14th International Conference on Greenhouse Gas Control Technologies, GHGT-14

21st -25

th October 2018, Melbourne, Australia

Application of inert gas tracers to identify the physical processes

governing the mass balance problem of leaking CO2 in shallow

groundwater system

YeoJin Jua, Seong-Sun Lee

a, Dugin Kaown

a, Kang-Kun Lee

a*

aSchool of Earth and Environmental Sciences, Seoul National University, 1 Gwanak-ro, Gwanakgu,

Seoul, 08826, South Korea

Abstract

The shallow groundwater is last trapping zone of leaking CO2 with measurable retention time. As the success of

Carbon Capture and Storage (CCS) project is defined by loss of injected CO2, therefore, it is important to understand the mass

balance problem of leaking CO2 in shallow groundwater system. The mass balance CO2 is largely governed by solubility

controlled trapping mechanism in shallow groundwater such as phase-partitioning and degassing process. Inert gases such as

sulfur hexafluoride (SF6) and noble gases are biochemically stable in groundwater system which can separate and explain the

solubility controlled physical process in complicated groundwater system. Two pilot tests were preceded by the artificial CO2

injection test. First injection test was made in Wonju, Korea. Three different tracers including chlorine, helium and argon tracer

were jointly injected in 7.5 – 10.5 m below water table and recollected at same point after 1 day drift time in groundwater system.

The mass recovery curve of the Single-Well Tracer Test (SWTT) was 49.3% of SF6, 58.1% of helium, 78.2% of argon, and

73.1% of chlorine. The recovery of inert gas tracer was relative to their solubility. Second injection test was made in Eumseong,

Korea. Four tracers such as chlorine, helium, argon, and krypton were released along an induced pressure gradient where

0.0133% of helium, 0.0162% of argon, 0.0528% of krypton, and 75.0% of chlorine were retrieved. In this Inter-Well Tracer Test

(IWTT), the recovery of inert gas tracer was also relative to tracer’s solubility. This study insights the possibility of inert gas

tracers to identify the mass balance problem of leaking CO2 in shallow aquifer system.

Keywords: Carbon capture and storage; CO2 leakage; noble gas; shallow aquifer; multi-phase plume

1. Introduction

The leaking CO2 is finally lost into atmosphere after undergoing several trapping mechanisms in subsurface

system such as structural/stratigraphic trapping, residual trapping, solubility trapping, and mineral trapping [1]. The

process which controlling the initial mass balance of leaking CO2 is solubility trapping mechanism where the initial

CO2 plume of high partial pressure is firstly phase-partitioned and degassed in relative to solubility until the plume is

stabilized in shallow groundwater system. The depressurized plume is gradually diluted with local groundwater

during transport. Therefore, physical processes such as degassing and dilution process should be thoroughly

reviewed to understand the mass balance of leaking CO2 in shallow aquifer system.

* Corresponding author. Tel.: +82-2-873-3647; fax: +82-2-873-3647.

E-mail address: kklee@snu.ac.kr

2 GHGT-14 Author name

The naturally occurring CO2 is the function of chemical, biological, and physical process having a wide

distribution in natural system. The variable sources and sinks of CO2 is an obstacle to detect the CO2 leakage as

various biochemical processes involving in CO2 anomaly diluting the signal for the CO2 leakage detection [2]. One

the other hand, noble gas is chemically and biologically inert and only constrained by physical process in shallow

aquifer system. Therefore, the noble gas can be used to separate and explain the physical process from biochemical

processes [3]. For example, the noble gas can selectively explain the phase partitioning or degassing process in

groundwater system as it has a fractionation according to solubility controlled process. In CCS project, the noble gas

has been applied for identifying a preferential pathway, but also for mass balance problem of leaking CO2 [4], [5],

[6], [7].

This study focused on mass balance problem of leaking multi-phase plume. We used inert gas tracers such as

sulfur hexafluoride (SF6) and noble gases to entangle and explain the physical process from chemically and

biologically complicated system. We monitored the noble gas concentration trying to constrain the major physical

theorem governing the CO2 mass balance.

2. method and materials

2.1. Site selection for the pilot test

Wonju and Eumseong, Korea were selected for the pilot test because they are similar in hydrogeological feature.

The aquifer of both sites consists of weathered and highly fractured rock overlain by soil and alluvial deposits. A

detailed description was made in [8] and [9] for Wonju site and [10] for Eumseong site. In Wonju site, the aquifer

thickness is 10 – 15 m and the observed water table was 2 – 13 m below ground surface. And the hydraulic gradient

varies from 0.008 to 0.023 m and the hydraulic conductivity values ranging between 2.0 × 10−4

cm/s and 4.2 × 10−3

cm/s. On the other hand, the water table is made at 16 – 17 m below ground surface and the hydraulic gradient is

ranging from 0.01 to 0.05 for Eumseong, Korea. Also, the hydraulic conductivity was identified from pumping test

ranging from 4.0 × 10−6

m/sec to 2.0 × 10−5

m/sec. The push-and-pull test was made in the site and identified the

groundwater linear velocity (0.06 – 0.44 m/d), effective porosity (0.02 – 0.23), and aquifer thickness of 47 m.

2.2. Injection of artificially enhanced tracers

Tracer-mixed groundwater was prepared and injected into groundwater. Firstly, tracer-infused groundwater was

made by blowing inert gases into local groundwater using gas bombe, regulator, ball-flowmeter, flexible lines,

silencer (for gas diffusion), and carboy bottle (Fig. 1). And each tracer-infused groundwater was gently mixed

together in the order of their solubility. The injection was made with a submersible and controllable quantitative

pump, MP1 (Grundfos, Bjerringbro, Denmark). The sample of initial tracer-mixed groundwater (C0) was collected

during injection period and salinity was measured in-situ using a portable equipment, YSI (YSI Inc./Xylem Inc.,

USA).

Fig. 1. Tracer infused groundwater.

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The first pilot test was made in Wonju, Korea as a single-well tracer test (SWTT). The SWTT is three-step

process consists of push, drift, and pull period. During push period, injection of tracer-mixed groundwater (200 L)

and chaser fluid (120 L) was made at the speed of 9.30 L/min (September 22, 2016). The injection well was a full-

screened well and 27 m in length. The target injection zone was 24 – 27 m below ground surface and 10 m below

water table. Packer was used to tightly seal the 1 m length above the injection zone. After the injection, the injected

tracer-infused water had a drift time of 1522 min in groundwater system, and then, tracers were recollected for 319

min at the speed of 3.87 L/min (September 23, 2016). The samples for inert gas tracers were collected every 2 – 10

min which was gradually increased. The salinity was measured in-situ using a portable equipment, YSI (YSI

Inc./Xylem Inc, USA) in every 2 – 10 min.

The second pilot test was made in Eumseong, Korea as an inter-well tracer test (IWTT). Before tracer injection,

induced hydraulic gradient of 0.93 was firstly made between two wells. The tracer-mixed groundwater of 1000 L

was injected into injection well (IW) at the speed of 4.74 L/min by tightly packing the 1 m length above injection

zone (October 13, 2016). The target injection zone was 21 – 24 m below ground surface and 4.5 m below water

table. The injection event was followed by 10 days of monitoring period (to October 22, 2016). The samples for

inert gas tracers were collected every 2 – 5 hour which was gradually increased. The salinity was measured in-situ

using a portable equipment, YSI (YSI Inc./Xylem Inc, USA) in every 2 – 5 hours.

2.3. Data acquisition

Samples were analyzed in Korea Polar Research Institute (KOPRI). 28cc of groundwater was sampled using

copper tube and stainless steel clamps to prevent the air contamination. Firstly, dissolved gas was extracted from

groundwater and stored in aluminosilicate ampoule. Therefore, most of water was removed during extraction

process. And some residual water vapor, active gases, and other abundant gases were additionally screened in

automated purification processing line [11] using cryogenic traps and getters pumps (hot and cold St 101 ZrAl alloy).

Then, each component of noble gas tracers was separated and drawn down into mass spectrometer, RGA200

(Stanford Research Systems, California, USA) starting from low to high mass. Helium, argon and krypton were

converted into concentration value using air standard from 0.9 cc to 2.7 cc considering the wide concentration range

of artificially enhanced tracers. The difference of duplicated samples was below 5% and, discrepancy with the result

from the University of Utah was below 7.8%. The water samples for Sulfur Hexafluoride (SF6) were collected in

glass bottles and sealed tightly using caps with metal liners. The sample analysis was automatically preceded at Core

Laboratory of Innovative Marine and Atmospheric Technology (CLIMATE), Pohang University of Science and

Technology (Postech) [12].

3. Results and Discussion

3.1. Observation of time-graph curve

The injected concentration for SWTT was 68.6 ppb of sulfur hexafluoride (SF6), 2.05E-06 ccSTP/g of helium,

3.06E-03 ccSTP/g of argon, 10.7 ppt of salt was injected in shallow aquifer. In Breakthrough curves (BTCs), all of

tracers were showing an identical pattern (Fig. 2). Double peaked shape in all of the tracers might be attributed to

stagnant mass of injected tracers where they didn’t move along the local groundwater staying around injection point.

The stagnant mass was firstly retrieved back to injection well during pull-period before the arrival of main plume.

That can be verified by the stagnant volume which calculated by multiplication of pumping rate and arrival time of

the first peak, and then, the obtained volume was comparable to injected chaser amount. In mass recovery curves,

injected tracers didn’t show 100% mass recovery where SF6 was 49.3%, helium was 58.1%, argon was 78.2%, and

salt was 73.1% (Fig. 2). The inert gas tracers clearly showed the mass recovery relative to their solubility. The

solubility controlled processes possibly affected the mass balance of gas tracers.

4 GHGT-14 Author name

Fig. 2. Breakthrough curves (BTCs) of SWTT.

The injection concentration for IWTT was 4.02E-06 ccSTP/g of helium, 2.29E-02 ccSTP/g of argon, 1.05E-05

ccSTP/g of krypton, and 5.87 ppt of salt was injected in shallow aquifer. In BTCs, the krypton followed the trend of

conservative salt tracer except mass deficiency around plateau. But the helium and argon tracers were irregular in

shape and few points even degassed below local background (Fig. 3). The degassing loss of gas tracers was

indicated as a retardation coefficient in solute transport problem [13]. And it was successfully identified with

partitioning and decay coefficient in [14] under two dimensionless zone (mobile zone of groundwater and immobile

zone of gas bubble). However, the partitioning behavior seemed irregular in this IWTT, therefore, not capable of

such a kinetic explanation. The mass recovery of IWTT was lower than 100% where helium was 0.0135%, argon

was 0.0137%, krypton was 0.0602%, and salt was 75.0% (Fig. 3). And the retrieved amount was again clearly in the

order of their solubility. Therefore, the solubility controlled processes needed to be identified.

Fig. 3. Breakthrough curves (BTCs) of IWTT.

4. Conclusion

Two pilot tests were preceded by the artificial CO2 injection test. Inert gas tracers were used to separate and

explain the major processes involved in mass balance problem of leaking CO2. In Wonju site, mass reduction of

injected tracers was attributed to solubility controlled process. In Eumseong site, retrieved tracer was also related to

the tracer’s solubility, however, far lower than the Wonju site. The enhanced tracers successfully produced a strong

signal far above natural background concentration and verify their potential for tracking the leaking CO2.

GHGT-14 Author name 5

Acknowledgements

This study was supported by the “R&D Project on Environmental Management of Geologic CO2 Storage” from

the KEITI (Project Number: 2018001810002).

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