Visual Investigation of Improvement in Extra-Heavy Oil Recovery by Borate-Assisted CO₂ CO 2 -Foam Injection

The significant reduction in heavy oil viscosity when mixed with CO₂ is well documented. However, for CO₂ injection to be an efficient method for improving heavy oil recovery, other mechanisms are required to improve the mobility ratio between the CO₂ front and the resident heavy oil. In situ generation of CO₂ -foam can improve CO₂ injection performance by (a) increasing the effective viscosity of CO₂ in the reservoir and (b) increasing the contact area between the heavy oil and injected CO₂ and hence improving CO₂ dissolution rate. However, in situ generation of stable CO₂ -foam capable of travelling from the injection well to the production well is hard to achieve. We have previously published the results of a series of foam stability experiments using alkali and in the presence of heavy crude oil (Farzaneh and Sohrabi 2015). The results showed that stability of CO₂ -foam decreased by addition of NaOH, while it increased by addition of Na₂CO₃ . However, the highest increase in CO₂ -foam stability was achieved by adding borate to the surfactant solution. Borate is a mild alkaline with an excellent pH buffering ability. The previous study was performed in a foam column in the absence of a porous medium. In this paper, we present the results of a new series of experiments carried out in a high-pressure glass micromodel to visually investigate the performance of borate–surfactant CO₂ -foam injection in an extra-heavy crude oil in a transparent porous medium. In the first part of the paper, the pore-scale interactions of CO₂ -foam and extra-heavy oil and the mechanisms of oil displacement and hence oil recovery are presented through image analysis of micromodel images. The results show that very high oil recovery was achieved by co-injection of the borate–surfactant solution with CO₂ , due to in-situ formation of stable foam. Dissolution of CO₂ in heavy oil resulted in significant reduction in its viscosity. CO₂ -foam significantly increased the contact area between the oil and CO₂ significantly and thus the efficiency of the process. The synergy effect between the borate and surfactant resulted in (1) alteration of the wettability of the porous medium towards water wet and (2) significant reduction of the oil–water IFT. As a result, a bank of oil-in-water (O/W) emulsion was formed in the porous medium and moved ahead of the CO₂ -foam front. The in-situ generated O/W emulsion has a much lower viscosity than the original oil and plays a major role in the observed additional oil recovery in the range of performed experiments. Borate also made CO₂ -foam more stable by changing the system to non-spreading oil and reducing coalescence of the foam bubbles. The results of these visual experiments suggest that borate can be a useful additive for improving heavy oil recovery in the range of the performed tests, by increasing CO₂ -foam stability and producing O/W emulsions.