Sustainable use of geothermal resources : renewability aspects

Abstract

The sustainable use of geothermal resources should secure longevity of production. This can be achieved by moderate production rates, taking into account the local conditions (field size, natural recharge rate, etc.). After production stops, the resources recover by natural processes; the production of geothermal fluid and/or heat successively 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 re-establish the pre-production state. The question of sustainability and renewal thus boils down to the rate of fluid/heat resupply. Unlike mining activities (although geothermal is often regulated in many countries by the mining law) where the mined-out ore is gone forever, the replenishment of geothermal resources always takes place, albeit at sometimes low rates. The recovery shows asymptotic behaviour, being strong at the beginning and slowing down subsequently, the original state being re-established theoretically only after infinite time. However, practical replenishment (e.g. 95% recovery) will already be reached much earlier, generally on a time-scale of the same order as the lifetime of geothermal production systems.