Molecular Insights into CO ₂ Flooding and Huff-and-Puff for Enhanced Shale Oil Recovery and Carbon Sequestration in Dead-End Nanopores

CO₂ injection for enhanced oil recovery (EOR) and geological carbon sequestration (GCS) in shale reservoirs offers a promising win-win strategy. Yet, the mechanisms by which CO₂ flooding and huff-and-puff (HNP) mobilize multicomponent shale oil in inorganic nanopores remain unclear, particularly regarding their respective contributions to oil recovery and CO₂ storage. Here, we employ molecular dynamics (MD) simulations to compare CO₂ flooding and HNP in calcite dead-end nanopores saturated with a multicomponent hydrocarbon mixture. We analyze interfacial and transport behaviors, extract localized molecular clusters to quantify self-diffusion coefficients, and examine the roles of flood and flowback pressures. Results show that flooding promotes greater CO₂ penetration into nanopores but suppresses counter-current hydrocarbon migration, limiting swelling-driven recovery. In contrast, HNP enables CO₂ diffusion during the huff stage and reverse pressure gradients during the puff stage, which enhance oil swelling and achieve higher recovery at the expense of lower storage efficiency. Elevated CO₂ density increases the diffusion coefficients of all components, underscoring the critical roles of pressure gradients and molecular transport. By extracting localized molecular clusters, we further obtained pressure-dependent diffusion bounds for CO₂ and hydrocarbons and quantify injection and flowback pressure windows that jointly optimize oil recovery and the CO₂ storage efficiency. On this basis, we propose a well-count-agnostic and field translatable hybrid method that achieves 60.44% oil recovery and 47.18% CO₂ storage efficiency. These findings provide molecular-scale guidance for full-process CCUS design in unconventional reservoirs.