Measurements of CO2 Carbon Stable Isotopes at Artificial and Natural Analog Sites

Carbon storage in geologic formations is one method to prevent carbon dioxide (CO2), produced by fossil fuel combustion, from entering the Earth's atmosphere. The monitoring, verification and accounting (MVA) of geologically sequestered CO2 is critical to the operation of a geologic storage site. Surface MVA techniques need to identify seepage from the sequestration reservoir at or below ambient CO2 concentrations. The CO2 carbon stable isotope ratio of is a sensitive diagnostic signature that can distinguish between anthropogenic and natural sources of CO2. Frequency Modulated spectroscopy (FMS) is an ultra-sensitive version of absorption spectroscopy that is capable of detecting the CO2 carbon stable isotope ratios. The technique involves phase modulation of the laser such that two side bands, spaced wider than the absorption feature of interest (in this case +/-2 GHz) are created. The signal is mixed with the local oscillator yielding a signal proportional to the species concentration. This FMS signature is recorded at multiple wavelengths to obtain the CO2 carbon isotope ratio.Two instruments using the FMS technique have been built and tested at LANL. One instrument draws ambient air into a multi-pass cell for a measurement, point source measurements. The other instrument uses an open-air path, tested up to 160 m (round trip), to measure the CO2 carbon isotopic ratio along the beam path, column average measurements. In this paper, results from multiple field deployments of one or both of the instruments will be presented. The Zero Emissions Research & Technology (ZERT) group at Montana State University established a field test site where controlled amounts of CO2 are released to test the performance of CO2 detection instruments and measurement techniques. The field site allows a controlled flow rate of CO2 to be released into the near surface through a 100 m long horizontal pipe. In July of 2009, a release was conducted, with a uniform flow rate of 0.2 tons per day, as the subsequent seepage was measured. There was a similar release, but at a flow rate of 0.15 tons/day, in July 2010. Stable isotope measurements have also been made at several natural analog sites. Two places of interest are the Valles Caldera National Preserve in NM and Soda Springs, ID. The Valle traps CO2 at night and can have very large swings in concentrations that test the instrument range. Soda Springs, ID has many carbonated natural springs and carbon isotope information from this site can provide information regarding CO2 from the deep subsurface, useful for future MVA work.