Few researchers are equally adept in both the field and the lab. Adam Hawkins, a graduate student in Earth and Atmospheric Sciences is one of those researchers, and rightly so. His work on enhanced geothermal systems (EGS) bridges the gap between lab-based experiments and widespread field commercialization. As a research team member in the Jefferson W. Tester lab, Smith School of Chemical and Biomolecular Engineering, Hawkins uses a combination of experimental field tracer tests, lab experiments, and computational models to predict the performance of geothermal reservoirs—hot water and steam trapped under impermeable rock within the Earth.
Everywhere in the world, geothermal energy is right below our feet. Conventional geothermal energy systems, however, only target hot, easily accessible fluids close to Earth’s surface. Since very few areas are suitable for the conventional systems, this sustainable energy source has limited application.
Mining Earth’s Heat: Promises and Challenges of EGS
EGS emerged out of the hope of extracting abundant heat from deep within the planet, without the strict geographical limitations of the conventional geothermal energy systems. In an EGS application, water is first injected through injection wells into the ground. Through contact with the hot rocks deep down, the water is heated and resurfaces at production wells, to be used to generate electricity or provide heating. Effectively, the system is mining Earth’s heat. Given the potential output, flexibility, and reliability of such systems, EGS may amount to a superior solution to our energy and climate dilemma.
While the promise is bright, the uncertainties surrounding EGS have resisted illumination. The biggest issue is in determining how fluids flow underground, specifically as they flow through interconnected pore spaces throughout rocks.
The performance of any geothermal system, natural or engineered, depends on the effectiveness of the heat exchange between the fluid and the rocks underground. For instance, the larger the surface area available for heat exchange between the rocks and the fluid, and the longer the fluid flows before reemerging to the surface, then the longer heat can be extracted from that reservoir at economic levels before a particular site needs to rest. Since engineering a reservoir involves connecting different parts of existing pores and fractures in addition to stimulating new ones, it is important to have a reliable method of understanding these intricate and elusive underground fluid flow paths.
Innovations for Illuminating Underground Paths of Fluid Flow
Hawkins hopes to advance EGS by using innovative tracer testing to illuminate fluid flow paths underground. These innovations will be an important set of tools in predicting the performance of a geothermal reservoir before massive investments are made to drill and utilize it.
A tracer test works by injecting a foreign solute into an injection well and detecting concentrations of this tracer at the production well over time. Conventional tracers are inert compounds, which simply flow along with the injected water. In this case, a tracer test is limited to measuring the amount of time the water spends underground before reemerging, often called its residence time.
While the residence time is an important characteristic of a reservoir, Hawkins wants to obtain more detailed reservoir information by injecting thermally-reactive tracers, such as phenyl acetate, into the ground. This tracer reacts over time, and at different rates depending on the temperature. As a result, the concentration of whatever tracer is left at the end allows him to quantify the temperature distribution throughout the subsurface. Therefore, thermal performance can be anticipated years in advance, which reduces performance uncertainty and financial risk.
In addition to thermally reactive tracers, Hawkins is also testing a carbon-cored nanoparticle tracer referred to as C Dots, which was invented at Cornell. While the particle is currently intended to be an inert tracer, adding functional groups to its surface can modify its properties.
“From your home, you won’t see giant wind turbines or solar collectors, lining your vista.”
Hawkins’ field experiments are the first to show that a nanoparticle tracer can be used to characterize fractures without sticking to materials underground. “Once we have fully understood how the C Dot tracer works as an inert tracer, we can add functional groups to try to introduce useful reactions,” he says.
Field Testing from Altona, New York to Iceland
Such unconventional tracers have not been proven useful beyond the confines of the laboratory, making Hawkins’ work both pioneering and salient. “It is difficult to reproduce field conditions at the lab-scale,” he says. “What I found was that the reactive tracer and the nanoparticle tracer in the natural environment undergo more complicated reactions and transport properties. For example, the reactive tracer in the field seems roughly 20 times more reactive than when measured in the laboratory.” He hopes that his meso-scale field tests will serve as a stepping-stone between idealized lab experiments and commercial applications.
Hawkins became truly immersed in all aspects of the works at the Altona Field Laboratory in Altona, New York. This field laboratory is a small but well-characterized site consisting of a single fracture connecting two wells 14 meters apart. There, he showed that his methods work, and he is now working on additional experiments at larger-scale sites. Ultimately, he plans to conduct tests in an actual geothermal reservoir in Iceland, a country where over 30 percent of its electricity is generated using geothermal energy.
Hawkins’ success in developing sophisticated field experiments stems from his unique work experiences. He worked in the plumbing and electrical industry back in high school and as an undergraduate. He also spent several years guiding large groups of students through the backcountry. Hawkins credits those experiences with preparing him to overcome the challenges in developing a remote site like the Altona Field Laboratory.
Back in the lab, Hawkins is also building on the computational modeling work of Don Fox, a former graduate student in the Tester lab. The modeling simulates the effect of complicated flow passes between two wells. Conventionally, the permeability of the subsurface—or the ability of the rock underground to let fluids flow through—is assumed to be constant between two wells, because it is difficult to identify an alternative distribution of aperture, or small pores in the rock, as they are deep underground.
Hawkins is helping to develop a more realistic approach that uses a statistical tool called Principal Component Analysis. This method identifies underground patterns that are most relevant to the performance of geothermal reservoirs, basing its model on well data from known fracture networks. Combining this statistical technique with an inversion procedure referred to as a genetic algorithm, he is able to infer representative flow paths that can have a tremendous impact on geothermal reservoir performance.
Geothermal, a Superior Energy Alternative
Hawkins is drawn by the advantages that geothermal energy holds over many other sustainable energy systems. “My main interest is in advancing sustainable energy technologies. But energy generated from other technologies such as solar and wind is very sensitive to natural variations in weather. In contrast, energy produced from geothermal can be adjusted as demand fluctuates, so it is more flexible and reliable,” he says. Geothermal systems also tend to have a smaller spatial footprint on the existing landscape. “From your home, you won’t see giant wind turbines or solar collectors lining your vista.” In his motivation, Hawkins looks to both ends of the spectrum—technical viability and aesthetic beauty.