Carbonate reservoirs, deposited in the Western Canadian Sedimentary Basin (WCSB), hold significant reserves of heavy crude oil that can be recovered by non-thermal processes. Solvent, gas, water, and water alternating gas injections are the main methods for carbonate heavy oil recovery in the WCSB. Due to the fractured nature of carbonate formations, many advantages of these production methods are usually contrasted by their low recovery factor. Alternative processes are therefore needed to increase oil sweep efficiency from carbonate reservoirs. Foam/polymer enhanced foam (PEF) injection has gained interest in conventional heavy oil recovery in recent times. However, the oil recovery process by foam, especially PEF, in conjunction with solvent injection is less understood in fracture heavy oil carbonate reservoirs. The challenge is to understand how the combination of surfactant, gas, and polymer allows us to better access the matrix and efficiently sweep the oil.
This paper introduces a new approach to access the unrecovered heavy oil in fractured carbonate reservoirs. CO2 foam and CO2 PEF were used to decrease oil saturation after solvent injection, and their performance was compared with gas injection. A specially designed fractured micromodel was used to visualize the pore scale phenomena during CO2 foam/PEF injection. In addition, the static bulk performances of CO2 foam/PEF were analyzed in the presence of heavy crude oil. A high definition camera was utilized to capture high quality images.
The results showed that in both static and dynamic studies the PEF had high stability. Unlike CO2 PEF, CO2 foam lamella broke much faster and resulted in the collapse of the foam during heavy oil recovery after solvent flooding. It appeared that foam played a greater role than just gas mobility control. Foam showed outstanding improvement in heavy oil recovery over gas injection. The presence of foam bubbles was the main reason to improve heavy oil sweep efficiency in heterogeneous porous media. When the foam bubbles advanced through pore throats, the local capillary number increased enough to displace the emulsified oil. PEF bubbles generated an additional force to divert surfactant/polymer into the matrix. Overall, CO2 foam and PEF remarkably increased heavy oil recovery after solvent injection into the fractured reservoir.
The results of these direct visualization experiments improve our understanding of the heavy oil recovery process by solvent alternating CO2 foam/PEF flooding in fractured reservoirs. Besides enhancing oil production, application of CO2 foam/PEF may require the injection of a lesser amount of solvent into the reservoir, providing economic and environmental advantages.