API Publication 4760
LNAPL Distribution and Recovery - Volume 1
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Overview
This document provides background information necessary to characterize the behavior of LNAPL in porous media with regard to performance of LNAPL liquid recovery technologies. The scope of information is selected to support model assumptions and development for the API LNAPL Distribution and Recovery Model (LDRM) which simulates the performance of proven hydraulic technologies for recovering free-product petroleum liquid releases to groundwater. This manuscript (Volume 1) and its companion manuscript (Volume 2) supersede API Publication Number 4682 and 4729, and document the LDRM software models for design and analysis of liquid free-product recovery systems using single- and dual-pump wells, vacuum-enhanced wells, skimmer wells, and trenches.
Contents
Section 1 The scenario-based models for recovery wells and trenches are described in Section 1.
Section 2 provides necessary background information for characterizing the vertical distribution of LNAPL located near the water table under conditions of vertical equilibrium. Capillarity, capillary pressure curves, and LNAPL residual saturation are discussed. The capillary pressure curve model presented by van Genuchten is presented, along with scaling relationships that allow the model representation to be applied with different multiphase fluid systems. Hubbert’s relationships for forces in multiphase fluid systems are presented. These are important in understanding how water-enhanced and vacuum-enhanced recovery systems create hydraulic gradient within the LNAPL phase causing its movement to a well or trench. The relationship between LNAPL thickness in a monitoring well and fluid pressure and capillary pressure within a formation is discussed in some detail. This relationship, when combined with a capillary pressure curve model, allow one to estimate LNAPL accumulations within the porous medium from monitoring well thickness measurements. Significant parameters are identified. Calculation of LNAPL specific volume and LNAPL recoverable volume as a function of LNAPL thickness in a well is discussed.
Section 3 concerns possible LNAPL movement. Darcy’s law is presented for LNAPL flow, and the Burdine and Mualem LNAPL relative permeability models are discussed. The effect of vertical hydraulic gradient in fine-grain soil on LNAPL saturation is described, and the critical vertical gradient at which LNAPL is displaced to accumulate beneath fine-grain soil is identified. Potential lateral migration of LNAPL is discussed. For lateral migration into pristine soils, capillary pressure curve models that include a finite displacement pressure should be used, and it is shown that LNAPL plumes are stable towards lateral spreading (an LNAPL plume will stop spreading even though LNAPL has a positive head). The vertical distribution of LNAPL mobility ratio is examined to show LNAPL is much more mobile in the upper part of the capillary fringe than groundwater. The LNAPL-layer transmissibility is introduced to calculate the LNAPL-layer volume flux. The lateral flow equations for LNAPL to wells and trenches are developed for water-enhanced and vacuum-enhanced systems, and for skimmer wells.
Section 4 shows how the continuity principle applied with regions of capture can be used, when combined with the LNAPL recoverable volume and transmissibility functions, to predict performance of LNAPL liquid recovery systems. Model parameterization and integration are discussed, and the basic equations of LDRM are presented.
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