Current Projects
Company Geological Rationale
The theory of hot dry rock geothermal energy has been well documented in many research papers in recent times. In essence it involves extraction of heat from deeply buried hot rocks via superheated water (Figure 1). The three key factors for a successful hot dry rock geothermal energy project are:
- A suitable hot rock source. Typically this is a so-called “hot” granite that is generating abnormally high amounts of internal heat from the natural radioactive decay of uranium, thorium and potassium bearing minerals.
- An insulating blanket of sediments, particularly shales, siltstones and coal seams, that effectively entraps the heat generated from the buried granite and prevents it being dissipated.
- Adequate fracturing of the hot dry rock source that allows circulation of a horizontal fluid flow regime.
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Figure 1 - 3D cutaway showing electricity generation from a hot dry rock
source - diagram modified from ANU Hot Rock Energy website
(http://hotrock.anu.edu.au)
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The idea is that water, circulated via injection and extraction boreholes, is super-heated by the hot rock and brought to a surface heat exchanger loop without boiling. Steam produced in a closed loop on the other side of the heat exchanger is used to generate electricity in a conventional steam turbine. By preventing the hot rock water from boiling, deposition of dissolved salts in the pipe systems is reduced to a minimum. The power generating ability of the hot rock circulation system is critically dependent on the hot rock temperature, the flow rate of water through the hot rocks and the actual volume of hot rock that the circulating water comes in contact with. The crucial factor therefore is adequate permeability in the hot rock, so that sufficient volumes of formation or injected water can circulate freely through a large mass of sub-horizontal hot rock to become superheated. Given
the huge downward pressures at these depths due to the sheer weight of overlying rocks, the hot rock source is unlikely to have very much natural open fracturing or permeability. Consequently, it is necessary to stimulate fracturing of the hot rock by injection of water from the surface under extremely high pressures. The water pressure jacks open existing fractures in the hot rock, which do not completely close again when the water pressure is removed. This hydro-fracturing stimulation technology is commonly used in the oil industry to improve flow rates, and with it, permeabilities in the host hot rock can be dramatically enhanced.
Over time the hot rock source will cool as heat is continually extracted from it by the circulating water. Calculations indicate that it will take 30 years to cool and then 20 years to regenerate sufficient heat again. In an operating hot dry rock geothermal system, this would necessitate shifting of the circulation cell to another part of the hot dry rock body to allow time for regeneration of the heat. In this sense, hot dry rock geothermal energy is truly renewable and sustainable. The quantities of energy available are potentially enormous, with a US Government energy agency estimating that total global geothermal energy resources are approximately 50,000 times the energy contained in all the known oil and gas reserves in the world.
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Figure 2 - Heat flow map of Australia
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Study of heat flow maps of Australia shows that South Australia is particularly well suited as a hot dry rock destination because of abnormally high heat flows, thought to be caused by the widespread abnormally radiogenic Precambrian granites (Figure 2). A high heat flow zone runs through the southeastern portion of the state encompassing Geothermal Resources’ tenements. It has been well known for many years that the Cooper Basin region in the far northeast of South Australia has abnormally high heat flows caused by radiogenic granite buried beneath several kilometres thickness of sediments. Recently, operators in this field have achieved astounding scientific success by demonstrating that granite at some 4000 metres beneath the Cooper Basin is capable of being fractured to successfully produce superheated water in quantities considered sufficient to operate a commercial power station. Other companies have also recently demonstrated abnormally high heat flows above an interpreted buried granite body near Lake Callabonna and also close to Olympic Dam. Thus the concept of hot dry rock geothermal energy in South Australia has now moved beyond theory to demonstrated fact.
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