Earth contains an enormous amount of heat, partly leftover from the formation of the planet and also from the decay of naturally radioactive elements such as potassium, thorium and uranium.

1. Geothermal energy
2. Non-volcanic geothermal energy
3. Low-temperature hydrothermal systems

1. Geothermal energy

Geothermal energy is derived from geothermal heat, which is concentrated in volcanic areas where magma nears the surface. Temperatures are extreme and the heat flows outward continuously.

At the surface, volcanic locations such as Iceland, New Zealand and California developed the classic geothermal systems that use natural water circulating near hot volcanic chambers, or similar bodies, to power geothermal electrical generation plants.

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2. Non-volcanic geothermal energy

As you go deeper and deeper inside the Earth the temperature increases at a rate of approximately 25°C per kilometre, but harnessing heat in non-volcanic environments is highly challenging.

Hot dry rock systems

In the 1970s and 1980s, hot dry rock (HDR) systems were started in the US and the UK to target dry granites located a few kilometres below ground. These rocks produce heat from the natural radioactive decay of some contained minerals in addition to the background heat flow, and can reach temperatures of up to 200ºC.

In an HDR system, an injection well pumps high-pressure water into the rock. A plumbing system allows the water to travel within the rock capturing the heat, and an extraction well allows the hot water that comes out of the system to be converted into electricity. The resulting cool water is used again to repeat the process.

The system is expensive owing to the necessity of drilling deep boreholes, and there are other drawbacks such as the possibility of micro-seismic events. These are caused by the fracturing of rock to allow water to travel and the need for a local source of water.

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3. Low-temperature hydrothermal systems

Another alternative emerged when densely spaced oil exploration drill holes were used on the western flank of the Rhine Graben in France.

What emerged was a distinctive pattern of adjacent hot and cold patches that can be explained as the up-welling and down-welling limbs of hydrothermal convection. To extract the heat, it is sufficient to drill to the top of this convection cell and let natural fluids extract the deep heat.

Such systems, called low temperature hydrothermal systems, occur in natural sedimentary strata and/or naturally fractured rock containing hot water.

Using technology to target low to intermediate temperature water motions

The technology that has emerged to target, and potentially enhance, low to intermediate temperature water motions is particularly suited for the direct use of heat from deep sedimentary basins such as the Perth basin.

Heat from the Perth sedimentary basin is possible because:

• Targeting low temperatures dramatically increases the available areas for geothermal energy extraction.
• Natural high permeability exists within sediments of the Perth basin, allowing water to flow while carrying its heat.
• No artificial permeability enhancement is required, making the system more cost-effective than classical geothermal targets.
• Low temperatures are available at a shallower depth, thereby sharply reducing drilling costs.
• Shallower geothermal systems are much easier to target than deep systems by geophysical methods.

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