## 4 Non-Condensate-Gas-Assisted Steam Drive
### CO\({}_{2}\)-Injection-Assisted Steam Flooding for Heavy-Oil Reservoir  
The performance of steam huff and puff and steam drive in the reservoir development is limited by steam overlap and steam channeling due to the heterogeneity of formation, and the differences between the steam and oil density and viscosity. This would result in a decrease in sweep efficiency for steam flooding and a low heat-utilization rate, and eventually cause low oil recovery. For these kinds of reservoir, there is only a 30-50% recovery degree, and more than half of the reserves remain untapped. The application of air, N\({}_{2}\), CO\({}_{2}\), flue gas [57], and other non-condensate gases to assist steam flooding can achieve the purpose of further enhanced oil recovery. Among many non-condensate gases, CO\({}_{2}\) has the most obvious viscosity-reduction effect. In recent years, due to the greenhouse effect on global warming and the more severe environment problems, it is necessary to find a feasible way to reduce CO\({}_{2}\) emission. Many researchers have suggested that, through the integration of CO\({}_{2}\) capture, storage, and oil displacement, the circular economy model can be realized [58; 59]. The application of CO\({}_{2}\)-injection-assisted steam flooding for heavy-oil reservoirs not only helps solve the problems of rapid production decline for the heavy-oil reservoir, but also greatly reduces the production costs. Therefore, a lot of scholars have conducted studies on the mechanism of CO\({}_{2}\)-assisted steam flooding and the optimization of the injection parameters [60]. Many scholars have analyzed the influencing factors of CO\({}_{2}\) injection, such as the injection timing, amount, and mode [61]. CO\({}_{2}\)-assisted steam flooding experiment results show that CO\({}_{2}\)-assisted steam flooding is better than conventional flooding in both vertical and horizontal wells, and the recovery efficiency increases more than 20%. The optimum injection mount is critical to the ultimate oil recovery. Therefore, it is necessary to conduct research to evaluate the optimal CO\({}_{2}\) injection amount [62]. The optimum study results suggest that the optimal CO\({}_{2}\)-steam ratio under a one-dimensional condition is 9.4, and the optimal CO\({}_{2}\)-steam ratio under a three-dimensional condition is 8.7. Experiment results show that a 0.5 PV CO\({}_{2}\) injection after steam injection would obtain the best effect [63]. Many scholars have also summarized and analyzed the oil-displacement mechanism in the process of CO\({}_{2}\)-assisted steam flooding [64; 65; 66]. CO\({}_{2}\) can significantly reduce the heavy-oil viscosity. CO\({}_{2}\) can extract light hydrocarbon components from ultra-heavy oil. CO\({}_{2}\) can replenish formation capacity. CO\({}_{2}\) has a demulsification effect. CO\({}_{2}\) can increase the elastic energy of crude oil. CO\({}_{2}\) can also reduce interfacial tension and residual oil saturation. As early as 1988, the Midway-Sunset field in California, USA, had conducted a pilot test to study the feasibility of a new steam-CO\({}_{2}\) injection technology. After two stages of testing, it was found that the cumulative oil production increased by 2.08 times and 0.44 times in nine months after three months of development in the new steam-CO\({}_{2}\) injection process compared with steam flooding alone [67]. The CO\({}_{2}\)- and surfactant-assisted steam flooding was implemented in the QI 40 block of the Liaohe Oilfield in 2011 [68]. Through an optimization study, the simultaneous injection of steam and CO\({}_{2}\) not only strengthened the viscosity reduction and thermal expansion of crude oil, but also maintained the dryness of steam, improved the heat-transfer efficiency of steam, and made up for the adverse effect of steam condensation in cold oil flooding. CO\({}_{2}\)- and surfactant-assisted steam flooding was implanted with an effective rate of 90% and an average oil production increase of 458 t per well.