Hot rock energy a likely source of baseload power

Australia is at the world forefront of the drive to develop commercial geothermal power plants from ‘hot rocks’ located deep underground, superheating water to drive turbines.

Geodynamics Ltd leads the way in Australia. It has been operating a hot fractured rock (HFR) geothermal energy project in granite rock in the Innamincka area of north-east South Australia since 2003 and is now achieving considerable success. The Innamincka granite is buried about four kilometres beneath Australia’s largest onshore petroleum basin, the Cooper Basin.

This process has been termed hot fractured rock (HFR) by Geodynamics, but is also known as the hot dry rock (HDR) process. The term ‘enhanced geothermal system’ (EGS) is applied widely throughout Europe and North America with the same meaning.

The aim is to extract heat from the granite rock by circulating water and then to generate large amounts of low-emission, baseload electricity from that heat (Figure 1, page 18).

One of the main attractions of the Cooper Basin site for large-scale geothermal development lies in the great extent of the known high temperatures in the basement granite rocks. These have been outlined by decades of oil and gas exploration in the cover rocks. But the location is remote from the electricity market: more than 400km from a strong connection to the Australian electricity grid. Large-scale development is required to satisfy connection to the electricity market, but such development appears probable based on the existing progress.

The positive results from Geodynamics’ Habanero project have led to a ‘heat rush’ and a newly emerging Australian geothermal exploration industry. The Australian Geothermal Energy Association (AGEA) has been formed, with a vision of geothermal energy “providing the lowest-cost, emissions-free, renewable baseload energy to Australian homes and businesses for centuries to come”. AGEA has nearly 20 members and held its first national conference for the geothermal industry in Melbourne in August.

There are 11 geothermal companies listed on the Australian Stock Exchange and 23 other companies exploring for geothermal resources.

State governments have issued 284 geothermal exploration licences, and exploration companies have committed more than $800 million worth of exploration in works programs by 2012.

The prize is a massive energy potential that could eventually provide all Australia’s energy needs into the foreseeable future. The many thousands of potential ‘clean’ megawatts of power should have a considerable positive impact on Australia’s greenhouse problem.

A recent study of the EGS potential of the US, The Future of Geothermal Energy (www1.eere.energy.gov/geothermal/future_geothermal.html), has determined a similar potential in the US. According to this study, “using reasonable assumptions regarding how heat would be mined from stimulated EGS reservoirs” it is estimated that the extractable portion of the US EGS resource base is “about 2000 times the annual consumption of primary energy in the United States in 2005.”

In recognition of the Australian potential, the Australian Government has recently announced a $50 million geothermal drilling fund to assist in bringing to fruition a range of geothermal projects distributed around the country.

The Geodynamics experience

To date Geodynamics has drilled three wells (Habanero 1-3) into the granite near Innamincka at depths of more than 4000 metres. A fourth well, Jolokia 1, the deepest so far, is nearing completion with a target depth of 5000m.

Rock temperatures of 250ËšC at 4300m have been confirmed in the Habanero field, making the region the hottest in the world at this depth in a non-volcanic environment. Jolokia 1, which is 9km from Habanero, is expected to be even hotter – 290ËšC at 5000m. At these temperatures a 1ËšC increase in temperature equates to about a one per cent increase in power generation output for the life of a power station, so the highest possible temperatures are targeted.

Hydraulic stimulation, a process of opening natural fractures using high-pressure water, has been carried out in the Habanero wells, monitored by a network of shallow borehole seismometers. The stimulations have been highly successful and have demonstrated that fracture systems can be opened to achieve more effective flow. The increase in permeability resulting from this process is estimated to be two orders of magnitude. The Habanero stimulations have extended over an area of more than 4km2, much larger than in any other project in the world. The reason for this achievement relates to the nature of the stress conditions and the resulting fracture networks in the granite mass.

Using this technique to enhance the capability of flowing water through the rock mass it is possible to extract heat from the rock by circulating between two or more wells drilled into the resulting reservoir. A continuously circulating system can be designed to operate for several decades, depending on the fracture complexity and well spacing before the temperature declines below some economically defined value.

Numerical models have been developed that show a temperature decline of approximately 12ËšC over 20 years and 40ËšC over 50 years with a well spacing of 1000m in a reservoir like that built at the Habanero field. The life of a power station would be greater than 50 years with this temperature decline.

At the Habanero field, flow testing has been in progress since the completion of the Habanero 3 production well in February 2008. Initially this was carried out by flashing the high-pressure water to atmosphere and re-pressurising it to get it back down the injection well. A high-pressure pipeline between Habanero 3 production well and Habanero 1 injection well was completed earlier this year, and now circulation will be carried out in a closed loop, with no release of fluid to the atmosphere.

An air-cooled heat exchanger removes heat from the fluid to simulate a power station before the water is injected. The power requirements to run the injection pump are much lower in a closed loop. In addition the benefit of having lower density hot water in the production well and higher density colder water in the injection wells provides a natural thermosyphon condition that reduces the pumping requirement.

A closed-loop test over a six-week period is about to commence at Habanero that incorporates the injection of benign chemical tracers. This will firm up the numerical fracture model so that a better prediction of temperature decline with time can be established and a geothermal reserve can be declared.

But the understanding is already adequate to show that a pilot power station can be built on the Habanero circulation. A small steam turbine with a 2MW capacity has been purchased, and will be operated at about one MW output to provide power to the existing Habanero infrastructure (field camp and workshop) including parasitic load and for distribution to the nearby village of Innamincka. The pilot power station will be operating by early 2009.

On the basis of the volume of rock stimulated, the ability to drill directional wells and the Habanero numerical fracture model, a scale-up program has been developed to implement, once the ‘proof of concept’ Habanero circulation test is completed.

The initial scale-up will be a nine-well program producing 50MW net. All nine wells will be drilled from the same well pad, minimising the surface pipework and overall footprint. The initial well is drilled vertically and the other eight wells are drilled directionally outwards in a spider pattern, so that the well is about one kilometre displaced by the time it reaches full depth.

An additional benefit of this scale of development relates to the power station itself. The maximum size of a single-axis steam turbine in the temperature range of 250ËšC to 280ËšC is about 60 to 70MW gross, or close to 50MW net, given the pumping requirements. This makes modules about this size the most cost-effective from both a drilling and power-station point of view.

Geodynamics envisages placing 50MW modules located two to three kilometres apart over the area where the buried granite resource is located. This area is at least 1000km2 – possibly up to 2000km2 – in the Innamincka area, so that several hundred power stations, each with 50MW output are possible.

The Minister for Resources, Energy and Tourism, Martin Ferguson, recently issued a Ministerial Statement, ‘The Vast Potential of Australia’s Geothermal Resources’.

Martin Albrecht AC FTSE was Managing Director of Thiess Pty Ltd, one of Australia’s largest engineering and construction companies, for more than 15 years. He is now Chairman of Thiess and a director of Leighton Holdings Ltd. He is also Chairman of Geodynamics Ltd, one of Australia’s leading geothermal explorers. Since his retirement he has taken up a number of board positions in companies including Queensland Gas Ltd and the Siemens Australia Advisory Board.

Dr Doone Wyborn is Executive Director and Chief Scientist for Geodynamics Ltd, having spent more than 25 years with the Bureau of Mineral Resources, Geology and Geophysics and the Australian Geological Survey Organisation. He has worked on the potential of hot fractured rock (HFR) geothermal energy since 1992 and is recognised as the leading Australian expert authority on this subject. He is a member of the Executive Committee of the International Energy Agency Geothermal Implementing Agreement and studied HFR geothermal projects in Japan, Europe and the US.


Editor's Note: This article was first published in Australian Academy of Technological Sciences and Engineering's (ATSE) Focus Magazine issue 152 (Green Power). This article is under copyright; permission must be sought from ATSE to reproduce it.