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The science of hot fluids in cracked rocks

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Fuseworks Media
Fuseworks Media

Research by a Victoria University of Wellington PhD graduate has improved understanding of how underground hot fluids flow through fractured rocks, which will help in the development of geothermal energy.

A study by Dr Cécile Massiot, who graduated last week with a PhD in Geophysics, focused on determining the nature of the cracks-or fractures-that control the circulation of fluids in the Earth’s crust.

"There are myriads of fractures underground, but a common challenge in geosciences is mapping where they are, how big they are, and which ones actually serve as pathways for fluids," says Dr Massiot.

"Identifying the characteristics of those fractures that guide fluids is critical for the exploration and management of geothermal renewable resources, which currently accounts for nearly a quarter of New Zealand's electricity supply."

Dr Massiot compared observations made at the surface of the Earth where fracture systems can be seen and touched, with measurements from boreholes where data is sparse but directly representative of underground conditions.

Part of Dr Massiot’s study focused on the fracturing adjacent to the Alpine Fault on the South Island’s West Coast, where scientists drilled a nearly 900 metre-deep borehole to measure subsurface conditions. The results of that project, published in Nature, reveal surprisingly high temperatures next to the Alpine Fault and the potential for large geothermal resources in the area.

"Determining the layout of fractures near the Alpine Fault is key to understanding the role fluids play in earthquake processes," says Dr Massiot. "The high temperatures found in the borehole also open new opportunities for exploring geothermal resources in Westland."

Dr Massiot, who is originally from France and came to New Zealand in 2010 to work at GNS Science, also looked at the fracture systems in volcanic rocks found at Mount Ruapehu and the Rotokawa Geothermal Field near Taupo.

"We’ve known for some time that current tectonic forces acting on New Zealand control the underground fracture systems," says Dr Massiot.

"My results show that, in addition, the original fracture networks that formed within lava as it cooled at the Earth’s surface millions of years ago, now also control the architecture of fracture systems steering geothermal fluids.

"These findings will improve the use of geothermal resources, in New Zealand and overseas, and help us to better understand pressure and temperature conditions in conventional geothermal settings such as those near Taupo, and unconventional geothermal settings such as the Alpine Fault."

She is now back at GNS Science and will pursue these themes of research, in partnership with geothermal operators who have shown strong interest in incorporating the new data into their models. In turn, this will help lift the operating efficiency of geothermal power stations.

Dr Massiot’s research was supervised by Professor John Townend of Victoria’s School of Geography, Environment and Earth Sciences, Professor Andrew Nicol, formerly of GNS Science and now at the University of Canterbury and Dr David McNamara, formerly at GNS Science and now at the National University of Ireland, Galway.

The research was funded by GNS Science via the Sarah Beanland Memorial Scholarship, and made use of data provided by the Rotokawa Joint Venture and Mercury New Zealand.

"I am grateful to the Department of Conservation, the Ruapehu Ski Field, Ngāti Rangi and Ngāti Hikairo for their support. Working with iwi introduced me to the spiritual significance of the mountain, and brought a new dimension to my studies," says Dr Massiot.

Dr Massiot’s thesis can be viewed online here: http://researcharchive.vuw.ac.nz/handle/10063/6194

For more information contact Dr Cécile Massiot on 04-570 4776 or c.massiot@gns.cri.nz.

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