Tracer solubility, stability, and reactivity in ultramafic rocks: Enhancing CCS and mineralization monitoring efficiency

Artificial tracers have been widely used in hydrogeology to identify groundwater flow pathways and to quantify mixing, and chemical and biological reaction processes. De- pending on the specific environment, tracers behave differently and can be subject to chemical interaction. This research investigated the adsorption characteristics of two common artificial tracers, fluorescein (FL) and Na-Naphthenate-Methylene Blue (Na-Napth-MB), dissolved in different types of water and on serpentinized peridotite rock under ambient conditions with emphasis on applications in carbon capture and storage (CCS) and CO₂ mineralization. Using the prepared 1 ppm fluorescein and 5 ppm Na- Naphthenate -MB solutions, investigations were carried out for their reactivity and stability in freshwater, sea- water, soda water, and serpentinized peridotite rock. The data indicate that lower concentration of FL promoted higher adsorption than in higher concentrations; this effect declined with higher concentration/dosage. Whereas Na-Napth-MB showed much higher adsorption even in high dosage concentrations. In addition, contrary to the assumption that such tracers would remain non-reactive with other rock types, it was observed that ultramafic rocks (e.g. serpentinized peridotite) were significantly adsorbing. When the adsorption of an in-situ condition is considered, it is rather assumed that the adsorption potential will be very high and might get even 10 times higher than the values determined in the laboratory. This indicates that there is a need for careful selection of appropriate artificial tracers, their concentrations, and their effective monitoring in subsurface applications such as CCS. This makes it necessary to consider alternative conservative tracers such as e.g. noble gases, perfluorocarbons or nanoparticles for CCS projects. The outcomes are important to im- prove the tracer-based monitoring techniques for ultramafic rocks to accomplish long-term tracking of CO₂ and mineralization.