It is challenging to enhance heavy oil recovery in the late stages of steam flooding. This challenge is due to reduced residual oil saturation, high steam-oil ratio, and lower profitability. A field test of the CO2-assisted steam flooding technique was carried out in the steam-flooded heavy oil reservoir in the J6 block of the Xinjiang oil field (China). In the field test, a positive response to the CO2-assisted steam flooding treatment was observed, including a gradually increasing heavy oil production, an increase in the formation pressure, and a decrease in the water cut. The production wells in the test area mainly exhibited four types of production dynamics, and some of the production wells exhibited production dynamics that were completely different from those during steam flooding. After being flooded via CO2-assisted steam flooding, these wells exhibited a gravity drainage pattern without steam channeling issues, and hence, they yielded stable oil production. In addition, emulsified oil and CO2 foam were produced from the production well, which agreed well with the results of laboratory-scale tests. The reservoir-simulation-based prediction for the test reservoir shows that the CO2-assisted steam flooding technique can reduce the steam-oil ratio from 12 m3 (CWE)/t to 6 m3 (CWE)/t and can yield a final recovery factor of 70%.
The steam injection is an effective way to development heavy oil reservoir. But for the high acidic heavy oil, due to the high amount of naphthenic acid, the steam and heavy oil can easily form W/O emulsion which possesses higher viscosity and lower temperature sensitivity than crude oil and increase the displacement difficulty. The introduction of high pressure carbon dioxide was able to destroy the W/O emulsion structure and decrease the viscosity of formation fluid. In this paper, based on the NS block reservoir condition, the physical simulation experiments were carried to investigated the demulsification of CO2 in the displacement process. The results showed that The high acidic heavy oil of NS block can be formed the W/O emulsion easily during the steam flooding process and the high viscosity of emulsion increased the difficulty of displacement. In the flooding process, the high pressure CO2 possessed the demulsification function for the emulsion and thus the lower viscosity of oil can lead to higher cumulative recovery. The alternative injection measures achieved the highest recovery and appropriate CO2/steam volume ratio is very important to the following flooding. The temperature of steam had the biggest influence on the final recovery. The demulsification effect of high pressure carbon dioxide has never been considered as the mechanism of displacement. For highly acidic heavy oil, if the generated emulsion was able to be demulsification by carbon dioxide during the flooding process, it will become one more advantage for carbon dioxide as the important displacing medium in EOR area.
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