| Abstract |
Within the renewable energy spectrum, geothermal heat deserves a special statute. Contrary to solar, wind and tidal, geothermal energy is not an inexhaustible energy source. Quoting Henry J. Ramey Jr. (1987), commenting, in a historic overview, on the fact that geothermal fluids were of meteoric origin and geothermal systems subject to transfer and recharge “A popular theory was that production of a geothermal system at the natural recharge rate would lead to an inexhaustible source of energy. This idea appeared often and since unchallenged, was eventually accepted as a fact”. Low grade heat geothermal deposits are exhaustible. Heat within the geothermal reservoir is resupplied by conduction and abstracted, for commercial development purposes, by convection, at least an order of magnitude higher, an imbalance, which causes heat to be mined. How to reconcile this “fatal” issue with sustainable heat extraction strategies, aimed at securing well and reservoir longevities, is what geothermal engineering and further resource management are all about. Optimum heat recovery, pressure maintenance and waste disposal concerns make (re)injection of the heat depleted brine into, preferably, the source reservoir a major engineering and environmental concern. This poses the problematic of reservoir life, assessed from production well thermal breakthrough times, and of sustainable resource extraction and reservoir management, which ambition at mining heat over significantly long, say at least fifty year, times. In so doing, it should be borne in mind that geothermal operators need to reconcile this farsighted objective with severe constraining factors, such as well lifetime and drill site limitations, the latter particularly acute in city geothermal district heating environments. The foregoing are discussed through case studies addressing the following issues: • heat extraction issues • sustainability: a reservoir engineering approach |