CONFERENCE PROCEEDINGS 

Monitoring Geologically Sequestered 

CO2 during the Frio Brine Pilot Test 

using Perfluorocarbon Tracers 

1

McCallum, S.D., 1Riestenberg, D.E., 1Cole, D.R., 2Freifeld, B.M.,

2

Trautz, R.C., 3Hovorka, S.D., and 1Phelps*, T.J. 

1 Oak Ridge National Laboratory, 1505 Bethel Valley Road, Oak Ridge, TN, 37831-

6036 USA
 2 Lawrence Berkeley National Laboratory, 1 Cyclotron Road, Berkeley, CA 94720-

90R1116, USA

3 Texas Bureau of Economic Geology, The University of Texas at Austin, University 

Station, Box X, Austin, Texas 78713-8924, USA
 

*Corresponding author and presenter (phelpstj@ornl.gov) 

FOURTH ANNUAL CONFERENCE ON CARBON CAPTURE AND SEQUESTRATION DOE/NETL 

May 2-5, 2005 
 

Abstract 

A suite of gaseous perfluorocarbon (PFT) conservative tracers were successfully employed to 

monitor migration of the CO2 plume during the Frio Brine Pilot Test. The conservative tracers enabled 

quantification of plume breakthrough and transport times, added value to model validity testing, and are 

providing insight into the saturation behavior of the CO2 plume. Specific pairs of tracers were 

introduced at chosen times alongside the injected CO2 stream. These tracers were transported through 

the formation and then collected at a monitoring well approximately 30 meters up-dip. A novel U-tube 

design allowed for high frequency sampling at the monitoring well. The PFTs introduced at the onset of 

CO2 injection appeared at the monitoring well 50.3 hours after injection and coincided with the initial 

CO2 breakthrough. A second and third set of PFT pairs were introduced 102 and 120 hours after the 

initial CO2 injection. The breakthrough times for the second (51.7 hours) and third (51.2 hours) PFT 

sets indicated little change in the overall saturation of the CO2 plume, between the injection and 

monitoring location. The rapid establishment of well developed flow paths demonstrates the importance 

in understanding preferential flow for assessing the storage efficiency of geologic CO2 sequestration 

sites. 
 

Introduction 

CO2 is a known greenhouse gas that has been released into the atmosphere in increased quantities since 

the industrial revolution. Geologic sequestration of CO2 has been proposed as a method for storing CO2, which 

could serve to alleviate some of the effects of global warming. Geologic formations have the ability to hold 

approximately 920 Gt in depleted oil and gas reservoirs, 400 – 10,000 Gt in deep saline aquifers, and 

greater than 15 Gt in unminable coal seams (IEA, 2001). The Frio Injection Test is a field scale experiment 

that aims to evaluate the capability of saline aquifers to sequester CO2. The CO2 was injected into an 

approximately 6 m thick high permeability sandstone interval within the Frio formation in south east Texas that 

was confined by low permeability shale layers. 

The objective of this research was to track the plume of injected CO2 using a suite of perfluorocarbon 

tracers (PFTs). In addition to monitoring the CO2 plume the PFTs also served to quantify CO2 plume 

breakthrough, added value to model validity testing, and provided insight into the saturation behavior of CO2. 

This paper describes the methods used to inject, collect, and analyze PFTs during the Frio Pilot CO2 injection and 

reports the results. 

Material and Methods 

PFTs are conservative gaseous tracers that are non-depositing, non-scavenged, non-reactive, have low 

atmospheric background, limited industrial use, and detectable at the femtogram (Dietz, 1986) to picogram level. 

Due to the properties of PFTs they have been used extensively for atmospheric (Dietz, 1986; Lagomarsino, 1996, 

Straume, 1998) and subsurface studies (Senum et al., 1990; McKinley and Colwell, 1996; Sullivan et al., 1998; 

Phelps, 2001). 

Of the nearly one dozen varieties of PFTs available for 

tracer studies four were selected to use during the Frio injection. 

The tracers used were perfluoromethylcyclopentane (PMCP), 

perfluoromethylcyclohexane (PMCH), 

perfluorodimethylcyclohexane (PDCH), and 

perfluorotrimethylcyclohexane (PTCH). These four PFTs were 

selected because of their volatility and their favorable gas 

chromatographic (GC) separation and elution time, which ranged 

from 1 to 2 minutes between each PFT (Figure 1). Using PFTs with 

spaced elution times allows multiple PFTs to be examined in a 

single GC injection. Detection of the PFTs was sensitive to the 

picogram level when using a Hewlett Packard 5890 GC equipped 

with an electron capture detector (ECD). 

After selecting the PFTs to be used for the Frio Test an 

injection scenario was devised that would allow the PFTs to be 

introduced during the CO2 injection at the injection well. In order to 

do this a high performance liquid chromatography pump (HPLC) 

that operated at pressures in excess of the Frio formation pressure 

(~13 MPa) was used. A total of 3 PFT injections were performed 

consisting of a pair of PFTs injected at a specific volume. Injection 

1 occurred just after the beginning of the CO2 injection (1.9 hrs, 10/04/2004), lasted for 3.9 hrs, and introduced 

900 mL (1.6 kg) of PMCH and 860 mL (1.6 kg) of PTCH. The second injection occurred just after the beginning 

of the second CO2 injection (102.8 hrs, 10/08/2004), lasted for 0.6 hrs, and introduced 100 mL (0.2 kg) of PMCP 

and 100 mL (0.2 kg) of PDCH. The third injection occurred at 120.1 hrs, lasted for 0.5 hrs, and introduced 90 mL 

(0.2 kg) of PMCH and 85 mL (0.2 kg) of PTCH. 

The injected PFTs were introduced via the injection well to an approximately 6 m thick interval of the 

Frio Formation. This interval was bounded by low permeability shale that constrained the vertical flow of CO2 

and PFTs. The lateral flow from the injection well followed an up-dip direction for at least 30 m where sampling 

occurred at a monitoring well. High frequency samples were collected at the monitoring well using a U-tube, 

described in detail by Freifeld et al. (manuscript submitted). The U-tube was a ‘U’ shaped tube that was inserted 

to the sampling depth (~1530 m) and was equipped with a series of one way check valves at the cusp of the ‘U’ 

bend in the tube. The pressure in the U-tube could be decreased below formation pressure which would permit 

Figure 1. Gas chromatograph showing 

the 4 PFTs used during the Frio Injection 

and their relative elution times 
 

 

sample fluids to enter the tube through the check valves. The U-tube pressure would then be increased using 

compressed nitrogen gas and the sample would be rapidly transported to the surface where subsampling would be 

accomplished. 

Samples that had been transported to the surface by the U-tube system were subsampled for PFTs by 

filling 150 mL Monel steel high pressure cylinders (Hoke Incorporated, Spartanburg, SC) with formation fluid 

(brine, CO2, and PFTs). The cylinders remained pressurized and 

were further subsampled using a 0.4, 1.0 mL, or 10 mL sample 

loop (Figure 2), which also remained pressurized. At each 

sampling time 58 and 158 mL serum vials were injected with 

between 0.4 and 10 mL of formation fluids from pressurized 

sample loops. The fluid was introduced to each serum vial by 

piercing the Teflon septa and injecting the fluids from the 

sample loop. The introduction of pressurized formation fluids 

Figure 2. Sample loop (1 mL) used to inject caused gasses to exsolve from solution at atmospheric 

fluid samples into glass vials for transport and conditions and pressurized the glass sample vial slightly above 

analysis atmospheric pressure (< 0.03 MPa excess of atmospheric 

pressure). After the injection an additional ~2 mL of distilled 

water was used to flush the sample loop into the vial and assist in sealing the vial. These samples were classified 

as pressurized samples. An alternate sampling technique used a 10 mL sample loop to collect a subsample from 

the Monel cylinder. The sample loop would then be vented into a 58 mL (and then 158 mL) glass serum vials at 

atmospheric pressure. The sample loop was flushed with distilled water and the glass vial was sealed with an 

aluminum cap and septa. These samples were classified as unpressurized samples. Therefore, at each sampling 

time four subsamples were collected, pressurized samples in 158 and 58 mL vials and unpressurized samples in 

158 and 58 mL vials. In total over 200 samples were collected over a 9 day period. The samples were 

transported off-site for PFT analysis using a GC-ECD. 

The GC used for sample analysis was equipped with a 50 m RT-Alumina (Restek Corporation, Bellefonte, 

PA) capillary column and an ECD. Each of the samples was analyzed in a GC using duplicate 100 uL injections. 

Standards were analyzed at the beginning and end of each GC working day to ensure accuracy throughout the 

analysis. GC detection of PFTs was reported as an area count which could be converted to a concentration based 

on standard curves. Due to variation in original PFT injection volumes at the injection well, variable sized sample 

vials, and multiple PFTs, it was necessary to standardize and display the data as concentration divided by 

maximum concentration (C/Cmax) for each injection. 

After GC analysis was complete the data were viewed graphically as C/Cmax versus time. Time was 

displayed in hours after the start of the first CO2 injection (0 hrs, 10/04/04). PFT breakthrough times were 

calculated based on the peak arrival time of a given PFT minus the injection time minus the mean injection 

duration minus the U-tube travel time (1.83 hr) (see eq. 1). As the U-tube travel time varied during the 

experiment it was necessary to subtract the U-tube travel time from the 2nd and 3rd breakthroughs, but not the 1st

. 

PFT breakthrough times were used in calculations of a simple radial flow model, which was used to estimate the 

level of CO2 saturation within the Frio and interpret the ability of CO2 to bypass the denser and lower mobility 

brine. 

B = p – i – d – u (eq. 1) 

B = PFT breakthrough time 

p = peak arrival time 

i = injection time 

d = mean injection duration 

U = U-tube travel time (2nd and 3rd breakthroughs) 

Results 

Breakthrough times for the 3 PFT injections are summarized in Table 1. The variability in breakthrough 

times was minimal for all three injections. However, the 12 hr perfluoroethylcyclohexane (PECH) injection 

performed by the National Energy and Technology Laboratory (NETL) was introduced prior to a 31 hour stand 
 

 

down in CO2 injection. Table 2 shows that the breakthrough time varied from 50.3 to 51.5 hours (except for the 

PECH breakthrough). Duration time was 3.9 hours during injection 1 because a total of 1760 mL (3.2 kg) of 

PFTs were injected at 6 to 8 mL/min compared to approximately 200 mL (0.4 kg) of PFTs injected at the same 

rate during injections 2 and 3. Approximately 500 mL (0.9 kg) of PECH were injected at a rate of < 1 mL/min 

which resulted in a 12 hour injection period. 

Table 1. Injection time, injection duration, peak arrival time, and 

breakthrough time for PFT injections. Injection time and peak arrival 

time is given in hours after the start of the first CO2 injection. 

Injection 

Injection # Injection 

time (hrs) Duration 

(hrs) 

Peak Arrival 

Time (hrs) 

Breakthroughi 

Time (hrs) 

#1 (PMCH/PTCH) 1.9 3.9 54.1 50.3 

#2 (PMCP/PDCH)ii 102.8 0.6 156.6 51.7 

#3 (PMCH/PTCH)ii 120.1 0.5 173.4 51.2 

i Breakthrough time is calculated by subtracting the injection time and 

mean duration from the peak arrival time. 

ii In order to calculate breakthrough time an additional 1.83 hours was 

subtracted from the peak arrival time to account for U-tube travel time. 

The breakthrough times for 

the complete dataset can be 

graphically viewed as a C/Cmax 

versus time graph (Figure 3) showing 

each PFT before, after, and during 

breakthrough. PMCH and PTCH 

were introduced during injection 1. 

However, PMCP and PDCH which 

had not been introduced appear at low 

levels during the first breakthrough 

due to the use of the technical grade 

PFT solution (data not shown). A 

stand down time occurred from hours 

72 to 103 in which both the injection 

and monitoring well were not 

operated in order to observe pressure 

transients and perform geophysical tests. The PECH breakthrough (NETL injection) was also detected. 

Breakthrough of injection 2 (PMCP/PDCH) appeared as a well defined peak that spanned less than 20 hours. The 

3rd injection breakthrough is partially overlain by the PFTs of injection 2, but also spans approximately 20 hours. 

Figure 3. Dataset showing C/Cmax versus Time (hours after first CO2 injection) for the first, second, and third 

injection breakthroughs as well as the PECH breakthrough (NETL injection). 
 

 

 

 

Figure 4. C/Cmax versus time in hours after the start 

of the CO2 injection. Area shaded in gray 

corresponded to the transition from liquid to gassy 

samples. 

Figure 5. C/Cmax versus time in hours after the start 

of the CO2 injection. PECH injected by NETL was 

identified. 

PMCH and PTCH were introduced during 

injection 1, and their travel time was 50.3 hrs. Analysis 

of the first injection breakthrough (Figure 4) revealed a 

minor arrival that appeared larger and followed the first 

breakthrough. It is likely that the first breakthrough 

was smaller than the minor arrival due to the transition 

from liquid to gassy production at the monitoring well. 

The gassy samples arriving during the minor arrival 

(hours 60 plus) provided more concentrated PFT 

samples than the liquid samples that arrived prior 

(hours 50 to 60). Mass spectrographic (MS) data 

provided a more complete record during the liquid to 

gas transition. Inspection of the MS results revealed a 

larger initial breakthrough followed by a minor arrival 

of the PFT injection. 

A stand down time lasting from hours 72 to 103 

persisted after the breakthrough of the 1st injection. The 

second injection introduced PMCP and PDCH and was 

initiated after the stand down (Figure 5). The travel 

time for these PFTs (51.7 hours) was similar to that of 

the first injection. The breakthrough of PECH was 

observed prior to the second injection breakthrough. 

PECH exhibited a similar and largely overlapping 

elution time as PDCH. Identification of the PECH 

breakthrough peak was based on 1) the recognition of 

the chromatographic profiles of the PDCH peak, 2) the 

simultaneous arrival off PMCP and PDCH versus a 

single peak for PECH, 3) the earlier arrival time of 

PECH at the monitoring well, 4) and the peak 

broadening of PECH. 

As with the first injection breakthrough, the 

breakthrough of the second injection also revealed a 

minor arrival at a later time. Minor arrival from the 

second injection overlapped the breakthrough of the 

third injection. For the sake of clarity the conflicting 

PFTs have been removed from Figure 6. Prior to hour 

179 a dashed line indicates a data point that was 

estimated based on the PFT ratios within 3 out of 4 

vials because one of the sample vials had been 

contaminated and skewed the original trajectory. 

The travel times of the 3 injections could be 

used to calculate CO2 saturation using a simple radial 

flow model. This model treated the CO2 flow as a 

uniform disk expanding outward in a geologic 

formation. The model was expressed by: 

SCO2 = (Q t)/(πr

2

lφf) (eq. 2) 

Where: 

‘SCO2’ is percent saturation of CO2 

‘Q’ is volumetric flow 

‘t’ is travel time 

‘r’ is distance from well to well 

‘l’ is formation thickness 

‘φf’ is percent porosity 
 

Using known values for Q (19.9 m3

/hr), r (30 m), l (6.1 m), and φf (0.35) from the Frio formation and the PFT 

travel times from the 3 injections CO2 saturation maintained approximately 17% saturation from the first 

breakthrough to the third breakthrough (Table 2). 

Table 2. Percent CO2 saturation calculated using 

a simple radial flow model and the travel time at 

each injection 

Travel Estimated CO2 Injection # Time (hrs) Saturation (SCO2) 

#1 (PMCH/PTCH) 50.3 17 

#2 (PMCP/PDCH) 51.7 17 

#3 (PMCH/PTCH) 51.2 17 

Discussion 

Multiple PFT tracer suites were introduced 

Figure 6. C/Cmax versus time in hours after the start of via an injection well at three separate times during the 

the CO2 injection. Circled area indicates data points Frio Test. The PFTs were then sampled at a 

that were calculated based on PFT ratios within 3 out of monitoring well 30 meters up-dip. GC analysis was 

4 vials. used to detect the injected PFTs and generate 

breakthrough profiles for each injection. The use of 

PFT suites provided data for identification of multiple breakthroughs. Multiple travel times for each injection 

varied between 50.3 and 51.7 hours. 

The travel times for each injection provided data to estimate the CO2 saturation using a simple radial flow 

model. The low variance in travel times led to a low variance in calculated CO2 saturation of approximately 17 %. 

The lack of variability in CO2 saturation from the first to third injection implied that saturation and CO2 flow 

paths were rapidly established and persisted throughout the injection. This flow model does not take into account 

parameters such as buoyancy, relative permeability, and capillary pressure-saturation which more sophisticated 

models incorporate (e.g. Bachu et al., 2004) and are being used by other contributors for more detailed analysis of 

the Frio project. 

Lessons Learned 

A variety of sample collection methods were used to increase the likelihood of capture and transport of 

PFT subsamples. Samples were collected in 58 and 158 mL vials and identified as pressurized or unpressurized. 

It was later found that the pressurized samples had a tendency to leak before they could be analyzed even though 

the pressure in the vials was less than 0.03 MPa over atmospheric pressure. This was in part due to the punctures 

in the Teflon septa of the pressurized vials. The septa were punctured when the sample was transferred from the 

sample loop to the serum via needles. The small pin hole created by the needle provided an escape route for 

fluids within the vial. Vials that contained leaks were identified by a fluid level below the mineral rim, or if no 

fluid was present, or if external mineral precipitation was identified. It was also found that the needles would 

often clog or partially clog with particulates, which would result in fluctuations in concentration between samples. 

Future experiments need not use pressurized vials or needles and 58 mL vials could be used exclusively and could 

be collected in triplicate. Unpressurized vials avoided these problems and showed less fluctuation between vials. 

The use of this sampling technique was efficient and has become more streamlined. This method could readily be 

scaled from the 200 samples collected to handle larger sampling scenarios in the range of greater than 1000 

samples. 
Acknowledgements 

We acknowledge the collaborative support of the entire Frio Project team especially the Texas Bureau of 

Economic Geology. We are particularly grateful to Edward A. Miller for the organization and oversight of the 

field operations. Oak Ridge National Laboratory is managed by UT-Battelle, LLC, for the U.S. Department 

of Energy under contract DE-AC05-00OR22725. 
References 

Dietz, R.N., 1986. Perfluorocarbon Tracer Technology. Regional and Long-range Transport of Air 

Pollution, Lectures of a course held at the Joint Research Centre, Ispra, Italy, S. Sandroni (Ed.), 

pp. 215-247. 

Bachu, S., J. Nordbotten, and M. Celia, 2004. Evaluation of the spread of acid gas plumes injected in 

deep saline aquifers in western Canada as an analogue for CO2 injection in continental 

sedimentary basins. Proceedings of the 7th International Conference on Greenhouse Gas Control 

Technologies, September 5–9, 2004, Vancouver, B.C., Canada. 

Freifeld, B.M., Trautz, C., Kharaka, Y.K., Phelps, T.J., Myer, L.R., Hovorka, S.D., and Collins, D., 

Manuscript submitted. The U-Tube: A Novel System for Acquiring Borehole Fluid Samples 

from a Deep Geologic CO2 Sequestration Experiment. JGR-Solid Earth. 

IEA Greenhouse Gas R&D Programme, 2001. Putting Carbon back into the Ground. ISBN 1 898373 28 

0. 

Lagomarsino, R.J., 1996. An Improved Gas Chromatographic Method for the Determination of 

Perfluorcarbon Tracers in the Atmosphere. Journal of Chromatographic Science, 34, 405-412. 

McKinley, J.P. and Colwell, F.S., 1996. Application of Perfluorocarbon Tracers to Microbial Sampling 

in Subsurface Environments using Mud-Rotary and Air-Rotary Drilling Techniques. Journal of 

Microbiological Methods, v. 26, pp. 1-9. 

Phelps, T. J., and J. K. Fredrickson. 2001. Drilling, coring and sampling subsurface environments. In: 

Manual of Environmental Microbiology (M. McInerney ed.) ASM Press. pp. 679-696.95. 

Senum, G.I., Dietz, R.N., D’Ottavio, T.W., Goodrich, R.W., Cote, E.A., and Spandau, D.J., 1990. A 

Perfluorocarbon Tracer Transport and Dispersion Experiment in the North Sea Ekofisk Oil Field. 

Paper prepared for U.S. Department of Energy Assistant Secretary for Fossil Energy, Prepared 

by Brookhaven National Laboratory Department of Applied Science Environmental Chemistry 

Division Tracer Technology Center, Upton, NY. 

Straume, A.G., Dietz, R.N., Koffi, E.D., and Nodop, K, 1998. Perfluorocarbon Background 

Concentrations in Europe, Atmospheric Environment, v. 32, n. 24, pp. 4109-4122. 

Sullivan, T.M., Heiser, J., Gard, A., and Senum, G., 1998, Journal of Environmental Engineering, June, 

pp. 490-497.