## 3 Enhanced Heavy-Oil Recovery by Gas Injection
### Review of CO\({}_{2}\) Huff-and-Puff Study in Heavy-Oil Reservoirs
Experimental studies of CO\({}_{2}\) stimulation have been carried out with physical model temperatures up to 90 \({}^{\circ}\)C, heavy-oil viscosity up to 28,646 mPa-s, and reservoir permeability ranging from 30 \(\times\) 10\({}^{-3}\)\(\upmu\)m\({}^{2}\) (core) to 24,200 \(\times\) 10\({}^{-3}\)\(\upmu\)m\({}^{2}\) (sand-pack model). Lab-scale experimental studies suggest that permeability is not the main parameter affecting CO\({}_{2}\) huff-and-puff productivity in heavy-oil reservoirs. In addition, CO\({}_{2}\) huff and puff could also be applied in reservoirs with low oil saturation (as low as 0.41), which means that CO\({}_{2}\) huff and puff can be performed in reservoirs with higher water saturation [25]. Other scholars have conducted core displacement experiments related to CO\({}_{2}\) huff and puff. The results show that the second cycle would achieve maximum production. Higher injection pressure would result in higher production due to more CO\({}_{2}\) dissolution in heavy oil under higher pressure to reduce the viscosity and increase the oil expansion coefficient. However, the increase in the soaking time does not significantly improve the recovery efficiency. Field injection pressures normally range from 1.7 MPa to 25.0 MPa. In the production stage, the pressure depletion rate has a significant impact on the productivity of heavy-oil reservoirs [26]. For the CO\({}_{2}\) huff-and-puff study using the sand-pack model, the experimental work results suggest that the recovery rate of the CO\({}_{2}\) huff-and-puff process is proportional to the injection pressure, soaking time, and pressure decay rate. CO\({}_{2}\) huff and puff can be used as a follow-up process of primary recovery [27]. In the past few decades, field tests of CO\({}_{2}\) huff and puff for enhanced oil recovery in heavy-oil reservoirs have been successfully applied in many oil fields, as presented in Table 1.  
The CO\({}_{2}\) huff-and-puff technology was first applied in the forest reserve oil field, and [PERSON] conducted research to evaluate the enhanced oil-recovery (EOR) mechanisms associated with CO\({}_{2}\) huff-and-puff applications, such as oil viscosity reduction, volume expansion, and gravity drainage [32]. The related influencing parameters, such as slug size, huff and puff period, CO\({}_{2}\) injection, and reservoir pressure, were also analyzed. Finally, parameters screening criteria for CO\({}_{2}\)-EOR were established based on the field test results [33]. The application of CO\({}_{2}\) huff and puff in the Bati Raman heavy-oil reservoir in Turkey is also concerned because of its good recovery effect [34]. In the CO\({}_{2}\)-EOR process of this oil field, a series of improved processes, such as a fracture-sealing polymer gel system, chemically enhanced water injection process, and alternate alkaline water gas injection (WAG) process, were used to improve the oil recovery. \begin{table}
\begin{tabular}{c c c c c c c c c c} \hline
**Official Name** & **Renewari Depth** & **Original Reservoir** & **Penewari (MPa)** & **Penewaribility (nm)** & **O1 Visually Reservoir** & **Injection Amount** & **Sanking Time** & **Performance Evaluation** \\  & **(an)** & **Pewariary (MPa)** & **(c)** & **Condition (mPa)** & **(c)** & **(c)** & **(c)** & **(c)** \\ \hline [PERSON] Coll  
These technologies successfully solved the problems of the strong heterogeneity of natural fractured carbonate rocks, the low CO\({}_{2}\)/crude oil mobility ratio, and the high gas-oil ratio (GOR) at the late stage of development. Therefore, the immiscible CO\({}_{2}\) injection project is widely recognized as one of the most unique and successful EOR applications in the history of heavy-carbonate reservoir development [35]. In recent years, field applications and field pilot tests of post-thermal CO\({}_{2}\) huff and puff have been conducted in major heavy-oil fields such as the Jidong, Dagang, and Liaohe Oilfields.