Hot fluids, cracked rocks and the path to geothermal energy

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

Dr Cécile Massiot, who graduated 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. She explained, “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.

“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. Her study examined the fracturing adjacent to the Alpine Fault on the South Island’s West Coast, where scientists drilled a nearly 900 m-deep borehole to measure subsurface conditions, as well as the fracture systems in volcanic rocks found at Mount Ruapehu and the Rotokawa Geothermal Field near Taupo.

The results of her project, published in the journal Nature, reveal surprisingly high temperatures next to the Alpine Fault and the potential for large geothermal resources in the area. Dr Massiot noted, “Determining the layout of fractures near the Alpine Fault is key to understanding the role fluids play in earthquake processes.

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

“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.”

Now working at GNS Science, Dr Massiot will pursue these themes of research in partnership with geothermal operators who have shown strong interest in incorporating the new data into their models. This will help lift the operating efficiency of geothermal power stations.