Non-condensable gas assisted steam, being recognized as a promising technique in enhanced heavy oil recovery, has been successfully employed in oilfield for many years. Nevertheless, the displacing mechanism of this technology is complicated and has not been thoroughly understood. The emphasis of this study lies in the performance of non-condensable gas assisted steam flooding in improving heavy oil recovery and its comprehensive mechanism has been discussed.
In the research, carbon dioxide (CO2) addition to steam injection process is proposed. The influence of CO2 dissolution on oil properties and interfacial tension (IFT) between oil and gas was investigated via PVT and IFT tests. Experiments on high temperature relative permeability were made to study the effect of CO2 on flow characteristic of oil and water in porous media. Based on above studies, the displacing performance of CO2 assisted steam process was investigated through one-dimensional sand-packed physical experiment. CO2 alternating steam flooding followed the constant CO2/steam volume ratio of 1:10.
The experimental results showed that CO2 dissolving in heavy oil was able to swell oil volume, reduce oil viscosity and IFT, and enhance oil phase relative permeability. Addition of CO2 effectively could maintain soaking pressure in steam huff and puff process and offered more oil production energy. CO2 alternating steam flooding could delay injection pressure drop and push the thermal front forward heating more heavy oil to be produced out. The performance of CO2 assisted steam process markedly outperformed single steam injection in oil recovery, which attributed to the increment of both microscopic oil displacement efficiency and macroscopic volume sweep efficiency. Oil swelling and low IFT were beneficial to reduce residual oil which was trapped in capillary resistance. The reduction of oil viscosity and the increment of oil phase relative permeability indicated that mobility of viscous oil was improved, which could enhance displacing efficiency in porous media. Meanwhile, maintenance of reservoir pressure through introducing CO2 to steam injection process could lead more oil to be heated, which contributed to improve sweep efficiency. Moreover, the dramatic decrease of oil viscosity lowered the mobility contrast between oil and water in porous media, probably mitigating viscous fingering and channeling to some extent.
This study has provided more powerful evidences to fully understand comprehensive mechanisms of non-condensable gas assisted steam process. Results may prove that this method is an efficient technology for improving oil recovery in heavy oil reservoir.