Boiling mudpots, steaming fumaroles, balmy hot springs. These hydrothermal features are reminders that heat lurks just below the Earth’s surface in and around northern New Mexico’s Valles Caldera—a 13-mile-wide depression in the otherwise rugged Jemez Mountains. The caldera was formed by several powerful volcanic eruptions, most recently at about 1.25 million years ago.
So perhaps it’s no surprise that, back in the 1970s, when researchers at Los Alamos Scientific Laboratory were searching for a location to test out their Hot Dry Rock Program—their idea to extract heat from deep inside the Earth to create energy—they looked no farther than the western edge of the Valles Caldera.
“Without the means to explore far and wide, they went looking ‘just over the hill’ west of Los Alamos, in the Jemez Mountains,” remembers Laboratory engineer Donald Brown in his 2012 book Mining the Earth’s Heat: Hot Dry Rock Geothermal Energy. “The Los Alamos team reasoned that recent volcanic activity … would have produced a region of elevated temperature that would extend radially outward from the caldera at least several miles.”
But the scientists weren’t interested in the mud pots, fumaroles, hot springs, or any other hydrothermal phenomena that bubbled up from natural reservoirs just below the surface. Instead, they were interested in engineering a geothermal reservoir, much, much deeper underground.
Why? Because they believed that one of the world’s greatest untapped energy sources was right beneath their feet.
Hot dry rock generally refers to a type of rock found almost everywhere below the Earth’s surface. As the phrase implies, this rock is hot—hundreds of degrees Fahrenheit due to its location deep underground—and dry, meaning that it is nearly impervious to fluids. Hot dry rock does not contain water nor does it absorb water that comes in contact with it.
Hot dry rock is found at different depths all over the world. In areas with high geothermal activity, such as parts of northern New Mexico, hot dry rock might be closer to the Earth’s surface and therefore more easily reachable to humans, should they choose to access it.
“The natural heat in hot dry rock at accessible drilling depths is one of the largest supplies of usable energy that is available to man,” Los Alamos scientist Morton Smith wrote in The Furnace in the Basement: The Early Days of the Hot Dry Rock Geothermal Program, 1970–1973. “It is potentially capable of satisfying the world’s total energy needs for thousands of years.”
Conceptually, extracting heat from hot dry rock is simple. A borehole is drilled a few thousand feet into the Earth. When the borehole is at a depth with sufficiently high temperature rock, water from the surface is injected into the borehole at a pressure high enough to fracture the rock at the bottom of the hole—a process called hydraulic fracturing. The resulting fracture system creates a fluid reservoir.
Although the location, shape, size, orientation, and growth of the reservoir are controlled by local geological conditions, they can be determined from monitoring the fracture process. As the rock fractures, microseismic waves (aka very small earthquakes) are generated. These microseismic waves can be detected and recorded by seismic instruments. Using the resulting seismic data, scientists can estimate the reservoir’s location and approximate dimensions.
After this analysis, a second borehole is drilled to intersect the fluid reservoir. If the drilling is successful, the two boreholes and the reservoir create an interconnected fluid system underground.
The next step is to extract geothermal heat. A high-pressure injection pump circulates water down the injection borehole, into the fluid reservoir. There, the water heats up through contact with the surrounding high-temperature rock. The water then flows up the second borehole, which is called the production borehole. En route to the surface, the hot water must be kept at a pressure high enough to keep it from flashing to steam.
At the surface, the hot water flows through a heat-exchanger. The heat is used as energy, and the cooled water is recirculated. The process repeats.
Hot dry rock contains “a truly international energy supply that, if it could be brought to the surface at useful temperatures and reasonable cost, would be a major source of energy for every country in the world—including those that lack all other indigenous energy supplies,” Smith summarized. “In an environmentally concerned world, this should make it a very desirable energy source.”
Hot Dry Rock at Los Alamos
The concept of mining heat from hot dry rock was devised by Smith and others from Los Alamos in the early 1970s. The initial Hot Dry Rock (HDR) Program was “informal in every respect, including financial support, and was carried on largely on a volunteer basis,” Smith wrote. Those who contributed did so out of pure curiosity and in their own free time. No one had any direct experience with geothermal energy, but they became almost-experts in geology, geophysics, hydrology, drilling, rock mechanics, hydraulic fracturing, and related areas. Even legendary Los Alamos physicist Frank Harlow (the founder of computational fluid dynamics) got involved by modeling the efficiency of heat extraction from a hot dry rock reservoir.
Eventually, their findings were sufficient to garner support and funding from the Laboratory and later from the Atomic Energy Commission (AEC, the predecessor to the current Department of Energy, or DOE).
In December 1971, with permission from the U.S. Forest Service, the not-yet-official Los Alamos “geothermal group” began drilling shallow (approximately 600-foot-deep) boreholes on national forest land near the western rim of the Valles Caldera. After measuring geothermal gradients—the rate at which temperature changes with depth—in these holes, they concluded that, using modern drilling equipment, hot dry rock should indeed be accessible anywhere in that area.
So, the group members decided to drill a deeper hole, which would help them further their understanding of local geology, hydrology, and heat flow. They chose Barley Canyon, near present-day Fenton Lake State Park, for Geothermal Test Well No. 1 (GT-1). Drilling began on May 9, 1972. “Because no deep holes had ever been drilled in the area in which we were drilling GT-1, nobody knew exactly how far down it was to the top of the [hot dry rock],” Smith wrote.
Fifty-two days later, the group had an answer. The hole was 2,575 feet (nearly a half mile) deep, with the bottom 470 feet in hot dry granitic rock, which was at a temperature of about 210 degrees Fahrenheit. GT-1 experiments, many of them having to do with hydraulic fracturing, began in February 1973. “We were handicapped by subfreezing temperatures, several heavy snowfalls, and the fact that—in undertaking novel and difficult experiments—not everything worked the first time we tried it,” Smith wrote. “However, we accomplished most of the things that we had set out to do, and our results were sufficiently encouraging to justify support for later and more elaborate experiments.”
Among the most important findings from the GT-1 experiments was that hot dry granitic rock is not homogeneous. The group had expected to hit only hot dry granite, but it was rather a patchwork of granite, gneiss, and amphibolite.
The group also discovered that hot dry rock is not isotropic—identical in all directions. The rock has joints—cracks—that have been sealed over the years by mineral deposits or heat. Instead of breaking the hot dry rock apart, hydraulic fracturing actually causes the joints to open. Therefore, the resulting reservoir is not a single fracture but rather a network of joints intersecting other joints—a “dendritic [branched] pattern of interconnected joints,” according to Brown.
Alternate energy needs
As the Los Alamos team was dipping its toes into hot dry rock, the United States was being subjected to an oil embargo by oil producers in the Middle East. “All of a sudden, there were lines at the gas station,” remembers Leigh House, a now-retired Laboratory seismologist who worked on the HDR Program. “It was seen as a time of energy shortage, and so there was interest in finding alternatives to oil and gas.”
In 1971, the United States Congress directed the AEC to assume responsibility for all research related to energy supply, conversion, distribution, and storage. “The U.S. was becoming aware of the limitations of its conventional energy supplies and the environmental problems that resulted from their exploitation; the AEC had been directed to do something about those limitations; and the AEC was now informed that one of its major laboratories was ready to undertake the development of a new, essentially inexhaustible, environmentally benign, domestic energy supply,” Smith explained. “So far as we were concerned, the timing of all this couldn’t have been better.”
An October 1972 letter from the AEC to Los Alamos Director Harold Agnew authorized the Laboratory “to initiate work in general energy development areas” with the broad goal of “turning over to industry viable technologies, or as mutually agreed to, the final development of technologies.” By fiscal year ’73, money for the Los Alamos Hot Dry Rock Program began to roll in, which meant more experiments and more drilling (at millions of dollars per hole, drilling was by far the most expensive part of the Hot Dry Rock Program).
Because of limited space and muddy conditions in Barley Canyon, a new location was necessary. Fenton Hill, less than two miles southwest of Barley Canyon, was chosen. The Forest Service transferred management of the 20-acre property to the AEC, which then contracted Los Alamos to operate it. The mesa-top site, located off paved State Highway 126, was accessible to heavy equipment and close to existing power lines. And because a wildfire in 1971 had destroyed most of the trees, no vegetation had to be removed to set up shop.
Reaching rock at a suitably high temperature at Fenton Hill involved first drilling through volcanic tuff near the surface and then through a layer of limestone before encountering granitic rock, which was the drilling target.
Drilling through any type of rock relies on pulverizing the rock below the drill bit. Getting the pulverized rock fragments out of the way of the drill bit requires circulating a fluid down the drill string and back up and out of the borehole. If fluid circulation is lost, the progress of the drilling stops, and the drill string may get stuck in the borehole.
Over centuries, the limestone beneath Fenton Hill has been partly dissolved by fluids (such as rainwater and snow melt) and is characterized as “cavernous.” And so, perhaps not surprisingly, on multiple occasions the drilling process lost fluid circulation while getting through the layer of limestone. “They were basically drilling into large caves,” recalls Los Alamos geophysicist Scott Phillips. “When you’re drilling, you have fluid moving around, and the fluid is contained. If you hit a cave, the fluid is gone, which is a tough drilling problem.” Stopping the drilling to fix a lost-circulation problem is expensive, as a drill rig costs thousands of dollars a day to lease and operate.
To solve the problem, everything from trees to walnut shells to shredded tires were thrown downhole to try to regain fluid circulation. “Drillers get desperate sometimes and pretty ingenious,” House says. “We might have done better pumping dollar bills down the well to fix the lost circulation zones.”
Eventually the drill pressed on, and on December 9, 1974, Geothermal Test Well No. 2 (GT-2) was completed. The hole stretched 9,619 feet (1.8 miles) into the ground, and the temperature at the bottom was 386 degrees Fahrenheit.
“Soon thereafter,” Brown wrote, “a favorably oriented joint was opened by hydraulic pressurization near the bottom of GT-2” and a second borehole, Energy Extraction Hole No. 1 (EE-1), was drilled.
However, 18 months after the second hole was completed, researchers had failed to establish a satisfactory connection between the two wells. They decided a new course of action was necessary.
“Thus, there was no longer any alternative: If an adequate flow connection was ever to be achieved, one or the other of the two deep boreholes would need to be redrilled,” Brown explained. Between December 1976 and March 1977, a series of experiments took place to determine which hole would be modified, and which direction it would go. (At this time directional drilling, especially in the hard granitic rock, was a relatively new technology. According to Phillips, “the strange combination of oil patch drillers and egg heads” figured it out as they went along.)
In the end, the lower portion of GT-2 was redrilled twice, directionally, giving it legs: GT-2A and GT-2B. With continued hydraulic fracturing and seismic monitoring, a reservoir connection was finally achieved, and the first hot dry rock system was established. A fluid circulation system operated until late 1979 and generated thermal power rates as high as 5 megawatts, which confirmed the basic concept of the HDR Program.
A second HDR system—called the Phase II system—was then planned to produce power at a larger scale, more similar to a commercial power plant. This meant drilling deeper (nearly three miles) to reach hotter rock and hopefully establishing a reservoir large enough to extract more heat.
However, around this time, “the oil and gas industry became very busy,” House explains. “Drill rigs became very expensive and very in demand. We worried if we released our leased drill from the Fenton Hill site, we might not be able to get it back in time to carry on the program,” House says. “So we decided to drill both holes early on and then try to establish the reservoir between them.”
He pauses. “That decision came back to bite us.”
Scientists expected that the hydraulic fractures would be simple, largely planar features that would intersect both holes, EE-2 and EE-3. After many unsuccessful attempts to create a fluid connection between the two wells, the HDR Program decided that better seismic monitoring of the hydraulic fracturing was needed.
With additional monitoring in place, in December 1983, 21,600 cubic meters of water (more than eight Olympic-size swimming pools) were injected during a hydraulic fracturing operation that still failed to establish the needed fluid connection between the two holes. However, analysis of the newly acquired microearthquake data finally showed the reason for the many failed attempts. Rather than the fracture system extending oblique to the trajectories of the two holes, it was oriented nearly parallel to them.
“The earth is a complicated system; it doesn’t always do what you want it to do,” says Mike Fehler, a seismologist who started working on the program as a graduate student in 1974. “That’s what makes it interesting, challenging, and frustrating.”
Results from analysis of the microseismic data were used to plan the path for redrilling one of the wells. After the redrilling, which required careful directional drilling, several more hydraulic fracturing attempts were carried out. Finally, the long-sought connection between the two wells was achieved in early 1985.
Despite the various setbacks, Fehler says the team’s spirits were always high. “We had parties all the time,” he remembers. In May 1985, Brown threw a “Thank God it’s Connected Party” at his home to celebrate the success of Phase II.
Both HDR systems at Fenton Hill operated for almost a year each. Thermal power production ranged from 4to 10 megawatts, “proving beyond a doubt that it is technically feasible to recover useful amounts of thermal energy from HDR,” wrote Brown, who noted that at the time of his writing in 2012, the reservoirs at Fenton Hill were the only true hot dry rock reservoirs anywhere in the world.
An important attribute of a usable hot dry rock reservoir, Brown explained, is that the reservoir is confined—the pressurized water is totally contained within it. This is in contrast to a natural hydrothermal system, such as California’s The Geysers, which is the largest complex of geothermal power plants in the world and draws steam from already existing natural reservoirs. The number and locations of natural geothermal systems world-wide is quite limited. A major appeal for an engineered system, such as Hot Dry Rock, is the ability to exploit geothermal energy in many more places.
The end of an era
“Hot Dry Rock went on for a long time, and it died a slow death,” Fehler says. “It would be slated for dissolution, and then New Mexico Senator Pete Dominici would rescue it.” DOE finally stopped funding the program in 1995.
After nearly a quarter century of existence, the boreholes at Fenton Hill were plugged and the site was remediated. House remembers the program with a mix of satisfaction and frustration. “In addition to the pioneering work on drilling and fluid technologies, we did something no one else had been able to do up to that point in terms of the rate we could collect seismic data,” he remembers. “We recorded and located more than 20,000 seismic events, most of which were about a magnitude –2 or –3. Even the tiniest events helped show the boundaries of the fractured fluid system.”
Geothermal energy today
In the years since the Los Alamos HDR Program, researchers in Australia, France, Germany, Japan, Sweden, Switzerland, and the United Kingdom have experimented with their own hot dry rock programs, all with positive results. (British, German, and Japanese scientists were involved in the Los Alamos HDR Program.)
Yet, “in spite of its enormous potential, the geothermal option for the United States has been largely ignored,” according to a 2006 study by researchers at the Massachusetts Institute of Technology (MIT). “This has led to the perception that insurmountable technical problems or limitations exist.”
The MIT team delved into these problems and limitations and concluded that “while further advances are needed, none of the known technical and economic barriers… are considered to be insurmountable.” Not to mention that drilling knowledge and technology have greatly improved since the Los Alamos HDR days, which would likely reduce costs. Seismic technology has also improved, so reservoir mapping would likely be more accurate.
Today, the conversation is starting to shift. Climate and clean energy are back on DOE’s radar. The hot dry rock—now called the enhanced geothermal system, or EGS—concept is back on the table as an option for sustainable energy.
At Los Alamos, current geothermal research includes EGS modeling and monitoring, imaging of hydrothermal reservoirs, and developing machine learning applications in support of geothermal energy.
Farther afield, the DOE Frontier Observatory for Research in Geothermal Energy (FORGE) initiative has awarded a $220 million grant to the University of Utah to establish an underground EGS laboratory in Milford, Utah. Researchers from all over the world can utilize the site at Utah FORGE to develop and test EGS technologies, including those having to do with drilling, instrumentation, reservoir stimulation, flow testing, data collection, and data dissemination.
“In a nutshell, we are working on procedures for creating a geothermal system where none exists naturally with the long-term goal of making EGS technologies both replicable anywhere in the world and commercially viable,” says Joseph Moore, the managing principal investigator for Utah FORGE. “Already, significant progress toward reservoir creation in low permeability rocks has been achieved by merging ideas developed at Los Alamos more than four decades ago with advances in drilling and fluid extraction technologies developed by the oil and gas industries.”
According to DOE, EGS could one day result in commercially deployable systems that might collectively produce more than 100 gigawatts of electrical power. For context, 1 gigawatt—or 1 billion watts—is the amount of energy generated by approximately 3 million solar panels or 430 wind turbines. One gigawatt will power 110 million LED lights or 9,000 Nissan Leaf electric automobiles. More than 100 gigawatts could easily power 100 million homes.
In 2012, Brown reflected that “HDR researchers over the next 20 years will look back at the pioneering work done at Los Alamos National Laboratory and ask only why it took so long for the country to finally discover HDR.”
Ten of those 20 years have passed. But HDR—now in the form of EGS—appears to be on the cusp of a renaissance. “The Los Alamos HDR Program was a pioneering effort that paved the way for all subsequent EGS projects,” Moore says. “If EGS can be developed, the potential for this type of renewable energy is multifold.” ★