Analytical solutions for NAPL recovery using complex multiple well systems

Abstract

Groundwater contamination with immiscible fluids known as non-aqueous phase liquids (NAPL) can lead to the formation of a separate phase in groundwater. The mobile part of NAPL may accumulate in the saturated zone as a continuous free-phase lens above the groundwater table (Light NAPL) or on the bottom of the impermeable ground layer (Dense NAPL). Solutions for modeling free-hydrocarbon recovery in steady-state flow using skimmer-, single- and dual pump wells, which are analyzed in technical literature, are limited to radial flow toward a single well. This paper extends these solutions through a new generalized calculation method for recovery systems using multiple wells, drains, etc. Simplified assumptions allow the introduction and use of potential functions for a mathematical representation of the coupled stratified immiscible fluid flow that satisfy partial differential equations with coupling conditions on the interface. The solutions are for successive steady-state flow based on the assumption of vertical equilibrium. The proposed method allows the estimation of the discharge or heads of the multiple well recovery system. Superimposing the effects of complex arrangements of recovery wells may influence changes on the free surface and the interface (NAPL/water) when NAPL is pumped alone or simultaneously with water. A coupling parameter is introduced that can be used to adjust and optimize the discharge rate of both phases to avoid smearing of NAPL below the water table. The proposed calculation methodology may improve the efficiency of the current state-of-the-art solutions to present a capable tool for designing complex clean-up plants consisting of a variable array of wells, and thereby offer a first step for the calibration of complex models.