Tracer-based characterization of a stimulation-enhanced rock volume and application of novel DNA nanotracers in fractured crystalline rock

Abstract

Geothermal energy is one of the renewable energy sources receiving growing interest as a consequence of increasing fossil fuel prices, low-carbon imperatives, and environmental awareness. As traditional hydrothermal systems are scarce, gaining access to a broader resource is fundamental in increasing the share of electricity generated from geothermal systems. Drilling down to greater depths of about 5 kilometers gives us access to ubiquitous hot rocks, where, however, permeability is low and there is scarcity of water in-place. Thus, to engineer a commercially viable heat exchanger in low- to zero-permeability rock, known as Enhanced Geothermal Systems (EGS), hydraulic stimulation technologies, such as hydraulic fracturing or shearing, must be applied. Regarding such geothermal reservoir creation and its subsequent characterization, there exists a knowledge gap at the intermediate deca-meter scale to understand i) the processes relevant for permeability enhancement, and ii) the properties of the subsurface heat exchanger and of the hydraulic connections. This thesis contributes to improving our understanding of the hydrodynamic changes in the fractured crystalline rock mass induced by hydraulic stimulation experiments and the injection of hot water. Firstly, in concert with solute tracers, I applied novel DNA-labeled silica nanoparticles to investigate their transport properties in fractured crystalline rock. These nanoparticles, with an approximate diameter of 166 nm, are labeled with unique DNA signatures and encapsulated into silica spheres. The resulting nanotracers are identified based on their DNA signature, but their transport properties can be equated with that of natural sand particles. I observed that the stability of the recovered tracer response curves, i.e., whether there are fluctuations between consecutive samples, is directly correlated with the injected tracer mass. It is also evident that size exclusion, and potentially density effects, attenuate the DNA nanotracer signal. These effects are manifested as a reduction in the following parameters, in comparison to solute dye tracers: tracer recoveries, swept volumes, mean residence times, and dispersion. However, lower detection limits and no susceptibility to background concentrations promote the use of DNA nanotracers in tracer tomography and in tracing particulate-bound contaminant transport. By applying solute dye tracers before and after the hydraulic stimulations and the start of hot water injection, I was able to place constraints on the evolution of preferential flow paths and determine the changes in the tracer-swept volumes resulting from the thermohydro- mechanical responses of the rock mass. Examining the tracer response curves showed that spatial heterogeneities in the fracture network result in fluid flow channeling and a wide distribution of residence times. As a consequence of the hydraulic stimulation programs, tracer swept volumes increased considerably, i.e., between 43% and 316%. The lack of a corresponding general trend in the other obtained moment analysis results, that is, recovery, mean residence time, Gini coefficient, and second moment, is indicative of spatial heterogeneities in the fractures that dominate fluid flow. The tomograms of hydraulic conductivity, K, derived from tracer peak concentration arrival times before and after the hydraulic shearing stimulation, showed that after the stimulation, fluid flow was accessing pathways with higher K values. Likely due to new hydraulic connections, the geometric mean of the computed K values increased. As the spatial distribution of flow properties is not obtainable from temporal moments, using a tomographic approach to complement the reservoir characterization can be decisive in estimating the performance of a geothermal reservoir. Finally, I observed that the thermo-mechanical response, induced by hot water injection, redistributed the fluid flow at the Grimsel Test Site (GTS) in Switzerland. This observation is supported by comparing solute dye tracer response curves and their temporal moments from before and two weeks after the start of hot water injection. It is important to note that, the total recovery of the tracers decreased significantly due to fluid losses to the far field. The first fractures to be affected by reservoir stimulation and operational activities are likely those carrying high flow rates and large fractions of injected fluid, so that, when the permeability of these key flow paths increases (e.g., due to stimulation) or decreases (e.g., via heat build-up), fluid flow is strongly redistributed. Understanding the evolution of the preferential flow paths is crucial for the sustainable management of EGS and other subsurface reservoirs. For that purpose, as it is shown in this thesis, the analysis of tracer tests, by estimating the temporal moments of tracer response curves, provides essential information on the hydrodynamic properties of geothermal reservoirs.