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Old 04-16-2010, 08:52 AM
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Default Article for Geothermal Newbies

I have posted the article below as although it doesn't offer anything new it is a useful summary

For a comprehensive look at Goethermal Energy (Hot Rock Energy) please visit HotRockEnergy.com

Turning geothermal energy from a concept to a reality


Filed under: Geothermal, Renewable Energy, Issue March | April 2010, Print Edition
by Paula Wallace ? created Apr 16, 2010 02:36 PM
Geothermal energy in Australia is currently delivering only 150 kWe of electricity (Birdsville, Queensland) and a limited amount of heat for direct use applications such as space heating, pool heating and hot springs (Chopra 2

By Dr Cameron Huddlestone-Holmes (CSIRO Earth Science and Resource Engineering, Petroleum and Geothermal Portfolio) and Professor Klaus Regenauer-Lieb (Western Australian Geothermal Centre of Excellence; CSIRO Earth Science and Resource Engineering, Petroleum and Geothermal Portfolio; The University of Western Australia)
However, geothermal energy is often discussed alongside solar thermal with storage, and wave power as a potential provider of clean base-load power in a low carbon economy. This article briefly describes the geothermal resource styles targeted in Australia and looks at some of the major technical challenges that need to be overcome to take geothermal from its current position and deliver on the potential it promises.
The resource
Geothermal energy can simply be described as heat extracted from the earth. The Australian geothermal industry is targeting two broad styles of geothermal resources; Hot Rock (HR) and Hot Sedimentary Aquifers (HSA). Hot Rock resources are typically in crustal rocks that are heated by the decay of radioactive potassium, uranium and thorium and buried under insulating sediments that trap the heat. For example, Geodynamics? geothermal play near Innamincka in South Australia?s taps radiogenic heat from the Big Lake Suite Granodiorite by the insulating carbonaceous muds, silts and coals of the overlying Cooper and Eromanga Basins. Here, the thermal gradient exceeds 50 degrees C/km and the heat flow exceeds 100 mW/m2, nearly double the average for continental crust (25-30 degrees C/km and 65 mW/m2 respectively).
The model for extraction of heat from these resources involves fracturing the reservoir to enhance its natural permeability and a working fluid is pumped down an injection well, through the rock, and up a nearby production well where the heat can be used to generate electricity. Temperatures of at least 160 degeres Celsius are targeted, however, higher temperatures improve the efficiency of power generation with reservoir temperature over 250 degrees Celsius being optimal.
Hot Sedimentary Aquifers are simply aquifers deep in sedimentary basins that contain hot water. These resources are currently exploited for swimming pool heating in Perth (Perth Basin) and small scale power generation in Birdsville (Great Artesian Basin). The heat source may be the same as for HR, or advected heat from deeper in the crust. HSAs are attractive because they have high volumes of water and relatively high permeability. For HSA, shallower and lower temperature resources are targeted because the permeability in aquifers decreases with depth.
HSA resources typically have temperatures between 90 degrees Celsius and 200 degrees Celsius. Power generation is only considered feasible at temperatures over 160 degrees Celsius. Lower temperature resources can be used for direct use applications including space heating, industrial processes, cooling (via absorption and adsorption chillers) and thermal desalination.
The key challenges
The challenge in the development of geothermal energy resources is getting the heat energy from the reservoir to the surface. To do this we need to be able to efficiently drill wells into the reservoir and produce a sufficient flow rate of working fluid to bring the heat energy to the surface.
Technological advancement in drilling and reservoir enhancement and management will have a significant impact on the success of the geothermal energy industry in Australia.
Geothermal resources, by their nature, will generally be deep; current targets in Australia are typically at depths between 3 km and 5 km. The geothermal industry is currently relying on technologies developed in the petroleum industry, where wells are routinely drilled to depths of 4 km to 5 km. However, there are several key differences between geothermal drilling and petroleum drilling.
Geothermal reservoirs are usually hotter than the petroleum reservoirs (although the petroleum industry is working in deeper and hotter reservoirs as shallow reservoirs are depleted). The adverse effects of the higher temperatures on the durability of downhole equipment such as drill bits and on geophysical logging tools need to be overcome. Casing has to be able to tolerate thermal expansion caused by the high temperatures.
Wells drilled in HR reservoirs will encounter crystalline (granitic/metamorphic) formations that are harder and more fractured than the sedimentary formations typically encountered in petroleum reservoirs.
Novel drilling techniques such as thermal spallation, which removes rock by thermal expansion (Augustine et al. 2007), are being developed to improve penetration rates in these harder formations. To achieve the required flow rates, geothermal wells are usually completed at larger diameters than typical petroleum wells. Drilling techniques that maintain the natural permeability in the near-well environment will be critical for HSA and HR resources.
Hot Rock resources do not normally have high enough natural permeability to allow the production of sufficient flow rate of hot fluid from the reservoir to be viable. The permeability of the reservoir must be enhanced so that high enough flow rates can be achieved. Hydraulic fracturing is a proven technology applied in the petroleum and minerals industries and has been demonstrated at some of the experimental HR geothermal sites around the world. The application of these technologies to HR is complicated by the strength of the rock, high confining stresses, the need to avoid thermal breakthrough, risks of induced seismicity and the effect of temperature on stimulation hardware.
Drilling and reservoir enhancement are two areas where technological advancement will be crucial to the success of geothermal energy in Australia. There are many other avenues of research that will improve the viability of the industry including the development of better exploration technologies, improving the efficiency of power generating equipment, development geophysical methods for reservoir characterisation and monitoring and development of direct use applications.
The challenges are not insurmountable and the Australian geothermal industry has grown to 47 companies holding over 384 geothermal exploration licence areas covering ~360,000 km2 (Goldstein et al. 2009). Significant research effort is underway with three Centres of Excellence in geothermal energy established in the last 18 months. The industry and research community are working toward demonstrating the viability of geothermal energy and have been supported by the award of over $250 million in government grants in 2009. The heat rush is well and truly on.
References:
Augustine, C., J. Potter, R. Potter, and J.W. Tester, 2007. "Feasibility of Spallation Drilling in a High Pressure, High-Density, Aqueous Environment: Characterization of Heat Transfer from an H2-O2 Flame Jet." Geothermal Resources Council Transactions, Volume 31, p. 241-245.
Chopra, P.N. 2005. ?Status of the Geothermal Industry in Australia, 200-2005.? Proceedings, World Geothermal Congress 2005. Antalya, Turkey, 24-29 April, 2005.
Goldstein, B.A, Hill, A.J, Long, A, Budd, A.R, Holgate, F. and Malavazos, M., 2009. ?Hot rock geothermal energy plays in Australia,? PROCEEDINGS, Thirty-Fourth Workshop on Geothermal Reservoir Engineering Stanford University, Stanford, California, February 9-11, 2009 SGP-TR-187
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