In Situ Characterization Of Dense Non Aqueous Phase Liquids Uusing Partitioning Tracers 1998 Annual Progress Report

Major advances have been made during the past year in research on interwell partitioning tracers tests (PITTs). These advances include: (1) progress on the inverse problem of how to estimate the three-dimensional distribution of NAPL in aquifers from the tracer data, (2) the first ever partitioning tracer experiments in dual porosity media, (3) the first modeling of partitioning tracers in dual porosity media, (4) experiments with complex NAPLs such as coal tar, (5) the development of an accurate and simple method to predict partition coefficients using the equivalent alkane carbon number approach, (6) partitioning tracer experiments in large model aquifers with permeability layers, (7) the first ever analysis of partitioning tracer data to estimate the change in composition of a NAPL before and after remediation, (8) the first ever analysis of partitioning tracer data after a field demonstration of surfactant foam to remediate NAPL, and (9) experiments at elevated temperatures. The authors have developed a new analytic approach that has several advantages over existing approaches for inversion of tracer data. First, the technique utilizes an extremely efficient three-dimensional multiphase streamline simulator as a forward model. Second, the parameter sensitivities are formulated in terms of one-dimensional integrals of analytic functions along the streamlines. Thus, the computation of sensitivities for all model parameters requires only a single simulation run to construct the velocity field and generate the streamlines. The inversion of tracer data is then performed using a two-step iterative linearization that involves first lining-up the breakthrough times at the producing wells and then matching the production history. Their approach follows from an analogy between streamlines and ray tracing in seismology. The inverse method is analogous to seismic waveform inversion and thus, allows them to utilize efficient methods from geophysical imaging. The new approach has been applied for estimating permeability distribution based on conservative tracer tests (D.W. Vasco and A. Datta-Gupta: ''Asymptotic Solutions for Solute Transport: A Formalism for Tracer Tomography, '''' Submitted to Water Resources Research, December 1997), and is currently being extended for analysis of partitioning tracer data.