Cost-effective, non-polluting enhanced geothermal systems

The project addresses some of the shortcomings of typical geothermal systems. In addition to the prospect of developing systems in areas of impermeable rock, enhanced geothermal systems also would not require that water already be present underground.

Because of the advantages, researchers see great potential for enhanced geothermal systems. Existing U.S. geothermal power plants generate up to 3.4 gigawatts of energy, making up about .4 percent of the nation’s energy supply. A 2006 Massachusetts Institute of Technology report estimates enhanced geothermal systems could boost the nation’s geothermal energy output to 100 gigawatts, enough to power 100 million American homes.

That potential has attracted the interest of the Department of Energy, which has funded five enhanced geothermal system demonstration projects in the United States. At one project in Nevada, enhanced geothermal methods increased a conventional geothermal plant’s productivity by 38 percent. However, technical challenges and concerns over cost and the amount of water used in these systems has limited their use.

Hydraulic fracturing processes, similar to those used in oil and gas production, also have been used in enhanced geothermal systems, but those processes have drawbacks due to the amount of water required, the potential toxicity of the chemicals used and the high costs of retrieving and treating the water.

In contrast, the NMSU/PNNL team’s fluid is a solution of water and 1 percent polyallylamine, a chemical made of carbon and nitrogen that is similar to polymers used in medicine. After it is pumped into a well at a geothermal hot spot, pressurized carbon dioxide is injected into the well.

Within twenty seconds, a chemical reaction causes a hydrogel to form, expanding the fluid up to 2.5 times its original volume, expanding existing cracks in the rock and creating new ones. The process is expected to cut in half the amount of water and time needed to open up an underground reservoir, lowering the cost of power generation.

Researchers are testing the fluid’s performance on cylindrical samples of impermeable rock. The samples are placed inside a high-pressure, high-temperature test cell created by the researchers. Small amounts of the fluid and liquid carbon dioxide are injected into the test cell, then pressure and temperature are adjusted to match the conditions of the underground geothermal reservoirs.

The researchers found that the fluid consistently created small but effective cracks in the samples that allowed water to flow through. This led them to believe that larger scale tests might produce larger cracks.

The research team also expects that the fluid could be inexpensively recycled.

NMSU notes that additional studies are necessary to more thoroughly evaluate the fluid’s performance. The team is planning lab studies to measure the level at which the fluid can be recycled as well as its ability to fracture larger pieces of rock. The ultimate goal is to conduct a controlled field test.

The team also is studying a similar fluid for unconventional oil and gas recovery. It would use a different polyamine that is similar to the chemical used in the geothermal extraction fluid. The fluids are stable and can withstand extreme temperatures, pressures and acidity levels, unlike some fluids used in oil and gas recovery, which degrade over time. The new non-toxic, recyclable fluid also would result in more efficient use of water.

— Read more in H. B. Jung et al., “Stimuli-responsive/rheoreversible hydraulic fracturing fluids as a greener alternative to support geothermal and fossil energy production,” Green Chemistry 17 (2015): 2799-812 (DOI: 10.1039/C4GC01917B); and Hongbo Shao et al., “Environmentally friendly, rheoreversible, hydraulic-fracturing fluids for enhanced geothermal systems,” Geothermics 58 (November 2015): 22–31 (doi:10.1016/j.geothermics.2015.07.010)