From The Sanford Underground Research Facility-SURF (US): “To advance geothermal systems EGS Collab maps the hidden fractures behind a wall of rock”

SURF-Sanford Underground Research Facility, Lead, South Dakota, USA.

From The Sanford Underground Research Facility-SURF (US)

Homestake Mining, Lead, South Dakota, USA.
Homestake Mining Company

November 29, 2021
Erin Lorraine Broberg

In 2021, the EGS Collaboration began outfitting a drift on the 4100 Level of Sanford Underground Research Facility for geothermal research. Photo by Adam Gomez.

Picture this: you’re standing in a drift, 4,100 feet below the Black Hills of South Dakota. Wrinkled rock arches over you. At first, the rock appears gray and featureless. But as you peer through the net-like metal mesh of ground support, you notice something interesting: a thick stripe of white quartz then faint, hairlike veins swirling like loose cursive against the dark, amphibolite rock.

The drift you’re imagining is a research testbed on the 4100 Level of Sanford Underground Research Facility (SURF) and home base for The Enhanced Geothermal Systems (EGS) Collaboration, a research group interested in extracting renewable energy from Earth’s deep, hot rocks.

“Geothermal energy extraction requires three things: hot rock; permeable pathways through the rock; and fluid to extract the heat,” said Tim Kneafsey, a staff earth scientist at DOE’s Lawrence Berkeley National Laboratory (Berkeley Lab) who leads the EGS Collaboration research group. “Hot rock is an abundant resource in the U.S., but it is often missing open pathways that allow you to extract the heat.”

Because pathways are often the limiting factor in geothermal systems, EGS Collaboration is investigating ways to maximize the usefulness of natural fractures that exist in rock formations.

While rock on the 4100 Level isn’t hot, it gives the EGS Collaboration a place to test a method called “hydraulic shearing” to open, or stimulate, a matrix of preexisting natural fractures in their testbed.

“By opening them and causing them to shift slightly, the roughness of the fractures keeps them propped open,” Kneafsey said. “This self-propping allows water to flow through and–in hot rock environments–transfer heat.”

Before natural fractures can be propped open however researchers first must locate them.

A library of rock

At their testbed on the 4100 Level of SURF, researchers drilled nine boreholes which will be used later in the experiment to stimulate and monitor the rock’s response to hydraulic shearing.

The boreholes, which measure four inches in diameter, vary in depth, the shallowest measuring 180 feet and the deepest measuring 265 feet.. When the rock, or core, is removed from these boreholes, it is carefully boxed and delivered to SURF’s Core Archive, a library of rock samples collected over several decades from underground.

The Core Archive was originally created by the geologists at Homestake Gold Mine. The core samples allowed them to probe the boundaries of gold, silver and copper ore bodies throughout the mine.

Earth scientists, however, are more interested in fractures than precious metals.

Megan Smith, an Earth scientist at The DOE’s Lawrence Livermore National Laboratory (LLNL), and Bill Roggenthen, a research scientist at The South Dakota School of Mines and Technology(US), laid out the recently drilled core samples, each representing the length of one borehole.

Megan Smith, an Earth scientist at DOE’s Lawrence Livermore National Laboratory (LLNL), examines recently drilled core samples. Each column represents the length of one borehole. Photo by Adam Gomez.

On the surface, Smith and Roggenthen spent two days meticulously inspecting more than 1,000 feet of core, seeking natural fractures in the rock. Optimally, they look for fractures that span the length of the testbed, cutting through several boreholes that could be propped open using the hydraulic shearing technique.

It wasn’t an easy task.

“The vast majority of the breaks are from the drilling process, and we are looking for the ones that aren’t. And that’s a difficult interpretation,” said Smith. “The drilling process will break rocks along weaker planes, right where natural fractures might be, too. When the breaks are perpendicular through the core sample, we can tell that the drilling process caused that break. But if there’s a break that follows along a mineralized zone, that’s something we have to pay more attention to.”

When cracks run perpendicular through the core samples, researchers can assume those breaks were created during the drilling process. Those breaks are ruled out as researchers search for patterns of natural fractures in the rock formation. Photo by Adam Gomez.

Turning the rocks inside-out

Back underground, other researchers use the now-vacant boreholes to further probe their testbed. Lowering a series of sensors called wireline geophysical logs, into the borehole, researchers explore the rock formation from the inside-out.

Researchers with the EGS Collaboration perform “logs” of the boreholes, gathering data to create a clear picture of the rock formations they will study. Photo courtesy Timothy Kneafsey.

Perhaps the most straightforward of these sensors is called an “optical log,” which takes images from inside the borehole. These high-resolution images offer a 360-degree view of the borehole. The optical log alone, however, yields limited information.

“The rock is nearly black in some areas, and when you have a dark, shadowed fracture in a section of dark rock, all you get is a dark image, which doesn’t tell us much,” said Jeff Burghardt, lead geomechanic at Pacific Northwest National Laboratory (PNNL).

Other wireline logs can complete the picture. The “acoustic log,” for example, uses ultrasonic frequencies to map the borehole.

“We are making a series of snaps—short, sharp sounds—that reflect off of different materials in the borehole,” said Paul Schwering, senior geoscientist at Sandia National Laboratories. “If the snap hits solid rock, it will reflect really fast. But it it’s softer rock or if it’s been gouged out, then that response slows down.”

These reflected acoustic signals can flag fractures the optical log cannot.

The list of wireline logs continues—and gets increasingly complex. The logs take measurements of geomechanical properties; monitor fluid and temperature conductivity; and even test electrical conductivity that yield insights into permeability, porosity and water quality. Using these tests, the researchers can infer information about the routes water takes as it flows through the rock.

Data from the wireline geophysical logs is represented on a computer screen. Photo by Adam Gomez.

Completing the map

After examining both the core and the void it left behind researchers combine the information to get a more complete picture of their testbed.

“If we see something promising when we evaluate the core, we can correlate it to the wireline logs that were performed after the cores were drilled,” Smith said. If they see a correlation, they can be relatively certain of a fracture in that location.

If a specific section of core is promising, they use photography of the core to render a 3D model of that section.

“With the 3D model, we can rotate the core, move it around and even orient it back into the borehole, the way it was originally oriented, and measure directions and angles from that core very easily,” said Roggenthen.

A 3D model of a core sample is examined on a computer screen. GIF courtesy Timothy Kneafsey.

The challenge

Having mapped the invisible landscape behind a wall of rock, what have researchers learned?

“These wells are what we would call very ‘tight,’ meaning they hold the water very well. The hydraulic permeability—how well water can flow through rock—is very, very low in most of these wells,” Burghardt said. “The challenge before us is to enhance the permeability of this testbed.”

To test hydraulic shearing techniques, the EGS Collaboration will need to zero in on the few fractures that were located.

The boreholes will be outfitted with equipment that stimulates the rock with pressurized water, opening and propping the fractures, in an attempt to create an interconnected network. All the while, researchers will closely monitor changes in the rock and water flow between boreholes.

The information EGS Collaboration gathers will has real-world applications beyond the 4100 Level.

Every byte of data will inform expansive field experiments, like the Department of Energy’s FORGE laboratory in Milford, Utah. This field laboratory hopes to develop techniques that will enable powering 100 million American homes through geothermal energy.

“Getting EGS [Enhanced Geothermal Systems] demonstrations all the way through to commercialization and understanding how geothermal energy can be used to produce electricity—that’s the bottom line,” said Hunter Knox, a geophysicist at PNNL.

By mapping, stimulating and monitoring the subsurface at SURF, the EGS Collaboration is contributing to the nationwide goal of extracting clean renewable energy from the ground beneath our feet.

The EGS Collaboration includes researchers from ten national labs—DOE’s Lawrence Berkley National Laboratory (US); DOE’s Sandia National Laboratory (US); DOE’s Lawrence Livermore National Laboratory (US); DOE’s Pacific Northwest National Laboratory (US); DOE’s Idaho National Laboratory (US), DOE’s Los Alamos National Laboratory (US); DOE’s National Renewable Energy Laboratory (US); DOE’s National Energy Technology Laboratory (US); DOE’s Brookhaven National Laboratory (US); and DOE’s Oak Ridge National Laboratory (US); and seven universities— The South Dakota School of Mines & Technology (US); Stanford University (US); The University of Wisconsin (US); The University of Oklahoma (US); The Colorado School of Mines (US) The Pennsylvania State University (US); Rice University (US), and The Texas A&M University (US).

This EGS Collaboration Project is supported by the U.S. Department of Energy, Geothermal Technologies Office; part of the Office of Energy Efficiency and Renewable Energy (EERE).

See the full article here .

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About us: The Sanford Underground Research Facility-SURF (US) in Lead, South Dakota advances our understanding of the universe by providing laboratory space deep underground, where sensitive physics experiments can be shielded from cosmic radiation. Researchers at the Sanford Lab explore some of the most challenging questions facing 21st century physics, such as the origin of matter, the nature of dark matter and the properties of neutrinos. The facility also hosts experiments in other disciplines—including geology, biology and engineering.

The Sanford Lab is located at the former Homestake gold mine, which was a physics landmark long before being converted into a dedicated science facility. Nuclear chemist Ray Davis earned a share of the Nobel Prize for Physics in 2002 for a solar neutrino experiment he installed 4,850 feet underground in the mine.

Homestake closed in 2003, but the company donated the property to South Dakota in 2006 for use as an underground laboratory. That same year, philanthropist T. Denny Sanford donated $70 million to the project. The South Dakota Legislature also created the South Dakota Science and Technology Authority to operate the lab. The state Legislature has committed more than $40 million in state funds to the project, and South Dakota also obtained a $10 million Community Development Block Grant to help rehabilitate the facility.

In 2007, after the National Science Foundation named Homestake as the preferred site for a proposed national Deep Underground Science and Engineering Laboratory (DUSEL), the South Dakota Science and Technology Authority (SDSTA) began reopening the former gold mine.

In December 2010, the National Science Board decided not to fund further design of DUSEL. However, in 2011 the Department of Energy, through the Lawrence Berkeley National Laboratory, agreed to support ongoing science operations at Sanford Lab, while investigating how to use the underground research facility for other longer-term experiments. The SDSTA, which owns Sanford Lab, continues to operate the facility under that agreement with Berkeley Lab.

The first two major physics experiments at the Sanford Lab are 4,850 feet underground in an area called the Davis Campus, named for the late Ray Davis. The Large Underground Xenon (LUX) experiment is housed in the same cavern excavated for Ray Davis’s experiment in the 1960s.

The LBNL LZ Dark Matter Experiment (US) Dark Matter project at SURF > , Lead, SD, USA </a.

In October 2013, after an initial run of 80 days, LUX was determined to be the most sensitive detector yet to search for dark matter—a mysterious, yet-to-be-detected substance thought to be the most prevalent matter in the universe.

The U Washington MAJORANA Neutrinoless Double-beta Decay Experiment Demonstrator experiment (US), also on the 4850 Level, is searching for a rare phenomenon called “neutrinoless double-beta decay” that could reveal whether subatomic particles called neutrinos can be their own antiparticle. Detection of neutrinoless double-beta decay could help determine why matter prevailed over antimatter. The Majorana Demonstrator experiment is adjacent to the original Davis cavern.

The LUX Xenon dark matter detector | Sanford Underground Research Facility mission was to scour the universe for WIMPs, vetoing all other signatures. It would continue to do just that for another three years before it was decommissioned in 2016.

In the midst of the excitement over first results, the LUX collaboration was already casting its gaze forward. Planning for a next-generation dark matter experiment at Sanford Lab was already under way. Named LUX-ZEPLIN (LZ), the next-generation experiment would increase the sensitivity of LUX 100 times.

SLAC National Accelerator Laboratory(US) physicist Tom Shutt, a previous co-spokesperson for LUX, said one goal of the experiment was to figure out how to build an even larger detector.

“LZ will be a thousand times more sensitive than the LUX detector,” Shutt said. “It will just begin to see an irreducible background of neutrinos that may ultimately set the limit to our ability to measure dark matter.”

We celebrate five years of LUX, and look into the steps being taken toward the much larger and far more sensitive experiment.

Another major experiment, the Long Baseline Neutrino Experiment (LBNE)—a collaboration with Fermi National Accelerator Laboratory (Fermilab) and Sanford Lab, is in the preliminary design stages. The project got a major boost last year when Congress approved and the president signed an Omnibus Appropriations bill that will fund LBNE operations through FY 2014. Called the “next frontier of particle physics,” LBNE will follow neutrinos as they travel 800 miles through the earth, from FermiLab in Batavia, Ill., to Sanford Lab.

FNAL DUNE LBNF (US) from FNAL to SURF >, Lead, South Dakota, USA

FNAL DUNE LBNF (US) Caverns at Sanford Lab.

U Washington MAJORANA Neutrinoless Double-beta Decay Experiment (US) at SURF.

The MAJORANA DEMONSTRATOR will contain 40 kg of germanium; up to 30 kg will be enriched to 86% in 76Ge. The DEMONSTRATOR will be deployed deep underground in an ultra-low-background shielded environment in the Sanford Underground Research Facility (SURF) in Lead, SD. The goal of the DEMONSTRATOR is to determine whether a future 1-tonne experiment can achieve a background goal of one count per tonne-year in a 4-keV region of interest around the 76Ge 0νββ Q-value at 2039 keV. MAJORANA plans to collaborate with Germanium Detector Array (or GERDA) experiment is searching for neutrinoless double beta decay (0νββ) in Ge-76 at the underground Laboratori Nazionali del Gran Sasso (LNGS) for a future tonne-scale 76Ge 0νββ search.

Compact Accelerator System for Performing Astrophysical Research (CASPAR). Credit: Nick Hubbard.

Compact Accelerator System for Performing Astrophysical Research (CASPAR). Credit: Nick Hubbard.

CASPAR is a low-energy particle accelerator that allows researchers to study processes that take place inside collapsing stars.

The scientists are using space in the Sanford Underground Research Facility (SURF) in Lead, South Dakota, to work on a project called the Compact Accelerator System for Performing Astrophysical Research (CASPAR). CASPAR uses a low-energy particle accelerator that will allow researchers to mimic nuclear fusion reactions in stars. If successful, their findings could help complete our picture of how the elements in our universe are built. “Nuclear astrophysics is about what goes on inside the star, not outside of it,” said Dan Robertson, a Notre Dame assistant research professor of astrophysics working on CASPAR. “It is not observational, but experimental. The idea is to reproduce the stellar environment, to reproduce the reactions within a star.”