Influence of Water Occurrence on CO2 Mass - Transfer, Diffusion, Oil Recovery, and Sequestration Mechanisms in Shale Reservoirs Under Coupled Thermal–Mass Effects

Abstract The presence of residual fracturing fluid and formation water in shale reservoirs significantly influences CO2 mass transfer, diffusion, and its mechanisms for enhanced oil recovery and storage. However, the impact of varying water saturation levels and coupled thermal-hydraulic effects on CO2 EOR and storage mechanisms remains unclear. This study investigates CO2 mass transfer and diffusion behavior through pressure-driven core flooding and high-temperature huff-and-puff experiments, complemented by NMR analysis, under varying temperatures and water saturation conditions in shale reservoirs. Results show that elevated temperatures shorten the initiation period of oil mobilization, accelerate CO2 diffusion, and enable the system to reach equilibrium more rapidly. Meanwhile, retained fracturing fluids and formation water create an aqueous barrier that slows down pressure drop rates as water saturation increases. At lower temperatures (<110 °C), oil recovery improvements are limited; however, at 140 °C, oil recovery increases linearly, indicating that CO2 exhibits both efficient displacement and favorable sequestration characteristics under these conditions. Although early-stage oil recovery is lower under aqueous conditions, subsequent cycles yield more sustained improvements. The presence of water amplifies the incremental benefits of CO2 huff-and-puff, ultimately enhancing oil recovery by 9.53% compared to dry conditions. This study provides new theoretical insights for optimizing CO2-based EOR and sequestration strategies in shale reservoirs.