Researchers working at the Energy Department's Pacific Northwest National Laboratory (PNNL) developed what they said is an eco-friendly fracturing fluid called StimuFrac that leverages lower pressures to fracture highly impermeable rocks in enhanced geothermal systems.

They said that current fracturing fluids used in enhanced geothermal systems (EGSs) are costly and inefficient for the temperature conditions. What's more, conventional fluids used in the oil and gas industry typically require large volumes of water and may pose risks to groundwater.

New technology from Pacific Northwest National Laboratory could benefit geothermal energy development. Source: National Renewable Energy LaboratoryThe fluid, called StimuFrac, is intended to address these issues by creating potentially safer, cost-effective fractures that penetrate high-temperature reservoirs.

Growth potential

Geothermal power is an energy source that captures heat below the Earth’s surface and turns it into electricity by using steam turbines. PNNL researchers said that although it is relatively easy to find high-temperature rocks, it is challenging to find geologic formations that can be developed into sustainable reservoirs. The mechanical process known as hydraulic fracturing creates a network of fractures through high-pressure fluid injection into target rock formations, allowing heat transfer from the rock to the injected fluid.

Last spring, the DOE outlined a path to expand the contribution of geothermal energy to U.S. electricity supply. The agency said that installed geothermal generating capacity could grow from the current total of about 3,500 MW to 60 GW by 2050, which would provide 8.5% of all U.S. electricity generation. Technology improvements could also enable more than 17,500 geothermal district-heating installations nationwide, and 28 million U.S. households could realize cost-effective heating and cooling solutions through use of geothermal heat pumps.

Process uses less water

PNNL researchers said they conducted a computational and experimental study of StimuFrac, determining that it consistently fractures rock material at lower pressures compared to conventional hydraulic and waterless fluids.

StimuFrac involves pumping a water-based solution of polyallylamine into the ground. It is then combined with injected carbon dioxide and forms a hydrogel. The resulting volume expansion creates pressure that causes rock fracturing.

The researchers said that StimuFrac could reduce operating costs by 60% by using less water and less pumping energy in comparison to conventional fracturing methods.

The researchers confirmed the two processes by conducting a series of experiments using fracturing fluids on fused silica, which has homogeneous “rock-like mechanical properties.”

A PNNL computational scientist used modeling based on the finite element method to determine the pressures required to fracture fused silica. The team observed a reduction of injection pressure required to generate fractures under conditions where volume expansion occurs. StimuFrac consistently fractured fused silica at 30% lower pressures than the conventional fluids, according to findings published in the American Chemical Society journal Sustainable Chemistry and Engineering.

Lower pressure means the technique is less energy-intensive and relatively inexpensive compared with conventional hydraulic fracturing. Source: NRELIn one of the processes, the researchers established that overpressure generated by the CO₂-triggered volume expansion of the water-polymer solution was responsible for the observed more efficient fracturing. The value of overpressure was then determined using an innovative high-pressure mixing vessel developed by the team.

In the second process, the StimuFrac fluid penetrated rock at lower pressures to fracture the rock. The lower pressure means it is a less energy-intensive and relatively inexpensive fracturing process.

Researchers are now evaluating different injection strategies in foot-scale rock samples to identify injection strategies that show promise for potential field deployment. PNNL said it is seeking industrial partners to transition this technology to the field within the next two years.