Foam placement for soil remediation

SPECIAL ISSUE CSIRO PUBLISHING

Environ. Chem. http://dx.doi.org/10.1071/EN17003

Research Paper

Foam placement for soil remediation Henri Bertin,A,C Estefania Del Campo EstradaA and Olivier AtteiaB A

Institut de M
ecanique et d’Ing
enierie (I2M), Centre National de la Recherche Scientifique (CNRS), Universit
e de Bordeaux, France. B G
eoressources et Environnement-ENSEGID, Bordeaux INP, France. C Corresponding author. Email: [email protected]

Environmental context. Soil pollution is an important concern and remediation techniques, especially in situ techniques, should be studied. We investigate a new technique based on foam generation and placement inside the porous soil to improve the pollutant extraction. This technique could be useful when the soils are heterogeneous because it allows a complete soil sweeping. Abstract. Foam can be generated in porous media, mainly by snap-off phenomena, by co-injecting gas and a surfactant solution. The liquid films that separate the gas bubbles, called lamellae, and gas trapping in small pores where capillary pressure is high generate a resistance to flow that drastically decreases fluid mobilities in porous media. Experiments performed with a 2D laboratory pilot consisting of two layers with different properties clearly highlight that foam is generated in the high-permeability layer and will divert flow towards the low-permeability region. This behaviour is of great interest for the remediation of heterogeneous polluted soils.

Received 4 January 2017, accepted 6 June 2017, published online 28 June 2017

Introduction

in order to recover the pollutant by a reduction of interfacial tension or biosparging.[9] Another way is to control the flow in heterogeneous aquifers by reducing gas mobility in highpermeability zones.[8,10–13] The objective of our study is to present experimental results obtained at the laboratory scale with a 2D heterogeneous pilot made of two layers of contrasting permeabilities (1 : 5), allowing a direct visualisation of flow and showing the ability of foam to control the advancing front in a layered system.

In situ soil remediation is of growing interest and several techniques have been developed (e.g. venting, pump and treat, sparging). Among all the available techniques, in situ remediation can be achieved by injecting fluids (liquid or gas) together with chemical products like surfactants and solvents.[1] However, the nature of the contaminants and the properties of soils make remediation a long and difficult process. One difficulty arises from the heterogeneity of soil. When two zones of soil present contrasting permeabilities, flow takes place preferentially in the high-permeability zone to the detriment of the low-permeability one. This leads to very low sweep efficiency in zones where contaminants are trapped owing to capillary forces. A way to circumvent this issue is to slow down the flow in the high-permeability zone and divert it towards the low-permeability one. Following research carried out in the field of petroleum engineering,[2–5] a way to divert flow in heterogeneous systems is the use of foam. Foam, which is a dispersion of gas in a surfactant-laden solution,[6] can be generated and transported in porous media. Gas bubbles are mainly generated by a snap-off phenomenon,[7] and the liquid films separating the gas bubbles, called lamellae, generate a resistance to flow that drastically decreases fluid mobility in porous media by an increase of apparent viscosity and reduction of gas relative permeability. Foam has been used in petroleum engineering to enhance oil recovery and to control the advancing front in heterogeneous reservoirs.[8] The in situ treatment of polluted soils using foam can be considered in two ways. Foam can be considered as a ‘delivering’ fluid for surfactant or hydrogen in the source zone Journal compilation
CSIRO 2017

Experimental Fluids The surfactant was Triton X-100, which is a non-ionic surfactant, purchased from Sigma–Aldrich (France). This surfactant, commonly used as a preservative and antiseptic agent, has also been used for soil remediation[14] even though its environmental safety is questionable. The value of its critical micellar concentration (CMC) is equal to 150 mg L
1 in pure water. In our experiments, we used the surfactant diluted in pure water at a concentration of 1.5 g L
1 (10 times the CMC). The gas was pure air, provided by Air Liquide. A dye, Erioglaucine disodium salt, provided by Sigma– Aldrich (France), was used for tracer studies. This tracer is considered a ‘perfect’ tracer; that is, it has no interaction with the solid phase and does not change the density and viscosity of the injected fluid. Porous media Two calibrated silica (more than 98 %) sands provided by Sibelco France were used to make up the unconsolidated porous A

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H. Bertin et al. 100

(2) Tracer test When the experimental pilot was full of water-saturated sand, we injected a dye tracer at a constant flow rate (Qw ¼ 11 mL min
1) and followed its displacement as a function of time through photos (Fig. 3a–h) while the pressure drop was recorded.

Transmittance (%)

90 80 70

Fine sand

60

Coarse sand

50 40 30

(3) Surfactant solution injection The surfactant solution (pure water þ Triton X-100) was injected into the pilot. The objective of this step was to displace the dye tracer and complete surfactant adsorption; three pore volumes of surfactant solution were necessary to complete this step.

20 10 0 10

100

1000

Particle diameter (µm) Fig. 1. Grain size distribution.

24 cm

(5) Tracer injection When the foam front reached the pilot outlet, gas injection was stopped and the dye tracer was injected under similar conditions to Step 2. The experiment was performed at room temperature (T ¼ 20  2 8C).

High permeability

24 cm

50 cm

Low permeability

(4) Foam generation Gas (90 % in volume) and surfactant solution (10 % in volume) were co-injected at a constant total flow rate of 2 mL min
1 into the bottom layer in order to generate foam to block the flow.

95 cm

Results Tracer experiments The time evolution of the tracer front is presented in Fig. 3a–h. We see clearly that the dye front is flowing faster in the highpermeability layer than in the low-permeability one. This expected result is consistent with Darcy’s law and clearly shows that the low-permeability layer remains poorly swept. It can also be noted that the front shape is influenced by the position of the injectors. The displacement was simulated using MODFLOW and MT3DMS software. MODFLOW is a 3D groundwater model developed by United States Geological Survey (USGS)[15] that allows simulation of groundwater flow in heterogeneous systems. In our study, the code was used to derive the water velocities in the vertical pilot from the injected water discharges at the injection points, using the outgoing boundary conditions and the permeabilities of the layers concerned. MT3DMS, developed by the University of Alabama,[16] allows simulation of solute transport from the velocities calculated by MODFLOW. MT3DMS solves the advection dispersion equation for the tracer transport in the same heterogeneous medium and with dispersion coefficients adapted to the scale of the medium. The solution used for the advection is TVD (total variation diminishing), which allows the modelled dispersion to be kept equal to the theoretical one. Here, the tracer transport was simulated with a transient injection of a known amount of tracer, before foam injection. Considering the different time positions of the average displacement front (Fig. 3) and the values of the pressure drop (530 Pa in the low-permeability layer and 275 Pa in the highpermeability one), we can simulate the dispersion process in the heterogeneous system (Fig. 4a, b). The dispersion zone is more spread out in the numerical simulation than in the experiment; however, we could fit the average displacement profiles

Fig. 2. Schematic diagram of 2D pilot. The red marks correspond to pressure measurement holes and black marks correspond to injection holes.

media. The high-permeability layer was made of grains with an average diameter of 450 mm and the low-permeability layer was made of calibrated grains with a diameter of 140 mm (Fig. 1). The density of the two sands was 2650 kg m
3. 2D pilot The experimental pilot was composed of a metallic frame and two lateral glass panes in between which the porous media were packed (Fig. 2). The dimension of the each porous layers was 95 cm  24 cm  2 cm. The fluids were injected from eight holes drilled in the left-hand side and pumped out from eight holes on the right-hand side of the pilot. Two holes (in red in Fig. 2) at the inlet and outlet of each layer were used for pressure measurement using liquid columns. Setup and procedure A peristaltic pump (Masterflex L/S model 7524–55) was used to inject and pump the liquid and a mass flow controller (EI-Flow Bronkorst F201CV; 0–20 sccm min
1) to inject the gas. The two devices were connected through a tee-junction before injection. (1) Preparation Water was injected first in the pilot up to 5 cm, then sand was slowly poured between the glass panes while the whole system was gently shaken to avoid trapping air. This process was continued, centimetre by, until half of the pilot height (24 cm) was filled up. The process was similar with the second sand until the whole pilot was fully saturated (48 cm). Porosity was determined by weighing the sand used to make each layer. B

Foam placement for soil remediation

(a)

(e)

t  0 min

t  74 min (f)

(b)

t  16 min

t  91 min (g)

(c)

t  32 min

t  105 min (h)

(d)

t  52 min

t  127 min

Fig. 3. Visualisation of the dye tracer displacement profiles at different times.

(a)

(b)

Fig. 4. (a) Simulated dispersion profile (t ¼ 16 min); (b) simulated dispersion profile (t ¼ 32 min). The red dotted line represents the experimental average displacement profile. The colour lines represent the tracer concentration during the dispersion process.

Foam displacement Gas and surfactant were co-injected at the bottom of the pilot. The time progression of the foam is visible in Fig. 5a–h. The detailed analysis of these photos is 2-fold. First, we clearly see that foam is generated in the high-permeability layer

(represented by the red dotted line in Figs 4a, b) and obtained the following values of the permeabilities (K): K1 ¼ 1.3  10
4 m s
1 (low-permeability zone, top of the pilot); K2 ¼ 6.1  10
4 m s
1 (high-permeability zone, bottom of the pilot). C

H. Bertin et al.

(a)

(e)

t  0 min

t  28 min

(b)

(f)

t  7 min

t  38 min

(c)

(g)

t  13 min

t  75 min

(d)

(h)

t  16 min

t  139 min

Fig. 5. Photos of pilot apparatus showing foam displacement front as a function of time.

as expected. Second, it appears that the front shape is affected by gravity. The ‘triangular’ shape of the foam zone is due to gravity override in the high-permeability zone. Moreover, the reasonably high value of the permeability is not favourable for the generation of a strong foam. However, foam is generated in the high-permeability layer and propagated all along the porous medium until the gas reaches the pilot outlet. Foam does not penetrate the low-permeability layer, owing to the high entry capillary pressure.

confirming that water mobility is significantly reduced when foam is present. In Fig. 7, we compare the dye front positions presented in Figs 3c and 6d (t ¼ 32 and 35 min) and in Figs 3g and 6h (t ¼ 105 and 115 min). We see in Fig. 7 that the dye fronts flowing in the foam zone are slowed down in the high-permeability layer but have travelled further in the low-permeability zone. The absence of foam at the bottom of the pilot results in the high velocity. It can be seen that the velocity of the front within the foam is the pore velocity, which increases in the presence of low water saturation. The tracer flux in this layer is thus much smaller.

Tracer displacement in presence of foam The dye tracer was injected (Qw ¼ 11 mL min
1) after foam placement. The time progression of the tracer is shown in Fig. 6a–h. We see in these figures that dye is flowing in all regions of the porous medium, including the zone where foam is present; moreover, in Fig. 6h, we see that the dye fronts in the lowpermeability zone and in the foam zone are very close,

Conclusions The objective of the present experimental study was to show foam behaviour in a heterogeneous porous system. For this, we built a 2D pilot made of two contrasting-permeability layers wherein the flow could be visualised. D

Foam placement for soil remediation

(a)

(e)

t  0 min

t  43 min

(b)

(f)

t  6 min

t  62 min

(c)

(g)

t  16 min

t  94 min

(d)

(h)

t  35 min

t  115 min

Fig. 6. Photos of pilot apparatus showing dye displacement front as a function of time.

A tracer injection in the water-saturated system shows that flow takes place preferentially in the high-permeability layer while the low-permeability one remains poorly swept. Co-injection of gas and surfactant solution leads to foam generation and propagation in the high-permeability layer and, despite its low density, foam does not enter in the low permeability layer. This behaviour confirms that foam is more stable in a high-permeability layer than in a low-permeability one owing to the capillary pressure difference. A tracer injection performed after foam placement showed that the presence of foam significantly decreased water mobility in the zone where foam is present, straightens the displacement front and allows a better sweep efficiency of the low-permeability layer. However, we clearly see that the foam zone develops a triangular shape due to gravity effects. This point highlights the difficulty in injecting gas in an open well owing to its low density. This difficulty could be overcome by the use of inflatable packers located in front of the target zone or by a

Low permeability

Foam zone

High permeability

Fig. 7. Comparison of the dye fronts (no foam in blue; presence of foam in red).

pregeneration of foam in the well before its injection into the porous medium. The advantage of this last option is to allow foam propagation in a porous medium whose permeability is so high that foam could not be generated in situ. E

H. Bertin et al.

This experimental study is a first stage before field application. The presence of pollutants will also be considered in future studies.

[10] J. J. Kilbane II, P. Chowdiah, K. J. Kayser, B. Misra, K. A. Jackowski, V. J. Srivastava, G. N. Sethu, A. D. Nikolov, D. T. Wasan, T. D. Hayes, Remediation of contaminated soils using foams. Land Contamination & Reclamation 1997, 5, 41. [11] G. J. Hirasaki, R. E. Jackson, M. Jin, J. B. Lawson, J. Londergan, H. Meinardus, C. A. Miller, G. A. Pope, R. Szafranski, D. Tanzil, M. D. Annable, J. W. Jawitz, P. S. C. Rao, R. D. Rhue, T. J. Simpkin, Field demonstration of the surfactant/foam process for remediation of a heterogeneous aquifer contaminated with DNAPL, in NAPL Removal: Surfactants, Foams and Microemulsions (Eds S. Fiorenza, C. A. Miller, C. L. Oubre, C. H. Ward) 2000, pp. 1–163 (CRC Press: Boca Raton, FL). [12] S. Wang, C. N. Mulligan, Rhamnolipid foam enhanced remediation of cadmium and nickel contaminated soil. Water Air Soil Pollut. 2004, 157, 315. doi:10.1023/B:WATE.0000038904.91977.F0 [13] H. Wang, J. Chen, Enhanced flushing of polychlorinated biphenyls contaminated sands using surfactant foam: effect of partition coefficient and sweep efficiency. J. Environ. Sci. 2012, 24, 1270. doi:10.1016/S1001-0742(11)60881-4 [14] S. Gitipour, K. Narenjkar, E. S. Farvash, H. Asghari, Soil flushing of cresols contaminated soil: application of nonionic and ionic surfactants under different pH and concentrations. J. Environ. Health Sci. Eng. 2014, 12, 129. doi:10.1186/S40201-014-0129-Z [15] A. W. Harbaugh, E. R. Banta, M. C. Hill, M. G. McDonald, MODFLOW-2000, The US Geological Survey Modular GroundWater Model – User Guide to Modularization Concepts and the Ground-Water Flow Process 2000 (US Geological Survey: Reston, VA). [16] C. Zheng, P. P. Wang, MT3DMS: a Modular Three-Dimensional Multispecies Transport Model for Simulation of Advection, Dispersion, and Chemical Reactions of Contaminants in Groundwater Systems; Documentation and User’s Guide. Alabama University report. Report SERDP-99 1999.

Acknowledgements This research was funded by the InnovaSol Foundation. The authors thank Marian Montbrun for assistance in conducting several of the experiments.

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