| Abstract |
The immense store of heat in the earth (~1013 EJ), provided mainly by decay of natural radioisotopes, is the ultimate source for geothermal resources. It results in a global terrestrial heat flow of 40 million MWt, which alone would take over 109 years to exhaust the earth’s heat. So, the geothermal resource base is extremely large and ubiquitous. Geothermal energy is 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 and those typical of technological/societal systems. 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. However, excessive production is often pursued, mainly for economic reasons, such as to obtain quick payback of investments, with reservoir depletion the result (e.g. The Geysers). An enhanced geothermal system (EGS) study showed that sustainable production can be achieved with lower production rates and can provide similar total energy yields as those achieved with high extraction rates. Regeneration of geothermal resources following exploitation is a process that occurs over various time scales, depending on the type and size of production system, the rate of production and the characteristics of the resource. It depends directly on the rate of fluid/heat re-supply. Time scales for reestablishing the pre-production state following the cessation of production are 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) conventional use of low-enthalpy resources, 4) the generation of electricity on a high enthalpy, two-phase reservoir and 5) an EGS. 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. It is concluded that: 1) “balanced” fluid/heat production that does not exceed the recharge can be considered fully sustainable, 2) production rates that persistently exceed the rate of recharge (natural or induced) will eventually lead to reservoir depletion, thus stopping economic production, 3) following termination of production, geothermal resources will undergo recovery towards their pre-production pressure and temperature states, 4) the post exploitation recovery typically exhibits an asymptotic behaviour, being strong at the start and slowing subsequently, and reaching a “practical” replenishment (~95% recovery) on time scales of the same order as the lifetime of the geothermal production system, 5) geothermal resources are renewable on timescales of technological/societal systems (~30-300 years), 6) sustainable production secures the longevity of the resource at lower production levels, 7) the level of sustainable production depends on the utilization technology as well as on the geothermal resource characteristics and 8) long-term production from geothermal resources should be limited to sustainable levels. |