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Chemical Tracer Detection Using Raman Scattering

Milestone 2 Report: Calibration and evaluation of water quality interactions, injection stream and rock reactions with tracers

Project # 7-1116-0300

Shavinka Fernando (WellDog Gas Sensing Technology Corporation), Trevor Brown (WellDog Gas Sensing

Technology Corporation), Grant A Myers (WellDog Gas Sensing Technology Corporation)

Revisions To be submitted by: 18 June 2020

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Acknowledgement and Disclaimer

The authors wish to acknowledge financial assistance provided through Australian National Low Emissions Coal

Research and Development (ANLEC R&D). ANLEC R&D is supported by Australian Coal Association Low Emissions

Technology Limited and the Australian Government through the Clean Energy Initiative.

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Executive Summary

This project is aimed at supporting CO2 storage projects in Australian sedimentary basins, specifically the CTSCo

Surat CO2 storage demonstration project. The goal of this project is the evaluation and testing of a field tool for use

on site either downhole or at surface, to monitor the footprint of the CO2 plume and to provide leak detection with the

use of tracers. The accurate subsurface distribution and impact of low concentrations of sulphur oxides (SOx),

nitrogen oxides (NOx) and CO2 impurities in groundwater will be a significant indicator of the extent of the subsurface

plume of an injected Greenhouse Gas (GHG) stream derived from coal-fired power stations. By use of tracers, a

significantly increased sensitivity can be realised for both plume monitoring and for Monitoring & Verification (M&V)

requirements for early detection of possible leakage of the injected GHG stream.

The CTSCo demonstration project seeks to identify, develop, and trial in-situ testing technologies that would provide

an alternative to the traditional tracer method used in a manner that is low cost, yet highly reliable and non-intrusive.

Some of the considerations for tracer selection include that they should be non-toxic, naturally absent from the

reservoir(s), thermally stable, non-reactive in the conditions of the reservoir(s), easily detectable, and readily

available.

WellDog has developed the Reservoir Raman System (RRS), a chemical sensing field instrument for use at the

surface and downhole, which includes a Raman spectrometer used to monitor reservoir gases, such as CO2 and

CH4, dissolved in the fluid. Because Raman scattering is a weak effect, Raman spectrometers are designed to detect

very low levels of light and therefore, can be extremely sensitive to fluorescence with detection limits down to ppb or

lower. Therefore, it is proposed the RRS could be used both for the analysis of dissolved gases and for fluorescent

compound detection and tracking of the GHG plume and leakage monitoring. The ability of the tool to perform at the

surface (in a non-intrusive manner with very little well-head preparation) and downhole (in-situ at reservoir conditions)

provides a low-cost and unique solution to detect and analyze these chemicals in near real-time.

In the Milestone 1 report, research and experimentation into the selection of a fluorescent tracer for this project were

outlined. Three front-runners, rhodamine-WT, eosin-Y and uranine were analyzed for calibration, limit of detection,

and basic clay compatibility testing. With no mechanical modifications to the RRS, the dyes performed significantly

better than anticipated, showing low limits of detection with or without clay.

For the present Milestone 2 report, the behaviors of the tracers under reservoir conditions were investigated,

including photostability, and interactions with salinity, pH, kaolinite clay, temperature, Precipice Sandstone rock and

greenhouse gas. During these tests, eosin and uranine exhibited significant shortcomings. Eosin proved difficult to

quantify because of poor photostability characteristics that contributed to erratic test results. Uranine performed

poorly in low pH buffers and consequently in GHG interaction tests, where carbonic acid formation reduces the pH of

the system. Rhodamine-WT, though not immune to the effects of environmental variables, emerges as the best

option for a hydrological tracer in reservoir flow studies.

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Table of Contents

Executive Summary ....................................................................................................................................................... 3

List of Tables .................................................................................................................................................................. 5

List of Figures................................................................................................................................................................. 5

Introduction .................................................................................................................................................................... 6

Background .................................................................................................................................................................... 7

Methodology................................................................................................................................................................... 9

Results and Discussion ................................................................................................................................................ 11

Conclusions and Future Work ...................................................................................................................................... 28

List of Abbreviations and Scientific Terminology .......................................................................................................... 30

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List of Tables

Table 1: Chemical references table ................................................................................................................................ 9 Table 2: Concentration of dyes used in experiments ................................................................................................... 10

List of Figures

Figure 1: Fluorescent intensity as a function of salinity .................................................................................................. 7 Figure 2: Chemical structures of tracers ........................................................................................................................ 8 Figure 3: Reaction between carbon dioxide and liquid water ......................................................................................... 9 Figure 4: Short-term eosin photostability experiment ................................................................................................... 11 Figure 5: Long-term photodegradation of eosin ........................................................................................................... 12 Figure 6: Long-term photodegradation of rhodamine-WT ............................................................................................ 13 Figure 7: Long-term photodegradation of uranine ........................................................................................................ 13 Figure 8: Eosin salinity testing...................................................................................................................................... 14 Figure 9: Rhodamine-WT salinity testing ..................................................................................................................... 15 Figure 10: Uranine salinity testing ................................................................................................................................ 15 Figure 11: Testing of pH response of eosin ................................................................................................................. 16 Figure 12: Testing of pH response of uranine .............................................................................................................. 17 Figure 13: Testing of pH response of rhodamine-WT .................................................................................................. 17 Figure 14: Eosin and kaolinite interactions ................................................................................................................... 18 Figure 15: Uranine and kaolinite interactions ............................................................................................................... 19 Figure 16: Rhodamine-WT and kaolinite interactions ................................................................................................... 19 Figure 17: Eosin stability at 70°C ................................................................................................................................. 20 Figure 18: Uranine stability at 70°C.............................................................................................................................. 21 Figure 19: Rhodamine-WT stability at 70°C ................................................................................................................. 21 Figure 20: Eosin cooling from 70°C.............................................................................................................................. 22 Figure 21: Uranine cooling from 70°C .......................................................................................................................... 22 Figure 22: Rhodamine-WT cooling from 70°C ............................................................................................................. 23 Figure 23: Eosin sandstone interaction ........................................................................................................................ 24 Figure 24: Uranine sandstone interaction .................................................................................................................... 24 Figure 25: Rhodamine-WT sandstone interaction ........................................................................................................ 25 Figure 26: Eosin carbon dioxide interaction ................................................................................................................. 26 Figure 27: Uranine carbon dioxide interaction .............................................................................................................. 27 Figure 28: Rhodamine-WT carbon dioxide interaction ................................................................................................. 27 Figure 29: High Pressure test apparatus set up ........................................................................................................... 29

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Introduction

This report is a continuation of the research work presented in the Milestone 1 report on the characteristics of

fluorescent dyes and their potential use in monitoring and verifying sequestered carbon. To review briefly, monitoring

and verifying sequestered carbon is a key component of any sequestration project. The injection plume should be

tracked in order to determine how efficiently the reservoir is being utilized, and the reservoir and surrounding area

need to be monitored to detect any leakage of greenhouse gases outside of the geo-sequestration sites. A key

challenge for large scale carbon geo-sequestration is to identify, research, and develop field-deployable Monitoring &

Verification (M&V) technology that can detect and measure very low concentrations of injected greenhouse gases for

early detection of breakthrough and to mitigate the risk of significant greenhouse-gas stream release from the

sequestration reservoir. Ideally, the sequestered carbon should be detectable at extremely low concentrations. Using

an additive tracer could make it possible to positively identify injected material at a much lower concentration than the

injected material alone.

Tracers are chemicals used to track the movement of liquids and gases [1, 2, 3, 4]. They are in some way distinctive

from the fluid they will be used to track. Ideally, tracers travel along with the material they are marking, allowing it to

be discriminated from unmarked material. They are commonly used to track the movement of water such as in

plumbing or groundwater systems. Tracers can be naturally occurring components or artificial additives.

Examples of common additive tracers include noble gases, sulfur hexafluoride [5], and fluorocarbons. They are stable

chemicals that are rare or nonexistent in the system they will be used to evaluate. Detection of these tracers is

typically ex-situ, for example with mass-spectrometry. Isotopic tracers are naturally occurring or artificially added

materials with isotopic ratios that are distinct from the naturally occurring isotopic ratios. Carbon-13, oxygen-18,

deuterium, and other uncommon stable isotopes may be used for this purpose. Detection of these tracers usually

relies on capturing samples and transporting them to a laboratory for analysis. Another class of tracers, radiological

tracers, use radioactive isotopes to emit radiation and show the presence of the tracer. Fluorescent tracers [6, 7, 8, 9,

10, 11] are dyes that absorb light energy at a specific excitation wavelength and efficiently re-emit new light at a

longer wavelength.

A novel option is to use the WellDog Reservoir Raman System (RRS) for in-situ monitoring. This system was

originally developed for Raman spectroscopy. This analytical technique is a type of vibrational spectroscopy that has

been employed to non-destructively analyze various materials. Raman spectroscopy operates on the Raman

principle, where photons of light occasionally strike a molecule, and rather than being reflected, they are absorbed

and re-emitted at a different wavelength. The pattern of wavelengths of emitted light forms a unique spectrum for

each compound, directly related to the molecular bonding and structure of the molecule. Higher concentrations of a

molecule result in the observation of more photons at the fingerprint spectral energies specific to that molecule.

Hence with correct calibration, the concentration of specific dissolved species can be determined. This sensing

technique has been extended by WellDog to down-hole in-situ analysis of methane and carbon dioxide species

naturally occurring in coalbeds [12]. In particular, for the past nine years, WellDog has performed commercial

downhole logging services using its downhole-RRS logging technology that measures solvated concentrations of

methane and carbon dioxide, and calculates related gas partial pressures, to inform production expectations for coal

seam gas operators [13, 14]. However, the system is several orders of magnitude more sensitive to a fluorescent

signature than a Raman signature. We hypothesize that the WellDog RRS can reliably detect fluorescent tracers in

field conditions during carbon dioxide injection and subsequent sequestration in porous water-wet sandstone for the

purposes of early breakthrough and leak detection.

While WellDog’s RRS instrument was developed for the capture and analysis of Raman spectra, many chemicals

also emit distinctive fluorescence signatures, which can also be characterized by the system. These manifest in

spectra as intense, broad features that, particularly for fluorescent dye tracers, are much stronger than the Raman

effect. Quantum efficiencies for fluorescence (i.e. number of photons emitted per number absorbed) can approach

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unity as compared to Raman efficiencies on the order of 1 emitted per 107 absorbed. Thus, the RRS can potentially

operate with a much higher sensitivity to fluorescent substances. Fluorescence spectroscopy can be performed with

the standard RRS without any mechanical changes or adaptations.

In the previous Milestone 1 report, we outlined research for the selection of a fluorescent tracer for the project. Five

aqueous tracer formulations were acquired and tested with a WellDog RRS for basic ease of use, solubility,

detection, and compatibility with clay. Three front-runners, eosin-Y, uranine and rhodamine-WT were analyzed for

calibration and limit of detection, with rhodamine-WT exhibiting parts-per-trillion levels of detection and the latter two

exhibiting parts-per-billion limits of detection. These fluorescence limits of detection are 5-6 orders of magnitude

better than for Raman scattering signals with no mechanical modification to the RRS. Basic clay compatibility testing

showed only modest effects on detection for rhodamine-WT and uranine, but significant effects on eosin.

In this milestone report, characterization is continued for the front-runner tracers, eosin, uranine and rhodamine-WT.

Behaviors of the tracers under reservoir conditions are investigated including photostability and interactions with

salinity, pH, kaolinite clay, temperature, Precipice Sandstone rocks and greenhouse gas.

Background

Photostability is a known issue with fluorescent tracers. When exposed to the sun or even to ambient light, tracers

tend to degrade or photobleach over time. For this reason, dye solutions are stored in dark bottles or in a dark place.

The excitation laser used in the measurement of fluorescence can also cause degradation over time, which can lead

to inconsistent measurements of concentration. For this report, we performed experiments to characterize the

photostability of the tracers over time as solutions of each dye are exposed to continuous excitation light.

Salinity is another possible variable that can affect fluorescent tracers. Magal et al. have experimented with the use of

fluorescent dyes as tracers in highly saline groundwater [15]. Solutions with high salinity may reduce fluorescence

signal. Figure 1 below adapted from Magal et al. shows the relative fluorescent intensity (FI) of the tested dyes at

high salinity. Rhodamine-WT was not a part of the study, however, sulforhodamine-B (SRB) is a derivative of

rhodamine and structurally similar to rhodamine-WT with substitutions of sulfonates for carboxylates.

Figure 1: Fluorescent intensity as a function of salinity [15]

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During the first milestone, tracers were mixed into buffer solutions containing 8.5 g/L sodium chloride. Reservoir

salinity is approximately 1.5 g/L. According to Magal et al., the signal is drastically changed only in solutions with

more than 50 g/L and therefore we expect the difference in intensities between 1.5 g/L and 8.5 g/L to be negligible

[15]. Nevertheless, experiments were conducted for this milestone report to observe any change in fluorescence

emission of the tracers under those conditions.

The Precipice Sandstone groundwater has a baseline pH of 7.8, but injection of large amounts of greenhouse gas will

likely reduce the pH significantly. Fluorescein and its derivatives, uranine (the deprotonated form of fluorescein),

eosin and rhodamine-WT have molecular groups that are ionizable under different pH values. Figure 2 shows the

molecular structures for these with ionizable groups positively or negatively charged as indicated. Just as pH is a

measure of the acidity/basicity of aqueous solutions, the pKa is a measure of the strength of an acidic or basic

ionizable chemical group. Acids have a pKa less than seven and bases have a pKa greater than seven. Functionally,

when the pH of a solution matches the pKa of a chemical group, half of the chemical groups will be protonated, and

half will be deprotonated. Slyusareva et al. investigated the effects of pH on the fluorescence of fluorescein/uranine

and eosin [16]. They demonstrate that below pH 7, the emission from fluorescein/uranine drops significantly, with a

~50% reduction at a pH of 6.2 corresponding with the major pKa for the protonation of the carboxyl (COO-) group. For

eosin, they report 50% reduction at pH 3.2, again corresponding with the pKa for protonation of the carboxyl group.

Rhodamine-WT has a reported first pKa of 5.1 [17], but doesn’t exhibit the same strong dependence of fluorescence

emission within this range of pH.

Besides scattering light through turbidity, clays can also directly absorb tracer molecules to reduce their apparent

concentration. In the first milestone report, the interactions of tracers with bentonite clay were studied. However, the

prevalent clay in the Precipice Sandstone is kaolinite. Although we expected little difference between bentonite and

kaolinite, tests were performed here to characterize the interactions of kaolinite clay with the tracers.

The Precipice Sandstone storage reservoir at Glenhaven has a temperature of around 70°C due to its depth and the

local geothermal gradient. Temperature is a crucial environmental factor that will affect the stability and utility of dyes

within the reservoir. Chemical reactions are very sensitive to temperature, so it is reasonable to assume that

fluorescent dyes will be temperature sensitive. It is important that the dyes are still usable under high temperature

conditions if they are to be used for tracking the movements of groundwater and carbon dioxide in situ. Al-Riyami

characterized the thermal stability of tracer dyes for groundwater and found that when 200 ppb solutions of

Figure 2: Chemical structures of tracers

On the left, fluorescein/uranine (R = hydrogen) and eosin (R = bromine); right, rhodamine-WT

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rhodamine WT and fluorescein were subjected to temperatures greater than 100°C, a reduction in fluorescence was

observed [18]. Rhodamine WT was more stable, but lost 50% of intensity after 2000 hours at 100°C. For this

milestone report, dyes were subjected to elevated temperatures expected in the Precipice Sandstone storage site

and monitored for changes in fluorescence emission.

Interactions of tracer solutions with samples of Precipice Sandstone rock were also characterized for this report. The

samples were offcuts of core leftover from a previous study [19]. The minerals and surfaces of the rock samples have

the potential to react with and adsorb tracer molecules. We performed optical and confocal scanning Raman

spectroscopy for basic mineralogy of samples prior to incubation with solutions of dyes.

The injection of carbon dioxide into water results in carbonic acid formation as shown in Figure 3. This would be

expected to lower the pH of the sample, which could indirectly affect the signal strength of some tracers. In this

milestone report, we examine the interaction of sub-critical carbon dioxide gas on solutions of tracers. In future work,

we will report on the interactions of tracer solutions with super-critical carbon dioxide at reservoir temperatures and

pressures including partitioning and transport.

Methodology

Table 1 shows a list of all chemicals used for Milestone 2 research including molecular formula, source, Chemical

Abstracts Service (CAS) number, grade and purity. CAS is a unique identifier assigned to each chemical and is

particular to its analytical grade, relevant salt and/or molecular group(s). All chemicals were used as supplied without

further purification.

Citrate-Phosphate buffer was chosen for this project due to its large working pH range that allows for testing the dyes

at a wide range of acidity. Tracer dye dilutions were then performed once the Citrate-Phosphate buffers were

prepared (See Milestone 1 report for details on tracer dilutions) [19].

Figure 3: Reaction between carbon dioxide and liquid water

Common Name Formulae Company/Seller CAS Grade Purity

Citric Acid C6H8O7 Research Products International 77-92-9 ACS ≥ 99.5%

Sodium Hydroxide NaOH Research Products International 1310-73-2 ACS (Supplied as Beads) ≥ 97.0%

Sodium Phosphate Na2HPO4 Research Products International 7558-79-4 ACS ≥ 99.0%

Sodium Chloride NaCl Millipore Sigma 7647-14-5 ACS ≥ 99.0%

Methanol CH3OH Millipore Sigma 67-56-1 ACS ≥ 99.8%

Hydrochloric Acid HCl Millipore Sigma 7647-01-0 ACS 37%

Fluorescein C20H12O5 Abbey Color 2321-07-5 N/A 94.60%

Rhodamine WT C29H29ClN2Na2O5 Abbey Color 37299-86-8 N/A 19.80%

Eosin C20H6Br4Na2O5 Abbey Color 17372-87-1 N/A 93.40%

Carbon Dioxide CO2 AirGas 124-38-9 Instrument Grade (Pressurized cylinder) ≥ 99.99%

Table 1: Chemical references table

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For each experiment, dye solutions were prepared from fresh raw-material dye obtained from Abbey Color and

diluted (using citrate buffer) to the final concentrations for experimentation. It is vital that new stocks are made in

order to minimize possible biological growth and bleaching effects due to ambient light.

1. Dye Stock A: Utilized for pH, salinity, dye-fluid-sandstone and dye-fluid-carbon dioxide tests

2. Dye Stock B: Utilized for kaolinite and thermal interactions, and initial trials for dual windowed cells

Spectrum Measurements

Measurements of the fluorescence of dilute tracer solutions were performed using the WellDog RRS, composed of an

excitation laser, an optical system, a spectrometer and a charge-coupled-device detector. Solutions were added to

1-cm quartz cuvettes and placed at the laser focus of the system. Each sample spectrum was captured in a series of

replicates at two integration times, 100 ms and 600 ms. 100 ms is the shortest integration time allowed by the current

RRS. Increased integration time increases the collected signal and reduces the level of noise in the measurement.

For quantification, the signal from fluorescence was calculated by integrating the counts between 545 nm and 630 nm

after background subtraction of a linear fit between the endpoints of the region. Similarly, the signal under the water

stretch bands was integrated between 630 nm and 660 nm after background subtraction of a linear fit between the

endpoints of the region. To normalize for differences in integration time, the measured fluorescence signal in a

spectrum is given by the ratio of the integrated fluorescence and water signals.

Experiments

Behaviors of the tracers under reservoir conditions were investigated including photostability and interactions with

salinity, pH, kaolinite clay, temperature, Precipice Sandstone rock and greenhouse gas. In general, for each

experiment, solutions of tracer dye were prepared according to Table 2. A portion of each solution was subjected to

the environmental variable for some length of time and another portion kept aside as a control for comparison. The

solution concentrations were selected from in the middle of the calibration range from the Milestone 1 report so as to

be easily detectable. Details of the experiments on each variable are provided in the Results section below.

Table 2: Concentration of dyes used in experiments

Dye Concentration

Eosin 50 ppb

Uranine (Powdered form) 50 ppb

Rhodamine WT 100 ppt

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Results and Discussion

Photostability of the dyes

It was recognized during testing that eosin could exhibit sporadic results. In Figure 4, the short-term photobleaching

behavior of 50-ppb eosin in a 3-mL cuvette was investigated. Spectra were acquired every twelve seconds with

continuous laser exposure until a total of 200 were collected. At first, the fluorescence intensity declines rapidly. After

about forty spectra have been acquired, the intensity stops falling and begins to rise again. The intensity continues to

increase until about the one hundredth spectrum, at which point the signal is about 20% higher than when the

experiment started. Next, the fluorescence intensity begins a steady, gradual decline through the end of the

experiment. A possible explanation for the observed behavior is that during the initial drop, photobleaching of tracer

molecules occurs in the small interrogation volume of the focused laser spot. This is followed by recovery as

absorption of laser energy heats the fluid in the cuvette to foment convective mixing that introduces fresh dye into the

interrogation volume. Also, as shown in the thermal stability experiments described in the section below, the

fluorescence intensity of eosin may also increase with temperature, which could contribute to the apparent increase

in fluorescence after 40 spectra. After recovery, the slow decline in the following 100 spectra could be attributed to

irreversible photobleaching of the molecules in the whole volume of the cuvette.

After recognizing this behavior, a continuous series of two thousand spectra were acquired over seven hours (Figure

5). The concentration of the dye was the same as for all other experiments in this report. Within three hours, the

intensity at the peak dropped by 50% and in the following four hours, the intensity dropped an additional 40%. When

characterized after the photodegradation test, the fluorescence of eosin did not recover, suggesting an irreversible

photochemical reaction. While the photobleaching effect was observed in all three dyes, the behavior is much more

pronounced in eosin and appears irreversible. The photostability of eosin presented challenges in all of the

experiments in this report that may render it unusable for quantitative analysis.

Figure 4: Short-term eosin photostability experiment

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Interestingly, it was observed in experiments described below that the photodegradation effect on eosin was reduced

when kaolinite clay was present in the sample. It could be that the suspended clay particles absorb, scatter, and

dissipate some of the laser energy, reducing the intensity at the focused laser spot, thus mitigating photobleaching.

Figure 6 below shows a long-term photodegradation study of rhodamine-WT. The inset to the figure shows the

intensity of rhodamine-WT near the peak emission wavelength of 583 nm, which unlike eosin did not exhibit the same

strong decline over time. Over the first 6 hours of continuous laser exposure, the intensity declines by just 5%. Thus,

it was concluded that rhodamine-WT does not have a strong bleaching effect due to laser exposure. A similar long-

term test was performed on uranine, which showed similar photobleaching behavior to rhodamine-WT in Figure 7.

Note that these long term experiments are not necessarily representative of the conditions in a downhole logging

situation. In reality, the effect of photobleaching would be minimal due to the large the volume of tracer used in the

reservoir. Furthermore, movement of the instrument through the fluids or the flow of fluids in the reservoir would

continuously present fresh molecules to the instrument.

Figure 5: Long-term photodegradation of eosin

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Figure 7: Long-term photodegradation of uranine

Figure 6: Long-term photodegradation of rhodamine-WT

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Salinity Tests

Figures 8-10 below compare eosin, uranine and rhodamine-WT spectra at 1.5 g/L and 8.5 g/L salinities. The variance

in the data for a given salinity is greater for eosin when compared with uranine or rhodamine-WT, showing a lack of

consistency due to photodegradation. It should also be noted that both salinities are significantly lower than the

hypersaline solutions tested by Magal et al. [15]. Salinity has minimal effect on rhodamine-WT, as shown in Figure 9

below, where the spectra from the two data sets are clustered together.

Figure 8: Eosin salinity testing

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Figure 9: Rhodamine-WT salinity testing

Figure 10: Uranine salinity testing

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Tests of pH

Four different buffers with pH levels 3, 5, 7.1 and 7.8 were prepared and mixed with tracer dye stocks to form

solutions with the concentrations in Table 2. Figures 11-13 shown below illustrate eosin (50 ppb), uranine (50 ppb)

and rhodamine-WT (100 ppt) at the four pH values tested. The pH values significantly affect the spectral intensity.

Eosin (Figure 11) was readily detectable at all pH levels. Eosin produces a slightly lower intensity spectrum for higher

pH values and is far more consistent across the range of tested pH values than uranine seen below in Figure 12.

Uranine performed poorly at low pH. This agrees with Slyusareva et al. who reported the pKa of uranine at a pH of

6.2 and 50% reduction in fluorescence [16]. The low detectability of uranine in acidic conditions is a major

shortcoming for usage in a CO2 injection site due to carbonic acid formation. It is worth noting that uranine produced

an extremely consistent result at each tested pH level and therefore may be used as a pH indicator. Rhodamine-WT

(Figure 13) also decreased in signal, but only at the lowest pH and not as dramatically as uranine. The pKa for the

first ionization of rhodamine-WT is at a pH of 5.1 [17] and this ionization appears not to affect the fluorescence

emission. The drop in fluorescence intensity at a pH of 3 may be due to the second ionization of the molecule. It

would be fair to say that rhodamine-WT is the most readily detectable dye under all conditions tested due to its

consistent nature.

Figure 11: Testing of pH response of eosin

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Figure 12: Testing of pH response of uranine

Figure 13: Testing of pH response of rhodamine-WT

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Kaolinite Interaction Results

Kaolinite clay is more prevalent in the Precipice Sandstone than bentonite, which was tested in the previous

milestone report. Two 50-mL samples of each dye were prepared according to Table 2. A mass of 2.0 g kaolinite

powder was added to one sample of each dye type. Magnetic stir bars were placed in each flask, then the flasks were

sealed with parafilm. The flasks were mixed on a stir plate in a darkened room for one week to ensure that the

kaolinite blended thoroughly. After two weeks, the kaolinite samples were passed through Fischer Q5 filter paper and

then loaded into quartz cuvettes.

Eosin with kaolinite produced spectra with higher intensity and consistency compared to the control sample without

kaolinite. As seen in Figure 14, the spectra for the control sample are inconsistent likely due to photobleaching

effects. Kaolinite seems to mitigate the photobleaching effect in eosin. This could be a result of the clay particles

scattering the laser light, slowing down dye degradation.

Kaolinite slightly reduced the intensity of uranine (Figure 16) and rhodamine-WT (Figure 15). The fluorescent intensity

of the two dye samples with clay are still acceptable since they can be easily distinguished from the buffer blank with

clay (data not shown).

Figure 14: Eosin and kaolinite interactions

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Figure 16: Uranine and kaolinite interactions

Figure 15: Rhodamine-WT and kaolinite interactions

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Thermal Interaction Results

In order to simulate reservoir conditions, the dyes were tested to determine their suitability in elevated temperature

conditions. 200-mL samples of eosin, uranine and rhodamine-WT were prepared in concentrations according to

Table 2. The three dye samples were transferred to screw top Nalgene bottles and placed directly in the reservoir of a

water bath chiller set to 70°C. 2-ml aliquots were then extracted periodically by pipette to be analyzed in cuvettes by

the RRS. The sampling interval was every 24 hours for the first three days and thereafter, the interval was variable.

Figures 17-19 show measured concentration (ppb or ppt) as a function of the number of days elapsed under high

temperature conditions. None of the dyes have shown permanent changes that can be attributed to temperature

since testing began. On this time scale, all three dyes showed no significant degradation and are reasonably stable at

the reservoir temperature of 70°C. However, the fluorescent characteristics of all three dyes were changed at the

higher temperature as compared to room temperature. Eosin (Figure 17) and uranine (Figure 18) became brighter at

elevated temperatures. Rhodamine-WT (Figure 19) showed the opposite behavior, with intensity dropping at high

temperatures.

Eosin showed extremely variable behavior during this test. The intensity is much less consistent than rhodamine WT

or uranine. This is at least in part due to the photobleaching phenomena that occurs in eosin. The immediate onset of

photobleaching makes it difficult to acquire consistent spectra in the first few minutes of laser exposure. Eosin does

not appear to have any long-term problems with exposure to reservoir temperatures. The results were erratic, but

they were always brighter than room temperature spectra.

The intensity of uranine was significantly increased compared to room temperature, up to five times brighter than

expected. The duration spent at high temperature does not have an effect on uranine over a two-week time scale.

Rhodamine-WT was unusual among the dyes tested. The intensity of rhodamine-WT was lower at high temperatures

than at room temperature. This effect should not hinder rhodamine-WT’s use within the reservoir since the intensity

was only reduced by a maximum of 20%. Rhodamine-WT does not appear to suffer any negative effects from

exposure to reservoir temperatures over a two-week time scale.

Figure 17: Eosin stability at 70°C

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Among the set of measurements from a given day, the results have significant variability in intensity. This has been

attributed to the sample in the cuvette cooling after being removed from the heated bath. For eosin and uranine, the

brightest spectra is the first one, when it is warmest. The spectra become progressively less intense over time as the

sample cools as shown below in Figures 20-22.

Eosin removed from a 70°C bath shows a dimming trend over four minutes time as the dye returns to room

temperature (Figure 20). The initial brightness is much higher than the same concentration would be at room

temperature. In this graph, the brightness is already well into its decline to room temperature levels. The cuvettes

used for this experiment were only 3mL in volume, so they cooled rapidly. By the time this cuvette was placed in the

Raman system, it had already cooled noticeably.

Figure 18: Rhodamine-WT stability at 70°C

Figure 19: Uranine stability at 70°C

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Uranine is quite similar to eosin in its thermal response (Figure 21). This sample shows an initial intensity 3.2 times

higher than an equivalent concentration at room temperature. By four minutes, this has dropped to 1.8 times higher.

The sample was tested later and returned to normal once the temperature reached equilibrium. This shows that the

thermal effects are reversible, and the dye is not being affected by reservoir temperatures over a two-week time

scale.

Rhodamine-WT shows behavior opposite of uranine and eosin (Figure 22). Rhodamine-WT presented less variability

with respect to temperature than the other dyes. The highest intensity recorded was 1.25 times the expected level,

and the lowest was 0.67 (Figure 19). The lowest readings for rhodamine-WT occurred at 70°C.

Figure 20: Eosin cooling from 70°C

Figure 21: Uranine cooling from 70°C

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Dye-Fluid-Precipice Sandstone Rock Interactions

The purpose of this experiment was to determine what interactions will take place between the fluorescent dye and

the surrounding environment of fluid and rock. A buffer solution mimicking the properties of the Precipice Sandstone

storage reservoir (pH of 7.8, salinity of 1.5 g/L) was prepared. The same dye concentrations from Table 2 were used.

One-centimeter cubes of Precipice Sandstone rock were placed in three jars along with the dye solutions. Aliquots of

fluid were periodically extracted from the jar with a pipette and placed in quartz cuvettes for spectral analysis. This

was performed every 24 hours for three days and thereafter, on an interval of every three days until 21 days after the

start of the experiment.

During this experiment, samples of Precipice Sandstone rock were also analyzed for changes. The samples used all

came from the same core at the same depth. Before being immersed in the dye solution, they were characterized

both using optical mineralogy and with a Renishaw Raman microscope. The rock sample analysis is ongoing. The

rock samples have not yet been analyzed after being removed from the dye solution. The results from these

analyses will be included in a later report.

Eosin shows dramatic changes in behavior over the two-week observation period (Figure 23). These changes are

attributed to photobleaching behavior. The pattern of changes is completely random. This is consistent with eosin’s

behavior in other tests. The difficulty of obtaining consistent measurements with eosin is a concern. This test was the

first incident where eosin was recognized as being susceptible to photodegradation.

Uranine (Figure 24) shows consistent results for all measurements over the two weeks. Uranine does not appear to

be reacting with Precipice Sandstone formation rock. As shown in Figure 25, rhodamine-WT is also consistent. There

does not appear to be any reaction between rhodamine-WT and the Precipice Sandstone rock.

Figure 22: Rhodamine-WT cooling from 70°C

24

Figure 23: Eosin sandstone interaction

Figure 24: Uranine sandstone interaction

25

Dye-Fluid-Carbon Dioxide Interactions

This experiment analyzed the interactions between fluorescent tracer dyes in Precipice Sandstone brine and carbon

dioxide gas. Dye solutions were prepared according to Table 2 as well as a buffer blank. The four solutions were then

introduced into sapphire-windowed pressure cells resulting in slightly more than half of the cell being filled with fluid.

Carbon dioxide gas was then introduced into the cell up to a pressure of 600 psi at 25°C. Once the pressures were at

equilibrium, spectra were acquired through the sapphire windows periodically for 6 days. Note that these pressures

represent an incremental step to the actual conditions at the Glenhaven storage site.

Eosin spectra showed wide variation due to the photodegradation effect over time (Figure 26). Uranine was

consistent over the six-day period, albeit with substantially reduced intensity due to pH reduction from carbonic acid

formation (Figure 27). The fluorescent intensity of uranine signal recovered (though not completely) after the pressure

cell was depressurized showing that the pH effect on the fluorescent signal is partially reversible. Rhodamine-WT

data is very consistent, showing minimal effects at elevated pressures (Figure 28). Rhodamine-WT is the best choice

for continuously measuring dye in the presence of high dissolved carbon dioxide concentrations.

Figure 26 below shows spectra collected from eosin solution. Thirty spectra per day were collected on days 0, 1, 2, 5

and 6 after pressurization. The inset shows the counts near the peak fluorescence emission at 550 nm. The spectra

are colored from blue to red according to the time collected as indicated in the inset plot. The effects of

photobleaching can be recognized during each day’s data set as well as over the course of the six days of the

experiment. Note the presence of Raman scattering signature from carbon dioxide dissolved in the fluid with the pair

of peaks located at 570 and 575 nm.

Shown in Figure 27 below are spectra collected from a solution of uranine in a pressure cell with 600 psi CO2. Thirty

spectra per day were collected on the day of pressurization, and 1, 2, 5 and 6 days after pressurization. The inset

shows the counts near the peak fluorescence emission at 550 nm. The spectra are colored from blue to red over time

as indicated in the inset.

Figure 25: Rhodamine-WT sandstone interaction

26

Shown in Figure 28 below are spectra collected from a solution of rhodamine WT in a pressure cell with 600 psi CO2.

The inset shows the counts near the peak fluorescence emission at 583 nm. The spectra are colored from blue to red

according to the day collected as indicated in the inset.

Figure 26: Eosin carbon dioxide interaction

CO2

27

Figure 27: Uranine carbon dioxide interaction

Figure 28: Rhodamine-WT carbon dioxide interaction

28

Conclusions and Future Work

In this Milestone 2 report, the top three picks of fluorescent tracers were further tested for photostability and

compatibility with reservoir conditions including salinity, pH, temperature, kaolinite, carbon dioxide and sandstone

interactions. During these tests, eosin and uranine exhibited significant shortcomings. Eosin proved difficult to

quantify because of poor photostability characteristics that contributed to erratic test results. Uranine performed

poorly in low pH buffers and consequently in GHG interaction tests. Rhodamine-WT, though not immune to the

effects of environmental variables, emerges as the best option among those tested for a hydrological tracer in

reservoir flow studies.

Future Work

The final Milestone report will focus on interactions of tracers with Precipice Sandstone rock, brine and supercritical

carbon dioxide at reservoir temperature and pressure under static and core-flood flowing conditions. Optical

characterization of Precipice Sandstone samples was performed before incubation with aqueous dyes and will be

performed after the incubation and these results will be detailed in the final report. Additionally, one point of feedback

from reviewers of the Milestone 1 report was the concern that aqueous tracers will not partition into and transport with

dense-phase carbon dioxide during injection and storage. We acknowledge the concern but suggest that the work

performed to date constitutes a (partial) proof-of-concept: the RRS can be used to detect fluorescent molecules in

fluids with no mechanical modification and very low limits of detection. The aqueous tracers tested here were easily

available, well-established hydrological tracers. However, they are by no means the only available fluorescent

tracers. Non-polar ‘solvent’-type dyes are also available which should much more readily partition into and travel with

super-critical CO2. For the final Milestone report, we will also report on characterizations of such dyes as neutral

fluorescein and Nile red using the protocols established to test them against environmental factors in the reservoir.

Preliminary testing of dyes with dense phase carbon dioxide have been performed to determine whether aqueous

rhodamine-WT dye will partition into dense phase CO2. Additionally, other dye alternatives such as non-polar Nile red

fluorescent dye have been acquired, which is more likely compatible with supercritical CO2. These will be

characterized using the protocols we have established in Milestone 1 and 2 to test them against environmental

variables of the Precipice Sandstone.

Work is underway to test the behavior of tracer dyes under temperature and pressure conditions that more closely

approximate the full reservoir environment. Reservoir pressure and temperature will be achieved using a different

pressure vessel system, the dual window cell apparatus adapted from previous work as pictured below in Figure 29.

In addition to static (non-flowing) experiments in the double windowed cell apparatus, a collaboration effort with the

University of Wyoming has been organized to test the tracer dyes in a core flood apparatus designed to

accommodate multi-phase flow. This will provide a better perspective into how the dyes will migrate within the

reservoir.

29

Figure 29: High Pressure test apparatus set up

30

List of Abbreviations and Scientific Terminology

ACS – American Chemical Society

ANLEC – Australia National Low-Emissions Coal

CAS – Chemical Abstract Service

CCS – Carbon Capture & Sequestration

CD4 – Deuterated Methane

CH4 - Methane

CO2 – Carbon dioxide

CTSCo – Carbon Transport and Storage Company

g – grams

GHG – Greenhouse gas

HCl – Hydrochloric acid

Kr - Krypton

L – liter

M – Molar

M&V – Monitoring & Verification

mL – milliliter

mM – millimolar

mol – mole

ms – millisecond

NaCl – Sodium Chloride

NaOH – Sodium Hydroxide

NOx – Nitrogen Oxides

nm – nanometer

pH – negative log of the concentration of Hydronium ions (It is a quantitative measure of acidity of a solution)

pKa – negative log of the acid dissociation constant (it is a quantitative measure of the strength of an acid in solution)

ppm – parts-per-million

ppb – parts-per-billion

ppt – parts-per-trillion

psi – pounds per square inch

RRS – Reservoir Raman System

SF6 – Sulfur Hexafluoride

SOx – Sulfur Oxides

SRB – Sulforhodamine-B

TRIS - tris(hydroxymethyl)aminomethane

WT – Water-tracer as in Rhodamine WT

31

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