Storing carbon as a supercritical fluid in deep geological reservoirs is crucial for reducing carbon dioxide (CO2) emissions from industrial processes. For the safe geological storage of carbon, strategies for detecting, quantifying, and managing carbon dioxide leakage are critical. Monitoring solution-phase chemistry in geological formations is one such strategy.
Despite developing several in situ and on-site environmental monitoring technologies, a cost-effective and affordable sensing platform that can remotely interrogate broad geographic regions with varying topography or locations inaccessible to traditional sensing technologies is still needed.
Different Systems Used for CO2 Detection
Optical Sensor Based on Non-Dispersed Infrared (IR) Absorption
One of the most commonly used systems for detecting gas-phased or dissolved carbon dioxide is an optical sensor based on non-dispersed infrared (IR) absorption. This system is advantageous due to its robust, sensitive, and very selective nature. The drawback of using such a system is that it is bulky and expensive and requires a large sensing volume, making it unsuitable for mobile sensing applications.
Fiber Optic Sensors
Waveguide-based fiber optic sensors (FO sensors) have distinct properties such as a flexible design for in situ and in vivo analysis, long-range remote readout capabilities, and potential miniaturization for a cost-effective mobile sensing system.
Due to these advantageous properties, various fiber optic sensors have been used for real-time monitoring of geological CO2.
Types of Fiber Optics Sensors
Several fiber optic sensors with different variations have been developed in the past by scientists. For example, distributed fiber optic sensors based on hybrid Brillouin−Rayleigh backscattering, fiber optic sensor system based on the refractive index of surrounding to differentiate between supercritical carbon dioxide and carbon dioxide saturated brine, and a hollow-core photonic crystal fiber optic sensor.
However, these fiber optic sensors have a significant drawback of using broad-band laser sources and an optical backscatter reflectometer. This makes them unsuitable as a portable field prototype of an optical system.
Similarly, in another fiber optic sensor for monitoring dissolved CO2 in groundwater, the presence of CO2 modified the color of a pH indicator dye bound in a polymer matrix, altering light propagation through total internal reflection.
How the Mixed-Matrix Composite Integrated Fiber Optic Sensor was Developed
In a natural environment, the reliable detection of carbon dioxide is ensured only when the cross-sensitivity of water vapors is minimized.
Hydrophobic zeolites have molecule absorbing ability in high humid conditions. Similarly, plasmonic nanocrystals (NCP), such as Indium-tin oxide (ITO), and integrated fiber optic sensors are beneficial for sensing optical gas due to their improved sensitivity compared with traditional spectroscopy methods.
In this study, the researchers developed a cost-effective real-time monitoring system, integrated with mixed-matrix composite, capable of monitoring carbon dioxide in natural water above carbon storage reservoirs.
For this purpose, solution-stable ITO NCP was synthesized, which was then combined with hydrophobic zeolite (ZHP) particles in a cross-linked polymer matrix (PCL).
This formed a novel mixed-matrix composite NCPZHPPCL able to detect a range of gas-phase and dissolved carbon dioxide in natural waters.
This sensor is ideal for applications requiring long-distance sensing since the wet deposition procedure can be applied to generate coated fiber optics in long lengths.
Advantages of Mixed-Matrix Composite Integrated Fiber Optic Sensor
Based on the testing outcomes in actual circumstances and under controlled laboratory conditions, the mixed-matrix composite integrated fiber optic sensor showed quicker reaction and recovery times than a commercial IR sensor and effectiveness when exposed to various water sources.
The researchers used a single NCPZHPPCL fiber optic sensor for all the testing, including laboratory and field tests, indicating the sensor’s exceptional environmental stability.
A mixed-matrix composite integrated fiber optic sensor system, capable of detecting and quantifying gas-phase and dissolved carbon dioxide, ensures safe and cost-effective monitoring of carbon storage areas. The coating material of this fiber optic sensor consists of plasmonic nanocrystals and hydrophobic zeolite embedded in a polymer matrix.
The mixed-matrix composites demonstrated remarkable stability and sensitivity over an extensive concentration range in high humid conditions. This exceptional sensing performance was possible due to the properties of plasmonic nanocrystals and hydrophobic zeolite, which significantly enhanced sensitivity and effectively mitigated interference from water vapor.
Over many cycles, reproducibility was verified in the field and lab environment. More significantly, the research illustrated the potential for online monitoring via a wireless telemetry device that sent data from the field to a website.
This mixed-matrix composite fiber optic sensor is a strong contender for real-world carbon storage applications due to its excellent CO2 detection capabilities, simple coating processability and cost-effectiveness.
Kim, K. J., Culp, J. T., Ellis, J. E., & Reeder, M. D. (2022). Real-Time Monitoring of Gas-Phase and Dissolved CO2 Using a Mixed-Matrix Composite Integrated Fiber Optic Sensor for Carbon Storage Application. Environmental Science & Technology. https://pubs.acs.org/doi/10.1021/acs.est.2c02723