Experimental study of thermally enhanced recovery of high-viscosity DNAPL in saturated porous media under non-isothermal conditions.

Thermal enhancement is known to be an efficient way to decrease the residual saturation of some common dense non-aqueous phase liquids (DNAPLs) after pumping. However, the effect of transient heat transfer during the recovery of a high-viscosity contaminant, such as coal tar, in highly permeable porous media is still unknown. A 2D tank experimental setup allowing monitoring of temperature and saturation fields during DNAPL pumping has been developed. Experiments were run under isothermal and non-isothermal conditions, at low and high flow rates. We investigated the presence of viscous fingering and how that influences the shape of the cone of depression, as well as the residual saturation. The saturation fields show that less viscous fingering occurs in pre-heated cases and that heating increases the recovery efficiency. Increasing the temperature increases the critical velocity and the viscosity ratio and helps to stabilize the interface between the non-wetting and wetting phase. Observations were first made on an oil and ethanol fluid pair because its properties were known, before extending the experiments to a coal tar and water fluid pair. Residual oil saturation after pumping was decreased by 6-16% in all pre-heated conditions. Pumping at low flow rate in these conditions leaves the smallest oil residual saturation (20%) after pumping. A low flow rate increases the recovery efficiency by reducing viscous fingering and by spreading the generated heat to a larger part of the tank. Finally, results on coal tar pumping show that the high thermal conductivity of water helps in keeping the temperature high during pumping. The residual coal tar saturation was reduced from 40% at 20 °C to 28% when pre-heating the tank. Operating at a low flow rate and with a uniform temperature is the key to recovering the highest amount of a viscous DNAPL such as coal tar from the soil and satisfying cleanup goals when using thermally enhanced pumping.