Partitioning Interwell Tracer Test Research Articles

Fluorescent Dyes as Partitioning Tracers for the Estimation of NAPL‐Mass Saturation in Porous Media

AbstractAccurate estimation of the nonaqueous phase liquids (NAPLs) saturation such as chlorinated organic compounds (COCs) in aquifers is crucial for the proper remediation of contaminated groundwater. A combination of conservative and partitioning tracers (PTs) are commonly used to assess NAPL saturations in the subsurface at COC release sites, using the partitioning interwell tracer test (PITT). In this study, five fluorescent dyes were assessed as PTs to estimate the saturation of octanol and 3 COC NAPLs in soil columns. PT experiments required an initial assessment of both partitioning (NAPL/water and octanol/water) and linear free‐energy relations. The predictability of the partition coefficients was correlated to the pH of the two‐phase fluids for both systems (NAPL/water and octanol/water). The COC NAPLs were acidic and some PTs with acid‐base properties, like fluorescein, are easily influenced by pH. The PITT experiments were performed in a column packed with glass beads, using rhodamine WT as PT because of its particular specificity for the complex mixture of NAPLs and sodium chloride as the inert tracer. Breakthrough curves of rhodamine WT were examined to estimate the saturation of a NAPL made of a complex mixture of COCs. The DNAPL residual saturation estimation accuracy was sensitive to both pH variations and the water velocity. The latter was represented by an exponential function which resulted from non‐equilibrium measurements, heterogeneous sweeping of the contaminated sample, and redistribution of the NAPL droplets in the medium.

Environmental risk assessment of inter-well partitioning tracer compounds shortlisted for the offshore oil and gas industry

Quantifying residual oil saturation (SOR) in the inter-well region of oil and gas reservoirs is key for successfully implementing EOR solutions. Partitioning inter-well tracer tests (PITTs) has become a common method for quantifying SOR. A new group of seven chemicals – pyridine, 2,3-dimethyl pyrazine, 2,6-dimethyl pyrazine, 4-methoxybenzyl alcohol, 3,4-dimethoxybenzyl alcohol, 4-chlorobenzyl alcohol, and 2,6-dichlorobenzyl alcohol – have been proposed as potential partitioning tracers for quantifying SOR. Using these tracers can lead to their environmental release in the marine environment through produced water discharges, with currently limited knowledge on impacts in the marine ecosystem. The primary objective of the present study is to assess the environmental risk of discharging the tracer compounds in the marine environment. We investigated the fate and effect of these tracers in the marine environment. Biodegradability in seawater was measured to understand the fate of tracers in the marine environment. The acute toxicity of tracers was measured in terms of the percent cell viability of a rainbow trout gill cell line (RTgill-W1) and growth inhibition of the algae Skeletonema costatum. The ecotoxicological information obtained from these experiments was used in the dynamic risk and effects assessment model (DREAM) to calculate the tracers’ contribution to the environmental impact factor (EIF). The results from the DREAM simulations suggest no contribution towards EIF values from any of the tracers at the expected back-produced concentrations. Results from simulations at higher concentrations suggest that both pyrazines have the lowest environmental risk, followed by 3,4-dimethoxybenzyl alcohol, 4-methoxybenzyl alcohol, and pyridine; while both chlorobenzyl alcohols show the highest environmental risk.

Variation of the partition coefficient of phase-partitioning compounds between hydrocarbon and aqueous phases: an experimental study

Many non-ionic chemical compounds will form real solutions in equilibrium between two immiscible phases in contact. The partition coefficient (K) is defined as the quotient between the equilibrium concentration of the substance in the hydrocarbon and the aqueous phase. The reversible partition of a compound between hydrocarbon and aqueous phases is the basis of the partitioning inter-well tracer test (PITT). PITTs are of high interest for the characterization of oil reservoirs, in hydrogeology for assessment of the contamination of soils by non-aqueous phase liquids (NAPLs), and in process technology. The K value of substance is influenced by the actual chemical and physical conditions of the system where it will be used as phase-partitioning tracer. Thus, it is important to evaluate the extent of variation that the K value can exhibit under different relevant conditions.In the present document, we report the methodology and findings from the experimental determination of the K values of 4-methoxybenzyl alcohol, 3,4-dimethoxybenzyl alcohol, 4-chlorobenzyl alcohol, 2,6-dichlorobenzyl alcohol, pyridine, 2,3-dimethylpyrazine and 2,6-dimethylpyrazine between different hydrocarbon and aqueous phases. These 7 compounds were previously identified as interesting PITT tracer candidates. Individual K-values were determined under different temperatures, compositions of the hydrocarbon phase (synthetic mixtures of toluene, isooctane, and 1-octanol and 5 real crude oils), and different ionic strengths (I) and ionic compositions of the aqueous phase. The reversibility of the partitioning phenomena was also evaluated. The composition of the hydrocarbon phase and the ionic strength of the water phase were found to influence the K values of all seven compounds. Results suggest that the substitution of monovalent ions with divalent ions in the aqueous phase, keeping the ionic strength constant, does not influence the K values. Temperature effects on the K-values are always visible for 3,4-dimethoxybenzyl alcohol, 2,6-dichlorobenzyl alcohol, pyridine, 2,3-dimethylpyrazine and 2,6-dimethylpyrazine. Dependent on the hydrocarbon phase composition, temperature also influences the K-values for 4-methoxybenzyl alcohol and 4-chlorobenzyl alcohol.

Integrating hydraulic tomography, electrical resistivity tomography, and partitioning interwell tracer test datasets to improve identification of pool-dominated DNAPL source zone architecture

High-resolution characterization of complex dense non-aqueous phase liquid (DNAPL) contaminated sites is crucial for developing effective remediation strategies. The DNAPL source zone is usually characterized by hydraulic/partitioning tracer tomography (HPTT). However, the HPTT method may fail to capture the highly saturated pool-dominated DNAPL source zone architecture (SZA), because partitioning tracers tend to bypass the low-permeability zones where the pool DNAPL accumulates, resulting in a low-resolution DNAPL estimation. With a limited number of measurements, the estimation errors may be significant. To overcome these difficulties, time-lapse electrical resistivity tomography (ERT) was integrated with the partitioning interwell tracer test (PITT) and hydraulic tomography (HT) to characterize the pool-dominated DNAPL SZA. Herein, we proposed an iterative joint inversion framework coupling the multiphase flow model with the ERT forward model to estimate the heterogeneous permeability distribution and DNAPL SZA. Under this framework, permeability was estimated using the hydraulic head data from HT in stage 1, and the DNAPL SZA was subsequently estimated by assimilating both the PITT and ERT observations in stage 2. The permeability estimated from stage 1 was used as prior information for stage 2, and the DNAPL saturation estimated from stage 2 was served as prior information for stage 1 in the next loop to form an iterative loop to improve the estimation of both permeability and DNAPL SZA. The iterative joint inversion framework was evaluated in two numerical experiments with different heterogeneous structures by assimilating multi-source datasets, including hydraulic head, partitioning interwell tracer concentration, and electrical resistivity. Results show that with limited measurements of HPTT method, one can roughly capture the DNAPL distribution, missing the fine structure of the DNAPL SZA. In contrast, by incorporating multi-source datasets, the heterogeneous permeability and DNAPL SZA can be reconstructed with a higher resolution. Furthermore, the inversion accuracy of the DNAPL SZA improves progressively as the iteration proceeds, which demonstrates the advantage of utilizing complementary information from permeability and DNAPL distribution through the iteration framework. Comparing with the results without loop iteration, the estimation error is reduced by 17.3% for permeability and 8.6% for DNAPL saturation in Experiment 1; by 14.7% for permeability and 11.2% for DNAPL saturation in Experiment 2 through the iterative framework. To further evaluate our framework, we preformed the prediction of the depletion process of the DNAPL source zone and plume based on the estimated DNAPL SZA. Results show that using the iterative framework, the prediction of the SZA depletion is greatly improved, i.e., the estimation error of the dissolved downstream plume from the DNAPL source zone after 3 years is reduced by 20.9% in Experiment 1, and by 43.2% in Experiment 2, respectively, through the iterative framework. This significant improvement is because the iterative method can better capture the spread of DNAPL pool.

Stability assessment of PITT tracer candidate compounds – The case of pyrazines

The determination of the residual oil saturation (SOR) in the volumes swept between injector/producer well pairs is a parameter of major importance for the design and/or evaluation of IOR projects. The number of IOR projects is increasing with the growing number of mature oilfields. The partitioning inter-well tracer test (PITT) is used to measure SOR in waterflooded reservoirs and relies on the use of partitioning and passive tracers. Many PITT were unsuccessful in the past due to a poor knowledge about the compounds used as tracers and to date, very few compounds were developed for this application. New partitioning tracers are needed, and a systematic qualification process is necessary to reduce the risk of costly unsuccessful field tests. In the present document we report and discuss the findings from stability experiments performed on 2 alkylpyrazines, 1 methoxypyrazine, and 2 halogenated pyrazines under investigation for use as oil/water partitioning tracers for determination of the SOR in the inter-well region of water flooded oil reservoirs.The stability of 2,3-dimethylpyrazine, 2,6-dimethylpyrazine, 2-methoxypyrazine, 2-chloropyrazine, and 2-fluoropyrazine in brine solutions was evaluated in batch experiments at temperatures ranging from 25 °C to 150 °C for 12 weeks, at 3 different pH values and in the presence of typical oil reservoir rock materials (sandstone, carbonate rock, and clay). Results suggest that 2,3-dimethylpyrazine and 2,6-dimethylpyrazine possess the required stability and absence of significant interaction with the rock substrates. 2-chloropyrazine and 2-fluoropyrazine degrade with a temperature and pH dependent rate but are not influenced by the rock materials. 2-methoxypyrazine exhibits temperature dependent degradation in the absence of any rock substrate and in the presence of sandstone and carbonate rock. A strong interaction was observed between this compound and the clay material (kaolinite) which is responsible for dramatically increasing its degradation rate.

Stability assessment of PITT tracer candidate compounds: The case of benzyl alcohols

The selection of compounds used as inter-well tracers has traditionally been done taking into consideration the goal of the test and the ease in analyzing the chemicals used. This often led to improper selection of the tracers and insufficient knowledge about their behavior under typical reservoir conditions, resulting in several unsuccessful inter-well tracer tests. One of the critical characteristics of any conservative tracer is its stability under various reservoir conditions. In this document we present the study methodology and the findings from stability experiments carried out on 5 benzyl alcohols investigated as oil/water partitioning tracers for partitioning inter-well tracer tests (PITT). This is the first of three documents reporting such studies on 3 families of chemicals.A PITT consists of the simultaneous injection of one or more mass-conservative passive and phase-partitioning (one type of active) tracers. PITT is primarily conceived for mature water-flooded oil fields and measures the residual (or even remaining) oil saturation (SOR) in the swept volumes between wells. Knowledge of SOR may be used to identify EOR targets, assess efficiency of EOR operations, and the efficiency of volumetric sweep between wells. This is important information, in particular for the increasing number of mature oil-fields where EOR projects are under consideration and/or development. A thorough knowledge about the static and dynamic properties of the tracers under reservoir conditions is required both to ensure successful field tests, and also to increase accuracy of the data obtained from them.4-Chlorobenzyl alcohol, 2,6-Dichlorobenzyl alcohol, 4-Methoxybenzyl alcohol, 3,4-Dimethoxybenzyl alcohol, and 4-Hydroxybenzyl alcohol were tested in brine under simulated reservoir conditions with temperatures up to 150 °C during 12 weeks, at different pH values, and in the presence of typical sedimentary basin rock materials, sandstone, carbonate rock (limestone), and clay (kaolinite). 4-Chlorobenzyl alcohol and 2,6-Dichlorobenzyl alcohol were found to be fully stable at all tested conditions. 4-Methoxybenzyl alcohol and 3,4-Dimethoxybenzyl alcohol exhibit thermally induced degradation above 125 °C, but may still be considered for use below this temperature. 4-Hydroxybenzyl alcohol exhibits thermally induced degradation which was found to follow a first order kinetics. The apparent activation energy and pre-exponential constant in the Arrhenius law for the degradation reaction were determined and a kinetic model built. This opens the possibility for use of 4-Hydroxybenzyl alcohol as a non-conservative reservoir tracer to measure temperature or detect thermal fronts.

Pushing the envelope of residual oil measurement: A field case study of a new class of inter-well chemical tracers

The success of any improved oil recovery (IOR) project is largely dependent on how much oil is remaining to be mobilized within the targeted area of the partially depleted or mature reservoir. Partitioning tracers are generally used to measure residual oil saturation (Sor) or remaining oil saturation (ROS) in the near wellbore region via a single well chemical tracer test (SWCTT) or in an inter-well region via a partitioning inter-well tracer test (PITT). There is a limited repertoire of nonradioactive and environmentally friendly inter-well partitioning tracers for measuring ROS. A new class of environmentally friendly partitioning tracers was field tested, in a giant carbonate reservoir undergoing peripheral waterflood, for measuring ROS in inter-well regions in a depleted area.The new partitioning tracers were qualified via laboratory experiments and are deemed to be very stable at reservoir conditions (213 °F and a salinity range of 60–200 kppm). The field pilot was conducted concurrently with a set of non-partitioning inter-well chemical tracer test (IWCTT) to determine reservoir connectivity, water breakthrough times, and injector-to-producer pair communication in an area selected for an IOR/EOR field pilot. An elaborate sampling and analysis program was carried out over a period of 30 months.This paper reviews the complete design and implementation of the test, operational issues, and the analyses and interpretation of the results. The breakthrough times of the passive and partitioning tracers are reported, and inter-well connectivity between the paired and cross-paired injectors and producers are analyzed. The ROS measured by a majority of the novel tracers is comparable to the saturations obtained via SWCTT, core and log derived saturations.The combination of conventional IWCTT and the novel partitioning tracers via PITT has been very useful in analyzing well interconnectivity, understanding the reservoir dynamics and quantifying remaining oil saturation distribution in the reservoir. This has led to better reservoir description and an improved dynamic simulation model.

Use of Partitioning Tracers To Estimate Oil-Saturation Distribution

This article, written by JPT Technology Editor Chris Carpenter, contains highlights of paper SPE 179655, “Use of Partitioning Tracers To Estimate Oil-Saturation Distribution in Heterogeneous Reservoirs,” by R.M. Dean, D.L. Walker, V. Dwarakanath, T. Malik, and K. Spilker, Chevron, prepared for the 2016 SPE Improved Oil Recovery Conference, Tulsa, 11–13 April. The paper has not been peer reviewed. Partitioning interwell tracer tests (PITTs) have been used to estimate remaining oil saturations (ROSs) during waterflooding. Compared with core tests, well logs, and single-well tracer tests, PITTs sample a much larger representative elemental volume (REV) and provide interwell estimates of remaining oil saturation. In this paper, the authors present the information gained from conducting a polymer PITT and the saturation estimated during the PITT. The polymer PITT allows characterization of polymer-flood efficiency and is a useful tool in polymer-flood evaluations in heterogeneous reservoirs. Introduction The difference in arrival time between conservative tracers has long been used to estimate the volume of oil remaining in a reservoir after tracer tests. The method-of-moments solution to calculate ROS from mean arrival times of conservative and partitioning tracers is based upon single-phase flow. When large changes of saturations occur in between the initial conservative and partitioning tracers, the volume calculated is hard to interpret. Use of equations with single-phase-flow assumptions makes it a challenge to understand the correlation between the volume calculated and its corresponding time. The method of moments has been used to calculate oil volumes and saturations. With the method of moments, the mean residence time of the tracer for a given slug duration is calculated from the concentration of tracer in the produced effluent. Oil volume is normally related to the difference in mean residence volume between a partitioning tracer and a conservative tracer, although two partitioning tracers can be related. The remaining oil volume can be calculated from the mean residence volumes of the conservative and partitioning tracers. When oil cut and saturation change are large, the volume becomes difficult to correct. Often, correcting for production on the basis of the mean residence time of the first tracer gives the right value if oil production is small, but this does not give the correct answer if oil cut is high. Conducting a PITT with a favorable mobility ratio helps alleviate the problem of high oil cut during tracer production because it allows the partitioning tracer to contact oil at residual saturation. In order to calculate the volume of oil and saturation more accurately during an active waterflood with mobile oil, a technique is required that estimates oil remaining behind the waterflood. To achieve that objective, modifications to the method of moments, known as the residence-time-distribution- analysis (RTDA) method, were made in order to account for two-phase flow. In this new derivation, oil and water can be produced simultaneously and are accounted for by the fractional flow of each phase.

지하수 유동 방향에 대한 관정배열이 분배추적자 시험에 미치는 영향 분석

Partitioning interwell tracer test (PITT) is a method to quantify and qualify a site contaminated with NAPLs (NonAqueous Phase Liquids). Analytical description of PITT assumes that the injection-pumping well pair is on the line of the ambient groundwater flow direction, but the test-well pair could frequently be off the line in a real field site, which could be an erroneous factor in analyzing PITT data. The purpose of this work is to study the influence of the angle of the testwell pair on the ambient groundwater flow direction based on the result from PITT. From the experiments, it was found that the obliqueness of the test-well pair to the ambient groundwater flow direction could affect the tracer test resulting in a decreased NAPL estimation efficiency. In case of an oblique arrangement of the test-well pair to the ambient flow direction, it was found that the injection of a chase fluid could enhance the estimation efficiency. An increase of the pumping rate could enhance the recovery rate but it cannot be said that a high pumping rate can increase the test efficiency because a high pumping rate cannot give partitioning tracers enough time to partition into NAPLs. The results have a implication that because the arrangement of the test-well pair is a controlling factor in performing and interpreting PITT in the field in addition to the known factors such as heterogeneity and the source zone architecture, flow direction should be seriously considered in arranging test-ell pair.

The use of mass depletion–mass flux reduction relationships during pumping to determine source zone mass of a reactive brominated-solvent DNAPL

Mass depletion-mass flux relationships usually applied to a groundwater plume were established at field scale for groundwater pumped from within the source zone of a dense non-aqueous phase liquid (DNAPL). These were used as part of multiple lines of evidence in establishing the DNAPL source mass and architecture. Simplified source mass-dissolved concentration models including those described by exponential, power, and error functions as well as a rational mass equation based on the equilibrium stream tube approach were fitted to data from 285 days of source zone pumping (SZP) from a single well which removed 152 kg of dissolved organics from a multi-component, reactive brominated solvent DNAPL. The total molar concentration of the source compound, tetrabromoethane and its daughter products was used as a single measure of contaminant concentration to relate to source mass. A partitioning inter-well tracer test (PITT) conducted prior to the SZP provided estimates of groundwater travel times, enabling parameterisation of the models. After accounting for capture of the down-gradient dissolved plume, all models provided a good fit to the observed data. It was shown that differentiation between models would only emerge after appreciably more pumping from the source zone. The model fits were not particularly sensitive to the exponent parameters and variance of groundwater travel time. In addition, the multi-component nature of the DNAPL did not seem to affect the utility of the models for the period examined. Estimates of the DNAPL mass prior to the start of SZP from the models were greatest where the log of the variance of travel time was used explicitly in the source depletion models (mean 295kg) compared to where the associated power exponent and variance was fitted freely (mean 258 kg). The estimates of source mass were close to that of 220kg determined from the PITT. In addition to the PITT, multi-level groundwater sampling from within the source zone provided important supporting information for developing the conceptual model of the source zone. It is concluded that SZP may be an effective and relatively simple means for characterising DNAPL source zones.

Characterization of a heterogeneous DNAPL source zone in the Borden aquifer using partitioning and interfacial tracers: Residual morphologies and background sorption

A partitioning interwell tracer test (PITT) was performed in the Borden sand aquifer to characterize an aged heterogeneous DNAPL source zone. This zone evolved during 5 years of natural groundwater flow following the infiltration of 50 L chlorinated solvents DNAPL. To assess the lateral variability of remaining DNAPL mass and morphology, four sweepzones were analyzed. The low saturation residual nature of the source zone required correction of tracer breakthrough data for natural background sorption. Corrected estimates of the DNAPL percentage remaining (total 13.2–16.6%), average saturation (0.05–0.18%) and distribution across the sweepzones were in good agreement with previous findings based on detailed transect monitoring, core analyses and ground-penetrating radar reflection. Using a newly defined metric “average spherical radius equivalent (ASRE)”, sweepzone estimates of the average size of DNAPL presence indicated the dominance of single pore DNAPL blobs and suggested the absence of DNAPL pools. Tracer tests indicated that DNAPL presence in the most DNAPL depleted sweepzone was potentially overestimated due to increased sediment sorption by residualized Sudan IV that was added to the DNAPL infiltrate. As hydrophobic compounds are normally present in spent solvent DNAPL, this suggests that additional sorption needs to be considered when using PITTs to characterize aged DNAPL source zones.

Characterisation of a DNAPL source zone in a porous aquifer using the Partitioning Interwell Tracer Test and an inverse modelling approach

In this paper, we discuss the results of a Partitioning Interwell Tracer Test (PITT) performed in a large scale experiment with a well-defined TCE spill, and present a novel combined analytical–numerical inverse modelling approach using measured concentration profiles within a TCE plume to predict the distribution of the DNAPL in a virtual vertical plane of the source. The proposed inverse modelling approach assumes local thermodynamic equilibrium of the distribution of TCE between the NAPL phase and the aqueous phase and no decay or sorption of the dissolved TCE concentrations downstream of the spill area. The analytical part of the inverse modelling approach contains two steps. As a first step, the location of the contaminant in a virtual vertical plane of a porous medium is fixed by using measured concentration profiles and considering the dissolution of the organic phase under equilibrium conditions. In the second step, the volume of contaminant entrapped in the source cells is estimated. A multiphase advective–dispersive transport model is used in the final step to adjust the volumes quantified in the second step. The predictions are highly dependent on the quantity and quality of the data in space and time. From the PITT-breakthrough curves measured at the pumping well, a mean TCE saturation in the sweep zone of 0.0004 was derived, which is very low compared to that determined at the local scale. In a second analysis, tracer breakthrough curves available at measuring points placed closely downstream and upstream of the presumed source zone, were used to explain why the globally obtained DNAPL saturation was very low compared to the “real”, locally evaluated TCE saturations in the source zone. This was principally caused by the overall travel time compared to the short travel time of the tracers in the source zone. Another reason is that due to bypassing, only part of the volume of tracer injected had been in contact and had eventually interacted with the DNAPL. Furthermore, the quantified TCE volume was nearly 30% higher than the spilled volume; this agrees with the conclusions from other studies emphasizing that calculated volumes can overestimate the measured volumes, particularly in the case of an inhomogeneous distribution of the DNAPL within the soil. A good agreement of the measured and inversed concentration profiles was obtained, highlighting that it is possible to determine the length-averaged distribution of a residual pollution source from dissolved concentration profiles measured downstream of the source zone. The numerically obtained non-uniform distribution of DNAPL entrapped in the vertical plane of the source zone was experimentally confirmed by the TCE saturation values derived from PITT-breakthrough curves at measuring points located 0.75 m downstream of the source zone. However, the sensitivity study showed that the inverse modelling approach provided a rather non-unique solution. More data available may reduce the number of possible representations of the estimated source zone.

Intermediate‐Scale Investigation of Nonaqueous‐Phase Liquid Architecture on Partitioning Tracer Test Performance

Partitioning interwell tracer tests (PITTs) have been proposed as a tool to characterize source zones of waste sites with chemicals entrapped in the form of nonaqueous‐phase liquids (NAPLs). This technique has the potential to be applied to NAPL source characterization of both the vadose and saturated zones, but the recent applications have been primarily focused on dense NAPLs (DNAPLs) entrapped below the water table. This study presents an intermediate‐scale experiment to investigate the applicability of the technique to heterogeneous sites with complex NAPL entrapment architecture. Tracer experiments were conducted in an intermediate‐scale test tank (4.87 by 1.21 by 0.05 m) with heterogeneity defined using the geostatistical parameters of a spatially correlated random field. Tetrachloroethene (PCE), a DNAPL, was spilled into the heterogeneous packing and was allowed to distribute naturally, creating zones of both residual and pooled entrapment. The saturation distribution of the DNAPL was determined in situ using a gamma attenuation system. Once the saturation distribution was characterized, a set of tracer tests was conducted. Results of this study demonstrate how the inversion of the tracer breakthrough concentrations provided valuable information to establish the validity of the use of equilibrium partition coefficients in situations where tracer partitioning might be controlled by nonequilibrium behavior. While our previous work quantified the estimation error introduced by the assumption of equilibrium partitioning in tracer data analysis, this study demonstrated that more accurate NAPL estimates can be obtained using effective partition coefficients in combination with inverse modeling methods upscaled for more realistic heterogeneous settings.

Effects of Tracer Flow Velocity on Partitioning Interwell Tracer Test (PITT) for Estimating the Volume of Non-Aqueous Phase Liquids

Effects of Tracer Flow Velocity on Partitioning Interwell Tracer Test (PITT) for Estimating the Volume of Non-Aqueous Phase Liquids

Simulation and Performance Assessment of Partitioning Tracer Tests in Heterogeneous Aquifers

Characterization of dense non-aqueous phase liquid (DNAPL) in heterogeneous media represents a major challenge in the remediation process due to the complexity of DNAPL distribution in the subsurface. The partitioning interwell tracer test (PITT), which evaluates the relative transport of DNAPL-phase partitioning and conservative (i.e., non-partitioning) tracers, has recently been promoted as a way to quantify entrapped DNAPL mass in source zones. In some cases, the technique has been successfully applied to sites where DNAPL is present primarily at residual saturations. However, the effects of the geologic heterogeneity and DNAPL architecture on the performance and reliability of this technique have not yet been thoroughly examined. A systematic two-dimensional simulation study was conducted to evaluate the influence of DNAPL vertical distribution, well location, and geologic heterogeneity on the performance of the PITT for a total of 60 stochastic aquifer realizations. The aquifer hydraulic conductivity (K) distribution was defined using three different geostatistical parameter values describing the variance of the log K (\batchmode \documentclass[fleqn,10pt,legalpaper]{article} \usepackage{amssymb} \usepackage{amsfonts} \usepackage{amsmath} \pagestyle{empty} \begin{document} \({\sigma}^{2}_{ln k}\) \end{document} = [1.22, 0.75, 0.25]). A multiphase flow simulator was used to generate the source zone, allowing the DNAPL to distribute naturally. The tracer test simulations conducted using these various synthetic aquifers were specifically designed to quantify PITT performance for different degrees of heterogeneity. For cases where the aquifer is relatively heterogeneous and the DNAPL architecture is complex, PITT estimation efficiency averaged 48 percent (decreasing to 20 percent in certain scenarios) as a result of bypass flow and the limited tracer accessibility to lower hydraulic conductivity regions. While other studies have suggested the PITT technique may underestimate DNAPL saturation for these conditions, potential errors of this magnitude have not previously been reported. The results of this work underscore the importance of geological heterogeneity on PITT performance.

The Measurement of DNAPL in Low-Permeability Lenses within Alluvial Aquifers by Partitioning Tracers

Research Article| April 01, 2005 The Measurement of DNAPL in Low-Permeability Lenses within Alluvial Aquifers by Partitioning Tracers R. E. JACKSON; R. E. JACKSON 1INTERA Inc., P.O. Box 818, Niwot, CO 80544 Search for other works by this author on: GSW Google Scholar M. JIN M. JIN 2INTERA Inc., 9111A Research Blvd., Austin, TX 78758 Search for other works by this author on: GSW Google Scholar Environmental & Engineering Geoscience (2005) 11 (4): 405–412. https://doi.org/10.2113/11.4.405 Article history first online: 09 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation R. E. JACKSON, M. JIN; The Measurement of DNAPL in Low-Permeability Lenses within Alluvial Aquifers by Partitioning Tracers. Environmental & Engineering Geoscience 2005;; 11 (4): 405–412. doi: https://doi.org/10.2113/11.4.405 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyEnvironmental & Engineering Geoscience Search Advanced Search Abstract The partitioning interwell tracer test (PITT) was introduced into the practice of contaminant hydrogeology in 1994, since which time about 50 PITTs have been conducted in the field. Confirmation of results of vadose-zone and ground-water zone PITTs by subsequent field testing of aquifers in Utah and New Mexico have demonstrated the reliability of the technique for operational purposes, i.e., the measurement of average inter-well NAPL (non-aqueous phase liquid) saturations that provide meaningful values upon which to base remedial decisions. However, the use of the PITT has become increasingly infrequent, partly because it has attracted criticism concerning its reliability to quantify DNAPL (dense non-aqueous-phase liquid) either in low-permeability units surrounded by high-permeability materials or present in depressions at the base of aquifers. In the vadose zone, where gaseous partitioning tracers are used, the criticism maintains that the partitioning tracers will necessarily follow cleaned pathways created during soil vapor extraction and will thus bypass DNAPL zones. In the ground-water zone, the criticism is that it fails to detect DNAPL in low-permeability lenses either in the middle or at the base of aquifers, again because of bypassing. This paper presents numerical simulations of two laboratory experiments that demonstrate that the criticisms are incorrect and PITT will accurately detect residual NAPL volume when proper design and field implementation are undertaken. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Water Saturation Measurements by Gas Tracers in Unsaturated Porous Media—Effect of Mass Transfer Limitations

Water saturation is a key parameter in studies of pollutant transport in the vadose zone and an important factor controlling the rate of waste degradation in municipal solid waste landfills. The partitioning interwell tracer test (PITT) has been suggested as a useful tool for measuring water saturation over large measurement volumes in both systems. However, heterogeneous water distributions may result in mass transfer limitations, which can affect the accuracy of the measurements. In this study, we examined the influence of water saturation, Henry's Law constant, injected tracer mass, tracer quantification limit, and local rates of mass transfer between air and water on PITT measurements. A single‐region model was used to describe transport of gas‐phase tracers in a one‐dimensional system and to investigate the effect of these factors on measurement error. To verify the conclusions drawn from this modeling exercise, laboratory‐scale partitioning tracer tests were conducted in four different sand packings. Finally, the results from the mathematical modeling and laboratory experiments were used to suggest guidelines for minimizing measurement error in field‐scale PITTs for water saturation measurement.

Impact of Salinity on the Air–Water Partition Coefficient of Gas Tracers

The use of a gas partitioning interwell tracer test (PITT) has been proposed as a standard approach to the measurement of field-scale vadose zone water saturation fractions. The accuracy of the saturation measurement is largely dependent on the determination of the air–water partition coefficient, K , of the tracers. In practice, K is strongly influenced by the physical and chemical properties of the site water. In this study, column tests were conducted to investigate the impact of salinity on tracer partition coefficients for two promising gas phase candidate tracers, dibromomethane and dimethylether. Sodium thiosulfate was used as a salinity surrogate. The dynamic K values of the two partitioning tracers were measured for sodium thiosulfate concentrations between 0% and 36% by weight. Methane was used as the nonpartitioning tracer for all experiments. K values were found to decrease significantly with increasing sodium thiosulfate concentration. Similar correlations between K values and sodium thiosulf...

Characterization of Tetrachloroethene DNAPL in Low-Permeability Coastal Plain Sediments, North Carolina

The geosystem approach to source-zone characterization was used at a dry cleaners contaminated with tetrachloroethene (PCE) as a dense nonaqueous-phase liquid (DNAPL) within a shallow, low-permeability aquifer. This comprehensive approach to source-zone characterization provided a design-basis level of knowledge of the site geosystem, which includes aquifer heterogeneity, and the volume and spatial distribution of DNAPL. DNAPL was found to occur beneath and adjacent to the dry cleaning building, with both residual and free-phase DNAPL in the bottom 4 ft (1.2 m) of the shallow aquifer. The bottom 2.5 ft (0.76 m) of the aquifer consists of a fining downward sequence that grades from fine sand to clayey silt, with a resulting decrease in permeability in the DNAPL zone by at least one order of magnitude. The DNAPL zone is bounded below by a clay aquitard that serves as an effective capillary barrier to downward DNAPL migration. A partitioning interwell tracer test (PITT) measured approximately 81 ± 7 gallons (280–330 L) of DNAPL in the test zone, with the majority of DNAPL located near the building and decreasing northward away from the building. Soil samples analyzed for PCE show that the DNAPL saturations are greatest vertically from 17 to 18.5 ft (5.2 to 5.6 m) below ground surface (bgs), in the fine-sand to sandy-silt portion of the DNAPL zone. DNAPL saturations generally decrease at greater depths, from approximately 18.5 to 20 ft (5.6 to 6.1 m) bgs, in a silt to clayey silt (i.e., lower permeability) zone, with little or no DNAPL penetration into the underlying clay aquitard. Postremediation soil core analyses indicate that low permeability at depth was a limiting factor in the detection of all the DNAPL within the targeted zone using the PITT. It is concluded that the partitioning interwell tracer test successfully detected DNAPL in the more sandy portions of the aquifer, while not detecting an estimated 23 ± 5 gallons (87 ± 19 L) of DNAPL in the clayey silts below 17.8 ft (5.4 m) bgs.

Influence of mass transfer resistance on detection of nonaqueous phase liquids with partitioning tracer tests

Partitioning interwell tracer tests (PITTs) are a relatively new technique for measuring the amount of nonaqueous phase liquid (NAPL) within saturated porous media. In this work we examined the influence of mass transfer limitations on the accuracy of measured NAPL from PITTs. Two mathematical models were used along with laboratory column experiments to explore the influence of tracer partition coefficient, tracer detection limit, and injected tracer mass on NAPL measurements. When dimensionless mass transfer coefficients were small, NAPL measurement errors decreased with decreasing tracer partition coefficient, decreasing tracer detection limit, and increasing injected tracer mass. Extrapolating breakthrough curves exponentially reduced but did not eliminate systematic errors in NAPL measurement. Although transport in a single stream tube was used in the mathematical models and laboratory experiments, the results from this simplified domain were supported by data taken from a three-dimensional computational experiment, where the NAPL resided as large pool. Based on these results, we suggest guidelines for interpreting tracer breakthrough data to ascertain the importance of mass transfer limitations on NAPL measurements.