Hydraulic Displacement Of Dense Nonaqueous Phase Liquids

Abstract

Hydraulic displacement is a mass removal technology that involves recovering dense nonaqueous phase liquids (DNAPLs) from either vertical wells or horizontal drains. Hydraulic displacement is capable of removing significant quantities of pooled DNAPL from the subsurface in either porous or fractured media, but it is not suitable for the removal of residual DNAPL. Hydraulic displacement can stabilize a source zone: DNAPL pools are removed, thereby reducing the potential for future undesirable mobilization of DNAPL during drilling, hydraulic testing and construction of subsurface barriers.In addition to mass removal and source zone stabilization, hydraulic displacement is a means of preconditioning a source zone for implementing mass transfer based remediation techniques. Technologies such as enhanced in situ bioremediation (EISB), surfactant flushing, cosolvent flushing, and in situ chemical oxidation (ISCO) rely upon mass transfer of components from the DNAPL to groundwater. The rate of mass transfer from pooled DNAPL is small compared to that from residual DNAPL. This stems from both the lower specific DNAPL surface area for pools compared to the residual and from the lower relative permeability to water within a pool compared to within a zone of residual DNAPL. Preconditioning a source zone with hydraulic displacement will allow for more efficient and cost effective application of subsequent, mass transfer based technologies.Hydraulic displacement is also referred to as waterflooding (Craig, 1971;Willhite, 1986) and dual phase extraction (Gerhard et al., 2001). The technology was first developed in the petroleum industry as a means of recovering oil from reservoirs. At DNAPL sites, hydraulic displacement can be implemented in several ways. The simplest implementation is to pump DNAPL from any well in which it has accumulated. This will lead to a gradual reduction of DNAPL saturations within the formation adjacent to the well. The rate of DNAPL recovery can be enhanced by simultaneously extracting water and DNAPL from the well (the so-called dual phase or multiphase extraction). Water extraction results in an increased hydraulic gradient across the DNAPL pool in the formation, which in turn results in a favorable capillary pressure gradient facilitating faster rates of DNAPL migration to the well. The hydraulic gradient across DNAPL pools in the formation can be further increased by injecting water into wells around the perimeter of the source zone. The advantages of water injection and withdrawal include faster rates of DNAPL recovery and ability to mobilize pools that were not initially in contact with recovery wells or drains. A disadvantage of water injection and withdrawal is that any extracted water may need to be treated prior to either reinjection or disposal.