Fiber Optic Distributed Sensors for Carbon Dioxide Detection in Carbon Capture and Sequestration (or Fiber Optic Distribution Sensors for CO2 detection in CCS)
Reliable and cost-effective monitoring is important to making gas sequestration safe. A wide variety of monitoring techniques are being evaluated, including laboratory analysis of groundwater (pH and alkalinity) and vadose zone samples. Current methods of the direct analysis of groundwater (sampling followed by laboratory analysis), or in the vadose zone, where chemical sensors monitor gas flux in only one location, are not cost effective for large areas and long-term monitoring.
Desirable analytical systems will:
- Collect reliable data
- Cover large areas
- Monitor continuously
- Operate for years with little or no maintenance
- Be cost effective.
Distributed Fiber Optic Sensor for CO2 in CCS
Gases leaking from a deep storage reservoir are likely to intercept groundwater resources. Monitoring dissolved carbon dioxide is the most direct way to detect and quantify a leak reaching underground sources of sensitive water. IOS has developed robust distributed fiber optic sensors for carbon dioxide that will eliminate the need for point sensors to protect groundwater.
The entire length of a segment of the optical fiber is the sensor, capable of covering a large area and depths ranging over hundreds of meters.
Fiber Optic Sensor Response to Carbon Dioxide
A silica glass core fiber is coated with a polymer cladding containing a colorimetric indicator. Upon exposure of any segment of the fiber, the CO2 diffuses into the cladding and changes the color of the indicator.
The segment of fiber sensor exposed to CO2 changes color.
When light passes through any optical fiber, an “evanescent field” of optical energy extends into the fiber cladding. In our sensors the evanescent field is used to interrogate the optical properties of the cladding. A light source is at one end of the fiber and a photodetector at the other end. The light transmitted through the fiber varies with the concentration of CO2.
Transmission as Function of Dissolved Carbon Dioxide
The transmission of light (at a specific wavelength) increases with the concentration of dissolved CO2 in water. The transmission change is reversible, so the varying concentration of dissolved CO2 can be tracked.
Shallow Water Monitoring: Controlled CO2 Release in the Field
Field demonstration of CO2 leakage detection was conducted with a pulse-like CO2‑release test in a shallow aquifer. We monitored alkalinity, pH, and dissolved inorganic carbon (DIC) in groundwater, using the periodic groundwater sampling method and a fiber optic CO2 sensor for the real-time, in-situ monitoring.
Measurements taken with the fiber optic CO2 sensor displayed obvious signals of leakage, demonstrating the potential of real-time, in-situ monitoring of dissolved CO2 for leakage detection at a geologic carbon sequestration site.
Measurements of groundwater pH, alkalinity, DIC, and dissolved CO2 clearly deviated from their background values, showing response to CO2 leakage. Quantitation of dissolved CO2 in the tests was much more sensitive than that of groundwater pH, DIC, and alkalinity.
Plots of (a) pH; (b) alkalinity; (c) DIC; and (d) dissolved CO2 compared with modeling results. Elapsed time represents time after first CO2 release was stopped. Error bars represent 95% confidence intervals, estimated on the basis of calibration data and duplicate measurements.