Numerical simulation and optimization of CO₂ enhanced shale gas recovery using a genetic algorithm

CO₂ enhanced shale gas recovery (CO₂-ESGR) is a promising technology for addressing the global energy and environmental concerns by promoting the shale gas production and simultaneously reducing the CO₂ emissions. To provide a greater understanding of the environmental benefits of CO₂-ESGR, a three-dimensional numerical model corresponding to the typical shale reservoir parameters for the Ordos basin in China is established. Results show that the shale gas production can be enhanced by 6.74% at a constant CO₂ injection rate of 0.03 kg/s for 50 years, while 9.467 × 10⁷ kg CO₂ can be stably stored. By analyzing the impact of CO₂ injection rate and well distance, it is found that the shale gas recovery has non-monotonous variation with CO₂ injection rate and decreases with increase in the well distance before the occurrence of CO₂-breakthrough. An optimization framework is also proposed to optimize the shale gas production as well as the CO₂ storage for different well distances for the constant rate injection (CRI) mode and the constant pressure injection (CPI) mode. When CO₂-breakthorugh is the only constraint on the CRI mode, larger well distance is better and the optimal injection rate at well distance of 390 m is 0.0609 kg/s with a total CH₄ production of 4.252 × 10⁶ kg and CO₂ storage of 96.08 × 10⁶ kg. When adding the constraint on injection pressure, the optimal scenario restricts the CO₂ injection rate at 0.031796 kg/s and the well distance at 360 m. On the other hand, with the same maximum injection pressure constraint, CPI mode shows superior performance compared to the CRI mode both in CO₂ storage and shale gas production. The results in this paper could serve as a reference for the selection of optimal operation parameters and strategies for industrial scale development of CO₂-ESGR technology.