Hot Dry Rock energy

Hot Dry Rock energy

Summary:

Since an international conference in Canberra in 1993 described huge energy resources locked up in hot granite bodies in Australia, there has been increasing interest in how we can tap these resources and where the best sites reside in terms of connection to national electricity markets. The best sites from a geological point of view have been delineated at remote localities during oil and gas exploration, but the hunt is on for a site where a pilot plant could be connected directly to the national grid.

Visit http://www.greenhouse.gov.au/renewable/recp/hotdryrock/index.html

Extracts:

One possible site lies in the Hunter Valley south of Muswellbrook, where the first tenement in Australia for the right to extract heat from 'hot dry rock' (HDR) was granted to Pacific Power in February 1999. Pacific Power and researchers at the Department of Geology, Australian National University, teamed up to investigate the Muswellbrook geothermal anomaly. With the help of $790,000 in funding under the Renewable Energy Commercialisation Program, the project has determined the areal extent of the geothermal anomaly, and the temperatures and rock properties at a depth of around two kilometres in the core of the anomaly.

A series of shallow (300 metre to 920 metre deep) boreholes were drilled over the anomaly and temperature measurements were made in each borehole. A larger 1946 metre deep hole was then drilled in the central region of the anomaly and more than 1km of continuous core samples were taken to identify the rocks present and their physical properties. Temperature logs were also run and these demonstrated that the temperature is at least 900C at the bottom of the borehole. This temperature is much higher than is normally expected in Australia at such depths, which confirms that this area in the Hunter Valley is highly prospective for geothermal energy.

A 19km long seismic reflection survey was then carried out along an east-west track over the anomaly. Analysis of the seismic results and those from a micro-gravity study along the same track suggested that a buried granite probably exists at a depth of at least 5km. The geothermal anomaly is apparently due to radiogenic heat production in this buried granite, which means that the granite represents a substantial source of energy. The results of the project stimulated commercial interest in Australia's hot dry rock resources to such an extent that a new company, Geodynamics Limited, was successfully floated on the Australian Stock Exchange in 2002. This company has now acquired the Hunter Valley geothermal tenement from Pacific Power in addition to an adjoining site and two sites in the Cooper Basin in north-east South Australia. Geodynamics has raised capital of $11.7 million from its initial public offering and has commenced an ambitious development program near Innamincka, South Australia that will see the drilling of the hottest borehole ever drilled in Australia. Temperatures of over 2700C are anticipated at a depth of 4.9km.

Successful development of the Cooper Basin and Hunter Valley resources would constitute a major source of green energy base-load power for Australia. At an average temperature of 2500C, approximately 180 petajoules of usable heat for electricity production is available per cubic kilometre of rock. The combination of Australia's predominant crustal shortening stress conditions and buried granite hot rocks opens the way for low-cost engineered geothermal reservoirs based on only one injection well and two production wells. Economic modelling, performed by the Energy Laboratory at Massachusetts Institute of Technology, USA as part of the International Energy Agency's Geothermal Implementing Agreement, suggests that such a system has very favourable economics and could surpass most other renewable energy options. Moreover, the huge scale of Australia's geothermal resources, particularly those beneath the Great Artesian Basin, could provide economies of scale to rival those of base-load generation from coal.

Under certain conditions, subsurface granites can reach 2500C and higher at depths of 3 to 5 kilometers. These granites are hot for a number of reasons. They are relatively high in decaying radioactive elements, and heat is conducted from very hot sources below. In most cases (and preferably), the granites are buried beneath thick insulating sedimentary rocks.

The aim of a hot dry rock (HDR) program is to harness the energy in these granites by injecting water into a borehole and circulating it through a permeable reservoir created by hydraulically fracturing pre-existing, minute cracks in the rock. Success primarily depends on the presence of these natural fractures. The injected water is superheated as it passes through the hot rocks and returned to the surface via adjacent boreholes, where it is converted to carbon-free generated electricity using conventional steam turbine technology. Extending the reservoirs and adding more boreholes (injection and production) can increase power output.

The University of New South Wales (UNSW) is highly regarded, both domestically and internationally, in the field of hot rock energy (HRE) and its exploitation as a viable resource. It began its work on HRE in 1993, and its intensive research and studies, particularly in respect to HRE geothermal reservoir behaviour, have continued unabated ever since.

A study funded by the Australian Government's Energy Research and Development Corporation and completed in 1994 concluded that Australia is probably the only country that has extensive HDR resources with the potential to generate electricity many times its current total annual electric power needs. A significant proportion of this resource resides in the Cooper Basin. The university has determined that the granites in the Cooper Basin have the most favourable characteristics for HDR development.

The university has an exclusive arrangement with Scopenergy Limited, which was recently granted a geothermal exploration licence to exploit HDR energy sources from Block HDR-2000C in the Nappamerri Trough Region of the Cooper Basin. Block HDR-2000C covers an area of approximately 500km2, which has an estimated geothermal potential of about 102,000PJ (equivalent to 17 billion barrels of oil). This is sufficient to produce 500MWe over 25 years.

With the support of a $1 million grant under the Renewable Energy Commercialisation Program and additional funding from industry, UNSW will evaluate the HRE reservoir potential of the granite at the base of Big Lake #60 in block HDR-2000C. This was drilled and suspended as a possible future gas producer in 1997. This well reached a total depth (TD) of 9,800 feet, and granite is expected to be encountered within approximately 100 feet of this depth. The project aims to develop assessment methodology and criteria, invaluable tools for characterising HDR resources and determining the most appropriate reservoir development process. Very significant greenhouse gas reductions will result from the widespread exploitation of these HDR resources.

Although overseas HDR programs have not been successful to date, it is considered that geological and related conditions in Australia are infinitely more favourable than those encountered in HDR projects in the USA, Europe and Japan.


 
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