and hence the performance of CO2 injection in these reservoirs has not been evaluated. Due to a much larger viscosity difference between CO2 and heavy oil, reservoir sweep efficiency is expected to be lower than that in conventional reservoirs. Moreover, CO2 is not expected to develop miscibility with heavy oil and hence displacement efficiency is also expected to be lower in heavy oil reservoirs. However, one major advantage of injection of CO2 in heavy oil reservoirs is the well-documented substantial drop in the viscosity of heavy oil when mixed with CO2 which is much larger than that in light oil.

To alleviate the problem of adverse mobility ratio, CO2 injection in heavy oil reservoirs needs to be augmented by other recovery techniques. The use of foam to improve sweep efficiency during CO2 injection seems a logical way to displace the CO2-diluted oil out of the porous medium. Foam has the potential to improve the sweep efficiency of the injected gas by reducing the effects of low gas viscosity and reservoir heterogeneity. Application of foam injection for increased heavy oil recovery has not been widely reported before and hence the ability of foam formation in the presence of heavy oil and its potential to increase heavy oil recovery is by and large untested.

In this paper, we present the results of a series of visualization experiments carried out in high-pressure transparent micromodels to investigate the performance of subcritical CO2 and CO2-foam injection in heavy oil. Pore-scale interactions of CO2-foam/surfactant/heavy oil and mechanisms of displacement, flow and oil recovery are presented through vivid micromodel images. The results show that in addition to increasing sweep efficiency, there are also microscopic mechanisms taking place during the displacement of oil by CO2-foam that can significantly impact the performance of foam injection in heavy oil reservoirs. Furthermore, we reveal that the displacement efficiency strongly depends on the stability of the foam and if strong foam is not formed, it will drop drastically. The results of these direct visualization experiments improve our understanding of the process of heavy oil recovery by CO2-foam injection under various injection strategies. Extended CO2 injection in heavy oil reservoirs can offer a viable method for increasing heavy oil recovery. It can also be used as a method for combining EOR and CO2 storage by capturing CO2 from nearby industrial sources (e.g., steam generator for nearby steam-based heavy oil recovery method).