Deep underground on the 4850 Level at Sanford Lab, engineer Paul Cook explains how the SIMFIP tool will be used to measure openings in hard rock.
On May 22, researchers with the SIGMA-V experiment worked in near silence in the West Drift on the 4850 Level. The locomotives sat quietly on the tracks, jack-leg drills rested against drift walls and operations ceased for several minutes at a time as the team began pumping pressurized water into the injection well, one of eight boreholes drilled for this experiment.
“We requested quiet because we use sensitive seismic monitoring equipment,” said Tim Kneafsey, earth scientist at Lawrence Berkeley National Laboratory (Berkeley Lab). “The signals we measure are very small and we don’t want vibrations from other sources overwhelming those signals.”
Kneafsey is the principal investigator for the Enhanced Geothermal Systems (EGS) Collab Project, a collaboration comprised of eight national laboratories and six universities who are working to improve geothermal technologies. The test featured the SIMFIP (Step-Rate Injection Method for Fracture In-Situ Properties), a tool that revolutionizes the way scientists can study geothermal energy, a process that pulls heat from the earth as it extracts steam or hot water, which is then converted to electricity.
Developed at LBNL, the SIMFIP allows precise measurements of displacements in the rock and, most importantly, the aperture, or opening, of a hydro fracture.
The extreme quiet paid off, Kneafsey said.
“Our goal was to create a fracture from a specific zone in our injection well that would connect to our production well—about 10 meters away. And we were successful in doing that,” Kneafsy said.
"People were excited when the connection between the boreholes was made and measured. But it took a while for the team to realize how far we had come and how much research, logistics, planning and collaboration went into that moment. It was gratifying to say the least, and there was certainly a sense of accomplishment.” —Hunter Knox
The experiment
Before the introduction of the SIMFIP, separate tools were used to create and measure hydro fractures. They work like this: “Straddle packers”—pipes with two deflated balloons on either end—are placed inside boreholes. Once inside, the balloons are inflated and water injected down the pipes to create an airtight section. They continue to pump water until the rock fractures, then remove the packers and insert the measuring tool. In the time it takes to do all that, much of the pertinent data is lost, leaving traces, but little else.
“Even if you did get the aperture, when you released the pressure, the hydro fracture was already closing,” said Yves Guglielmi, a geologist at LBNL who designed the tool. “You don’t have the ‘true’ aperture and you also don’t know how the aperture might vary during the test.”
With the introduction of the SIMFIP, a small device that sits between the two packers, they can the aperture in real-time.
“This is really a new way to do the work,” Guglielmi said. “It will help us understand the whole process of initiating and growing hydro fractures in hard rock, which is kind of new. This is fundamental science. If we understand how hydro fractures will behave in this kind of rock, we can begin to make intelligent, complex fractures that can capture more heat from the earth.”
The device is “bristling with sensors and other instrumentation that give us a close-up view of what happens when the rock is stimulated—all in real-time,” said Paul Cook, LBNL engineer.
The SIMFIP measures fracture openings in hard rock in the EGS Collab test site. The team had drilled eight slightly downward-sloping boreholes in the rib (side) of the West Drift: The injection hole, used for stimulating the rock, and production well, which produces the fluid, run parallel to each other through the rock. Six other boreholes contain equipment to monitor microseismic activity (rock displacement); electrical resistivity tomography (subsurface imaging); temperature; and strain (how rocks move when stimulated).
Nestled between the straddle packers in the injection hole, the SIMFIP measured the rock opening as the team looked on.
The SIMFIP difference
The SIGMA-V team hoped to see signals as small as a few microns of displacements in the rock. As they watched data accumulate in real time over a two-day period, the excitement in the West Drift was palpable.
“People were excited when the connection between the boreholes was made and measured,” said Hunter Knox, the field coordinator with Sandia National Laboratory, “But it took a while for the team to realize how far we had come and how much research, logistics, planning and collaboration went into that moment. It was gratifying to say the least, and there was certainly a sense of accomplishment.”
Measurements from the SIMFIP could remove barriers that stand in the way of commercializing geothermal systems, which have the potential to provide enough energy to power 100 million American homes.
"We know fracturing rock can be done. But can it be effective for geothermal purposes? We need good, well-monitored field tests of fracturing, particularly in crystalline rock, to better understand that,” Kneafsey said.
With the first test under its belt, the EGS Collab just moved a step closer to that goal.
