Study On Flowing Features And Mechanism Of CO₂ Dissolution-driven Convection In Porous Media

As the global warming intensifies,the pressure of CO₂ emission reduction is increasing.The CO₂ storage technology can solve the problem of CO₂ emissions from the source,and has gradually become a research hotspot in recent years.In CO₂ geological sequestration,when CO₂ migrates to the difficult-to-penetrate cap rock,the injected CO₂ will accumulate to form a tank of CO₂ and keep a long-term stable interface between CO₂ and brine.As CO₂ gradually dissolves into the brine,a denser mixture will be generated on the interface to trigger convection flow,which can significantly accelerate the dissolution of CO₂ in brine.Since the driving force for dissolution-diven convection is the weak density difference between the mixture and the underlying fluid,a high-precision numerical simulation method is required to clearly capture the plume phenomenon during convection.Previous numerical simulations of dissolution-driven convection in porous media are performed in homogeneous porous media with numerical methods that two fluids are miscible with each other.In this work,the characteristics of dissolution-driven convection flow in heterogeneous porous media will be studied,based on the high-precision numerical simulation method,and experimental studied in heterogeneous Hele-Shaw Cell.Besides,a numerical method that two fluids are partially miscible with each other will be established to study the flow characteristics and mechanism of dissolutiondriven convection generated on the free interface,which will provide guidance for the evaluation of sequestration effect and potential in CO₂ geological storage.In the study of dissolution-diven convection characteristics in a heterogeneous porous media,a heterogeneous permeability field with random distribution is produced by mathematical methods,with the permeability distribution has specific horizontal and vertical correlation scales statistically.A numerical method that two fluids are miscible with each other is employed to simulate the dissolution-diven convection generated at the top boundary in a heterogeneous porous media with different correlation scales.The results show that the homogeneous porous media will have a significant effect on the pattern of plumes that formed during convection.In the isotropic heterogeneous porous media,structures such as split,turn,bump and tip will appear on the plume,while ring structure will appear on the plume in an anisotropic heterogeneous porous media.Through the statistical analysis of a large number of numerical simulation results in heterogeneous porous media,it is concluded that the influence of heterogeneous porous media on plume mainly includes the following three points: when the ratio of horizontal correlation scale to plume width is close to 1,a “resonance effect” occurs that statistical parameters are reached the extreme value;the permeability distribution with small correlation scale promotes the plume;and the laminar permeability distribution inhibits the plume.These three effects work together to make the convective flow in the heterogeneous porous medium shows rich characteristics.In order to validate the results of numerical simulation in heterogeneous porous media,a heterogeneous Hele-Shaw Cell was used to reproduce the flow in the heterogeneous porous media.At first,Hele-Shaw Cells with specific gap thickness distribution were built with the help of 3D printing technology,based on the isotropic heterogeneous permeability fields in the simulation.The gap thickness was detected with photometry and the gap thickness distribution was consistent well with the heterogeneous permeability field.In the experiment,by using propylene glycol and water to form a dense mixture,convection is trigged in a heterogeneous Hele-Shaw Cell and the evolution of plumes was record with camera to adopt statistical analysis.The experimental results show that plume morphological characteristics in the heterogeneous Hele-Shaw Cell are very similar to numerical simulation results that the plume structures,such as split,turn,bump and tip,appearing in the numerical simulation are reproduced.Meanwhile,when the correlation scale approximates to the width of plume,the statistical parameters of the plume also showed “resonance effects”.The results of experiment are consistent with the results of numerical simulation and the accuracy of the numerical simulation was verified.Considering the solubility of CO₂ in brine is very low,a numerical method that two fluids are partially miscible with each other is established,by including chemical potential energy into the governing equation.The dissolution-diven convection flow generated on the interface between supercritical CO₂ and brine are simulated.The results show that the partially miscible numerical method has a very good performance in capturing the two-phase interface that the interface remains clear and stable and rises slowly as the amount of dissolution increases.The plumes generated on the free interface in convection has fewer numbers than that generated the boundary,the root of plumes are slender,the horizontal movement of plumes is more obvious,and the evolution of the plume is slower.But the convection decays more quickly,due to the low solubility.The evolution of the solute flux can be clearly divided into five stages,and the solute flux of different stages can be modeled and analyzed.The diffusion stage is modeled along and the modeling formula of solute flux is set up with time and Rayleigh number.In the three stages from the flux growth stage to the constant flux stage,the time averaged solute flux is used in the modeling and the formula is set up with Rayleigh number.The dissolution flux of the convection shutdown stage is almost independent with Rayleigh number and the modeling formula is set up with time.In this paper,the dissolution-diven convection in heterogeneous porous media is studied with numerical simulation and experiment,while the partially miscible dissolution-diven convection on two-phase interface in homogeneous porous media is studied with numerical simulation,and abundant and important results obtained.The morphological characteristics and evolution characteristics of the plume during convection are obtained and analyzed,influences of heterogeneous permeability field on plume are summed up and modeling formula of solute flux in partially miscible dissolution-diven convection are summarized,which will provide reference and guidance for CO₂ geological sequestration process.