(713f) Prototype LIBS Sensor for Sub-Surface Water Quality Monitoring

“Prototype LIBS sensor for sub-surface water quality monitoring with applications in carbon storage”
D.A. Hartzler^1,2, J.C. Jain^1,2, D.L. McIntyre¹

¹National Energy Technology Laboratory, U.S. Department of Energy, Pittsburgh, PA 15236, USA.
²AECOM, National Energy Technology Laboratory, Pittsburgh, PA 15236, USA.

Geologic carbon storage (GCS) involves injection of CO2 into rock formations often containing high salinity brines. In the event of CO2 leakage, freshwater above the storage sites can potentially be contaminated with metals either originating from the brine or released by leaching of the formation rocks. An increase in the concentration of these proxy metals would warn of leakage well before the CO2 has reached the surface. Therefore, a Laser Induced Breakdown Spectroscopy (LIBS) sensor is being developed for sub-surface water quality monitoring. The sensor head is built using a low cost passively Q-switched (PQSW) laser, which is fiber coupled to a pump laser and a gated spectrometer. The ability to separate the LIBS excitation laser from the pump and spectrometer would allow the expensive and fragile components to remain at the surface while only the sensor head needs to be exposed to the hostile downhole environment. Spectrometer gating is achieved by triggering from a backwards traveling, nanosecond pulse coming to the surface through the pump fiber, eliminating the need for any electronic components to be located downhole. With the relative low cost and simplicity of the (laser) head, the sensor could be left in place for continuous long-term monitoring of a GCS site without the need to retrieve samples or equipment from downhole. A prototype has been constructed using off the shelf components along with a custom, monolithic PSQW laser. Laboratory testing of the prototype shows Limits of Detection (LODs) comparable to those of actively Q-switched systems and has allowed verification that the fiber coupling and triggering schemes perform as desired. Work to produce a miniaturized submersible version is in progress.