| Title | Comparison of novel synthetic DNA nano-colloid tracer and classic solute tracer behaviour |
|---|---|
| Authors | Kittilä, A; Deuber, C; Mikutis, G; Evans, K; Puddu, M; Grass, R M;Stark, W J; Saar, M O |
| Year | 2016 |
| Conference | European Geothermal Congress |
| Keywords | tracer, DNA nanoparticle, solute tracer, groundwater flow |
| Abstract | Tracer experiments are commonly used in hydrogeological and geothermal reservoir studies to determine attributes of fluid flow paths. In complex fractured media, it is often necessary to use several different tracers, injected at different locations, to determine hydraulic connections and preferential flow paths. A relatively recent development is the use of tracers in tomography studies, requiring a large number of tracers. However, the number of suitable tracers is reduced when their combined effects and interferences are taken into account and particularly when temperatures in the studied media are high, as elevated temperatures can degrade the artificial tracers. Consequently, the scientific community is continuously attempting to increase the number of available artificial tracers that are environmentally benign, can be easily detected in dilute concentrations, are typically conservative, and do not degrade (or degrade at known rates) at elevated temperatures. A new class of tracers are DNA nanotracers, which are environmentally friendly, sub-micron scaled silica particles encapsulating small fragments of synthetic DNA. Using synthetic DNA in tracer tests enables the development of a virtually unlimited number of distinct tracers, and consequently, “contamination” of the studied system by one specific environmentally benign tracer type is not a limitation anymore. In addition, these DNA nanotracers can potentially be used to determine the approximate maximum temperature the tracer has encountered, which is of particular interest in geothermal systems. However, as these tracers are new and constitute colloids, rather than solutes, their transport behaviour within the fluid flow path is not well understood. As colloids tend to move preferentially within the main fluid flow paths, they typically move faster through the porous or fractured medium than the mean fluid velocity, emphasizing preferential flow paths. Thus, classic tracer test analyses, developed for solute tracers, may not be completely applicable to the DNA colloid tracers. In this study, we test these novel DNA nanotracers in both a sedimentary porous medium setting, where a classic tracer test has been performed before, and in a fractured rock system, where heat and solute tracer tests are also conducted. Additionally, comprehensive column experiments will complement the study. This allows us to compare the new DNA nanotracers to classic solute tracer behaviour within these two fundamentally different settings. The fracture-based system investigated here is the Deep Underground Geothermal (DUG) Laboratory at the Grimsel Test Site in the Swiss Alps.. |