The European Hot Dry Rock-Programme: Assessments and Prospects for Exploiting the Energy Potential of Hot Dry Rock

Mr Paul-Henri Bourrelier, the main speaker of the evening, Ingénieur Général des Mines and member of the Policy Group which sets the strategic orientation of the research carried out at Soultz-sous-Foret, explained that hot springs have been used since ancient times and that in this century people began to exploit reservoirs of steam or hot water in order to produce electricity or to provide heating for towns.

He stressed that in all these cases where energy has been obtained there was a source of water or steam located in some volcanic or some particular interesting area from a thermal point of view. That means that it was a question of good luck in getting the two things at the same time: interesting thermal conditions and water. Until now only a tiny proportion of the vast resources of geothermal energy in the earth’s crust had been used; in particular natural water sources hot enough for power generation were limited geographically although heat was present everywhere.

He pointed out that for these reasons another solution had to be found and this was the idea of extracting geothermal energy from rocks which did not naturally contain useful amounts of water. Some American physicists pioneered plans to inject water under high pressure through boreholes drilled into compact granite massifs that had been fractured artificially.

These tests were then picked up and taken further in Great Britain and Japan. Mr Bourrelier explained that the ideas developed from these projects, especially the recognition that virtually all deep crystalline rocks are naturally fractured and that only limited artificial fracturing is possible, were then included in a European Programme. The European programme in Soultz is focused on an area where the network of natural fractures in the deep rocks contains a small amount of flowing water.

Deposits of this kind provide three very major advantages: drilling is (relatively) easy, temperatures are favourable due to the natural upward flow and it is possible to operate using a low-pressure closed loop without the need for an external water supply. He mentioned that these ideas have been carried further at the Soultz Project with a European team, drawing on all the technologies that have been developed in the US, UK and elsewhere and co-ordinating the efforts of teams from several countries.

In the latest tests, the work on the Soultz project has shown that water can be made to flow at a steady, constant rate of 90m3/h inside a closed loop made up a network of natural fissures with enhanced permeability between two boreholes about 450 metres apart, reaching a depth of about 3500 metres. The water is heated inside the rock from 70¡C to 145¡C and transfers 10 MW of thermal energy to the surface, at a pumping cost of only 0.2 MW of electricity. In contrast to earlier results from other projects, water losses were zero and the resistance to flow was small. This represents a great step forward and could be the starting point for an industrial project.

The basic unit at a modern industrial exploitation site would consist of one injection borehole between two extraction boreholes and the double loop could produce 120 MW of thermal energy over a period of 20 years. The corresponding reservoir of heat would amount to more than 1 million tonnes of oil equivalent (TOE). As an order of magnitude one could imagine that with the operation of several loops 20 million TOE can be achieved.

Mr Bourrelier explained that before launching a project of this kind, which is founded on a reliable assessment of technical and financial feasibility, a pilot unit must first be set up for a sufficient period of time to determine the conditions for complete extraction of the geothermal field. This stage will be a considerable operation itself taking place over several years and requiring financial support of the order of 70 million ECU and will need to be run by industrial firms.

He emphasized that a lot of research has still to be done and that with the help of geological and topographical knowledge other areas with fractured granites could be found in the whole of Europe in order to have access to a considerable amount of energy.

The speaker stressed that the most important reasons to take an interest in this Hot Dry Rock Technique are ecological. The closed loop, the relatively small surface area occupied and the possibility of setting up on-site units near to the end-user indicate that geothermal production of this kind is more or less ideal environmentally. From a strategic point of view, the technique could provide power from geothermal sources almost anywhere in Europe. For these reasons, geothermal energy could be extremely valuable in helping the European Union to reach its target of reducing CO2-emissions.

Mr Bourrelier finally expressed his hope that he could convince decision makers that there are good reasons to go further with the European project in Soultz. Until now DG XII of the European Commission played a remarkable unifying role. As geothermal energy has a capital-intensive nature and is therefore treated with suspicion by established forms, it need to be given more publicity and strong political backing.

Dr Garnish, the scientist in DGXII responsible for this work, supported this view. He complimented the various European teams on their achievements, both technical and – in their willingness to co-operate on a single test site – political. He reminded the audience that this was not a totally new, science-fiction-inspired technology. Conventional geothermal resources already contribute more than 8000 MW of power generation world-wide, and this HDR technology represents a logical (if difficult) extension. Considerable progress had been made towards achieving the conditions that would be required for commercially-viable and competitive power generation, but it would be essential to prove these findings in the planned pilot plant before commercial confidence could be established.

It was particularly appropriate for such work, involving large expenditures and considerable investment in technical expertise but – in principle – of almost universal application if successful, to be supported by the European Union. The timescale of each step, however, was ill-suited to the regular cycle of EU R&D programmes and it may be necessary to adapt some of the procedures if support were to continue in the future.

He stressed also that, as part of this development process, it was planned that both the management and the financing of the work would become increasingly the responsibility of the industrial partners. Realistically, however, there would be a continuing need for a certain level of public funding until the initial pilot plant came into operation and this, in itself, would need a strong degree of political support.