An integrated multi-scale model for CO₂ transport and storage in shale reservoirs

Shale reservoirs have gradually been recognized as promising potential candidates for CO₂ geological storage due to their wide geographic distribution and enormous storage potential. CO₂ sequestration in shale is a complex multi-scale process with spatial sizes ranging from nano-scale to kilometer-scale. In this study, an integrated multi-scale model, considering CO₂ adsorption, dissolution, slip flow, and diffusion, is developed to describe CO₂ transport and storage in shale reservoirs with a multi-stage fractured horizontal well. The computationally efficient semi-analytical solution of the multi-scale model is obtained via Laplace transformation and Pedrosa's substitution. Based on the established model, a workflow for evaluating the CO₂ storage capacity of shale reservoirs is proposed. Taking the Longmaxi shale reservoir in the Sichuan Basin as an application case, the results show that the storage capacity can reach up to 29.97 × 10⁸ m³ at a high constrained injection pressure. In addition, sensitivity analysis suggests that CO₂ storage capacity is strongly influenced by the adsorption index of clay minerals, kerogen content, solubility coefficient, and inter-porosity flow coefficient of kerogen and inorganic matrix. Most reservoir parameters have negligible effects on storage capacity at low injection pressures, yet significant effects at high injection pressures. More specifically, the CO₂ storage capacity can increase by 6.8 folds when the constrained injection pressure is increased from 5.5 to 8.5 MPa. The findings obtained in this study further expand the description of CO₂ transport in the shale formation, laying the foundation for highly efficient CO₂ sequestration in shale reservoirs.