A Novel Experimental Study on Density‐Driven Instability and Convective Dissolution in Porous Media

Geological carbon dioxide (CO2) sequestration (GCS) in deep saline aquifers is a promising solution to mitigate the impact of anthropogenic CO2 emissions on global climate change. CO2 dissolved in formation water increases the solution density and can lead to miscible density‐driven downward convection, which significantly accelerates the dissolution trapping of injected CO2. Experimental studies on miscible density‐driven convection have been limited. In the laboratory, we found an empirical linear correlation between reflected green light intensity and solute concentration, which enabled in situ measurements of solute concentrations in the spatial and temporal domains and consequently the mass flux across the top boundary of the porous medium. Using the novel experimental techniques, we determined the critical Rayleigh‐Darcy number and critical time scales for the onset of density‐driven instability and convective dissolution. This is the first study to determine these critical system parameters using laboratory experiments. Plain Language Summary Long‐term storage of carbon dioxide (CO2) in geological formations, such as deep saline aquifers, is a promising solution to mitigate the impact of anthropogenic CO2 emissions on global climate change. CO2 dissolved in formation water increases the solution density and can lead to miscible density‐driven downward convection, which accelerates the dissolution of CO2 in formation water and thus improves the long‐term security of the system. However, investigations of the critical system parameter and critical time scales for triggering downward convection have relied heavily on numerical simulations because of the challenges associated with laboratory experiments. In this study, we used experimental methods to find an empirical linear correlation between reflected visible light intensity and solute concentration, which enabled in situ measurements of solute concentrations in the spatial and temporal domains. Using these novel experimental techniques, we determined the critical Rayleigh‐Darcy number and critical time scales for the onset of density‐driven instability and convective dissolution. The findings from this experimental study have practical applications in many other engineered and natural processes, such as geothermal convection, heat transfer due to subsurface nuclear waste disposal, and variable‐density groundwater flow. Key Points Empirical linear correlation between reflected visible light intensity