Numerical Simulation Study On CO₂/brine Migration In Heterogeneous Pore Media

The greenhouse effect causes natural disasters occur frequently,which seriously affects the production and life for human society.CO₂ is the primary component of greenhouse gases.Reducing CO₂ emission is the primary goal to reduce the greenhouse effect.CO₂ sequestration in brine aquifers is one of the most effective and potential methods.Understanding the process of migration and the natural convection is important to evaluate the safety and effectiveness of CO₂ sequestration.Heterogeneity of the storage structure determines that the CO₂/brine migration is a complicated hydrogeological process.Therefore,it is of great value to study the influence of structural heterogeneity on CO₂/brine drainage and convection mixing process.The heterogeneity of porous media is an important factor affecting the drainage of CO₂/brine.Phase field method can effectively capture the interface of CO₂/brine in porous media.In this paper,the phase field method is used to establish a numerical model to study the effect of pore structure heterogeneity on the two-phase drainage of CO₂/brine.The results show that:1)CO₂saturation and the development of CO₂ fingers are influenced by the heterogeneity of porous medium.2)The existence of cements changes the connectivity of porous medium,prompting the development of fingers and increasing the instability of drainage.3)The dead-end pore results in the increase of residual brine saturation and the decrease of CO₂ saturation.4)The development of fingers is suppressed in the region where wettability heterogeneity,which results in a steep increase of CO₂ saturation.Convection mixing can effectively promote the dissolution rate of CO₂.Fluid dynamics and Darcy seepage describe the phenomenon of different scales.The former is used to pore-scale to describe the interaction among the fluids and Darcy seepage is used to core-and larger scales to describe the average flow velocity in porous medium and macro properties of porous media relation.In this paper,the sensitivity analysis is performed on different scale models to study the convection characteristics at different scales and the influence of pore structure heterogeneity on the convection mixing of CO₂/brine.The results show that:1)The initial time step size and the grid number have an effect on the simulation,which lead to the change of convection intensity and the time for the fingers to reach the bottom.2)Onset time,the time for fastest finger reach the bottom is decreased and the amount of CO₂ dissolved is increased on pore scale.3)The existence of cements and enlarged grains have an unfavorable effect on the fastest finger but narrow grains accelerate the development of the fastest finger.