Study On The Convective Mixing Characteristics Of Promoting Dissolution In CO₂ Geologic Sequestration

Global warming has become one of the biggest environmental problems facing mankind.CO₂ capture and storage?CCS?can effectively delay the issue of climate change.Considering the time scale and the storage capacity,the CO₂ storage in saline aquifers is considered to be the preferred technology for reducing the amount of CO₂ and mitigating the greenhouse effect.Density-driven-convection mixing in the process of CO₂ storage in saline aquifers reduces the time scale of dissolution by accelerating the dissolution of CO₂ into brine and,consequently,increases the effective storage capacity of CO₂ and the long-term security of storage.This is very important to accurately assessing the storage capacity and the security of storage in real case of CO₂ geologic storage.This paper studied the convective mixing process of fluid pairs with different density difference in Hele-Shaw cell and a vertical tube which is filled with glass sands or plastic sands by the visual experiment systems.The convective mixing experimental results of fluid pairs with different density difference in Hele-Shaw cell at room temperature and pressure showed that,as the density difference increases,the number of density fingers increases and the density fingers merge more quickly.As the density difference increases,the initial time of convective mixing decreases first and then tends to be stable,which is inversely proportional to the square of Rayleigh number,Ra.As the density difference increases,the finger growth rate increases linearly and is proportional to the characteristic speed,U.As the density difference increases,the initial Sherwood number,Sh,increases,and follows a power law relationship with Rayleigh Number.And as the density difference increases,the mixing time decreases first and then tends to be steady,which is inversely proportional to Ra.The convective mixing experimental results of fluid pairs with different density difference in a vertical tube which is filled with glass sands or plastic sands at room temperature and pressure showed that,for the same kind of fluid pairs,with the increase of permeability,the initial time of convective mixing decreases first and then becomes stable,which is inversely proportional to the square of Ra.With the increase of permeability,the wavelength of initial finger decreases and then tends to be stable,and is inversely proportional to Ra.With the increase of permeability,the finger growth rate increases linearly,and is proportional to the characteristic velocity U.With the increase of permeability,the dimensionless mass flow,Sherwood number,increases,and follows a power law relationship with Rayleigh Number.With the increase of permeability,the convection mixing time decreases first and then tends to be stable,which is inversely proportional to Ra.In the same type of sand packing,as the density difference increases,the initial time of convection mixing decreases,but the rate of decrease decreases.As the density difference increases,the wavelength of initial finger decreases,but the rate of decrease decreases.The finger growth rate and the dimensionless mass flow,Sherwood number,increases with the increase of density difference.And the convection mixing time decreases with the increase of density difference.The increase of the density difference/permeability promotes the onset of convection mixing,finger growth,mass transfer,the development and termination of the convection mixing process,but this promotion is diminished with increasing Ra.In addition,the increase of the density difference weakens the influence of the permeability on the initial time of convection and the wavelength of initial finger.In this paper,the parameters,N,t_onset,?,v,Sh,t_mix,are used to quantitatively analyze the influence of density difference and permeability on the onset and development characteristics of convective mixing,which provides new ideas and methods for reducing dissolution trapping time of CO₂ geological storage.