Research On Capillary Trapping And Convective Mixing Characteristics Of CO₂ Storage In Saline Formations

CO2 storage in deep saline aquifer has the advantages of wide geographic distribution and tremendous storage potential.It is an optimized solution for reducing carbon emission and promoting the deployment of low carbon strategy.It has important theoretical significance and engineering application value for reservoir capacity assessment and storage path prediction by carring out the trapping experiments under reservoir pressure and temperature conditions in laboratory.The purpose of this paper was to investigate the gas/liquid flow characteristic in porous media,study the capillary trapping mechanism and density driven convective mixing behavior.The key parameters such as permeability and fluid wettability of porous media were obtained.The visual observation of the density-driven convection fingers growth process in porous media was achieved.The variation of CO2 saturation in the drainage and imbibition process was calculated.The effects of injection conditions and injection modes on the capillary trapping process were clarified.The specific research contents are as follows:Design and build an experimental platform for simulating CO2 storage in saline formations,conducted parameter measurement of fluids flow in porous media.Physical property parameter such as porosity,permeability,gas-liquid two-phase saturation,interfacial tension and contact angle were measured.Combined with magnetic resonance(MRI)imaging,a semi-dynamic capillary pressure in-situ measurement method was established.It was found that fluid interfacial tension and porous media permeability are the key factors affecting capillary pressure.Based on the Burdine theory,the relative permeability curves of the gas-liquid two-phase flow were obtained.Capillary trapping experiments in sand packed porous media were carried out to study the effects of heterogeneity and wettability on gas-liquid two-phase distribution.In the stratified heterogeneous sand packed porous media,the higher porosity,the higher residual saturation of the displacing phase.The wettability of the porous media affects the moving velocity of the displacement front.The larger the contact angle,the faster the displacement front move.In addition,the influence of injection conditions on capillary trapping is studied.Compared with gaseous CO2 injection,supercritical CO2 injection can improve drainage efficiency,and the finally CO2 storage amount increased with the injection volume.When the injection volume is greater than 6 times pores volume,the amount of CO2 storage tends to be stable.The relationship between the initial displacing phase saturation and the residual displacing phase saturation is established.Through the dimensionless analysis,the critical gravity number is used as the criterion for determining the stability of the displacement front.When the gravity number of the CO2 drainage process is less than the critical gravity,an unstable gas-liquid two-phase interface will be generated.As capillary number changes,the initial displacing phase saturation is affected by the injection flow rate and fluid properties.The convective mixing experiments of two fluids with different densities in porous media were carried out.The occurrence and development of convective mixing process for fluids with different density,viscosity ratio and different porous media permeability were observed.The duration of convective onset and convective mixing was explored.The typical stages of finger formation and growth regime were described quantitatively,and the images of finger appearence,propagation,coalescence and regeneration were obtained.Through the analysis of the change of flux,it is revealed that the higher permeability and higher fluids density difference,the lower fluids viscosity ratio,will promote the convection mixing process.The research results of this thesis can provide theoretical basis for site and injection conditions selection,build forecasts for CO2 migration and distribution in saline aquifer.