HOT ROCKS FOR A COOL CLIMATE

Latest information on Hot Rocks pilot project plant in Australia - has now powered up and is producing electricity - Geodynamics Habanero pilot plant

A really interesting concept with CO2 ramifications - Power from "Hot Rocks" aka "Hot Fractured Rocks" - in some cases the "hot rocks" will be dry and water pumped in, in other cases there will already be a brine present - refer New Scientist Abstracts - even Google is investing in this technology through its philanthropic arm, GoogleOrg.

There are many projects being developed to proof of concept stage in Australia - Queensland's Cooper Basin, South Australia - (more), Flinders Rangers, Western Australia, Perth Basin, Tasmania, NSW, Hunter Valley & Victoria's Otway Basin. One of the most important recent developments has been recognition of massive areas of extremely hot rocks in a discrete zone extending from South Australia to Queensland. Called the South Australian Heat Flow Anomaly (SAHFA), it was first described by Neumann, Sandiford and Foden, in their paper ‘Regional geochemistry and continental heat flow: Implications for the origin of the South Australian heat flow anomaly.’ Also Geodynamics reported they were nearing completion of their Hot Fractured Rock ‘Proof of Concept’ to economically extract geothermal energy from a fracture network with artificially enhanced permeability.

Geothermal Resources - There are four types of geothermal resources: hydrothermal, geopressured, hot dry rock and magma. Of the four types, only hydrothermal resources are currently exploited commercially.

  • Hydrothermal, or hot water, resources are found in places where hot water and/or steam is formed in fractured or porous rock at shallow to moderate depths (100 m to 4.5 km). The ground water is heated either by molten magma coming close to the Earth’s crust or by the deep circulation of water through a fault or fracture [2]. High temperature hydrothermal resources, with temperatures from 180°C to over 350°C, are usually heated by hot molten rock. Low temperature hydrothermal resources, with temperatures from 100°C to 180°C, can be produced by either process.
  • Hot dry rock (HDR) resources are found in areas where the flow of heat from the interior of the earth to the surface is higher than average but there is no water because no aquifers or fractures are present. These are required to conduct water to the surface. It may be necessary to create these fractures so that water can be pumped down a borehole and allowed to circulate through cracks in the hot rock several kilometres below the surface. The hot water is then brought back up to the surface through another borehole and used to generate electricity. The water can be reused over and over again.

    Constraints to Geothermal Energy Use - High grade geothermal resources are limited to small areas of the world.

  • Geothermal fluids (steam or hot water) usually contain gases such as carbon dioxide (CO2), hydrogen sulphide (H2S), ammonia (NH3) and methane (CH4). These gases, if released, can not only add to greenhouse warming but can also be toxic and smelly. Geothermal fluids also usually contain dissolved chemicals, which commonly include sodium and potassium chlorides, arsenic or mercury. These fluids would be a source of pollution if discharged into the environment. Modern emission control techniques and re-injection of contaminated fluids back into the ground is needed to minimise the impacts of these pollutants.
  • The waste waters from geothermal plants have a higher temperature and even if pure need to be cooled before discharge.
  • Geothermal production may cause ground subsidence. This is rare in dry steam resources but possible in liquid dominated fields (eg Wairakai, New Zealand). The technique of injecting the geothermal fluid back into the ground can effectively remove this risk.
  • Geothermal energy production has been associated with increased seismic activity. This is a debatable issue as most geothermal fields are located in regions that are already prone to earthquakes. Seismic activity has not increased significantly close to production plants where the geothermal fluid is injected back into the ground and maintains reservoir pressures, (United Nations Environment Programme)
  • Geothermal energy resources will be depleted if used beyond their natural recharge rate.
  • Geothermal plants produce noise pollution during construction (eg drilling of wells, and the escape of high pressure steam during testing) but this is not significant once the plants are operating.

    Geodynamics is one company involved in Hot Fractured Rocks - YouTube

    Scaling and corrosion - Geodynamics is testing for scaling and corrosion by analysing the samples of water extracted by the steam separator. Corrosion in the wells is generally considered less of a problem than in normal volcanic geothermal developments. The corrosion issues will not apply to the turbine loop as the heat exchanger at the surface transmits the heat to demineralised water through the turbine. If it is necessary, the cooled fluid can be chemically treated to remove damaging pollutants prior to re-injection to reduce damage to the well casing. The life of a well is dependent on the life of the thick steel casing that is cemented in place following the drilling process, but can be extended through workovers. Additionally, as reported in their submission to the Garnaut Climate Change Review, Geodynamics collaborated with the University of Auckland - considered a world authority on Geothermal Scaling & Corrosion

    Likewise Goldstein et al have identified corrosion & scaling issues as areas for research focus. - See also AGEG submission to Garnaut Climate Change Review. Sometimes high chlorides, hydrogen sulphide and carbon dioxide may be present in a pH of <6 - raising concerns about corrosion challenges - so it is not surprising that research is being carried out on mitigation of scale and corrosion. In fact some information suggests that salt could be generated as a by product of hot rock projects.

    Australia’s geothermal energy resources - The accessible geothermal energy in Australia exists in two forms, which present different technical and economic opportunities and challenges. The first type is Hot Rock (HR), which includes Hot Fractured Rock (HFR) and Hot Dry Rock (HDR). HR resources are found mainly in high heat producing granites. These granites accumulate heat from the decay of radioactive elements or from the absorption of heat from material below. The temperature of these granites can reach 250°C at economically drillable depths.

    eg from www.pbworld.com granites at about 4200 m (13,800 feet) below ground may contain super-pressured saline water (brine) and yield well head pressures and temperatures of approximately 33 MPa and 250°C (4,790 psi and 482°F). Possible technology options include

  • Kalina cycle. This technology uses a varying mix of ammonia and water around the cycle to maximise the energy recoverable. The boiling water and ammonia mixture can closely follow the brine temperature in the high-pressure heat exchangers. Organic Rankine cycle. This technology uses an organic fluid, such as pentane, which is preheated and then vaporized in a series of heat exchangers. The high-pressure vapour can then be used to drive an expander in a manner similar to a conventional steam turbine plant.
  • Flash steam cycle. A new concept developed from early modelling showed that a conventional steam power plant in which the water is preheated and then boiled in a series of heat exchangers could not realise the full potential for heat recovery from the brine. Under the flash steam cycle, circulating water is heated so that there is a constant temperature differential between the brine and the circulating water. The high temperature clean water can then be flashed down in two stages to produce steam that passes to a conventional steam turbine. The exhaust from the turbine is then cooled in an air cooled condenser, and this condensate is returned to the circulating water flow.

    Other interesting technical aspects ..

  • High Pressure Heat Exchangers The brine heat exchangers have a very high design pressure that is in the range used for super-critical boiler feed pre-heaters. Unlike these heaters, however, the brine heat exchangers will require full access on the tube side so the tubes can be mechanically cleaned to remove scale that can result from the brine flowing through. The design developed uses heavy forged tube plates and channels with pressure seal style, flat plate closures to the channels.
  • Reinjection Pumps A review of available pumping technology for reinjection of the high pressure brine revealed that the multi-stage centrifugal pumps used in the oil industry represent the best available technology. These pumps have been developed to handle gassy hot fluids contaminated with sand, and they have given very good service. These pumps are offered conventionally in a submersible pump configuration that makes the seal design comparatively simple.

    There are Drilling and reservoir issues ....

    The nature of the environments in which HR geothermal resources are found provide a range of challenges & issues that can increase the comparative cost and risks of drilling,raise environmental concerns etc. These include:

  • Drilling depths. HR systems in Australia may require drilling to significantly greater depths than conventional geothermal project, possibly down to 6 km. While the technical limit for today’s petroleum drilling technology is to depths greater than 10 km, the cost of drilling rises sharply with increasing depth.
  • High temperatures. The pressure rating of wellheads and casing is significantly degraded at higher temperatures. Cooling of drill bits is much less effective in the HR situation, which results in a more rapid failure of seals and bearings.
  • High pressures. The combination of pressure and temperature places unusual demands on well design, casing, wellheads and heat exchangers and complicates the drilling process, for example in controlling pressures while running casing.
  • Lost circulation and corrosive fluids. HR environments can create conditions where there is lost circulation, excessive drill bit wear and corrosive fluids. These conditions mean that the correct design of drill bits, corrosion resistant casing, drilling mud and cement slurries, is critical in achieving successful geothermal well completion.
  • Downhole communication. Real time downhole communication and instrumentation, including collection of subsurface data on temperature, stress regimes and other drilling and rock parameters. While relatively mature in the oil and gas industry, the high temperatures encountered in the geothermal sector often cause major problems for the electronics.
  • Gaseous and liquid emissions. There is a possibility of increase release rates of dissolved gases (carbon dioxide, hydrogen sulfide and ammonia and minerals (silica, carbonates, metallic cations, sulphides and sulphates etc) over those naturally present.

  • Science Alert Jan 2009- Ministerial Statement - Martin Ferguson

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