Enhancing pollutant removal from contaminated soils using yield stress fluids as selective blocking agents.

This work presents a novel technique consisting in the use of yield stress fluids as blocking agents in porous media presenting pore-scale heterogeneities. The key feature of this method is that yield stress fluids only flow through the pores having a minimum size that depends on the applied pressure gradient. These fluids remain immobile in more and more pores as the pressure gradient is decreased. Therefore, the dimension of the pores which are invaded by the yield stress fluid can be controlled by adjusting the applied pressure gradient. Moreover, yield stress fluids are highly suitable blocking agents given the extremely high viscosity values that they exhibit in the pores. This allows for the diversion of the flow from greater to smaller pores during subsequent waterflooding stages, thus enhancing pollutant removal from the flow paths of small hydraulic conductance. A series of multiphase flow experiments were conducted in this study using well-characterized cores of artificial A10 sintered silicate. In these experiments, semidilute aqueous solutions of xanthan gum biopolymer were used as yield stress fluids to block the greatest pores. By doing so, considerably more pollutant was recovered by waterflooding. Furthermore, it was shown that an increase in polymer concentration does not always lead to a decrease in the size of the pores invaded by the blocking agent. Indeed, concentrated polymer solutions generate higher pressure gradients throughout the porous medium, which facilitates the invasion of small pores. Nevertheless, depending on the value of the yield stress-pressure gradient ratio, they may also develop extremely high viscosities that slow down their flow through such small pores. This work also presents a method to measure the volume of blocked pores using the results of tracer tests. The reported results suggest that using a polymer solution developing a yield stress as a selective blocking agent is a promising technique for soil remediation.

Estimation of Local Equilibrium Foam Model Parameters as Functions of the Foam Quality and the Total Superficial Velocity.

In this paper, the behavior of foam in a porous medium is studied in order to understand the effect of the fluid velocity on foam properties. This aspect is crucial during foam injection, as due to radial effects the foam velocity largely decreases around the injection well. The foam properties are detailed through the use of a new local equilibrium foam model parameter estimation approach using an improved new shear function and based on the most widely used STARS model developed by the Computer Modeling Group (CMG). A new mode of calculation of the STARS model parameters is then presented in order to allow both a semiautomated fitting of several quality scan pressure curves and a consideration of the role of the total velocity. The approach is tested through column experiments done at various velocities and gas fractions. Furthermore, the proposed model is also tested on literature results in order to validate it for very different experimental conditions. This study and the fitted results are then used to understand, on both our column experiments and the literature data, the existence of two shear effects and their origins.

Experimental study of foam propagation and stability in highly permeable porous media under lateral water flow: Diverting groundwater for application to soil remediation.

Foam propagation and stability in highly permeable porous media, encountered in soil pollution applications, are still challenging. Here, we investigated the application of foam for blocking the aquifer to divert the flow from a contaminated zone and, therefore, ease the remediation treatments. The main aim was to better understand the critical parameters when the foam is injected into a highly permeable aquifer with high groundwater flow velocity (up to 10 m/day). A decimetric-scale 2D tank experimental setup filled with 1 mm glass beads was used. The front part of the 2D tank was made of transparent glass to photograph the foam flow using the light-reflected method. The water flow was generated horizontally through injection and pumping points on the sides of the tank. The pre-generated foam was injected at the bottom center of the tank. Water streamlines (using dye tracing) and water saturation were investigated using image interpretation. Results show that 100% of the water flow was diverted during the injection of the foam. Foam stability in porous media depends significantly on the horizontal water flow rate. Recirculating water containing the surfactant increases foam stability. The main mechanism of destruction was identified as the dilution of the surfactant in water. However, the head-loss measurements showed that despite foam destruction, the relative permeability of the water phase in the media remained quite low. Injection of foam increases the radius of gas propagation, thanks to foam's high viscosity, compared to a pure gas injection case. These results are new highlights on the efficiency of foam as a blocking agent, showing that it can also serve as a means for gas transport more efficiently in porous media, especially for soil remediation applications.

Use of saponin foam reinforced with colloidal particles as an application to soil remediation: Experiments in a 2D tank.

Foam can be used to achieve environmental remediation in case of contamination caused by light non aqueous phase spills. However, when it comes in contact with oily pollutants, foam becomes weaker and its life time is greatly reduced. Such weakening can be dampened by using silica particles -together with saponin surfactant- which were shown to reinforce foam in bulk and 1D sandpack experiments. Here is addressed both foam propagation in a 2D porous media when buoyancy and gravity interfere, and foam behaviour when in contact with floating oil. Therefore, macroscopic foam displacement, and specific liquid and gas phases behaviours were studied in a 2D-tank. A piston-like displacement was observed during foam propagation in the absence of oil, while foam liquid phase was influenced by gravity and did not propagate homogeneously on entire tank height. In the presence of oil, foam was partly destroyed, which increased the local permeability of gas and created new preferential paths for gas flow. This effect was partially avoided via a surfactant concentration increase, but solid colloidal particles turned out to be a more efficient stabilizing agent, by significantly increasing foam strength and its oil-tolerance.

Estimation of Local Equilibrium Model Parameters for Simulation of the Laboratory Foam-Enhanced Oil Recovery Process Using a Commercial Reservoir Simulator.

An accurate determination of the foam simulation parameters is crucial in modeling foam flow in porous media. In this paper, we present an integrated workflow to obtain the parameters in the local equilibrium foam model by history matching a series of laboratory experiments performed at reservoir conditions (131 F and 1500 psi) on Estaillades limestone using a commercial reservoir simulator. The gas-water and water-oil relative permeability curves were first validated after history matching with the unsteady-state flooding experiments. The modeling parameters for foam generation and foam dry-out effect were obtained by history matching with the gas/surfactant coinjection experiments at varying foam quality and injection rates. Moreover, the modeling parameters for the destabilizing effect of oil on foam and foam shear thinning effect were derived after history matching with the foam-enhanced oil recovery process and oil fractional flow experiments in the laboratory. In practice, the calculated results reproduce the experimental outputs reasonably well. Furthermore, sensitivity analysis of foam modeling parameters is investigated to determine the most dominating parameters for accurate simulation of foam-enhanced oil recovery process in porous media. In this work, an efficient parameter estimation approach is developed from reliable foam flooding experimental data, which may be further applied to field-scale simulation. Moreover, the simulation approach can also be utilized to facilitate our interpretation of complex lab foam flooding results.

Non-Fickian dispersivity investigation from numerical simulations of tracer transport in a model double-porosity medium at different saturations.

It is generally admitted that dispersivity is an indicator of the heterogeneity scale of porous media. This parameter is assumed to be an intrinsic property which characterizes the dispersive behavior during the transport of a tracer in a porous medium. When the medium is saturated by two fluid phases (water and air), dispersivity depends strongly on saturation. "Double-porosity" medium concept can be attributed to a class of heterogeneous soils and rocks in which a strong contrast in local pore size characteristics is observed. In this work, we characterized non-Fickian dispersivities of a double-porosity medium at different saturations, by performing numerical simulations for a series of one-dimensional experiments of tracer dispersion under different initial and boundary conditions. The physical double-porosity model was composed of solidified clayey spheres, distributed periodically in a more permeable sandy matrix. Using a two-equation macroscopic model, numerical simulations reproduced very well the experimental data, thus allowing to determine the dispersivity for different transport scenarios. For the first time, the existence of a unique dispersivity of a double-porosity medium at a given saturation was demonstrated for different transport scenarios of initial and boundary conditions. The saturation dependence of the dispersivity in the double-porosity medium was established and compared with the trends obtained for the single-porosity soils in previous studies.

Saponin foam for soil remediation: On the use of polymer or solid particles to enhance foam resistance against oil.

Foams can be used to remediate aquifer pollution due to industrial leaks. However, when in contact with oily pollutants, foams may collapse and thus have a very limited life-time. A suitable formulation of biodegradable foam that resists oil contact is therefore needed. Hence, the ability of xanthan polymer and silica colloidal particles to stabilise foam against oil was investigated. Their performance in terms of stabilisation was evaluated via foam generation experiments in columns of porous medium, conducted with and without oil. The results show that the addition of xanthan polymer led to an increase in the viscosity of the solution, which thwarted the formation of foam. It did not improve the resistance of foam to oil, but increased altogether the resistance factor up to more than twice the original value. Concerning silica particles, it was demonstrated that they both noticeably increased resistance factor and moderately stabilised foam against oil by 20% at optimum concentration. As such, this study presents a new way to reinforce foam against oil for soil remediation issues.