Numerical Study On The Dissolution Trapping Mechanism Of Convective Mixing Process In CO₂ Sequestration At Deep Saline Aquifers

Dissolution of carbon dioxide(CO2)could increase the density of saline brine during the underground storage of CO2.Then the system will be unstable under the influence of perturbations and the convection will be further generated.This density-driven convective mixing process greatly accelerates the transformation of CO2 from free state to solution state,which reduces the leakage risk of CO2.However,the study of this problem is facing great challenges due to the interaction and dynamic coupling of fluid flow,multi-component solute transport and reactions,as well as pore structure changes in porous media.In this thesis,based on the lattice Boltzmann equation(LBE)method,numerical simulations are performed to investigate the influence mechanism of various factors on the convective mixing process.The main work of the thesis includes the following aspects:(1)The multi-relaxation time LBE models are introduced for incompressible miscible flow to investigate the density-driven convective mixing process caused by dissolution of CO2.An exact non-equilibrium extrapolation boundary scheme as well as a local reactive boundary scheme are constructed for velocity/pressure and fluid-solid reactive boundaries conditions,respectively.The accuracy and validity of these two boundary schemes are then verified by a series of numerical experiments.(2)The effects of CO2 solubility on convective mixing process are investigated on porescale using LBE method.The numerical results show that the finger-like density distribution caused by miscible interface instability becomes small with the increase of solubility of CO2.When the solubility is large enough,finger-like flow would occur in the pore space,then further develop and merge together to form larger plumes.Since the Darcy-scale investigations cannot describe the fluid flow in the pore space,so the predictions on Darcy-scale could overestimate the dissolution rate of CO2.(3)The LBE models combined with the exact non-equilibrium extrapolation boundary scheme are used to investigate the influences of impurities on convective mixing on porescale.The numerical results show that the density of the system will form a layered structure in the direction of gravity due to the different properties of impurities and CO2,which could affect the stability and the convective mixing process in system.In order to characterize the stability and convective mixing intensity of the system under the influence of different impurities,the effective Rayleigh number is defined and validated.(4)Linear stability analysis and LBE method are used to study the convective mixing process of H2S impurities coupled with fluid-solid reactions on Darcy-scale and pore-scale,respectively.The results show that the overall stability of the system is the results of the competition between gravity density instability and reaction-permeability instability.The reaction consumption of CO2 and the dissolution of H2S could inhibit the gravity instability of the system,while the reaction between solutes and solid will increase the permeability and enhance the reaction permeability instability of the system.In conclusion,based on the LBE method,an exact non-equilibrium extrapolation scheme and a local reactive boundary scheme are developed for velocity and pressure boundary conditions and linear reactive boundary condition,respectively.Based on the proposed schemes,the effects of CO2 solubility,impurities and fluid-solid reactions on the system stability and convective mixing mechanism are investigated.This thesis provides effective methods for studying the reaction transport process in porous media and enriches the current understanding of the mechanism in CO2 sequestration in deep saline brine.