Dustin McIntyre, of the National Energy Technology Laboratory, US Department of Energy in Morgantown, West Virginia, has been exploring the use of laser-induced breakdown spectroscopy (LIBS) to measure subsurface gases, liquids, and solids at subsurface conditions. He recently shared with Spectroscopy a look at some of the work he’s been doing and his thoughts on ways in which his technique can be used.
What sparked your idea for an improved LIBS elemental composition detection system?
In my graduate work, I focused on using a miniaturized diode pumped solid state passively Q-switched laser as an engine ignition system. While discussing laser spark plug technology with a colleague, we realized that we could use a similar concept to place a miniaturized laser below ground to produce a LIBS spark for atomic chemical analysis. We also realized that we could easily adapt the laser for Raman analysis to run molecular chemical analysis as well. My current area of research is carbon capture, utilization, and storage (CCUS), in which CO2 is captured from an electric power plant, compressed, and subsequently injected underground for permanent storage. The questions we are investigating are related to storage permanence and how to ensure that, once injected, CO2 will remain in its intended location underground. This new LIBS system will enable monitoring of downhole fluids to detect any chemistry changes that would be characteristic of undesirable CO2 migration.
What is innovative about your approach and how does it differ from currently used methods? What are its advantages?
Our system uses a remotely diode pumped solid state passively Q-switched laser to produce output for Raman and LIBS analysis. Other systems designed to provide these types of measurements rely on an above-ground, laboratory-scale laser to deliver either Raman excitation energy or the high peak energy LIBS pulse through optical fibers to the subsurface. Delivering the LIBS high peak energy can easily destroy the fiber and complicate refocusing of the laser beam. Using Raman in a long optical fiber can significantly interfere with the stimulated response primarily due to the Raman response of the optical fiber material itself. Our system uses a remotely positioned laser diode that is coupled to an optical fiber where the peak power is only a few hundred watts instead of megawatts. The light from the diode travels through the optical fiber where it is then focused into a monolithic laser gain medium to form an end-pumped laser system. The passively Q-switched laser, once properly excited by the fiber-delivered light, produces a high peak power pulse that is directed and focused at the location of interest. The light from the spark is then collected and either transferred back up the fiber for analysis by a laboratory-scale spectrometer or simply directed into a miniaturized narrow-band gated spectrometer that is situated alongside the solid-state laser.
Why are subsurface brine measurements important in environmental applications, and what challenges did you face in applying LIBS to these measurements?
As CO2 is injected underground it mixes with and displaces the native fluids. Monitoring devices placed around an injection site would enable us to detect the presence of leachates from the original formation. We would also be able to determine if the injected CO2 is remaining in its intended storage zone or if it is leaking into other formations where it could potentially impact ground water resources. The primary challenge for a LIBS monitoring system is keeping up with the cost and adoption of newer technological components. A few years ago many of the key components cost tens of thousands of dollars; now they cost just a few hundred. Automation and improved optical and semiconductor fabrication techniques have made many of the system components more accessible. A good example is the popularity and proliferation of laser pointers, in particular green laser pointers. The end-pumped laser system is essentially a green laser pointer on steroids. The green laser pointer is a diode end-pumped laser that is passively Q-switched to produce a high-frequency laser pulse train that is then frequency doubled. Our laser system is simply designed with gain, cavity, and Q-switch components that produce one large pulse capable of spark production when properly pumped.
Can you describe some other possible uses for this technique?
One exciting potential application is municipal water treatment and filtration systems. The laser system could provide an on-line quantitative measurement for contaminants, or it could act as an active feedback control sensor for water treatment systems.
What are your next steps with this work?We are currently fabricating a prototype for demonstration and designing a system that will provide for multiple lasers distributed over a wide measurement area, such as different locations above an injection formation for CO2 storage.