The ideal shape and orientation of an underground heat exchanger for the extraction or mining of the heat is a horizontal lens shaped body with a more or less uniform temperature.

Such an orientation also allows for the development of several stacked underground heat exchangers, (one on top of the other) all constructed from a single central development well. More importantly, a horizontal orientation allows the underground heat exchangers to be linked up horizontally, enabling the extraction of heat over a large flat lying horizon.

The current well locations identified in the Cooper Basin Project as at October 2008

Stacked heat exchangers in small scale demonstration
plant

To develop horizontal underground heat exchangers at depth, the horizontal stresses in the Earth's crust need to be greater than the vertical stresses. When hydraulic pressures are built up at the bottom of a development well, water flows along fractures which are most easily parted. When the vertical stress is lower, the horizontal fractures will open up.

Many studies have shown that the rocks in much of the Cooper Basin (South Australia) and in the Hunter Valley (New South Wales) are subject to high horizontal stresses. This bodes well for the development of horizontal underground heat exchangers and large multi-reservoir developments.

This is in contrast to underground heat exchangers previously developed in, for example, France and Japan. Underground heat exchangers in these countries have a semi-vertical orientation because the Earth's crust in these locations is being pulled apart, so vertical fracture sets have the least resistance and open up more easily. Semi-vertical underground heat exchangers have varying temperatures at the top (cooler) and bottom (hotter) and are difficult to link up into large multi-reservoir developments. Volcanic areas of the world have low horizontal stresses so the underground heat exchangers developed in these areas have been vertically oriented.

The proof of the optimal stress orientation in the Cooper Basin came when Geodynamics carried out its hydraulic stimulation of Habanero 1 well in November and December 2003. The reservoir that was developed extended close to horizontally over an area of more than 3 km² considerably larger than expected when compared to the extent of overseas developments previously.

Acoustic emissions (blue dots) related to the hydraulic stimulation progran at Habanero 1 in 2003 drapped over an aerial photo of the area (metric grid squares are 500m).