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

Laboratory 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 in situ 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. (C) 2011 Elsevier Ltd. All rights reserved.