## 3 Enhanced Heavy-Oil Recovery by Gas Injection
### Review of CO\({}_{2}\) Huff-and-Puff Study in Heavy-Oil Reservoirs  
Many researchers have conducted experiments of enhanced heavy-oil recovery study using CO\({}_{2}\), and suggest that CO\({}_{2}\) has a significant effect [15; 16; 17; 18; 19]. The process of CO\({}_{2}\) huff and puff could be divided into three stages: the injection stage, soaking stage, and production stage [20; 21]. The process of CO\({}_{2}\) huff and puff is presented in Figure 3. More detailed processes are as follows: (_a_) oil droplets are pushed to the depth of the reservoir by CO\({}_{2}\), (_b_) the light component is extracted by CO\({}_{2}\), (_c_) CO\({}_{2}\) is dissolved in the heavy components, (_d_) oil droplet expansion, (_e_) Flow direction of liquid, (_f_) residual oil due to changes in rock wettability, (_g_) oil droplets generated by crude oil expansion during production, and (_h_) oil droplets are displaced from the deep formation by the water phase. During the injection stage, CO\({}_{2}\) is injected to the reservoir. As a results, low-viscosity oil and water are pushed by the injected CO\({}_{2}\) to the deep reservoir, and more viscous, non-flowing oil is remained in the reservoir. This results in a significant increase in the heavy-oil relative permeability due to lower water phase saturation in the near-wellbore area, and a higher resistance that prevents the flow of low-viscosity oil mixed with CO\({}_{2}\) into the wellbore. CO\({}_{2}\) diffusion is neglected during the injection stage due to the short time of CO\({}_{2}\) injection, the high injection pressure compared to the reservoir pressure, and the low diffusion coefficient of CO\({}_{2}\) in heavy oil. The injection stage is shown in Figure 3A. In the soaking stage, the oil production is shut in. The CO\({}_{2}\) diffuses through the formation, causing the oil to expand and reduce viscosity, which is the main enhanced oil-recovery period in the entire huff and puff process. In the meantime, mass transfer occurs between CO\({}_{2}\) and heavy oil, and the light components of crude oil are extracted by CO\({}_{2}\), which increases the volume of crude oil and decreases the viscosity. The soaking stage is shown in Figure 3B. In the production stage, the CO\({}_{2}\) that is not mixed with crude oil is first produced, then the crude oil mixed with CO\({}_{2}\) is produced in large quantities. Finally, the pressure gradient increases, and the crude oil with higher heavy components is produced through waterflooding. The production stage is shown in Figure 3C. Figure 3: Process of CO\({}_{2}\) huff and puff for a heavy-oil reservoir: (a) oil droplets are pushed to the depth of the reservoir by CO\({}_{2}\); (b) the light component is extracted by CO\({}_{2}\); (c) CO\({}_{2}\) is dissolved in the heavy components; (d) oil droplet expansion; (e) flow direction of liquid; (f) residual oil due to changes in rock wettability; (g) oil droplets generated by crude oil expansion during production; (h) oil droplets are displaced from the deep formation by the water phase. Generally, laboratory studies are carried out before CO\({}_{2}\) huff-and-puff processes are implemented in the field. These laboratory studies include the experimental study and parameter measurement of the fluid characteristics of a heavy-oil-CO\({}_{2}\) system, such as viscosity, CO\({}_{2}\) solubility, swelling coefficient, interfacial tension, diffusion coefficient, and miscibility pressure. The adaptability of reservoir characteristics such as porosity, permeability, oil saturation, and water saturation is also studied experimentally, and the laboratory-scale optimization of process parameters such as cycle times, injection pressure, and soaking time are also studied [22; 23; 24]. Many laboratory studies on enhanced oil recovery using CO\({}_{2}\) for the heavy-oil reservoirs have been conducted by researchers from China, Canada, and the United States.