Investigation On Two-Phase Flow And Mass Transfer Characteristics Of CO₂ Sequestration In Saline Aquifer

CO2 sequestration in the saline aquifer has been proven to be an effective way to reduce CO2 emissions.During the CO2 storage in the reservoir,capillary trapping and dissolution trapping are critical mechanisms to achieve long-term storage of CO2.To evaluate the efficiency and safety of the CO2 storage,the CO2-brine two-phase flow characteristics,two-phase interface evolution and CO2-brine local mass transfer should be investigated.In this study,the two-phase flow and mass transfer characteristics of CO2-brine in porous media were investigated.Through the CT visualization,the characteristics of two-phase flow under the mixed-injection mode were clarified,and the major impact factors of CO2 displacement efficiency were explored.The evolution mechanisms of the two-phase interface during CO2 dissolution were revealed,and the quantitative descriptions of the interface evolution were realized.The local mass transfer characteristics during the non-equilibrium CO2 dissolution were analyzed,and the mass transfer models for stable and unstable flow based on the local mass transfer coefficient at the pore scale were established and evaluated.The main research contents of this article are as follows:Based on microfocus X-ray CT,an experiment system for gas-water two-phase flow was designed and constructed.The CO2 and gas mixture were used to conduct the gas-water co-injection experiment.The effects of the gas fraction,flow rate,N2 impurities in CO2,and temperature&pressure conditions on the displacement efficiency were explored.It is found that the displacement efficiency of the co-injection is similar to that of 1-to-1 displacement at lower flow rates,while more isolated CO2 clusters are generated during the co-injection at higher injection flow rates,decreasing the displacement efficiency.The CO2-N2 gas mixture could migrate more effectively than CO2 in the porous media,and its spatial distribution is more uniform,which improves the displacement efficiency.With the decrease of temperature and pressure,the influence of porosity distribution on the displacement efficiency of the gas mixture increases.The brine imbibition experiments were performed in the sand-filling core.The CO2-brine interface was dynamically tracked with the calculation of the interfacial area.The evolution mechanisms of the two-phase interface during CO2 dissolution were revealed.Three categories of the interface evolution in the porous media were proposed.It was found that the coalescence,detachment and fragmentation of the two-phase interface occur during the interphase mass transfer.The effect of heterogeneity on the interface evolution is greater than the gravity effect.The interfacial area follows the power-law distribution during the interphase mass transfer.Using the smaller cut-off threshold could reduce the error of the cumulative ratio correlation of the interfacial area.The specific interfacial area decreases linearly with the increase of the brine saturation.In the heterogeneous porous media,the number of smaller interfaces increases in the later stage of the mass transfer process,and the specific interfacial area increases.The pore-scale mass transfer processes in stable and unstable flows were compared,and the local mass transfer characteristics during the non-equilibrium CO2 dissolution were analyzed.It was found that as the gravity number increases,the number of small CO2 bubbles and the duration of the mass transfer process decreases in stable flow,while in unstable flow more CO2 bubbles appear in the early stage of the mass transfer process,and CO2 clusters with irregular shapes are more difficult to dissolve.During the mass transfer,the CO2 concentration gradually decreases in stable flow,while the CO2 concentration could increase by 3 times in unstable flow.The distribution range of the local mass transfer coefficient increases with the average size of the CO2 cluster.The mass transfer efficency is influenced by both specific interfacial area and flow development due to the slow local mass transfer under the effect of the bypass flow.Mass transfer models for the stable and unstable flow were established and different models were compared and evaluated.These theoretical knowledges support the predictions on the gas-liquid mass transfer in the reservoir.