Absorption and diffusion process at the CO2-decane interface considering density fluctuations near the critical CO2 point

The injection of CO2 into geological formations for storage and for efficiency enhancement of oil production is considered a promising technology. Fundamental is an understanding of the absorption and diffusion process at the CO2-decane interface and its pressure and temperature dependence when approaching the critical CO2 point (31 degrees C, 7.38 MPa). Based on a new conceptual model, which considers CO2 density fluctuations in the critical range, we were able to consistently explain the time dependence of the volume increase for the respective thermodynamic state. The experimental results confirm the new conceptual model that volume swelling in the non-critical pressure range (< 6 MPa) is a surface effect with limited penetration depth. In contrast, the volume increase in the critical pressure range (6 - 8 MPa) is caused by mixing of liquid CO2 droplets and the oil phase, i.e. to a level increase of the liquid mixed phase. Our experimental contact angles confirm this. The measured minimum miscible pressures (MMP) are 5.6 and 6.5 MPa at 20 degrees C and 30 degrees C, respectively. The CO2 absorption process at the CO2-decane interface is studied independently with pressure decay experiments. Our model results show excellent agreement (relative error congruent to 1 %) with the experimental results for both the non-equilibrium and equilibrium model, and the early-time diffusion coefficient is 4.7 +/- 1.9x10(-7)m(2)/s, confirming a fast CO2 mass transfer process.