Determining CO₂ diffusion coefficient in heavy oil in bulk phase and in porous media using experimental and mathematical modeling methods

In this research, experimental and mathematical modeling studies were implemented to determine the CO₂ diffusion coefficients in bulk phase (nontransparent high-pressure cell) and in porous media (real reservoir core). Experimentally, pressure decay was investigated in bulk phase and in porous media using the same initial injection pressure (5000 kPa). In the bulk phase, a sealed high-pressure cell was used to test the CO₂ diffusion coefficient, with the heavy oil and CO₂ contacting each other without the appearance of porous media. At the end of the diffusion process, CO₂ concentrations were measured in different sections of the CO₂-dissolved heavy oil to determine the CO₂ diffusion profile along the heavy oil column. In the porous media, a new experimental approach was generated to mimic the CO₂ diffusion process in real reservoir conditions. A half-saturated core using heavy oil was located in the core holder, and then CO₂ was introduced into the core, with the gas–liquid phase interface in the middle of the core. Pressures were monitored in the two experiments and were compared after a 200-hour diffusion process. A mathematical model was developed to determine the CO₂ diffusion coefficients and predict the time at which the diffusion process would reach the equilibrium condition in both experiments. Where the non-equilibrium boundary condition was considered at the gas–liquid phase interface, the CO₂ diffusion coefficients in the target heavy oil in the bulk phase and in porous media were measured as 5.778 × 10⁻⁹ and 3.222 × 10⁻⁹ m²/s, respectively. The tortuosity of the core was calculated using the measured CO₂ diffusion coefficient as 1.79, and the CO₂ concentrations at different test times were predicted.