In situ Raman characterization and visualization of microscopic dissolution and extraction of a CO2 + alkane + aromatics system

During CO 2 flooding, the bidirectional mass transfer between CO 2 and hydrocarbon affects the efficiency of CO 2 -enhanced oil recovery (CO 2 -EOR), yet their coupled effects on oil-phase behavior remain insufficiently understood. In this study, a novel microfluidic visualization platform coupled with in situ Raman characterization was established to investigate the interactive mass transfer and phase behavior in the toluene + n-decane system, representing crude oil. For the first time, the integration of Raman quantitative analysis and microfluidic visualization precisely enables characterization of aromatics concentration distribution and oil-phase volume. The results show that the phase behavior during miscibility processes depends strongly on pressure and contact time. Under constant-pressure conditions near the minimum miscibility pressure (MMP), at 91.0 % MMP, the relative CO 2 dissolution increased by only 31.50 %, while extraction-induced volume loss dominated, resulting in continuous oil-phase shrinkage. At 96.7 % MMP, rapid CO 2 dissolution induced swelling within 1 min, accompanied by a decrease in toluene volume fraction ( X T ) from 30.00 to 13.22 vol%; after 6 min, X T further declined to 1.40 vol%, with the toluene depletion ratio exceeding 90 % and slow shrinkage dominating. During continuous pressurization from 68.3 % to >100 % MMP, the system exhibited three stages with transition points at 75.3 % and 92.8 % MMP. Under low-pressure experiments at 5.7 % MMP, toluene extraction caused shrinkage of the toluene-containing oil phase, whereas pure n-decane showed slight swelling of 3.67 %. The innovative techniques and critical findings on oil-phase behavior, governed by the interplay between CO 2 dissolution-induced swelling and toluene extraction-induced shrinkage, provide insights into CO 2 -EOR processes.