Читать книгу Geophysical Monitoring for Geologic Carbon Storage - Группа авторов - Страница 51

Before conducting field experiments, the in situ instrument is calibrated and validated with ¹²CO₂ and ¹³CO₂ reference standards. This involves flowing reference CO₂ through the White cell as it would be used in the field. The CO₂ also flows from the White cell and into a LICOR instrument. The FMS trace similar to those in Figure 3.7 was recorded and the peak‐to‐peak intensities were extracted from the trace. Calibration curves were generated by plotting the peak‐to‐peak intensities against the CO₂ concentration in the White cell as depicted in Figure 3.8. The response over this concentration range is very linear, as expected.

Finally, the instrument was deployed at many locations over many years. By far, most of the data were collected during an annual trip to the Zero Emission Research and Technology (ZERT) field site on the Montana State University campus in Bozeman, Montana (Spangler et al., 2010). The ZERT field site consisted of a horizontally drilled well about 6 ft deep. The ZERT facilitators would use mass flow controllers to deliver a specific amount of CO₂ through the well that would leak to the surface simulating a leak from a geologic sequestration site.

The FMS instrument used a small pump to deliver atmospheric gases through a flexible hose into the White cell. The hose was routinely moved from locations over and around the well where leaks were identified. The hose was also placed in locations away from the well as a background. The instrument could collect and analyze a sample every hour autonomously. However, some experiments involved collecting gases in a sealed glass flask for isotope ratio mass spectrometer (IRMS) analysis at LANL. These samples were designed to validate the isotopic ratios recorded in the field. Figure 3.9 contains a plot of the FMS results observed against the IRMS results. The plot demonstrates that most of the results are within ± 20% relative agreement while there were a few data points each year that were just outside the ± 20% relative agreement. These data demonstrate that the FMS instrument is capable of making real‐time stable isotope measurements as accurately as the IRMS at these observed concentration levels. These experiments also suggest that stable isotope ratios below ‐15% represent seepage from the reservoir.