Ambitious energy and environmental policy goals are creating new challenges for energy suppliers: The energy mix of the future will have to be ecologically friendly, secure in resources, competitive, and last not least sustainable. The goal of the European Union to at least double the proportion of renewable energy sources in the overall energy consumed in the EU by the year 2010 makes it clear just how high the expectations towards the energy producers are. Geothermal energy is becoming more and more interesting in the course of this process. Energy in the form of technically usable heat or electric power can be produced from geothermal sources as needed. However, a European Geothermal Deep Drilling programme is necessary to trigger a significant contribution to the goal of the EU.
The Earth contains a high potential for supplying heat to the energy economy. The occurrence of geothermal heat sources is therefore not limited to regions with noticeable volcanism. In principle, there is geothermal heat everywhere, including Central Europe. In such areas with a lower temperature gradient depths of four to five kilometres are required to reach temperatures high enough to effectively generate electric power using steam turbines. This potential can be utilised only after the costs and risks of its development have been effectively reduced. The challenge lies in the establishment of technologies which improve the yield of geothermal repositories and reduce the risks associated with their exploration and exploitation.
The development of suitable technologies for the exploitation of underground heat has been one of the central research goals of the EU in the past years, for example at Soultz-sous-Forêts (Alsace) for application to HDR processes. The research and development projects within a forthcoming European Geothermal Deep Drilling programme require the combination of inter-disciplinary basic research with economic and technological planning concerning the operation of geothermal installations. The research has to address all aspects of geothermal energy production: the characterisation of potential geothermal repositories, for which we need expertise in the investigation of the geological, geochemical, geophysical, and geomechanical aspects of geothermal site development, all the way up to power plant efficiency. In addition, an analysis and evaluation of the overall systems has to be carried out.
However, as drilling is normally the one factor that consumes the single-most financial resources in the development of geothermal power production, a special effort to reduce the risks involved is not only justified but urgently needed. A geothermal drilling programme would be a suitable instrument to significantly lower the costs of drilling and would help to reduce technical and economical risks. The overall goals of such a programme are the improvement of the reliability of geothermal drilling, the standardization of geothermal drilling and stimulation operations, system studies with focus on operational issues, the development of drilling instruments and tools and completion components based on innovative 3D seismic geothermal exploration technology.
To reach these goals, standardised procedures have to be developed that exist for traditional geothermal areas such as Larderello/Italy, Icleand or the Philippines as well as for the hydrocarbon industry. For most of Europe, however, such standardised procedures with all aspects of drilling large diameters, directional drilling, and drilling mud technologies are yet to be established. Exploration for geothermal resources requires drilling solutions and machineries that cause the lowest possible pay-zone formation damage, maximum characterization of the target geology, high deviation and large drill holes directional drilling as well as intelligent well completion designs. A general task for all geothermal sites is the development of reliable drilling technologies into the reservoir minimizing formation damage, and providing other techniques to monitor drilling in situ downhole , the testing, stimulation, and operation of a thermal water loop.
A group of drilling projects in areas representing European sites has to address two major aspects, one geological/technical and one political. The latter is the careful assessment of the potential drilling project would have to trigger regional or even national developments and support for further geothermal activities. From the geological point of view, the specific setting should be somehow representative for other sites such that the experiences are to some degree transferrable. In addition, each drilling project should be accompanied each applying at least one innovative approach addressing improvement in drilling, mitigation of formation damage, or installation of new monitoring techniques downhole. The research program should be focused as follows:
• The preferred way to drill a geothermal payzone would be with low pump volume at under-balanced conditions. Aerated mud systems operated in counter-flush mode do provide such minimized impact on the formation, even with large bit sizes at penetration rates equivalent to oilfield rotary drilling. Research will have to be done, how to advance this drilling method for depths beyond 3000 m and hole deviation angles up to 90 degree and long horizontal as well as multi-lateral well sections.
• The optimal formation evaluation and geologic characterization of the geothermal reservoir would be to drill it with core drilling techniques. Wireline coring systems can provide an efficient and safe way to recover hundreds of meters of large diameter (94 mm) cores at close to 100% recovery. Its low mud circulation characteristics, combined with an under-balanced aerated hole opening thereafter, will assure maximum information recovery from the payzone under least reservoir damage and maintenance of best borehole stability conditions for later complementary logging runs. Research efforts will be needed of how to advance such core drilling system for high angle and horizontal drilling performance.
• The provision of the largest possible last production casing string has a significant impact on geothermal well economics. Research will be needed into the area of new high grade tubular materials combined with slim premium connections, and alternatively with new technology field welding techniques in order to avoid diameter constraints in geothermal wells from conventional oilfield casing string geometry. Expandable tubulars, casing while drilling techniques and other novel procedures will have to be evaluated for geothermal applications.
• High angle and horizontal well profiles do provide large exposures to a producing reservoir and can therefore maximize water productivity and hence improve financial performance of a project. Such wells will however require in addition selective and intelligent well completions, in order to maintain a uniform productivity over its entire length in a heterogeneous reservoir over time. Research will be needed into downhole controlled flow-control valves, multi-lateral spider well layouts and/or distributed submersible pump configurations in order to insure a uniform and uninterrupted water production over the entire life cycle of a geothermal production well.