Tuesday, April 30, 2013: 1:35 p.m.
Regency West 6 (Hyatt Regency San Antonio)
A multi-phase vacuum extraction system is used for LNAPL recovery at an industrial site consisting of a heterogeneous urban fill. The system shows limitations due to particular soil properties: low permeability soil layers (K < 10-6 m/s), high content in organic matter (foc =1.6%), carbonate minerals (up to 30%-mass), and high water retention in the dewatered horizon. A research project was carried out to use soil washing to improve overall LNAPL recovery of the vacuum extraction system, through three potential application scenarios. The first application consisted in the use of a low-concentration surfactant solutions (<0.5%-mass) to improve water drainage in the dewatered horizon by lowering the water surface tension by 50%. However, the use of two types of surfactants did not modify the shape of the soil water retention curve, suggesting that water was retained by forces other than capillarity, such as adsorption. The second scenario consisted in the use of a low-concentration surfactant solution (<0.5%-mass) to enhance the mobility of free-LNAPL phase and residual LNAPL trapped in the saturated zone by lowering the LNAPL-pore fluid interfacial tension (IFT) by a factor of 200. However, a field pilot test showed that even with such an IFT reduction, capillary forces controlling LNAPL trapping remained higher than the viscous and gravitational forces acting to displace them. The in situ increase of viscous forces during soil washing was limited by the unconfined nature of the aquifer. Finally, a third scenario involving miscible flooding with a high concentration alcohol solution was tested inside a 3D physical model (4 m3 sand box) simulating a 5-spot injection/extraction pattern. Results indicated that miscible flooding is possible for that site, even though improvements of the sweep efficiency of the alcohol solution using a mobility control technique (such as a polymer) would be required prior to a field application.
