Determination of diffusion coefficients of carbon dioxide in water between 268 and 473K in a high-pressure capillary optical cell with in situ Raman spectroscopic measurements

Accurate values of diffusion coefficients for carbon dioxide in water and brine at reservoir conditions are essential to our understanding of transport behavior of carbon dioxide in subsurface pore space. However, the experimental data are limited to conditions at low temperatures and pressures. In this study, diffusive transfer of carbon dioxide in water at pressures up to 45MPa and temperatures from 268 to 473K was observed within an optical capillary cell via time-dependent Raman spectroscopy. Diffusion coefficients were estimated by the least-squares method for the measured variations in carbon dioxide concentration in the cell at various sample positions and time. At the constant pressure of 20MPa, the measured diffusion coefficients of carbon dioxide in water increase with increasing temperature from 268 to 473K. The relationship between diffusion coefficient of carbon dioxide in water [D(CO2) in m2/s] and temperature (T in K) was derived with Speedy–Angell power-law approach as:D(CO2)=D0[T/Ts-1]mwhere D0=13.942×10−9m2/s, Ts=227.0K, and m=1.7094. At constant temperature, diffusion coefficients of carbon dioxide in water decrease with pressure increase. However, this pressure effect is rather small (within a few percent).