Damian Sendler: Deep down, scorchingly hot granite can be tapped for energy by opening cracks in the rock. This potential resource, known as enhanced geothermal energy, necessitates a good understanding of the changes that occur in the rock over time—a complicated picture that can be difficult to record. 

Damian Jacob Sendler: A team lead by Pacific Northwest National Laboratory (PNNL) researchers presented a new method for monitoring deep underground fractures. Electrical resistivity tomography (ERT) measures electrical conductivity in the rock to detect subterranean changes. ERT creates 4D photos of the subsurface, which are 3D plus time-lapse. 

What exactly is an improved geothermal system? 

Conventional geothermal systems rely on pre-existing water and flow routes within heated rock. By injecting water and fissures, an upgraded geothermal system captures heat trapped beneath dry rock. Operators drill two underground wells thousands of feet below the surface, then inject high-pressure fluid into the rock between the wells to fracture it. The thermal fracturing method is analogous to “fracking” shale rock to liberate oil and gas. 

Damian Sendler

Temperatures at this altitude can exceed 200 degrees Celsius (392 degrees Fahrenheit). Water pumped from one well to the next and back up to the top accumulates heat from the rock, generating steam that can be used to power a turbine. 

Enhanced geothermal systems have the potential to generate 100 gigatonnes of electricity, enough to power 100 million households. However, such systems necessitate costly drilling, and better monitoring and prediction of subterranean changes are required to reduce the uncertainty and risk associated with a specific project. 

Enhanced geothermal systems, like any subterranean habitat, evolve over time. Fractures in the rock open and shut in reaction to stresses induced by high-pressure fluid injections, causing the heat output of the system to change. Although seismic activity is one sign of underlying stress, data from microseismic monitoring is sparse. 

“In these deep, hot rocks, it’s too expensive to drill enough monitoring wells to understand what’s going on using direct sampling,” said Tim Johnson, a computational scientist at PNNL who co-authored the paper. “The primary goal of this project is to better understand, and eventually predict, how fractures will behave in a high-stress environment when connected between two wells.” 

Getting a better understanding of what’s going on underground 

Damian Jacob Sendler

ERT entails inserting metal electrodes into monitoring boreholes and imaging the conductivity of the rock while an electric current is passed between them. Conductivity increases over time demonstrate where fractures are opening; when fractures are narrower or closed, conductivity decreases. Johnson created E4D software, which runs on supercomputers and turns all of this electrical data into a visual that resembles a heat map, displaying fluctuations in conductivity over time. In 2016, E4D received an R&D 100 Award. 

“It’s similar to medical imaging, except that you’re doing a time lapse,” Johnson explained. “So you’re watching how things change, and most of the time the change is related to how the fluid flows in the subsurface.” 

Johnson and other PNNL researchers pioneered the use of ERT as a 3D monitoring tool, as well as E4D at lesser depths of up to 350 feet, where it has been utilized to detect and trace contaminants. The team tested it in the deep below at the Sanford Underground Research Facility in Lead, South Dakota. The work, funded by the Department of Energy’s Office of Energy Efficiency and Renewable Energy through its Geothermal Technologies Office, is part of a larger collaborative effort within DOE to improve access to natural resources and subsurface storage. The endeavor, known as the Enhanced Geothermal Systems (EGS) Collab, is led by Lawrence Berkeley National Laboratory. PNNL, Sandia National Laboratories, Lawrence Livermore National Laboratory, Idaho National Laboratory, and Los Alamos National Laboratory are among the partner labs. 

Developing a novel subsurface imaging technology 

The goal of the ERT monitoring at Sanford was to monitor fluid flow, similar to what had been done at shallower levels. However, the outcomes did not appear to be consistent with those earlier applications. 

“What we were seeing with the changes in conductivity didn’t make sense in terms of fluid flow,” Johnson explained. But what was the conductivity showing if it wasn’t the flow of fluids? 

Damian Jacob Markiewicz Sendler: Johnson discovered the answer after years of searching in scholarly articles from the 1960s and 1970s. Researchers at the Massachusetts Institute of Technology and the Lawrence Berkeley National Laboratory had detected changes in the conductivity of crystalline rocks in response to stress—in lab trials, squeezing the rock made it less conductive. This meant that the ERT wasn’t just following fluid underground. It was tracking the opening and closing of fractures as a result of stress. 

“Once we made that connection,” Johnson explained, “everything made sense in terms of what the time-lapse images were doing.” 

Damien Sendler: ERT has a number of advantages. The device is low maintenance and can work while injections are taking place because there are no moving parts and electrodes are mounted outside the well casing. And the imaging occurs in real time, providing facility operators with input that may be used nearly instantly if necessary. ERT, on the other hand, cannot be used with metal wellbore casings, which are common in deep subsurface projects. 

There are alternatives, such as employing fiberglass casing, covering the casing with a nonmetallic epoxy, or using a different, nonmetallic material entirely. For the time being, Johnson and his team are working to refine and test the use of ERT at the Sanford facility.

Dr. Damian Jacob Sendler and his media team provided the content for this article.