Multiple Flow Mechanism-Based Numerical Model for CO2 Huff-n-Puff in Shale Gas Reservoirs with Complex Fractures

CO2 huff-n-puff has been proven to be an effective method for enhanced gas recovery. Besides, considering the storage capacity and existing infrastructure, injecting carbon dioxide into shale gas reservoirs is a feasible geological storage option for carbon dioxide. However, the current huff-n-puff simulation model must consider the comprehensive flow mechanism crucial for accurate reservoir and huff-n-puff simulation. Besides, the feasibility of effective CO2 storage by CO2 huff-n-puff in shale gas reservoirs is still being determined. Therefore, a CO2 huff-n-puff numerical model considering gas adsorption/desorption, diffusion, dissolution, stress sensitivity, and stimulated reservoir volume (SRV) is established in this paper. The model’s reliability is verified by matching the production history with commercial software. Finally, the effects of various flow mechanisms on CO2 huff-n-puff and storage in shale gas reservoirs are analyzed. The results show that CO2 huff-n-puff can significantly improve the gas recovery of fractured shale reservoirs and realize the effective storage of part of CO2, where the storage factor can reach 0.62. Considering adsorption/desorption, diffusion, stress sensitivity, dissolution in water, and SRV is beneficial to storing CO2 during huff-n-puff. Adsorption/desorption, diffusion, and SRV can enhance the huff-n-puff effect, while stress sensitivity will weaken. For different mechanisms, adsorption/desorption and SRV can most affect the huff-n-puff and storage effect in shale reservoirs. This work provides a new approach for the simulation of CO2 huff-n-puff and storage, which is beneficial for exploiting the shale gas reservoirs and decreasing CO2 emissions.