Measurements of CO2 molecular diffusion coefficients in crude oils from swelling-time curve and estimation using viscosity from the Stokes-Einstein formula

The CO2 diffusion coefficients in crude oils were measured from curves of oil-swelling vs. elapsed time with CO2 dissolution in the oils. The oil swelling was measured with CO2 up to a pressure of 10 MPa at 50 °C. The swelling coefficient and diffusion coefficient of CO2 and CH4 gases in the oil column were compared with CH4 gas. The diffusion coefficient of the heavy oil was evaluated as approximately 1.1–1.6% of that of bitumen. The swelling factors increased with pressure, and the diffusion coefficients in CO2 supercritical range were more than twice as high as those in the CO2 gas phase. An empirical equation to estimate gas solubility in crude oils vs. API gravity has been presented as well. Gas diffusion coefficients were shown to relate to oil viscosity based on the Stokes-Einstein formula and a new correlation between the two with the absolute average deviation (AAD%) of about 15.5%, derived.