Laboratory experiments on DNAPL gravity fingering in water-saturated porous media

International audienceLaboratory experiments were carried out at the Darcy scale to investigate the gravity-driven fingering phenomenon of immiscible two-phase flow of water and a dense nonaqueous-phase liquid (DNAPL) such as trichloroethylene (TCE). Rate-controlled displacement experiments were performed on a homogenous sand-filled column under various displacement conditions. Several system parameters (e.g. flow rate, flow mode (upward flow, downward flow) and mean grain-size diameter of the porous medium) were varied in the experimental programme. Optical fiber sensors were developed to quantify the spatial distribution of the advancing displacement front in a given control section of the experimental device. Following each experiment, multi-point measurements of the remaining TCE saturation were obtained by insitu soil sampling. The resulting DNAPL distribution was heterogeneous even though the medium was homogeneous sand. Higher DNAPL injection rates and lower medium permeability both reduced gravity fingering. This is because viscous forces stabilize the advancing front with pressure gradients increasing as function of the injection rate and decreasing as function of the permeability. Average residual TCE saturations obtained by mass-balance in the experiment after a complete drainage-imbibition cycle were influenced by the mean grain-size diameter of the porous medium but were not affected by the flow mode of the primary drainage process

Étude expérimentale et numérique de la migration de polluants non miscibles dans un milieu poreux sature a l'échelle de Darcy

Le processus de déplacement vertical d'un fluide non miscible et plus dense que l'eau (DNAPL) en milieu poreux saturé a été étudié à l'échelle de Darcy. Les expériences sur colonnes de laboratoire ont été conduites en utilisant deux modes d'écoulement : vThe immiscible displacement processes of a dense non-aqueous phase liquid (DNAPL) were studied in a sand-filled column. The considered pollutant is the Trichloroethylene, volatile and soluble chlorinated solvent, usually found in soils and ground water i

The ghost component of the mass balances at the Critical Zone scale: the chemical reactivity of immobile water

International audienceThe critical zone (CZ) is characterized by the duality between mass transport processes such as diffusion and flow of gases, fluids, and solids and localized bio-geochemical interactions especially linked to the "immobile" capillary/adsorbed water. Open questions are towards the role of capillarity onto reactive mechanisms: is there a geochemical signature of capillary water that can change the bio-geochemical balance? The research efforts focused on this issue to develop modeling tools integrating capillary effects at the soil-profile or CZ scale. Water suction gains geochemical significance when ranging from 20 to 200 MPa, meaning high tension and low amount of stretched water with its specific thermophysical properties (Mercury and Tardy, 2001; Mercury et al., 2003; 2004; Pettenati et al., 2008). Therefore, our interest is directed to the dry end of the water retention curve (WRC). The recent model from Silva and Grifoll (2007) proposed a full-range soil-water retention functions, extending the description to the adsorbed films down to the monolayer thickness. At this stage, it becomes possible to evaluate the role of both capillary pockets and adsorbed films at all water content in the geochemical dynamics of non-saturated soils. We developed from that point by fitting the WRC not through a continuous porous network, but through a decomposition into two porous domains (immobile/mobile domains), each with its own potential-water content law. This amounts to treat the WRC according to an intrinsic dual porosity scheme, and make easier to involve chemical effects at each potential-water content couple. A simple test simulation is developed with calcite rock kinetically interacting with immobile and mobile water, themselves connected by diffusive and advective gradients in the limits of the immobile-mobile contact area. The reactive transport simulations are run with HP1 (Jacques and Šimůnek, 2005). The mass balance exporting toward groundwater is calculated with and without capillary effects, according to a wide range of conditions (climate, recharge rate, water potential in each domain, etc). It demonstrates how the capillary component acts on the chemical dynamics at the CZ scale. Keywords: critical zone, unsaturated zone, capillarity, reactive transport modeling,"stretched water"

Approche expérimentale et numérique des instabilités de déplacement de DNAPL en milieu poreux saturé à l’échelle de Darcy

Les recherches portent sur une quantification des instabilités de déplacement d’un DNAPL en milieu poreux homogène et saturé. L’approche expérimentale repose sur la mesure des vitesses d’arrivées du DNAPL dans une colonne de laboratoire à différents points d’une section de contrôle au moyen des fibres optiques et sur la cartographie in situ des saturations locales. La simulation numérique du déplacement du front de polluant est abordée par une approche de type réseau des pores et capillaires

Pore scale modelling of DNAPL migration in a water–saturated porous medium

International audienceA numerical simulator based on the discrete network model approach has been developed 12 to simulate drainage processes in a water-saturated porous medium. To verify the predictive 13 potential of the approach to simulate the unstable migration of adense nonaqueousphase liquid 14 (DNAPL) at the pore scale, the numerical model was applied to laboratory experiments 15 conducted on a sand-filled column. The parameters relative to pore body size and pore throat 16 size used in the construction of the equivalent network were derived from discrete grain-size 17 distribution of the real porous medium. The observed water retention curve (WRC) was first 18 simulated by desaturation of the network model.The good agreement of the modelled WRC 19 with the experimental one highlights that the applied approach reproduces the main 20 characteristics of the real pore space. The numerical model was then applied to rate controlled 21 experiments performed on a homogenous sand-filled column to study the gravity-driven 22 fingering phenomenon of immiscible two-phase flow of water and a DNAPL. The numerical 23 results match within 10% based on the standard deviation with the experiments. They correctly 24 reproduce the effect of several system parameters, such as flow mode (upward flow and 25 2 downward flow) and the flow rate, on the stability of the water/DNAPL front in a saturated 26 porous medium