at different phases*

Quantifying the mass transport of CO₂ at pore-scales accurately is crucial but challenging for successful technological deployment of Carbon capture and sequestration (CCS) in a deep saline aquifer. We carry out high-pressure microfluidic experiments, mimicking reservoir conditions up to 9.5 MPa and 35 ^oC, to elucidate the microfluidic mass transfer process of CO₂ at different thermodynamic phases into water. We measure the size change of CO₂ micro-bubbles/droplets generated using a microfluidic T-junction to estimate the volumetric mass transfer coefficient (k_La), quantifying the rate change of CO₂ concentration under the driving force of concentration gradient. The results show that bubbles/droplets at high-pressure conditions reach a steady-state faster than those at low-pressure. The measured volumetric mass transfer coefficient shows an increase with the Reynolds number (based on the liquid slug) and is nearly independent of the injection pressure for both the gas and liquid phases. In addition, k_La, significantly enlarges with increasing high-pressure at the supercritical state.  Our microfluidic results with a small hydrodynamic diameter (of ≈ 50 μm) show a significantly increase in volumetric mass transfer of CO₂ into water by two to three order of magnitudes, O(10²-10³), compared with typical applications using millimeter-sized capillaries.