Potential for deep geothermal energy in Scotland: study volume 2

Appendix 3 Examples of EGS development projects

Three of the most important EGS development projects, each representing a different geological setting, are described briefly below.

Fenton Hill, New Mexico

The world's first attempt to demonstrate the feasibility of generating power from HDR began in New Mexico in the early 1970's, with support from the US Atomic Energy Commission. The chosen site, at Fenton Hill around 70 kilometres west of Los Alamos, sits on the western flank of the Valles Caldera (a supervolcano of around 22 km diameter) in an area associated with geologically recent volcanism and ongoing hydrothermal activity; the local geothermal gradient is approximately 65 ºC/km, more than twice the global average. The HDR 'target' was a large intrusion of granodiorite.

Work progressed in several phases ( e.g. Brown, 1995; Duchane and Brown, 2002; Tester et al., 2006), beginning with the creation of a simple fracture system ( i.e. a small HDR reservoir) between two boreholes in the depth range ~2.5 to 3.0 km (1974-79). This successfully proved the validity of the engineered reservoir concept, so work began to create a larger, deeper (~ 3.5 to 4.0 km) and more complex reservoir dominated by multiple vertical fractures. Finally, a surface plant was constructed (1987-91) and the reservoir was flow-tested (1992-95) in a manner simulating the operation of a commercial HDR facility.

The pioneering work carried out at Fenton Hills revealed, and to a large degree overcame, many of the technical obstacles associated with creating and maintaining a fit-for-purpose engineered fracture system several kilometres below ground. The key challenge lies in ensuring the fracture system satisfies several requirements simultaneously: it must have sufficient permeability to permit an adequate flow of water, and sufficient surface area to permit the required amount of heat exchange; it must connect the injection and production wells but remain effectively a closed-system ( i.e. a significant amount of heated water must not 'bleed' from the reservoir); and it must be stable over the projected lifetime of the EGS (probably several decades).

Important conclusions from the project included:

• hydraulic fracturing in competent crystalline rock does not create new, disc-shaped fractures (as was conceived in the original concept), but rather causes existing natural joints to open
• joints oriented roughly orthogonally to the direction of least principle stress typically open first, followed by those in other orientations as hydraulic pressure increases
• the combination of stress regime and pre-existing joint structure leads to ellipsoidal reservoirs
• a system of three wells, with an injection well in the centre and production wells at both ends, allows the highest production rate from a reservoir
• the life-span of a reservoir will be maximised by separating the wells by as great a distance as possible.

As well as proving the potential of the HDR concept, the work at Fenton Hill stimulated interest in several other countries. Germany and Japan contributed funds and personnel to the project during the 1980's, and the expertise and technology were subsequently exported to Europe and Japan.

• The European Deep Geothermal Energy Programme

A European research programme initiated in 1987 has been developing the concept of heat and electrical power generation from a deep geothermal energy reservoir at Soultz-sous-Forêts, France, near the western border of the Rhine Graben. The consortium, made up of funding agencies (the European Commission and several French, German and Swiss government departments), and a range of industrial and scientific research partners from France, Germany and Switzerland, has effectively integrated at one site a substantial proportion of the European research activity into deep geothermal energy resources. Gérard et al. (2006) and Tester et al. (2006) have recently provided summaries of the geological setting, EGS design, and progress.

The site comprises a 1.5 km thick layer of sediments overlying three different, vertically stacked, masses of granite. The shallowest granite mass is highly fractured and hydrothermally altered in the vicinity of numerous large faults in the depth range 2.7-3.2 km. Deep hydrothermal convection cells in the fractured granite account for an abnormally high geothermal gradient of ~100 ºC/km within the sedimentary rocks. Radiogenic heat supplied by the granite makes only a minor contribution to the geothermal resource in this setting.

The project at Soultz began as the European Hot Dry Rock project. However, early drilling revealed large volumes of fracture-hosted, hot saline fluid in the targeted granite reservoir rocks. The fracture system at Soultz still requires artificial stimulation to create the level of permeability required for economic heat recovery, so the project was reclassified in 2001 as an Enhanced Geothermal System (this is not currently mentioned on the project website). The concept at Soultz now involves drilling at least two boreholes into the deep fractured rock, stimulating (by hydraulic fracturing) the rock around the base of each well to enhance the permeability of the natural fracture network and connect the two wells to it, then extracting hot fluid from one well, generating power from the heat it contains, and injecting the cooled fluid back into the reservoir through the other well.

The Soultz EGS departs from the classic HDR concept in several ways. For example, the fracture network connecting the wells is part of an open system, making it more difficult to control flow rates and predict sustainability. Also, the water supplying the geothermal energy is highly saline (unlike the fresh water that is pumped through an HDR system), so the technical challenges include dealing with corrosion of metallic components in the boreholes and power generation facility, and disposing of excess saline water at the surface.

In the first ten years of the programme two boreholes were drilled (to 3.6 and 3.9 km) and stimulated, a series of geological, geophysical and hydraulic investigations was undertaken, and a long-term (4 month) circulation test was performed between the two boreholes. In a second phase of work (1997-2005) the boreholes were deepened to 5 km, where they encountered temperatures of 200 ºC, and further stimulation and circulation tests were performed. A commercial-scale EGS prototype potentially generating up to 25 MW of electrical power was developed, but this is currently struggling to generate 1 MW.

• Cooper Basin, Australia

The recent discovery (Neumann et al., 2000) of the South Australian Heat Flow Anomaly ( SAHFA), a zone of unusually high heat flow extending from Queensland to South Australia (and possibly continuing as far south as Tasmania), has sparked significant interest in geothermal energy exploration in Australia. More than a dozen companies are currently exploring tenements across an area of around 62 000 km ², mainly in South Australia, and at least An $800 million worth of exploration and proof-of-concept investment is forecast up to 2013.

The most advanced, and probably the most exciting, prospect is in the Cooper Basin, in the north-east part of South Australia ( e.g. Chopra and Wyborn, 2003). Here, gas exploration wells have penetrated granite in the basement underlying 3.5 km of late Carboniferous to Permian sediments. Samples of the granite have HP values in the range 7-10 μW m ^-3, and the well data, together with seismic and gravity data, indicate that granite underlies the deepest part of the basin over an area of approximately 1,000 km ². The top of the granite, at 3.7 km, is at approximately 240ºC, equating to an average geothermal gradient of >50 ºC/km. The rock temperature is expected to increase by ~30 ºC for every additional kilometre of depth. The unusually high temperature at this relatively shallow depth is attributed to the HHP character of the granite and to its location beneath a thick layer of low conductivity sediments (including coal, which has very low thermal conductivity).

The sedimentary rocks and the granite are also currently under high geological stress, which has reduced permeability and limited the loss of heat through fluid flow. The granite pluton contains a network of fractures, dominantly in a sub-horizontal orientation, which formed when the rocks overlying it were removed by erosion, causing it to be exposed before being buried by Carboniferous sediments. These fractures are expected to make ideal pathways for circulating fluid and ideal surfaces for heat exchange, particularly following hydraulic stimulation.

Geodynamics Ltd, a private company, is leading the exploration of this potentially very significant resource. The Cooper Basin is in a remote location, and the cost of building infrastructure and transmitting power to major load centres will be considerable. Despite this, the company has been able to publish an attractive economic analysis and has raised a substantial sum of private money to prove the technical and economic viability of the resource. Geodynamics has proven the connection between two wells spaced 500 m apart, and has extracted water and steam from the production well at up to 210ºC. As of April 2009, the company had drilled its fifth deep geothermal well, completed a four-month closed loop circulation test between two of its deep wells, and built a 1 MW power plant and visitor centre.

The size of Australia's geothermal resource potential is enormous. According to the Geodynamics company website, "in Geodynamics tenements in the Cooper Basin a thermal resource equivalent to 50 billion barrels of oil is estimated. For comparison, Australia's current total oil reserves are 2.9 billion barrels, and the US oil reserves are 20 billion barrels". Identified HHP granites in Australia have the potential to meet the total electricity demand of the country for hundreds of years.

The 'buried hot granite' setting that is being exploited in the Cooper Basin has a significant advantage over the 'classic' HDR concept illustrated in Figure 1: instead of rising to the surface and being lost to the atmosphere, much of the radiogenic heat generated by the granite over many millions of years is trapped beneath the overlying low conductivity rocks, which act like a thermal blanket. Compared to sites where HHP granite crops out at the surface, 'buried hot granite' settings theoretically allow larger heat reservoirs to develop at the same or shallower depths, from rocks of similar or lower HP capacity.