Dynamic stability characteristics of fluid flow in CO2 miscible displacements in porous media

Abstract

The dynamic characteristics of fluid flow are important in miscible displacement processes in carbon dioxide enhanced oil recovery (CO₂-EOR) projects. And the stability of the in situ mixing zone greatly influences the oil recovery factor, which deserves further research. We investigated CO₂ miscible displacement processes using magnetic resonance imaging (MRI) apparatus. The CO₂ miscible displacement flows were performed at a low injection rate of 0.1 ml min⁻¹ with reservoir conditions of 8.5 to 9.5 MPa and 37.8 °C. The oil saturation evolution, the length of the in situ mixing zone, and the mixing-frontal velocity and CO₂-frontal velocity were quantified. The experimental results showed that the residual oil saturation decreased with pressure and the mixing zone length was independent of pressure. The mixing-frontal velocity and the CO₂-frontal velocity were nearly the same and increased with pressure. The critical velocity of the CO₂/n-decane (CO₂/nC₁₀) system was 1.105 × 10⁻⁵ m s⁻¹. Although the whole mixing zone length had no obvious change with pressure, a higher pressure compressed the mixing zone and led to an unstable mixing front above the critical velocity. The longitudinal dispersion coefficient was calculated by fitting the experimental data with an error function, which had no obvious change with pressure. Additionally, a three-dimensional lattice-Boltzmann method (LBM) was used to simulate pore-scale miscible fluid flows in upward vertical displacements. A front fingering occurred at a low kinematic viscosity ratio (ν_Co2 : ν_o = 1 : 1). At a large kinematic viscosity ratio (ν_Co2 : ν_o = 1 : 15), the high kinematic viscous oil restrained the buoyancy of supercritical CO₂, but also impeded the displacement with a pore-scale backflow which might lead to a low oil recovery factor.