Development of functionalized silica-based nanoparticle tracers for geo-reservoirs

Tracer tests are essential tools for assessing hydraulic reservoir conditions in both shallow groundwater aquifers and deep geothermal reservoirs. However, the current conventional tracer technology is limited in the range of measurable reservoir properties, the conditions under which it can be applied, and the accuracy and reproducibility of the results obtained. Among the main problems that hinder better tracer performance are lack of reference values and uncontrollable interaction of the tracer molecules (e.g., fluorescent dyes, ions) with the reservoir environment. A recent promising approach progressing the state-of-the-art tracer technology originates from the synergy of geoscience with nanoscience comprising nanoparticle-based tracers. These aim to take advantage of the modular and adaptable nature of nanoparticles to extend the toolbox of available tracers and enable customized behavior, increasingly elaborate functions, more measurable properties, higher accuracy in quantitative analysis and better predictability. The focus is mainly on silica nanoparticles, as silica is an abundant and natural geomaterial that is facile to synthesize and modify. ... mehrA necessary aspect in developing functional and applicable nanotracers concerns ensuring the transferability of these silica nanoparticle-based tracers to geological-relevant environments. Further remaining open questions concern the (thermal) stability, integrity, functionality and detectability of the nanoparticle tracers.

In this thesis, these research questions are addressed by an interdisciplinary combination of geoscience with nanoscience, chemistry and physics. More precisely, silica-based fluorescently labeled nanoparticle tracers are developed, enhanced and adapted to provide high stability and integrity, and their functionality, detectability and transportability are tested and proven under geological-relevant conditions.

The main challenges and limitations of conventional fluorescent molecular tracers are attributed to the interaction of the tracer with the environment, namely sorption tendency, (thermal) degradation, detection limitations and instability of the measurement function. To overcome these restrictions, the approach pursued in this thesis encompasses the encapsulation of molecular tracers inside silica nanoparticle carriers in order to restrict the interaction of the tracers with the environment. To fulfill their task the silica-based nanoparticles should endure harsh conditions (e.g., high salinities, elevated temperatures), possess long-term integrity, be identifiable and have favorable transport properties. It is first shown that existing approaches on silica-based nanoparticle tracers are not viable as they are prone to disintegration as a result of silica dissolution under reservoir conditions, similar to amorphous silica. This challenge is then overcome by applying chemical procedures to enhance the stability of the silica nanoparticles. To further
improve the protective function of the nanoparticles, mesoporous silica nanoparticles (MSN) with a
stable pore-blocking and surface coating made of titania are used as carriers for fluorescent dyes. This procedure prevents both dye leaching and silica nanoparticle dissolution. With this method, the toolbox of available tracers is augmented as the encapsulated dyes are shielded and protected from the environment. Hence, also molecules that are normally unusable as tracers, such as certain rhodamines, are made applicable as their unfavorable properties (e.g., sorption tendency) dissipate upon encapsulation, thus improving their performance and long-term stability.

Nanoparticles, unlike molecular tracers, offer the possibility of adjusting their interaction with the environment through chemical surface modifications. Applying physi- and chemisorbed surfactant and polymers to the silica-based nanotracers enhances transportability, improves dispersion stability and reduces sorption and retention within geological media. These positive effects are attributed to the formation of a steric layer on the surface of the particles that impacts the particle-particle and particle-surface interactions by affecting the ζ-potential and giving rise to an additional repulsive force: the steric force. Theoretical predictions conducted here based on the (extended) Derjaguin-Landau-Verwey-Overbeek theory (XDLVO theory) and filtration theory reaffirm the benefits surface modifications provide to the functionality of the nanoparticle tracers.

To transfer this system to geological applications, special considerations must be made of the complex and challenging environments – reservoirs differ from each other in parameters such as pH, salinity level, rock charge and chemical composition. However, nanoparticles stand out from other tracers in that they are adaptable and offer a wide possibility of surface modifications, thus enabling application under different reservoir conditions, as well as performing more reliable tracer tests or multi-tracer tests. Further, due to the ease of tailored design, using different particle sizes could help gain information about pore throat sizes and pore size apertures. The studies presented in this thesis lay the ground to allow further progress toward novel smart and functional tracers, thus paving the way to holistic and customized reservoir management.