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| Titel |
Rapid detection and characterization of surface CO2 leakage through the real-time measurement of δ13C signatures in CO2 flux from the ground |
| VerfasserIn |
Samuel Krevor, Sally Benson, Chris Rella, Jean-Christophe Perrin, Ariel Esposito, Eric Crosson |
| Konferenz |
EGU General Assembly 2010
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 12 (2010) |
| Datensatznummer |
250032974
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| Zusammenfassung |
| The surface monitoring of CO2 over geologic sequestration sites will be an essential tool in
the monitoring and verification of sequestration projects. Surface monitoring is the only tool
that currently provides the opportunity to detect and quantify leakages on the order of 1000
tons/year CO2. Near-surface detection and quantification can be made complicated, however,
due to large temporal and spatial variations in natural background CO2 fluxes from biological
processes. In addition, current surface monitoring technologies, such as the use of IR
spectroscopy in eddy covariance towers and aerial surveys, radioactive or noble gas isotopic
tracers, and flux chamber gas measurements can generally accomplish one or two of the
necessary tasks of leak detection, identification, and quantification, at both large spatial scales
and high spatial resolution. It would be useful, however, to combine the utility of
these technologies so that a much simplified surface monitoring program can be
deployed.
Carbon isotopes of CO2 provide an opportunity to distinguish between natural biogenic
CO2 fluxes from the ground and CO2 leaking from a sequestration reservoir that has ultimate
origins in a process giving it a distinct isotopic signature such as natural gas processing. Until
recently, measuring isotopic compositions of gases was a time-consuming and expensive
process utilizing mass-spectrometry, not practical for deployment in a high-resolution survey
of a potential leakage site at the surface. Recent developments in commercially available
instruments utilizing wavelength scanned cavity ringdown spectroscopy (WS-CRDS) and
Fourier transform infrared spectroscopy (FT-IR) have made it possible to rapidly measure the
isotopic composition of gases including the 13C and 12C isotopic composition of CO2 in a
field setting.
A portable stable carbon isotope ratio analyzer for carbon dioxide, based on wavelength
scanned cavity ringdown spectroscopy, has been used to rapidly detect and characterize an
intentional leakage of CO2 from an underground pipeline at the ZERT experimental
facility in Bozeman, Montana. Rapid (Â 1 hour) walking surveys of the entire 100m x
100m site were collected using this mobile, real-time instrument. The resulting
concentration and 13C isotopic abundance maps were processed using simple yet powerful
analysis techniques, permitting not only the identification of specific leakage locations,
but providing the ability to distinguish petrogenic sources of CO2 from biogenic
sources.
At the site an approximately 100-meter horizontal well has been drilled below an alfalfa
field at a depth between 1-3 meters below the surface. The well has perforations along the
central 70 meters of the well. The overlying strata are highly permeable sand, silt, and topsoil.
For 30 days starting July 15, 2009, CO2 was injected at a rate of 0.2 tonnes per day. The
injection rate is designed to simulate leakage from a mature storage reservoir at an
annual rate of between .001 and .01%. The isotopic composition of the gas from
the tank is at δ13C signature of approximately -52 parts per thousand (per mil),
far more negative than either atmospheric (approx. -8 per mil) or CO2 from soil
respiration (approx. -26 per mil) at the site. The CO2 isotopic and concentration
measurements were taken with a Picarro WS-CRDS analyzer with 1/8” tubing
connected to a sampling inlet. Simultaneous with CO2 concentrations (including
13C), position data was logged using a GPS receiver. Datapoints are taken around
every second. The analyzer was powered using batteries and housed in a mobile
cart.
The surveys consisted of traverses of the site along the length of the pipeline and
extending out 100 meters on either side of the pipeline with the end of the gas inlet tube
approximate 9 cm above the ground at a speed of 1-2m/sec. This simulates the type of survey
that could be easily performed if the actual or potential site of a leak was known to within an
area on the order of 100 square kilometers or less, the scale of expected industrial CO2
sequestration operations. The surveys were performed both during the day and
during the evening when CO2 flux due to respiration from the soil is markedly
different.
Keeling plots were used to characterize the spatially varying 13C composition of ground
source CO2 across the site. A map constructed from this data shows that CO2 flux from
sources of leakage was characterized by a δ13C of -40 per mil or less whereas
locations away from the leakage spots had much higher δ13C signatures, -25 per mil or
higher.
The distinct isotopic signature allows for a clear discernment between leakage of
petrogenic CO2 and that of natural CO2 fluxes from soil respiration. This is particularly
valuable in the circumstance where the leak is slow enough that it could not be
identified from CO2 concentration or flux changes above the natural background
signal alone. Moreover, this detection took place both rapidly and at high spatial
resolution. Samples collected from a mobile platform moving at the rate and with the
sampling frequency used in this study could provide a 1000 km of survey traverses over
an area of 100 km2 within 2-3 weeks. This provides a powerful tool for surface
monitoring, combining the utilities of leak detection, characterization, and source
identification with rapid deployment across large spatial scales and high spatial
resolutions. |
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