Geothermal Sustainability

Abstract

Geothermal energy is a classified as a renewable resource, where ?renewable? describes a characteristic of the resource: the energy removed from the resource is continuously replaced by more energy on time scales similar to those required for energy removal. Consequently, geothermal exploitation is not a ?mining? process. Geothermal energy can be used in a ?sustainable? manner, which means that the production system applied is able to sustain the production level over long times. The longevity of production can be secured and sustainable production achieved by using moderate production rates, which take into account the local resource characteristics (field size, natural recharge rate, etc.). The production of geothermal fluid/heat continuously creates a hydraulic/heat sink in the reservoir. This leads to pressure and temperature gradients, which in turn ? after termination of production? generate fluid/heat inflow to reestablish the pre-production state. The regeneration of geothermal resources is a process, which occurs over various time scales, depending on the type and size of the production system, the rate of extraction, and on the attributes of the resource. In general, geothermal production occurs over a certain length of time. Time scales for re-establishing the preproduction state following the cessation of production have been examined using numerical model simulations for: 1) heat extraction by geothermal heat pumps, 2) the use of a doublet system on a hydrothermal aquifer for space heating, 3) the generation of electricity on a high enthalpy, twophase reservoir and 4) an enhanced geothermal system. The results show that after production stops, recovery driven by natural forces like pressure and temperature gradients begins. The recovery typically shows asymptotic behaviour, being strong at the start, and then slowing down subsequently, and theoretically taking an infinite amount of time to reach its original state. However, practical replenishment (e.g. 95%) will occur much earlier, generally on time scales of the same order as the lifetime of the geothermal production systems.