New approach would boost use of geothermal energy

Creating enhanced geothermal systems requires injecting millions of gallons of water — a valuable resource in the arid American West, where enhanced geothermal has the most potential. That water is sometimes mixed with a very small amount of chemicals to help the fluid better create and spread tiny cracks underground, which ultimately extends the life of a geothermal power plant.

Some geothermal reservoir stimulation fluids are similar to oil and gas hydraulic fracturing fluids in that a small percentage of their volume can include proprietary chemicals, according to a 2009 paper in Geothermics and other sources. These chemicals can be toxic if ingested, leading geothermal developers to retrieve and treat used fluids. This protects aquifers, but increases the cost of power generation as well. Environmental reviews must be conducted to receive permits for enhanced geothermal injections.

A better solution
PNNL’s fluid is a solution of water and 1 percent polyallylamine, a chemical made of a long carbon chain with nitrogen attachments that’s similar to well-understood polymers used in medicine. The fluid is pumped into a well drilled at a geothermal hot spot. Soon after, workers also inject pressurized carbon dioxide, which could come from carbon captured at fossil fuel power plants.

Within twenty seconds, the polyallylamine and carbon dioxide link together to form a hydrogel that expands the fluid up to 2.5 times its original volume. The swelling gel pushes against the rocks, causing existing cracks to expand while also creating new ones. The expansion is expected to cut in half the amount of water and time needed to open up an enhanced geothermal reservoir, which shrinks the cost of power generation.

Passing the test
To test the fluid’s performance, geophysicist and co-author Alain Bonneville led the development of an experimental set up. Five cylindrical samples of rocks, about the size of C batteries, taken near a working enhanced geothermal power plant in California, were placed inside a high-pressure, high-temperature test cell created by the PNNL team. Small amounts of the fluid and liquid carbon dioxide were injected into the test cell. Pressure and temperature were gradually adjusted to match the conditions of underground geothermal reservoirs.

The researchers found their fluid consistently created small, but effective cracks in rock samples. Some of the new fractures were too small to be seen with a high-resolution imaging method called X-ray microtomography. When they watched fluids such as water or carbon dioxide being injected, however, the team saw liquids moving through the previously impermeable rock samples. Moving liquids did not pass through rock samples that were injected with plain water or the common hydraulic fracturing chemicals sodium dodecyl sulfate and xanthan gum.

The team reasoned larger-scale tests might produce bigger cracks.

— 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 (25 March 2015) (DOI: 10.1039/C4GC01917B)