Supercritical geothermal fluids (>5 km, >400°C) are expected to offer more energy than geothermal fluids found at current depths (~3.5 km) and reservoir temperatures (<350°C). To utilise these in a sustainable way, a greatly enhanced scientific understanding is needed to realise the potential of supercritical resources, and offer industry-ready solutions.
Earth energy is renewable heat, created when the planet formed. Groundwater percolates downwards towards the heat source and becomes heated to high temperatures before buoyancy returns these fluids to shallower levels, or even the surface. In the upper few kilometres of crust, faults and fractures in the rock act as natural channels.
Permeable rock formations store some of the rising fluids, forming geothermal reservoirs of heat. Drilling into these reservoirs is a renewable, reliable and secure source of energy. At around 2.5 km deep, a geothermal well can produce water and steam at about 300°C, resulting in about 5 megawatts of electricity generation – enough to power over 3,000 homes, or to run a large-scale timber drying kiln.
Our position along the Pacific – Australian tectonic plate boundary means high temperature fluids run through natural pathways close to the surface, which makes accessing the fluid and their heat an economically viable option – unlike many places around the world. Geothermal energy supplies over 17% of the nation’s electricity, as well as heat for domestic, commercial and industrial applications – from timber drying and milk processing to greenhouses and bathing.
Supercritical conditions occur near the brittle–ductile transition zone in the Earth’s crust, where cooler brittle rock overlays hotter plastic rock, and superhot fluids circulate. Above 374°C and 221 bars of pressure, water transforms into a supercritical fluid, where distinct liquid and gas phases don't exist. These fluids have a higher heat content and lower density than liquid water, so have the potential to generate much more energy than the same volume of a conventional geothermal fluid.
International projects aim to harness the power of the Earth’s supercritical heat. Innovative drilling and well completion techniques are needed to deal with extreme temperatures and aggressive fluid chemistry.
Supercritical resources offer a near-limitless energy supply that will fundamentally transform New Zealand’s energy sector. This future includes a decarbonised electricity system with vastly improved environmental performance.
Supercritical provides for new investment opportunities. Economic development will be supported through commercial applications of supercritical geothermal resources for material processing, industrial scale forestry, dairy and other industrial uses.
Conventional geothermal wells in New Zealand tend to be 1.5 km – 3 km deep. We are searching for supercritical conditions at 4 km – 6 km depth, where magmatic heat encounters permeable rocks, allowing hot fluids to pass through relatively easily.
The tools usually used to probe beneath the Earth’s surface depths have not been calibrated for rocks at extreme temperatures and pressures. We don’t know the geology nor how the rocks and fluids interact under supercritical conditions. The drilling, piping and electronic materials were never designed to handle the temperatures nor corrosive chemistry of supercritical fluids.
It is essential to define the heat transfer mechanisms from magma to surface, and understand the behaviour of magma bodies, their high temperature envelop, and the integrating fluid and heat transfer at these supercritical conditions. Then to translate this science into real world application and thinking. The Geothermal: The Next Generation research programme aims to do just that.
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