Application of inert gas tracers to identify the physical ...
Transcript of Application of inert gas tracers to identify the physical ...
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: [email protected]
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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.
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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.
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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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