Summary
Carbonate reservoirs, deposited in the Western Canadian Sedimentary Basin (WCSB), hold significant reserves of heavy crude oil that can be recovered by nonthermal processes. Solvent, gas, water, and water-alternating-gas (WAG) injections are the main methods for carbonate-heavy-oil recovery in the WCSB. Because of the fractured nature of carbonate formations, many advantages of these production methods are usually in contrast with 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 fractured 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 study introduces a new approach to access the unrecovered heavy oil in fractured-carbonate reservoirs. Carbon dioxide (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 used 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.
