Gravity assisted miscible CO2 injection into oil reservoirs is an efficient method of enhancing oil recovery. CO2 injection into aquifers for sequestration purposes is another application of miscible displacement under gravity control. This paper reports pore scale simulation studies to determine the role of different parameters on the frontal stability of the miscible displacement process under gravity. The simulation studies were performed using the Finite Element Analysis technique. The simulation model was initially validated by matching results with flow visualization experimental studies using glass micromodels. The Navier-Stokes, continuity and convection-diffusion equations were used in the simulation instead of ideal Darcy law. Wide ranges of parameters applicable for Enhanced Oil Recovery and CO2 sequestration have been used in the sensitivity study. Dip angles (Θ) between 0o and 180o (for updip and down-dip situations), different domain velocities, density differences of 50 to 900 kg/m3 between the injecting and displaced fluids and viscosity ratios from 1 to 50 (to include light and heavy oils) have been investigated. Snapshots were captured in each simulation case for visual comparison of the frontal advancement. In addition, breakthrough saturation was plotted against cos (Θ) to quantify the competition between viscous and gravity forces in gravity dominated miscible displacement process. The pore scale study suggests that stability of a miscible process can be influenced by several factors. When gravity acts in favor of displacement and there is a moderate to large density difference, angular tilt is the most important parameter influencing displacement. When the density difference is small then the mobility ratio and flow velocity also play a role. When gravity opposes displacement and buoyancy forces are dominating, results show little sensitivity to the actual tilt angle. Better displacement is seen for lower density difference and for higher flow velocity, while, again, the mobility ratio only impacts on displacement when the density difference is quite small.Zeeshan Mohiuddin, Yvonne Stokes and Manouchehr Haghigh
