X-Ray Computed Tomography Study of Foam Injection for the Enhanced Recovery of Diesel Fuel in Contaminated Soils

ABSTRACT: Surfactant foam injection is a promising in situ remediation technique for heterogeneous aquifers impacted by NAPLs (Non-Aqueous Phase Liquids). Foams have been shown to improve the sweep efficiency of gas flooding processes in enhanced oil recovery. In an environmental remediation (ER) context, foams also appear to be less sensitive to variations in soil hydraulic conductivity, which can be attributed to shear-thinning behaviour (exhibited under specific conditions) and to crossflow effects. The viscous properties of foam are mainly the result of bubble deformation caused by shearing of the gas/liquid interface against pore walls during flow. The increase in viscous force provided by foam, in addition to the introduction of surfactant molecules into the source zone resulting in lowered interfacial tension between aqueous and organic phases, promote NAPL mobilization. Our research uses X-ray Computed Tomography (CT) to image foam injection experiments in contaminated sand columns. Together with liquid effluent sample analytical results, instantaneous three-phase saturation profiles (aqueous, gaseous, organic) extracted from CT images provide important insights into NAPL desaturation dynamics, the evolution of foam morphology, as well as the recovery mechanisms involved (mobilization, solubilization). Foam injection experiments were carried out in a custom-made Teflon column (405 cm3) packed with a medium Flint silica sand (d50 = 0.53 mm, d60/d10 = 1.6) contaminated with diesel fuel at residual saturation (Sor = 0.19 +- 0.02). An experiment conducted using a high injection rate (Qfoam) resulted in a final induced pressure gradient of 736 kPa/m across the column and successful removal of 96 % of the initial contaminant mass. However, high foam viscosity is associated with poor injectivity in shallow unconsolidated sediments, where injection pressures are limited due to soil heaving concerns. Future laboratory studies will focus on optimization of the surfactant foam technology for ER applications.