Direct Quantification of CO2 Diffusion Behavior in Porous Media Using Raman Spectroscopy

Abstract

The diffusion behavior of CO2 in porous media is fundamental to geological carbon storage and enhanced oil recovery, yet its dynamic evolution remains inadequately resolved by conventional methods. This study presents a novel methodology combining Raman spectroscopy with high-pressure fixed-point sampling to directly quantify spatiotemporal CO2 concentration evolution during diffusion in porous media. Experiments were performed in kerosene-saturated systems of quartz sand and glass beads under simulated reservoir conditions (75 °C, 4 MPa). Variable diffusion coefficients were determined by fitting concentration profiles to Fick’s second law and compared with constant coefficients from pressure-decay experiments. Results reveal significant spatiotemporal heterogeneity: CO2 concentration declines sharply from the gas–liquid interface, while the diffusion coefficient decreases progressively with distance and time. The coefficient in 80–120 mesh quartz sand (∼10–8 m2/s) is two orders lower than in 5 mm glass beads (∼10–6 m2/s), underscoring the dominant influence of pore structure on mass transfer resistance (Figure 1). Diffusion front speed declines from 7.5 cm/d initially to 1.5 cm/d after 15 days, confirming that attenuation of the CO2 concentration gradient is the primary driving force. COMSOL simulations align with experimental results. System-averaged coefficients from Raman and pressure-decay methods agree within 5% error, validating accuracy. This work establishes Raman spectroscopy as a powerful tool for resolving CO2 diffusion dynamics in porous media. It provides critical insights for optimizing carbon capture, utilization, and storage (CCUS) applications in sandstone reservoirs and provides a methodological foundation applicable to a broad range of porous media.