Experimental study of the temperature effect on two-phase flow properties in highly permeable porous media: Application to the remediation of dense non-aqueous phase liquids (DNAPLs) in polluted soil

摘要

The remediation of aquifers contaminated by viscous dense non-aqueous phase liquids (DNAPLs) is a challenging problem. Coal tars are the most abundant persistent DNAPLs due to their high viscosity and complexity. Pumping processes leave considerable volume fractions of DNAPLs in the soil and demand high operational costs to reach cleaning objectives. Thermally enhanced recovery focuses on decreasing DNAPL viscosity to reduce residual saturation. The oil industry has previously applied this technique with great success for enhanced oil recovery applications. However, in soil remediation, high porous media permeabilities and product densities may invalidate those techniques. Additionally, the impacts of temperature on coal tar's physical properties have not been thoroughly discussed in available literature. Here, we investigated how coal tar's physical properties, the capillary pressure-saturation curve and the relative permeability of two-phase flow in porous media depend on the temperature and flow rate experimentally. Drainage and imbibition experiments under quasi-static (steady-state) and dynamic (unsteady-state) conditions have been carried out at 293.15 K and 323.15 K in a 1D small cell filled with 1 mm homogeneous glass beads. Two different pairs of immiscible fluids have been investigated, coal tar-water and canola oil-ethanol. Results demonstrated similar trends for temperature effect and values of fluid properties for both liquid pairs, which backs up the use of canola oil-ethanol to model coal tar-water flow. It was found that there is no temperature effect on drainage-imbibition curves or residual saturation under quasi-static conditions. In dynamic conditions, the DNAPL residual saturation decreased by 16 % when the temperature changed from 293.15 K to 323.15 K. This drop was mainly linked to decreasing viscous fingering, as well as the appearance of wetting phase films around the glass beads. Both phenomena have been observed only in dynamic experiments. A high enough pumping flow rate is needed to generate dynamic effects in the porous medium. Ethanol and oil's relative permeabilities also increase with temperature under dynamic measurement conditions. Our findings indicate that flow rate is an important parameter to consider in thermal enhanced recovery processes. These effects are not taken into account in the classically used generalized Darcy's law for modeling two-phase flow in porous media with temperature variation.