REPORT

The geothermal revolution

Clean, renewable, constant and available worldwide, geothermal energy is already being used in a large number of thermal and electric power plants. Researchers are currently developing new techniques that will enable geothermy to be extended over a wider geographical area. We take a tour of the Soultz-sous-Forêts pilot powerstation in Alsace (FR).

Les trois puits géothermaux de la centrale. GPK2 (à droite), est pourvu d’une pompe à arbre long LSP dont le moteur est en surface et la pompe 350 m en contrebas. Pour déterminer quel sera le système le plus résistant aux conditions extrêmes des forages, une pompe électrosubmersible ESP, où moteur et pompe se trouvent tous deux dans le puits, sera testée sur GPK4. © Qwentes/JVR
The power plant’s three geothermal wells. GPK2 (on the right) is equipped with a line shaft pump, where the motor is above ground and the 350-metre pump, below. To determine which will be the most robust system for the extreme conditions of the boreholes, an electrosubmersible pump, where both the motor and pump are situated inside the well, will be tested on GPK4. ©Qwentes/JVR
Flow diagram of geothermy at Soultz
Flow diagram of geothermy at Soultz
© GEIE Exploitation Minière de la Chaleur
Hydraulic stimulation principle. It involves injecting pressurised water or de-scaling. The water causes the rocks to slip a little along the fractures (Figure 2). When the pressure is released, the fractures are no longer enmeshed, leaving enough space for water to circulate (Figure 3). © GEIE Exploitation Minière de la Chaleur.
Hydraulic stimulation principle. It involves injecting pressurised water or de-scaling. The water causes the rocks to slip a little along the fractures (Figure 2). When the pressure is released, the fractures are no longer enmeshed, leaving enough space for water to circulate (Figure 3).
© GEIE Exploitation Minière de la Chaleur.

At first sight, Soultz-sous-Forêts is nothing special. Situated alongside the French-German border, it looks like any other little Alsace village, suffused with a rural calm barely ruffled by the bustle on the nearby hill. For the past two decades, an ambitious research project has been burgeoning on the hill. The project objective? To set up the world's first geothermal power plant, or Enhanced Geothermal System (EGS). The basis of this revolutionary concept, devised in the USA in the 1970s, is to extract terrestrial heat from places that were unexploitable in the past.

Even though the Soultz project has a core of only 15 permanent staff, the site witnesses a constant coming and going of professionals with very different skills from all sorts of backgrounds: engineers, geologists, geophysicists, seismologists, forklift truck operators, crane operators, electrical engineers and many more. This hive of activity became even more frenzied in January 2008, when work began on the above-ground installations required to convert terrestrial heat into electrical power.

We are now at the end of May. The project is celebrating the culmination of 20 years of frenetic research. At last, this pioneering geo - thermal power plant, built and funded by a public/private European collaboration project, began generating electricity. This really is a world first.

Exploiting a little-known environment

The geothermy concept - to extract underground heat stemming chiefly from the disintegration of radioactive elements in the rocks of the Earth's mantle - is nothing new in itself. Development was stepped up when the oil crisis struck in the 1970s. Although numerous geothermal power plants all over the world are already generating electricity or supplying district heating systems, Soultz has one fundamental element that distinguishes it from all the other sites - underground water. Existing techniques(1) are limited to pumping hot water from an aquifer and injecting it directly into a heating system or using it to drive turbines to generate electricity.

The originality of the Soultz concept is precisely the fact that it requires no local hydrogeological resources. Water is injected from ground level into natural fractures crisscrossing crystalline rocks situated deep enough to allow a sizeable amount of useful heat to be extracted from them. In the case of the Rhine Graben - the geological area where the Soultz pilot site has been built - the rock which the researchers have been studying for 20 years is granite.

Albert Genter, from the French Geological Survey (Bureau de Recherches Géologiques et Minières, BRGM), is a structural geologist. Although he has been scientific coordinator of the Soultz project since September 2007, his knowledge of the site dates back many years, having written his doctoral thesis on Soultz granite. "Field tests began in 1987, when the GPK1 well was bored, enabling us to take the first core samples and to determine the characteristics of the rock fractures using a variety of acoustic imaging techniques," he explains, pointing out an old borehole situated just in front of the offices of the project's lead organisation, the European Economic Interest Grouping (EEIG) Exploitation Minière de la Chaleur (Heat Mining).

"This provided us with a more accurate picture of the subsoil. The data gathered during past oil extraction programmes had provided us with very little information about the crystalline rocks underlying the sedimentary layers because, as they were unsuitable for oil exploitation, they attracted little attention from geologists. By contrast, the data did inform us about the region's atypical geothermal gradient, where the temperature increase with depth is much greater than elsewhere."

"The American researchers who had originally invented the EGS concept dubbed it Hot Dry Rock Geothermy. However, the Soultz experiments showed that the granite on the site is not in fact dry. We found it to contain natural water, only in small quantities, but enough to be tapped for a geothermal power plant. So this saline aquifer was used as a reservoir from which to pump water for reinjection into the fracture system."

Does the fact that the power plant draws on an aquifer make the project any less original? "Not in the least," Genter reassures us. "We are quite simply opportunists. Although water is pumped on the site, it is re-injected into a fracture system that contained practically no water at the start."

Prising open the rock

Exploratory research revealed the existence of a fracture system that was sufficiently developed to serve as a geothermal circulation system. The problem was that water could not be injected directly into it, as the granite fractures were obstructed by natural deposits of calcite and other siliceous, argillaceous and ferrous deposits. Before circulation tests could be conducted to confirm the system's feasibility, the right environment had to be created to make it suitable for exploitation.

"We used two techniques to enlarge the fractures and to make bore holes to improve connectivity in the natural system. The conventional method - hydraulic stimulation - consists of injecting thousands of cubic metres of fairly fast-flowing water to re-open the rock. The snag is that hydraulic stimulation causes mini-earthquakes. While most of these seismic impulses were extremely weak, some were large enough to be felt (roughly 2 on the Richter scale)." (2) Hydraulic stimulation is very tricky to do. In 2006, researchers working on a similar project in Basel (Switzerland) managed to trigger an earthquake of 3.4 on the Richter scale.

"From a scientific standpoint, such microseismicity phenomena are a positive sign, because they prove that stimulation is effective. However, some practical problems have arisen. One is that many dwellings border the site and, of course, they have to be taken into account. Second, hydraulic stimulation failed to produce the desired results, as the connectivity between wells had not been adequately improved. So we decided to use chemical stimulation. Weak acids were diluted in water, then injected into the subsoil to dissolve the remaining hydrothermal deposits."

Bingo, it worked! In 2006, circulation tests showed that this combination of chemical and hydraulic stimulation had improved the system's hydraulic performance to a satisfactory degree. The Soultz project was then shifted up a gear and construction of the power station began.

Above and below ground

On a small hilltop around 1 km from the EEIG offices stands the hard core of Soultz - the site where the actual power station is being erected. It consists of an inextricable tangle of piping surrounded by large structures: two red funnels, the separators and an enormous green platform, the cooler. "The separators are designed to separate liquid water from vapour. As the well has been left to rest for several months, the geothermal water being pumped still contains numerous rock particles so it cannot be re-injected into the injection well in its current state. Such impurities could choke the filters and damage the power plant equipment."

"The cooler is used to liquefy isobutane - the heat transfer fluid that recovers the heat from the geothermal water in heat exchangers to drive the power plant's turbine. As no sufficiently cold water source is available near the site, we opted for an air cooling system equipped with nine fans."

Downstream of the cooler, the power plant's key element, the turbine, is carefully isolated inside a special box. The turbine is coupled with the generator to produce electricity, which the former then feeds into the national grid. Alongside it lies the heat exchanger, a system of interlaced cylinders and tubes in which the geothermal water and isobutane circulate.

Standing at the centre of these above-ground facilities is the heart of the power station, the geothermal triplet: three boreholes sunk some 5 000 metres into the earth. They are the oldest structures on the site, on which all the researchers' attention had been focused before the power plant's above-ground equipment came to be added. GPK3 is the injection well, which is used to introduce water into the subsoil. The water is then recovered by the production wells, GPK2 and GPK4, which transport the geothermal water into the aboveground facilities. Although at ground level the wellheads are only 6 metres apart, at depth the three boreholes are around 650 metres apart.

"This enables the water to circulate in the fractures long enough to heat up. At first, we planned for a depth that would enable us to reach a temperature of 200 °C, the boiling point of the heat transfer fluids used in those days.However, as a result of heat loss in piping the water to the surface, the water recovered was no hotter than 170-180°C. Fortunately, "binary" organic fluids now exist, such as isobutane, which have a lower boiling point. When we bored the three wells, we also discovered that the geothermal gradient was not constant. The deeper we bored, the less sharply the temperature increased. We now know that the optimal depth is between 3 000 and 3 500 metres."

Future challenges

Apart from the three wells used to recover underground heat, two further wells have been bored at Soultz: the 3 600-metre deep GPK1 well used for exploratory research and, most important of all, the 2 200-metre ESP1 well designed to monitor the smooth operation of the power plant. It is equipped with a myriad of thermal and hydraulic sensors. "At the outset, ESP1 was meant to be much deeper but it veered horizontally as it was bored and we had to stop work. Although this was disappointing from a geothermal standpoint, from a geological standpoint it was a stroke of luck. We use this borehole to extract whole core samples of granite and are gaining a much more accurate picture of the rock's structure and nature. The samples collected from the other wells consist of debris, which means that we ca only infer the rock's original composition."

ESP1 is not the only monitoring tool at Soultz. Since the early 1990s, a system of seismic observation wells has been built all around the site. Just like ESP1, these 1 500-metre boreholes are in fact former oil wells appropriated for the research. "The data from these seismic stations is supplemented with data from France's Strasbourg-based national seismic monitoring network (Réseau National de Surveillance Sismique, ReNaSS)."

Having fashioned an effective fracture system, completed assembly of the power plant and started to generate the first kilowatts of electricity in June 2008, the Soultz project has now achieved its principal objective. Future challenges are no less momentous. "Even though we have already conducted a great many injection and production trials, they have never lasted for more than a few months," explains Marion Schindler, geophysicist at Germany's Federal Institute for Geosciences and Natural Resources (Bundesanstalt für Geowissenschaften und Rohstoffe, BGR), who is responsible for collecting and centralising the site's hydraulic and thermal data. "In the years to come, we plan to gather a lot of data on the seismicity, temperature, pressure and quality of geothermal water. All this should enable us to gauge the behaviour of fractures over the long term", she adds enthusiastically. "It is essential to have such information for the geothermal power plants of today, which are being developed all across the globe, as well as those of the future."

Julie Van Rossom

  1. Here we refer to low- and high-energy geothermal systems.

  2. All unattributed quotations are from Albert Genter.

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