Injecting n-BuOH to achieve density conversion of dense non-aqueous phase liquid: Pore-scale experimental simulation

Abstract

Dense non-aqueous phase liquids(DNAPLs) with a density greater than water have a tendency to migrate to the depth of the underground medium under natural conditions and under the action of gravity in the process of artificial remediation, thus increasing the risk of contamination.Previous experiments on one-dimensional sand column and two-dimensional sand box have found that the density-modified displacement(DMD) could reduce the risk of downward migration of DNAPLs, but there is a lack of quantitative observation on the effect of DMD on DNAPLs migration at the pore scale.In this study, simulated n-BuOH injection into microvoids was used to achieve PCE density conversion.By establishing the quantitative relationship between the PCE concentration, density and grayscale of dyeing PCE in non-aqueous phase(NAPL), the dynamic change of PCE-butanol density in microvoids after n-BuOH injection was quantitatively monitored.The migration state was analyzed based on the stress of representative PCE-butanol blobs in voids.The influence of the properties of void-scale media and density regulation on DNAPLs migration was revealed.The experimental results show that after n-BuOH injection, the concentration and density of PCE decreased rapidly, resulting in the reduction of the combined force of buoyancy and gravity and the friction of tube wall in the PCE-butanol blobs.The effective contact area between discrete PCE and n-BuOH is large and the effect is fast.When the density of PCE-butanol blobs decreases to slightly higher than that of water phase, the PCE-butanol blobs stop migrating downward under the influence of capillary force and friction.When the density of PCE-butanol blobs is less than that of water phase, the PCE-butanol blobs migrates upward, overcoming the capillary force, gravity and friction under the action of injection pressure and buoyancy.Injection pressure, friction, capillary force, buoyancy and gravity influence the migration behavior of PCE-butanol blobs in microvoid.The larger the microvoid radius, is the less influence the capillary force has on regulating the upward migration of PCE.n-BuOH with lower density than water migrates upward after being injected into the microvoids, so the n-BuOH injection from the lower end could improve the repair efficiency.The experiment confirmed that the injection of n-BuOH into the porous medium could significantly reduce the density of the DNAPLs and the risk of downward migration, providing information on the microscopic mechanism for the DNAPLs remediation program in the actual site.

Keywords

• microvoids
• n-buoh
• density-modified displacement method
• dense non-aqueous phase liquids