As part of the SCCER-SoE strategy (Evans et al. 2014), the Deep Underground Geothermal Laboratory (DUG-Lab) at Grimsel in the Swiss Alps hosts experiments to demonstrate permeability enhancement and to characterize the created reservoir as a result of experimental enhanced geothermal system (EGS) development. Although the ambient temperature in the DUG-Lab of 13 °C is much less than the temperatures of EGS reservoirs, it allows controlled and densely monitored experiments to be conducted on a decameter-scale to study permeability enhancement in granite. This project contributes to these studies by characterizing the flow paths in the test volume using tracer tests, and by placing constraints on heat transfer efficiency through simulations and borehole measurements. In concert with the tracer tests, we evaluate the performance of novel DNA nanotracers in fractured media for hydrogeological applications.
Fractures constitute secondary porosity in crystalline rocks with typically low primary porosity. These discontinuous structures often exhibit very complex geometries in which fluid flow and mass and heat transport take place. Although the characterization of fractured systems is challenging due to the complex geometries and natural heterogeneity of the properties of the fractured media, their characterization is a key factor for the successful development and management of enhanced geothermal systems (EGS). In EGS systems sufficient heat transfer between the reservoir and the circulating fluid, typically water, is required, which is achieved by hydraulically activating existing fractures and by generating new connections and promoting fluid flow through the system.
In this project a combination of field work and modeling is used to characterize connected flow paths and heat transfer efficiency both before and after the rock mass stimulation/permeability enhancement experiment in the DUG-Lab to address the question of how the hydraulic properties change as a result of the stimulation. Tracer tests play an important role in this, but the results will also be compared with other results obtained from geophysical methods. In the tracer experiments, in addition to conventional solute tracers, we also use novel DNA nanotracers. The goal is to evaluate their transport properties and suitability as tracers, and the field tracer experiments in fractured media conducted with the DNA nanotracers in this project are the first of its kind.
DNA nanotracers are environmentally friendly, sub-micron sized spherical silica particles encapsulating small fragments of DNA (Paunescu et al. 2013a, b). DNA is nature’s own information storage system, and the encapsulated DNA can be designed with a unique signature for each test. Thus, the transport properties of the DNA nanotracers are determined by the silica particles, whereas the identification and detection of the tracers from samples is determined by the differently encoded DNA fragments. This property allows repeat tracer tests with differently-encoded DNA fragment than that used in the original to be conducted without suffering from interference from the earlier test. It also has applications in tracer tomography, as multiple similarly behaving but uniquely distinguishable DNA nanotracers can be injected simultaneously.
Additionally, for constraining the efficiency of heat transfer in hydraulically active structures and the surrounding rock mass, we take temperature measurements at discrete distances from the conducting fractures to define the temporal and spatial evolution of temperature changes during post-stimulation circulation phase, when water that is cooler or hotter than the ambient rock is injected.
Comparison of DNA nanotracer and conventional solute tracer behavior
The ongoing research with DNA nanotracers has so far included column and field experiments in porous medium and field experiments in fractured rock system (DUG-Lab), and sand column experiments. During preliminary investigations of the breakthrough curves obtained from the fracture-dominated system in this project we observe that the DNA nanotracers were transported through the fractured media with faster mean transport velocities and that they experienced less hydrodynamic dispersion than the solute tracers.
Detailed analysis, results and discussion of the tracer comparison will follow later.
Quantification of pore space connectivity and heat transfer enhancement during EGS development at the DUG-Lab
Results and discussion will follow later in the project.
Evans, K., Wieland, U., Wiemer, S. and Giardini, D. 2014. Deep Geothermal Energy R&D Roadmap for Switzerland, 2014. SCCER-SoE, 43 p.
Paunescu, D., Fuhrer, R. and Grass, R.N. 2013a. Protection and deprotection of DNA—High-temperature stability of nucleic acid barcodes for polymer labeling. Angewandte Chemie International Edition, 52, 4269-4272.
Paunescu, D., Puddu, M., Soellner, J.O.B., Stoessel, P.R. and Grass, R.N. 2013b. Reversible DNA encapsulation in silica to produce ROS-resistant and heat-resistant synthetic DNA ‘fossils’. Nature protocols, 8, 2440-2448.