Development and Evaluation of Partitioning Interwell Tracer Test Technology for Detection of Non-Aqueous Phase Liquids in Fractured Media

by
Neil Edward Deeds, Ph.D.

University of Texas at Austin, 1999
Supervisors: Gary A. Pope
Daene C. McKinney

The objective of this research was to evaluate the partitioning interwell tracer test (PITT) for characterizing non-aqueous phase liquids (NAPLs) in fractured media and to provide new experimental and theoretical research to help adapt the technology to fractured media.

Tracer mass transfer between water and NAPL in porous media was evaluated using an existing aqueous phase mass transfer coefficient correlation. The NAPL phase mass transfer resistance was shown to be negligible for typical porous media. A 1-D tracer transport model showed that Damkohler numbers greater than approximately three resulted in an adequate PITT NAPL saturation estimate. The same mass transfer correlation was implemented in UTCHEM, and 3-D field scale PITT simulations showed that mass transfer limitations had virtually no effect on the tracer response at the field scale.

A 1-D parallel fracture model was developed and used to study the sensitivity of the PITT saturation estimate in fractured media to both the Damkohler number and the matrix number. Matrix diffusion was shown to have a significant and adverse impact on the results for some cases, due in part to the increased retardation of the more diffusive conservative tracer compared to the less diffusive partitioning tracer.

Single fracture experiments were completed in Berea core, both with and without NAPL. Matrix diffusion was shown to be the dominant process for causing retardation and increased tailing in the tracer response curves. An increased flow rate decreased the effects of matrix diffusion. Partitioning tracer response curves in a NAPL saturated single-fracture core showed effects of both water-NAPL partitioning and matrix diffusion.

The dual porosity feature of UTCHEM was used to simulate field scale NAPL spills and PITTS. The UTCHEM results indicated that for some types of fractured media, a PITT would fail to provide an accurate estimate of the NAPL volume. Increasing the flow rate of the PITT improved the NAPL volume estimate. No tracer mass transfer correlation was available for the fractured media simulations, so no upper limit to the flow rate could be established. In general, PITT technology was shown to effective in certain types of fractured media given a well-engineered design.

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