Investigation On Multiphase Flow In Porous Media Under Carbon Geological Sequestration Conditions

Geological sequestration and utilization is one of the effective way to mitigate the global climate change.In this thesis,the multiphase flow behavior of supercritical pressure CO₂ and aqueous phase were investigated under the CO₂sequestration reservoir conditions.The investigations were conducted based on the theoretical analysis,pore-scale experiments and flow numerical modeling,and the core flooding experiments.When the porous media is water-wet,the jumping events occurs when CO₂phase displaces water.In general,the wetting phase preferentially occupies the throats and the non-wetting phase preferentially occupies the pores.During同the secondary imbibition,the interconnected large CO₂ phase ganglia were snapped off and altered to the isolated small ganglia.The isolated ganglia are difficult to be mobilized due to the capillary entrance pressure at the throats.In a porous media with throats and pores,the value of capillary pressure is dominated by the size of throat,and the corresponding fluid saturation profile is dominated by the size and geometry of the pores located in the downstream of the throat.The depressurization induced exsolution and swelling behavior of CO₂ was investigated at pore-scale using micromodel experiments and lattice Boltzmann model.The results showed that the amount of exsolved CO ₂ nuclei was influenced by the fluid temperature,depressurization rate and the pore-wall wettability.Smaller ganglia size and more dispersed CO₂ phase morphology was observed when more nuclei appeared during the depressurization.In porous media with hydrophilic pore-wall,the pore-scale morphology of CO₂ showed significant similarity with the residually trapped CO₂.The Ostwald ripening phenomenon was observed when the sizes of exsolved CO2 bubbles were small.Due to the swelling effect,the initially isolated small CO₂ ganglia can be re-connected and the mobility CO₂ phase increased due to the re-connection of the small ganglia.Based on the pore-scale mechanisms observed,the quantitative core-scale experimental investigations were obtained focusing on supercritical pressure CO₂-water two-phase displacement,depressurization behavior and the residual oil recovery performance using CO₂ displacement.The results of CO₂-water core-flooding experiments showed that capillary pressure increases in presence of the particle retention due to the change of the throat size.The observation on the depressurization behavior showed that the exsolved CO₂ phase shows low mobility similar to the residually trapped CO₂.The residually trapped CO₂ has less security concern on the unfavorable migration in presence of depressurization comparing with the highly interconnected CO₂ phase with high mobility.The oil recovery experiments showed that the injection supercritical pressure CO₂ after water-flooding can significantly improve the oil recovery performance even when the reservoir showed strong heterogeneity.