Abstract
A newly developed isotope ratio laser spectrometer for CO2 analyses has been tested during a tracer experiment at the Ketzin pilot site (northern Germany) for CO2 storage. For the experiment, 500 tons of CO2 from a natural CO2 reservoir was injected in supercritical state into the reservoir. The carbon stable isotope value (δ(13)C) of injected CO2 was significantly different from background values. In order to observe the breakthrough of the isotope tracer continuously, the new instruments were connected to a stainless steel riser tube that was installed in an observation well. The laser instrument is based on tunable laser direct absorption in the mid-infrared. The instrument recorded a continuous 10 day carbon stable isotope data set with 30 min resolution directly on-site in a field-based laboratory container during a tracer experiment. To test the instruments performance and accuracy the monitoring campaign was accompanied by daily CO2 sampling for laboratory analyses with isotope ratio mass spectrometry (IRMS). The carbon stable isotope ratios measured by conventional IRMS technique and by the new mid-infrared laser spectrometer agree remarkably well within analytical precision. This proves the capability of the new mid-infrared direct absorption technique to measure high precision and accurate real-time stable isotope data directly in the field. The laser spectroscopy data revealed for the first time a prior to this experiment unknown, intensive dynamic with fast changing δ(13)C values. The arrival pattern of the tracer suggest that the observed fluctuations were probably caused by migration along separate and distinct preferential flow paths between injection well and observation well. The short-term variances as observed in this study might have been missed during previous works that applied laboratory-based IRMS analysis. The new technique could contribute to a better tracing of the migration of the underground CO2 plume and help to ensure the long-term integrity of the reservoir.
Highlights
Stable isotope analyses provide a powerful tool to trace the fate of CO2 in the subsurface.[5−8] Stable isotopes of injected CO2 can act as useful tracers in carbon capture and storage (CCS) because the CO2 itself is the carrier of the tracer signal and remains unaffected by sorption or partitioning effects
Carbon stable isotopes can be used to unravel the role of subsurface processes such as carbonate mineral dissolution or dissimilatory bacterial sulfate reduction (BSR) that can influence the isotope geochemistry of the injected CO2.9 isotope techniques can help to improve our understanding about processes in the subsurface during CO2 injection and to increase the applicability of reservoir safety monitoring programs
Optical instruments are available that use quantum cascade laser absorption spectroscopy (QCLS; Aerodyne Research Inc., Billerica, MA, U.S.A.), Fourier transform infrared spectroscopy (FT-IR; Ecotech Pty Ltd., Knoxville, Victoria, Australia), or the older, discontinued tunable-diode laser absorption spectrometer TGA200 (TDLAS; Campbell Scientific Inc., Logan, UT, U.S.A.)
Summary
Stable isotope analyses provide a powerful tool to trace the fate of CO2 in the subsurface.[5−8] Stable isotopes of injected CO2 can act as useful tracers in CCS because the CO2 itself is the carrier of the tracer signal and remains unaffected by sorption or partitioning effects. The instrument recorded a continuous 10 day carbon stable isotope data set with 30 min resolution directly on-site in a field-based laboratory container during a tracer experiment. To test the instruments performance and accuracy the monitoring campaign was accompanied by daily CO2 sampling for laboratory analyses with isotope ratio mass spectrometry (IRMS).
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