Underground Fiber-Optic Differential Absorption Instrument for Monitoring Carbon Dioxide Soil Gas Concentrations for Carbon Sequestration Site Monitoring
Abstract
The burning of fossil fuels has resulted in higher carbon dioxide (CO2) concentrations in the atmosphere with potential impacts on the Earth's climate. The use of fossil fuels is predicted to grow over the next several decades with the potential for further increasing the atmospheric concentration of CO2. A proposed method of diminishing the impacts of increased CO2 on the Earth's climate is to capture and store the CO2 in geologic storage sites. One issue with underground sequestration of CO2 is the ability to monitor sequestration sites to verify the integrity of the storage of the CO2. An underground fiber optic differential absorption instrument based on a tunable distributed feedback (DFB) diode laser is being developed at Montana State University to detect small changes in CO2 soil gas concentration in an effort to monitor the overall integrity of the sequestration storage site. The fiber optic instrument exploits the 2003-2006 nm region of the spectrum which contains four CO2 absorption lines. Light from the DFB laser is delivered to an underground absorption cell one meter in length via a single mode optical fiber. The normalized transmission is measured by tuning the DFB diode laser across these four absorption lines and the results are used to determine the CO2 soil gas concentration. A description of this instrument will be presented including the instrument design, operation, and performance characteristics. A field site for testing the performance of CO2 detection instruments and techniques has been developed by the Zero Emissions Research Technology (ZERT) group at Montana State University. The field site allows a controlled flow rate of CO2 to be released underground through a 100 m long horizontal pipe placed below the water table. Two release experiments were performed this past summer with flow rates of 0.1 and 0.3 tons CO2/day. The first release experiment lasted ten days while the second release lasted seven days. Measurements taken with the underground fiber optic differential absorption instrument 2.5 feet below the surface of the soil during these release experiments showed an absolute CO2 soil gas concentration exceeding 100,000 parts per million, an increase by a factor of greater than ten over the background levels of CO2 soil gas concentrations. These results indicate the capabilities of the below ground differential absorption instrument for carbon sequestration site monitoring.
- Publication:
-
AGU Fall Meeting Abstracts
- Pub Date:
- December 2007
- Bibcode:
- 2007AGUFM.H13F1658N
- Keywords:
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- 0480 Remote sensing;
- 0649 Optics (4264);
- 3934 Optical;
- infrared;
- and Raman spectroscopy