Geothermal energy researchers search for the ‘supercritical’ source

Geothermal energy provides about 18% of New Zealand’s electricity, and that could rise substantially by drilling into deeper, hotter fluids that are in a “supercritical” phase.

Researchers have been working to try to find the best place to drill that hole, and expect to publish their results in a year or two.

But before then an economic analysis will be completed looking at whether supercritical geothermal is economically viable, and what part it might play in meeting this country’s future energy needs.

The possibility of using supercritical geothermal fluids to produce electricity is being pursued in several countries, but it’s still in the ideas stage.

READ MORE:
* Taupō Volcano chills out as scientists reveal what caused spate of quakes
* Searching for New Zealand’s electricity future in the deep heat
* Geothermal energy critical if NZ is to reach carbon-zero by 2050

In this country, GNS Science has a key role in the research, which has concentrated on the Taupō Volcanic Zone (TVZ). GNS scientists provided an update on the work on Wednesday, ahead of NZ Geothermal Week being held in the Taupō area next week.

Supercritical fluids are under high pressure and at such high temperatures that the way they behave changes. Supercritical water is already used to drive turbines in some power stations, but the water is put into a supercritical state as part of the operation of the station, not drawn from deep underground.

It’s expected a well drilled to get to supercritical fluids in the TVZ will be 3-6km deep, with temperatures getting to around 400-450C.

In comparison, the conventional geothermal power stations in this country have wells down to about 3km. The highest temperature geothermal fluid found at a conventional station in this country has been 336C, while the average was about 290-300C.

“So we’re not that far,” GNS energy futures theme leader Dr Isabelle Chambefort said. Pure water became supercritical at 374C.

Researchers had been working to refine their understanding of the structure of the Earth’s crust in the TVZ, and those results would be published in the next year or two, she said.

The area being studied was in the central North Island between the Okataina caldera, east of Rotorua, including Tarawera, and the Taupō caldera.

In the TVZ the crust had been split apart and covered by volcanic deposits. The aim was to see through those volcanic deposits to where contact was made with the basement rock, Chambefort said.

That had important implications for how the geothermal fluids were circulating in the crust.

“We need to understand this deep circulation, and to do this that’s why we needed to understand the basement.”

Researchers had identified such things as where the magma was, where there was unrest, and where there was latent mature heated crust, which was the best target for supercritical drilling.

The two main active magma systems in the area were at Okataina and Taupō.

But in between at greater depth – around 8km – there was a zone where the rock was partially molten, with great heat, Chambefort said.

“This heat can conduct to the shallower level and then be transported by geothermal fluid.”

The aim was to not drill into magma. Doing so would mean more acidic volcanic fluid, and problems maintaining wells and ensuring magma didn’t come up the well.

A magma body could be around 750-800C. “So you’re too hot,” Chambefort said.

“So we’re targeting, really, hot water.”

The sort of imaging work done so far refined what was known about the crust in the area, but was never going to be perfect. “The best way to test is to drill.”

Choosing the best place to drill wasn’t just about geology, with other factors including access to the electricity grid.

Along with the scientific work, she was now busy on the challenging job of combining the information available to reduce the risk that would be involved in investing in supercritical geothermal energy.

“Right now we have two problems. People don’t necessarily see the benefits in terms of the economy, and they’re thinking the risk is too high,” Chambefort said.

An economic advisory group that specialised in renewable energy was producing a report looking at what role supercritical geothermal might play in New Zealand’s palette of energy possibilities. Hopefully a final report would be available by the end of the year.

“What is the market proposition for supercritical in the light of the wind, the solar, the gas, and the battery? That’s the key aspect of it.

“Why we have so much put into batteries in energy is because we are expecting a growing of intermittent renewables, like wind and solar,” Chambefort said.

“Supercritical geothermal is not intermittent, so that diminishes the need for a huge amount of batteries to the grid.”

Balancing energy sources was complex, and analysis needed to nail down how the grid would be affected if supercritical geothermal could be developed, or even conventional geothermal just increased.

“If you want supercritical to be a viable solution by 2050, 2040 you need to drill before 2030.”

A conventional geothermal development could take 10 years just to get consented, before building started.

“So we absolutely need to put all those different aspects together.”

Work was under way to determine the key actions needed for that to happen, with a strategy expected to be ready within a year.

“It’s coming along quite nicely, actually. The geology, it’s almost not that significant anymore. It’s all the things around it,”

Drilling down to supercritical fluids had been carried out in some countries, including Iceland, where the intercepted fluids were about 550C, Italy, Kenya and the US. Japan had also done it in the past.

A key problem still to be solved was how to maintain a supercritical well to keep it open. Iceland was working on a plan to drill a third well, but it wouldn’t go ahead until an improved type of casing for the well had been developed.

She expected a solution within three years.

In Japan work on supercritical geothermal energy was fully government-funded and had a short timeframe, with aggressive targets, and with plans to drill in two places in 2024.

“It needs to be economic… Maybe it’s not economic now, maybe it’s going to economic in 20 years. The work will have been done.”