Tracer tomography with DNA nanotracers

 

Tracer test at the DUG-lab

This novel tracer was developed at the Functional Materials Laboratory in the Institute of Chemical and Bioengineering at ETH Zürich, Switzerland, and is currently provided by the company Haelixa. The DNA nanotracers are spherical silica particles encapsulating sequences of DNA, greatly improving the stability of the DNA even in harsh conditions. For underground hydrogeological studies, DNA nanotracers provide an extremely fascinating tracing option. Information, as a unique signature, can be stored to a sequence of the DNA, enabling repeat tracer tests with different signatures to be conducted without suffering from interference from earlier tests. This property also enables detailed tracer tomography, as multiple uniquely distinguishable DNA nanotracers can be injected simultaneously. Additionally, encapsulating the DNA in silica not only allows the size of the particles to be modified, but also gives almost identical appearance for every DNA nanotracer. As a result, the transport properties of the DNA nanotracer are solely determined by the silica particle, whereas the identification and detection is determined by the differently encoded DNA.

With the goal of evaluating their transport properties and suitability as tracers, we have conducted column and field experiments in porous media. In column experiments we used DNA nanotracers modified in size together with a classical fluorescent solute uranine, and we could observe faster average transport velocity, lower mass recovery, and smaller swept volume as the particle size was increased. Additionally, compared to the solute, the DNA nanotracers had faster average transport velocity. These observations, also often made in literature comparing colloid and solute transport, were also made in the field experiment conducted in porous media.

In the Deep Underground Geothermal Laboratory (DUG-Lab) at Grimsel, where an in-situ stimulation and circulation experiment is taking place to improve our understanding of the processes related to permeability enhancement, a pioneering tracer experiment with the DNA nanotracers in fracture-dominated medium was conducted. In this decameter-scale field experiment we used both DNA nanotracers and fluorescent solutes, and it enables us to i) study the transport of the DNA nanotracer in fractures and ii) characterize the pre-stimulation connected pore volume. Our next tracer experiment in the DUG-Lab will include tomographic approach with the DNA nanotracers, and will aim at characterizing the post-stimulation flow path geometries and volumes.

Magnetotellurics (MT) is a geophysical sounding method which uses natural variations of the Earth’s electromagnetic field for imaging the electrical conductivity distribution in the subsurface. It is a well-established and proven method in geothermal exploration and has been deployed successfully in many geothermal regions as cases around the world show.
The electrical conductivity of rocks is closely linked to key geothermal parameters such as the hydraulic connectivity of conductive constituents like geothermal fluids, thereby making MT an important method for geothermal exploration.

We also work on synthetic studies on controlled source electromagnetic methods for downhole and cross-well monitoring. Placing electromagnetic sources downhole closer to the target allows a better monitoring of processes within a geothermal reservoir or in hydroshearing experiments like in the DUG lab.

Equipment
Our group owns three ADU07e broad-band Metronix MT stations. This commercial system is also used by other Swiss universities, which promotes collaborations during joint projects.