BESTEC

Geothermal Energy and its Potential for Future Energy Supply

by Dr. Jörg Baumgärtner, former Socomine, Soultz-sous-Forêts, France

Blue lagoon in Iceland (Bestec)In Japan and Iceland, of course; maybe even in Tuscany and the US - but not here! We certainly don’t have any volcanoes or hot springs. We may have hot water running in the underground, but this is already being used widely in thermal baths. But how about using hot underground water for heating and for feeding larger energy grids? That’s all well and good, but isn’t hot water already being used as an effluent in conventional power plants? The same hot water that is hardly available to the industry because the maintenance and installation of appending piping systems are too expensive…

These types of questions and arguments frequently arise when geothermal energy is proposed as a possible source of energy in our region. What is often ignored in this context is that the larger part of underground energy is stored in hot, but more or less dry deposits, so that it cannot be extracted by means of conventional methods of production.

The idea

So far, the development and commercial use of geothermal energy has been tied to readily identifiable deposits that can be accessed with simple technology, as is common use with other mineral resources. However, as information about local underground conditions became more easily available, and as production methods grew more advanced, deeper, hotter and energetically more efficient steam reservoirs could be tapped. Currently, in Europe, geothermal energy is extracted in depths of up to 3,500 meters in Italy, to be used for power generation.

Conventional extraction of geothermal energy requires large water or steam reservoirs in the underground, a necessary prerequisite that has impeded its recovery in the past. This naturally hot water or formation fluid serves as a carrier medium that transports the heat to the surface and to the respective power plant. However, high rock temperatures and underground water are rarely found together. This is the primary reason for why the use of geothermal energy remained restricted to a few regions only.

The challenge to develop the almost inexhaustible potential of terrestrial heat stored in deposits that carry no or almost no water was first picked up in 1970 by physicists at the Los Alamos Scientific Laboratories, a nuclear research center in the United States. Said scientists based their research on the assumption that deeper undergrounds must be largely free of cracks due to existing pressure and temperature conditions. They first presented a plan for the extraction of heat contained in impervious undergrounds, which relied primarily on the creation of an extensive fracture that was to be driven into the basement in between two neighboring boreholes. The extraction of underground heat was to be effected by circulating water through an artificial fracture. For this purpose, cold water is pumped into an injection well, from which it runs through the fracture, where it heats up and turns into superheated steam. It then rises to the surface through a second borehole, the production well. The extracted heat enters a closed system that is connected to one or more heat exchangers, which, in turn, drive a turbine. In this process, the water is cooled off and can be pumped back into the underground. The plan has been known under its English name, Hot Dry Rock, and its abbreviation, HDR. An advantage of this technology is that it is environmentally friendly: no pollutants (or carbon dioxide, for that matter) are released in the process.

The first HDR project: Los Alamos or Fenton Hill

In 1973, the year of the first oil crisis, work began on the first HDR Project near Fenton Hill in New Mexico, USA, on the fringes of an extinct volcano. Since this project has been crucial for subsequent HDR research in Germany, a few aspects of the project’s evolution should be addressed.

Between 1973 and 1979, the project’s central idea, to establish a connection between two boreholes through the creation of an artificial fracture, was tested. For this purpose, two wells were drilled to a depth of 3,000 meters. After following an almost vertical trajectory for 2,000 meters, both wells were then inclined at angle of 35°. At a depth of 3,000 meters, a temperature of 190°C was measured. Subsequent hydraulic testing showed that the injection of water under high pressure (up to 400 bars) does not only produce one artificial fracture, but activates as well, and surprisingly so, a series of natural joints, which were not expected in such depths. Circulation between the two boreholes was only successful after the lower end of one of the wells had been redrilled several times in what was a rather costly process, repeatedly inclining the borehole in the indicated direction. For the purpose of locating the hydraulically created fractures, a passive acoustic ranging method was used, which picked up fracture noises in the underground with the help of highly sensitive seismic detectors.

The HDR concept (Public Services, Bad Urach)

In the following months, the fracture system underwent a series of testing, in a section of no more than 90 meters. The results exceeded all previous expectations:

• The loss of thermal water was extremely low.
• The testing showed that in basement rock (such as crystalline rock, granite, or granodiorite as in Fenton Hill) fractures do not require to be kept open artificially in a tedious and costly process, but that they remain open naturally due to the roughness and the unevenness of the surface.     

The attempt to expand the distance between inlet and outlet to an estimated 300 meters in order to maximize the exchange surface, led to first difficulties. At this juncture, a surprisingly high flow resistance was observed. It became also evident that only part of the newly created fractures could be used for heat exchange. Nevertheless, and despite these difficulties, it was possible to establish a circulation system (with a thermal output of 2.3 megawatt) which surprised, again, with an extremely low loss of water.

As a consequence, the next step in this project was to increase the thermal output, which required larger exchange surfaces and a fracture system containing several parallelly arranged fractures. With the purpose of investigating such a multi fracture system in Los Alamos, a joint US-American, German and Japanese research initiative began its work in 1980. Two new research wells reached a depth of 4,500 meters and achieved temperatures of up to 327°C, a result which was far too high for the drilling technology and the instrumentation of the time. After a short period of hydraulic testing, which led to considerable complications, the wells were plugged with sand and operations continued at lower temperatures (232°C) and at a depth of 3,600 meters. Unfortunately, this joint venture of US-American, German and Japanese scientists was only little successful. The local underground conditions in depths of 3,600 meters (which did not coincide with the area to be investigated) did not suit the type of testing previously planned. Even though this test was the biggest injection experiment in the history of hydraulic fracturing in the United States, the envisioned connection between the two neighboring wells could not be established.

In retrospective it can be concluded that instrumentation as well as the ranging methods used for evaluating the fracture zone were still underdeveloped compared to those available today. Many of the experiments conducted at that time were blind testing. What is more, the scientists’ assumptions with regard to the local underground conditions proved to be fundamentally wrong. Later research showed that the Fenton Hill underground was not entirely free from cracks, as previously assumed, but contained, even in depths of 3,500 meters, a number of irregularly arranged natural joints and fractures, in which thermal water had been injected. Under these circumstances, the approach of drilling the wells first and fracturing the underground in a second step, turned out to be inadequate, as it was based on the assumption that single fractures could be controlled according to known physical laws. The evaluation of the seismic data showed that a convoluted system of fractures had been hydraulically opened, resulting in a fracture system of 800 (length) by 150 (width) by 800 meters (height), which ran through the underground at an angle of 30°. Given this situation, the measure to be taken was to redrill the lower end of one of the wells and to correct its trajectory, a solution that turned out to be very expensive. In 1986, six years after drilling had begun; the second circulation system in Los Alamos was finally completed. At this point, the patience of the German and Japanese partners was, however, exhausted. While the scientists from abroad had reached their own conclusions, the political pressure resulting from of the oil crisis was decreasing. Germany was to leave the Fenton Hill project first, only to be followed by Japan. It was left to the US-American scientists who remained at the nuclear research plant in Los Alamos, to prove — despite adverse political circumstances and with small means — that the hydraulic and thermal properties of this new circulation system were indeed favorable (with an output of 10 MW). Nevertheless, these results barely gained any international attention.

Subsequent projects

Even amongst experts, the Fenton Hill project was of little importance in the years after 1986, mainly because after the disintegration of the multi-national project, the former partners now kept to themselves. A series of so-called HDR projects sprung up in Japan, France, Germany, Great Britain and Sweden, many of them still based on the “Single Fracture Method.” Consistent research on the flow characteristics of water in fractured underground reservoirs (or, as geologists would have it, in “jointed” underground) was conducted in a larger project in Cornwall, located in the outermost southwestern part of England. Hydraulic circulation testing in neighboring wells was conducted over a period of several years, between 1980 and the spring of 1994, in depths of 2,200 to 2,600 meters, and at temperatures that did not exceed 90°C. Testing led to two crucial insights:

• The diffusion of water in the underground is bound to certain laws. In this environment, the spread of water is controlled by the forces that act on the surrounding medium, that is, by forces that exist in addition to the load of horizontal compressive forces, which move our continents and are responsible for all processes of mountain building.
• The seismo acoustic ranging method and related technology for locating fracture noises during the fracturing and opening of existing joints in the underground, was considerably improved.

  England’s Prince Phillip visits an HDR project in Rosemanowes    (Dr. Roy Baria)The project in Cornwall was discontinued in the spring of 1994, mainly because in this location commercially interesting temperatures of more than 180°C could only be reached in greater depths and at very high costs. The infrastructural isolation of the site was another disadvantage.

At the same time, in Europe, a German project, albeit of smaller dimensions, was under way in Falkenberg, in the Upper Palatinate region. Between 1978 and 1986, near-surface testing (in depths of 300 to 500 meters) focused on circulation experiments on an artifical single fracture. In an area of 100 by 100 meters, geologists succeeded in connecting seven boreholes to a previously created fracture zone. In this way, scientists were able to collect important data about pressure conditions in the fracture zone, at the fracture opening, and concerning the flow conditions in the fracture zone. All of this information is a necessary prerequisite for understanding flow phenomena in a multiple fracture zone.

Already in 1974, one year after the Fenton Hill project had begun its work, a national HDR research programme was launched in Japan. Japanese projects stand out because of their long-term planning. Their research comprises everything from purely theoretical research, laboratory tests on smaller rock samples and larger models (blocks of 10 x 10 x 10 meters), and field studies in deeper boreholes. Currently, active field studies are under way in two locations, in Hijori and Ogachi. Given the favorable geothermal conditions in Japan, temperatures of 200°C are reached in depths of 1,000 to 2,000 meters. The Ogachi project in particular, is a purely commercially oriented project, both founded and operated by the national power industry (Central Research Institute of Electric Power Industry).

The HDR programme in Soultz-sous-Fôrets

Parallel to ongoing HDR research, during the 1980, a series of geological deep drilling projects was conducted worldwide. In addition to the insights gained in projects such as Fenton Hill or Cornwall, scientists were able to secure detailed data about the general composition of basement rock. A particularly significant insight was that, unexpectedly, in many locations, basement rock showed natural joints even in greater depths, which were often times water-bearing.

This meant, however, that in these locations, the option of establishing a closed circulation system comprising several single and artificially created fractures (in which consisted the original Fenton Hill concept) had to be dropped.

A modified HDR project, which intentionally uses and integrates the natural jointing and permeability of crystalline rock as well as the water contained in these deposits, has been under way since 1987 in Soultz-sous-Fôrets (Alsace), in the Upper Rhine Rift near Germany. The project has been supported by the European Union, Germany, France (by the government as well as by local municipalities in the Alsace), Great Britain (from 1989 to 1993) and lately also by Italy. At this point, more than ten different European research institutes and companies are involved in the project. The on-site project management team consists of a group of international scientists and engineers, and is funded by both the European Union and a local French company, Socomine (Group BRGM).

The so-called “Soultz concept” is based on the assuption that, in this area, the basement of the Upper Rhine Rift carries a network of water-bearing joints and fractures that are hydraulically connected. By means of injecting water, the network is artificially opened (“extended”) and penetrated with a series of boreholes, of which some are used as injection and others as production wells. The injected water does no longer circulate through single fractures but spreads through a spacious network of opened joints and artifical fractures. The water produced by such a system is then a mixture of formation fluid and reinjected water.

The primary purpose of the Soultz project is power generation. While using geothermal plants for heating purposes may be teachnically indicated, the generally difficult economical conditions in district heating limit such a use of geothermal energy to favorably located individual units.

Milestones in the Soultz project

I. Prospecting of the underground structure

In 1987, a group of German and French scientists proposed a joint geothermal energy project in the Upper Rhine Rift. Why? In this region, geologist suspected the largest heat anomalies in central Europe. The term heat anomaly refers here to the particularly high heat increase rate in the local underground.

In the years between 1988 and 1992, initial prospecting focused on the specific geothermal, geological and hydraulic conditions in the Rhine Rift, in depths of up to 2,200 meters. This type of undertaking was new in many ways, since previous drilling in the area had never reached comparable depths. The region has a very interesting history of drilling, which dates back to the 18th century and was mainly motivated by the intent to find pitch or oil, but all of these activities were limited to sedimentary layers, that is, to a depth range of 400 to 1,400 meters.

By deepening old and abandoned boreholes, a so far unique seismo acoustic ranging network was installed in depths of 1,400 to 1,600 meters in four deep wells, all drilled in granite underground. Towards the end of 1992, a prospect well was drilled to 1,390 meters. Said well reached crystalline underground (granite) at a depth of 1,377 meters. As already expected, the granite samples were continously jointed and water-bearing joints were found down to the bottom of the wells. This natural thermal water is generally very saline, a condition which does, however, not pose a problem to HDR projects. After reaching the surface, the extracted hot water remains in a closed cycle system, where it cools off and is then reinjected into the formation. In Soultz, a temperature of 160°C is measured at a depth of 3,590 meters. In comparison, under ordinary geothermal conditions, the same temperature is reached in depths of more than 5,300 meters. The advantage of the Soultz site can be defined in terms of 1,700 meters of difference, which amounts to a considerable head start in drilling.

In the case of the Soultz site, it is generally very difficult to predict temperatures in greater depths, since the local heat increase rate or thermal gradient does not remain stable. While the temperature gradient is still comparatively high in depths of up to 1,000 meters (105°C/km), it then levels off abruptly and reaches a minimum temperature at an approximated depth of 2,400 meters (almost no temperature increase to be observed), only to rise continiously above 30°C towards the bottom of the well. This type of behavior indicates the existence of an active and natural water circulation system in the underground, and supports the current hypothesis that heat anomalies in the Upper Rhine Rift are of hydraulic origin, that is, they are caused by an extensive groundwater circulation system.

Lateral support measured in the wells in the Upper Rhine Rift was extremely low, which led, in turn, to accordingly low injection pressures (and a reduced energy consumption during the circulation phase).

2.The heat exchanger

 The HDR project in Soultz. location of the second well GPK 2, and the first borehole GPK 1 in the left upper corner (EWIV "Wärmebergbau")

In the summer of 1993, during a large-scale experiment, a fracture system of a size of 1,200 (length) by 300 (width) by 1,500 meters (height) was hydraulically opened. To this effect, 45,000 m³ of water, with a flow rate of up to 51 liters/second, were injected into the fracture system, under pressures that did not exceed 100 bars. In the process, highly senstive seismic detectors identified more than 16,000 fracture noises, which allowed to determine the correct location of the opened fracture system. This experiment was the first of its kind to verifiably activate a fracture system with a total surface area that met industrial standards.

The hydraulic properties of this system, which was to serve as a heat exchanger in the future, were investigated in detail in the summer of 1994. In an initial production test, 6,200m³ of artesian water (steam) with temperatures of up to 122°C were first extracted and then cooled off in a closed cycle system, in order to be reinjected into the fracture system through a second well. The extracted water contained a surprisingly high percentage of formation fluid (85 to 90 percent), even though, in the previous summer, another 45,000 m³ of surface water had already been injected into the underground. The short interval, in which the water exchange in the underground had occurred, suggested both high permeability of and high flow rates in the fracture system. The most significant information resulting from the experiment, was, however, the insight that, in Soultz, an estimated 10 liters/second (approx. 36 m³/h) of hot artesian formation fluid could be extracted from the crystalline underground without the help of additional production pumps. With regard to the envisioned circulation system, this was crucial information.

A second injection experiment showed that the fracture system activated the year before had been opened permanently. Compared to the previous year, injection pressures had dropped more than fifty percent:

• 1993: injection overpressure at 18 liters/second: 95 bars
• 1994: injection overpressure at 18 liters/second: 34 bars

Evidently, the energy required to keep circulation in the undergound active, had reduced considerably.

Contributing to future energy supply

The complete headline should end in “here in central Europe.” As we have seen, the use of geothermal energy, particularly for power generation, is no longer a utopia in (in Europe), but state of the art technology. Even though in our native central Europe, geothermal conditions may not be as favorable as they are in the United States or in Italy, in the future, geothermal energy should still be factored in as an important source of energy, albeit its contribution will have to be —initially, at least— restricted to few, favorably located regions. Of this, the geologists and engineers involved in the HDR project in Soultz are convinced. Compared to other, alternative methods of power generation, the power output of the Soultz project is no longer described in terms of watts and kilowatts, but in megawatts.

Nevertheless, as promising as previous experiments in Soultz may have been, it still is a long way until the technology tested in this project can actually be applied.

The prospecting and use of mineral resources is a technology that will never be entirely predictable and which relies heavily on insights and experiences that are the result of laborious testing, a lesson miners and oil engineers had to learn a long time ago. Progress is always made at the expense of effort, an effort that can best be implemented in joint multinational projects that are funded and supported by the industry, the state, and research institutions. With the support of the European Union, the HDR project currently under way in Soultz, could lead the way in this regard. For this to be possible, regional energy suppliers should be involved in the process early on, mainly to convince authorities that geothermal energy is, indeed, a technologically, commercially and environmentally meaningful natural resource in our region. Only the active participation of local energy suppliers makes it possible to adjust such a project to the specific requirements and conditions of local markets.

The future of the Soultz project

1994/1995: In the fall of this year, a second well was drilled to a depth of 3,600 meters. With the purpose of conducting a further circulation test, the new well was connected to the existing fracture system at the beginning of 1995. The planned testing forsees a circulation experiment over a distance of approximately 400 meters, (which would be a world record) and with a flow rate of an estimated 20 liters/ second.

After 1995: At the end of the year, the Soultz project is to be evaluated as a possible location for subsequent research on a scientific prototype. Given the currently projected annual investment volume of 16 to 18 million DM for the entire research programme (which includes basic research and technological development), each of the four sponsors (three nations and the European Union) would support the project with an annual sum of 4.5 million DM, assuming the total is divided evenly. The expected duration of the project is five to eight years. Current planning foresees the installation of a test plant comprising one injection well and two to three production wells. The thermal power of such a plant could reach 20 to 30 megawatts (with an estimated electricity output of 2 to 3 MW).

The considerable dimensions of the envisioned project suggest that it does does no longer serve for demonstration purposes only. The installation of a scientific prototype is supposed to furnish important data with regard to its operation and the long-term behavior of such a heat exchanger. It is further expected to help improve the hydraulic as well as thermal output of the plant and to establish a basis for calculating the future costs of this new technology.