Enhanced Geothermal Systems (EGS) are a new type of geothermal power technologies that do not require natural convective hydrothermal resources. Until recently, geothermal power systems have only exploited resources where naturally occurring water and rock porosity is sufficient to carry heat to the surface.[1] However, the vast majority of geothermal energy within drilling reach is in dry and non-porous rock.[2] EGS technologies "enhance" and/or create geothermal resources in this hot dry rock (HDR) through hydraulic stimulation.
When natural cracks and pores will not allow for economic flow rates, the permeability can be enhanced by pumping high pressure cold water down an injection well into the rock. The injection increases the fluid pressure in the naturally fractured granite which mobilizes shear events, enhancing the permeability of the fracture system. Water travels through fractures in the rock, capturing the heat of the rock until it is forced out of a second borehole as very hot water, which is converted into electricity using either a steam turbine or a binary power plant system.[3] All of the water, now cooled, is injected back into the ground to heat up again in a closed loop.
EGS / HDR technologies, like hydrothermal geothermal, are expected to be baseload resources which produce power 24 hours a day like a fossil plant. Distinct from hydrothermal, HDR / EGS may be feasible anywhere in the world, depending on the economic limits of drill depth. Good locations are over deep granite covered by a thick (3–5 km) layer of insulating sediments which slow heat loss.[4] HDR wells are expected to have a useful life of 20 to 30 years before the outflow temperature drops about 10 degrees Celsius and the well becomes uneconomic. If left for 50 to 300 years the temperature will recover.
There are HDR and EGS systems currently being developed and tested in France, Australia, Japan, Germany, the U.S. and Switzerland. The largest EGS project in the world is a 25 megawatt demonstration plant currently being developed in the Cooper Basin, Australia. The Cooper Basin has the potential to generate 5,000–10,000 MW.
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A 2006 report by MIT,[5] and funded by the U.S. Department of Energy, conducted the most comprehensive analysis to date on the potential and technical status of EGS. The 18-member panel, chaired by Dr. Jefferson Tester of MIT, reached several significant conclusions.
In addition the report has a wealth of data on EGS technology, economics and prior experiments.
Commercial projects are currently either operational or under development in Australia, the United States, and Germany.
The largest project in the world is being developed in Australia's Cooper Basin by Geodynamics.[6] The Cooper Basin project has the potential to develop 5–10 GW. Australia now has 33 firms either exploring for, drilling, or developing EGS projects. Australia's industry has been greatly aided by a national Renewable Portfolio Standard of 25% renewables by 2025, a vibrant Green Energy Credit market, and supportive R&D collaboration between government, academia, and industry.
Germany's 23 cent/kWh Feed-In Tariff (FIT) for geothermal energy has led to a surge in geothermal development, despite Germany's relatively poor geothermal resource. The Landau partial EGS project is profitable today under the FIT.
The AltaRock Energy effort is a demonstration project being conducted to prove out the company's proprietary technology at the site of an existing geothermal project owned and operated by NCPA in The Geysers, and does not include power generation. However, any steam produced by the project will be supplied to NCPA's flash turbines under a long-term contract.[7]
| Project | Type | Country | Size (MW) | Plant Type | Depth (km) | Developer | Status |
|---|---|---|---|---|---|---|---|
| Soultz | R&D | France (EU) | 1.5 | Binary | 4.2 | ENGINE | Operational |
| Desert Peak | R&D | United States | 11-50 | Binary | DOE, Ormat, GeothermEx | Development | |
| Landau | Commercial | Germany (EU) | 3 | Binary | 3.3 | ? | Operational |
| Paralana (Phase 1) | Commercial | Australia | 7-30 | Binary | 4.1 | Petratherm | Drilling |
| Cooper Basin | Commercial | Australia | 250-500 | Kalina | 4.3 | Geodynamics | Drilling |
| The Geysers | R&D; Commercial | United States | (Unknown) | Flash | 3.5 - 3.8 | AltaRock Energy, NCPA | Drilling (June 2009) |
- "There are some technical difficulties and challenges there, but those people who are keen on getting Australia into geothermal say we've got this great access to resource and one of the things, interestingly, that's held them back is not having the capacity the put the drilling plants in place. And so what we intend this $50 million to go towards is to provide a one for one dollars. Match $1 from us, $1 from the industry so that they can get these drilling rigs on to site and really get the best sites identified and get the industry going."[8]
The United States pioneered the first EGS effort--then termed Hot Dry Rock--at Fenton Hill, New Mexico with a project run by the federal Los Alamos Laboratory.[9] It was the first attempt anywhere to make a deep, full-scale HDR reservoir, and efforts there spanned 1974 through 1992, in two phases. Ultimately, the project was unable to generate net energy, and the project was terminated.
EGS funding languished for the next few years, and by the next decade, U.S. efforts focused on the less ambitious goal of improving the productivity of existing hydrothermal resources. According to the Fiscal Year 2004 Budget Request to Congress from DOE's Office of Energy Efficiency and Renewable Energy, [10]
EGS are engineered reservoirs that have been created to extract heat from economically unproductive geothermal resources. EGS technology includes those methods and equipment that enhance the removal of energy from a resource by increasing the productivity of the reservoir. Better productivity may result from improving the reservoir’s natural permeability and/or providing additional fluids to transport heat.[11]
In Fiscal Year 2002, this vision translated into completing "preliminary designs for five competitively selected projects employing EGS technology," and the selection of one project for "full-scale development" at the Coso Hot Springs geothermal field at the U.S. Naval Weapons Air Station in China Lake, Calif., and two additional projects for "preliminary analysis from a new solicitation" at Desert Peak in Nevada and Glass Mountain in California. Funding for this effort totaled $1.5 million. [12]
In Fiscal Year 2003, $3.5 million was appropriated to launch the Coso project, with the aim of improving the permeability of an existing poorly performing well, and to complete the conceptual design and feasibility studies at the Desert Peak and Glass Mountain sites.[13]
The Fiscal Year 2004 request for $6 million was to "[s]tep up work on EGS cost-shared projects' at the three sites, to include "drilling and reservoir stimulation experiments" at one and drilling a production well at another.[14]
The U.S. Department of Energy USDOE issued two Funding Opportunity Announcements (FOAs) on March 4 2009 for enhanced geothermal systems (EGS). Together, the two FOAs offer up to $84 million over six years, including $20 million in fiscal year 2009 funding, although future funding is subject to congressional appropriations. [15]
The DOE followed up with another FOA on March 27, 2009, of stimulus funding from the American Reinvestment and Recovery Act for $350 million, including $80 million aimed specifically at EGS proejcts,[16]
Cornwall is set to host a 3MW demonstration project that could pave the way for a series of 50MW commercial-scale geothermal power stations in suitable areas across the country.[17][18]
The EU's EGS R&D project at Soultz-sous-Forêts, France, has recently connected its 1.5 MW demonstration plant to the grid. The Soultz project has explored the connection of multiple stimulated zones and the performance of triplet well configurations (1 injector/2 producers).[19]
Portugal - Portuguese government has awarded, December 2008, an exclusive license to Geovita Ltd, to prospect and explore geothermal energy in one of the best areas in continental Portugal. An area of about 500 square kilometers that is being studied together by Geovita and Coimbra's University — Science and Technology Faculty — Earth Sciences Department, and foresees the installation of an Enhanced Geothermal System (EGS).
Some seismicity is inevitable and, indeed, expected in EGS, which involves pumping fluids at pressure to enhance or create permeability through the use of hydraulic fracturing techniques. Depending on the rock properties, and on injection pressures and fluid volume, the reservoir rock may respond with tensile failure, as is common in the oil and gas industry, or with shear failure of the rock's existing joint set, as is thought to be the main mechanism of reservoir growth in EGS efforts.[20] [21]
In several cases, significant events have occurred.
| Site | Maximum Magnitude |
|---|---|
| Cooper Basin, Australia | 3.7 |
| Basel, Switzerland | 3.4 |
| Rosemanowes, United Kingdom | 3.1 |
| Soultz-sous-Forêts, France | 2.9 |
Experience in the field has shown that seismicity associated with hydraulic stimulation can be mitigated and controlled through predictive siting and other techniques. According to the MIT report,[23]
With current technology, it appears feasible that the number and magnitude of these induced events can be managed. In fact, based on substantial evidence collected so far, the probability of a damaging seismic event is low, and the issue – though real – is often one more of public perception.
The case at Basel bears special mention because the induced seismicity led the city (which is a partner) to suspend the project and conduct a seismic hazard evaluation, which resulted in the cancellation of the project in December 2009.[24]
Although the Basel project had established an operational approach before proceeding with the geothermal stimulation for addressing induced earthquakes of concern, it had not performed a thorough seismic risk assessment beforehand,[25] even though most of the city was destroyed in 1356 by a magnitude 6.5 earthquake. Basel is in a known earthquake zone (see Basel earthquake) and sits atop a historically active fault.
On 8 December 2006, only 6 days after the main stimulation started on 2 December, the HDR project in Basel was suspended when an earthquake tripped a 4-level "traffic light" scheme established for halting operations in the event of unacceptable induced earthquake occurrences.[26] Trip points of Richter Magnitude ML 2.9 and a peak ground velocity of 5 millimeters per second were established by the project as independent criteria for a "red alert" that entailed halting fluid injection and bleeding-off to minimum wellhead pressure. Lesser operational curtailments were triggered for lower magnitude and peak ground velocity thresholds.
Earlier that day, a "yellow alert"--the second level--was called at 03:06 local time after a 2.6 ML event with peak ground velocity of 0.55 mm/s, which exceeded the "soft" 2.3 ML and 0.5 mm/s thresholds. As a precaution, the injection rate was reduced at 04:04.[26]
Following further events that were larger than 2.0 ML, a level-three "orange alert" was declared—the injection was stopped at 11:34 and the well shut-in, maintaining the pressure. However, a 2.7 ML event occurred at 15:46, followed by a 3.4 ML event at 16:48, and so in accordance with the response strategy, the well was bled off as soon as practicable.[26]
The largest event [27] [28] prompted concern from local residents. Further tremors exceeding magnitude 3 were recorded on 6 January (measuring 3.1),[29] 16 January 2007 (3.2), and 2 February 2007 (3.2).[30][31][32]
The six borehole seismometers installed near the Basel injection well to monitor the natural background seismicity and the geothermal stimulation recorded more than 13,500 potential events connected with the geothermal project, from which 3,124 were of sufficient quality to permit [hypocenter] determinations in the period 2-12 December 2006[26], which spanned the main stimulation and the decline in the event rate. During the post-stimulation period from 13 December 2006 onward, a further 350 locatable events were detected up to 2 May 2007, by which time events were occurring sporadically at around one per day. In all, locations for more than 3,500 events were determined.
Of these more than 3,500 events, only the 200 largest (magnitudes between 0.7 and 3.4) were also observed by the earthquake networks of the Swiss Seismological Service and the Seismological Service of Baden-Wuerttemberg,[33] The remainder were too small to be observed or felt at the surface. For the period through 24 January 2007, there were 168 earthquakes with magnitudes> 0.6, 15 with ML >2, and three with ML > 3. All of these were within 1 km of the wellbore, and at depths between 4 and 5 km, near the well bottom[34]. There were only 9 events with an ML of 2.5 or larger in the borehole vicinity for the period through 2007. Five occurred in December 2006,[35] two in January 2007, and one each February and March.[36]
Damage claims arose from the largest of the events. Eventually, some 2,700 claims were processed by the project's insurer for an estimated 7 million - 9 million Swiss francs (about 6.5 million to 8.3 million U.S dollars)[37]
The Basel HDR project was under review[38] which resulted in a cancellation of the project in December 2009.[24]
The recently established Center for Geothermal Energy Excellence at the University of Queensland, has been awarded $18.3 million (AUS) for EGS research, a large portion of which will be used to develop CO2 EGS technologies.
Research conducted at Los Alamos National Laboratories and Lawrence Berkeley National Laboratories examined the use of supercritical CO2, instead of water, as the geothermal working fluid with favorable results. CO2 has numerous advantages for EGS:
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