Study On The Interface Instability Of Fluids Flow During CO₂ Geological Sequestration

The greenhouse gas emissions?mainly CO₂?are increasing year by year which have become the focus of all countries in the world.It is considered that the permanent sealing of CO₂ in deep saline aquifer is one of the most effective ways to achieve carbon emission reduction.The injected CO₂ displacing the in-situ brine by percolation in deep saline aquifer,following a part of the CO₂ trapped in pores for CO₂ capillary trapping.Then a large amount of CO₂ gathers below the cap rock.CO₂ diffuse and dissolve into brine,which leads to physical structure trapping and dissolve trapping.With time goes by,the density differences between brine-saturated CO₂ and in-situ brine result in convection mixing,which further promotes dissolve sequestration.Both the process of CO₂ displacement and the convection mixing involve the fluids interface characteristic,and the interface stability has a crucial impact on the efficiency and safety evaluation of CO₂ storage.In this paper,Magnetic Resonance Imaging?MRI?visualization experiment device and microfluidics experiment device are set up aiming at the problem of fluid interface instability,which to study the interface instability of gas-water displacement process and convection mixing progress involved in deep saline aquifer storage respectively.The specific research content are as follows:MRI technology is used to visualized the seepage progress at core scale,observing the interface instability characteristics of CO₂ flooding in porous media.The results show that displacement front is prone to instability under low capillary number and viscosity ratio due to the CO₂ injection rates and effect of CO₂ phase states.The effect of capillary numbers?injection rates?on CO₂ breakthrough time and sweep area in the displacement process is analyzed,the maximum of CO₂ sweep efficiency and pore space utilization is reach at 0.03mL/min.At low rates,the saturation gradient decreases gradually along the direction of the porous media,however,it remains constant at high flow rates.There is a linear relationship between CO₂ saturation and the maximum gradient of saturation at breakthrough or steady state.Microfluidic technology is used to visualizing the pore scale seepage process in this study and discussing the influence of gravity and flow rates on the micro-interface instability.The results show that capillary finger is dominant with stable displacement interface in initial stage of horizontal displacement,and viscous finger is dominant in the later stage.CO₂saturation increases with the decrease of flow rates,and CO₂ saturation in the case of vertical direction is larger than horizontal direction.The binding and flow characteristics of CO₂ in the pores are analyzed quantitatively.CO₂ preferring to pass through the large size under the gravity direction,also,heterogeneous structure have a greater impact on the flow path.The interface instability of convection mixing in porous media have been studied.Dynamic parameters such as initial time,finger number density,fingertip concentration and finger growth rate are described.The effects of Rayleigh number?Ra?and Peclet number?Pe?on dynamic parameters are analyzed.The results show that the initial time of convection mixing decreases with the increase of Ra.The finger number increase first and then decrease at the same cross-section and the maximum finger number increases with Ra.Finger growth rate increases with Pe and the fingertip concentration gradient is larger under higher Pe.The characteristic parameters of mass transfer such as Sherwood number?Sh?,mass flux and dispersion flux are obtained.Sh increases with the increase of Ra in this experiment and follow the power law,Sh and mass flux increase with the injection rates.