| Abstract |
The non-renewability, at human time scale, of geothermal energy sources arises the problematic of reservoir longevity and sustainable mining of heat in place. Clearly, well and reservoir life are the core of sustainable development and management strategies. These key issues are reviewed and discussed in the light of pertinent methodologies among which reservoir and production engineering, water injection and risk assessment take an important share. The impact of the so called externalities is also discussed. The foregoing are illustrated in a selected case study addressing the Paris Basin geothermal district heating scheme, which consists of seventy production and injection wells operating to date, supplying geothermal heat to ca. 150,000 dwellings. Here, systematic water injection practice, exploitation monitoring and periodical well inspection set the bases of a relevant reservoir management protocol, highlighted by the coupling of a reservoir/exploitation database to various (scale dependant) reservoir simulation tools. The paper is concluded by the modelling, on typical district heating systems, of the future exploitation trends and reservoir pressure/temperature patterns, using the TOUGH2 and SHEMAT simulation codes, over a seventy five year life projection. Model runs show that neither cooling nor severe pressure depletions occur on the production wells after seventy five years, provided the producer/injector wells be periodically (every 25 years) (re)completed and drilled at adequate reservoir locations. Hence, the contemplated scenario achieves sustainability. The exercise proved rewarding in that it convinced the operators and energy planners that geothermal reservoir and system life could extend far beyond project life (twenty to twenty five years) expectations. Incidentally, it shows that lifetimes nearing one hundred years cannot be regarded any longer as utopia. |