Measurement of CO2 Diffusivity for Carbon Sequestration: A Microfluidic Approach for Reservoir-Specific Analysis

Predicting carbon dioxide (CO2) security and capacity in sequestration requires knowledge of CO2 diffusion into reservoir fluids. In this paper we demonstrate a microfluidic based approach to measuring the mutual diffusion coefficient of carbon dioxide in water and brine. The approach enables formation of fresh CO2-liquid interfaces; the resulting diffusion is quantified by imaging fluorescence quenching of a pH-dependent dye, and subsequent analyses. This method was applied to study the effects of site-specific variables-CO2 pressure and salinity levels-on the diffusion coefficient. In contrast to established, macro-scale pressure-volume-temperature cell methods that require large sample volumes and testing periods of hours/days, this approach requires only microliters of sample, provides results within minutes, and isolates diffusive mass transport from convective effects. The measured diffusion coefficient of CO2 in water was constant (1.86 [+/- 0.26] x 10(-9) m(2)/s) over the range of pressures (5-50 bar) tested at 26 degrees C, in agreement with existing models. The effects of salinity were measured with solutions of 0-5 M NaCl, where the diffusion coefficient varied up to 3 times. These experimental data support existing theory and demonstrate the applicability of this method for reservoir-specific testing.