Nature and assessment of geothermal resources

Abstract

The potential of the Earth’s geothermal resources is enormous, compared to the energy needs of mankind, and they can play a role in sustainable development. They are variable in nature, classified as (a) volcanic systems with the heat sources being hot intrusions or magma, (b) fracture-controlled convective systems with deep water circulation, (c) sedimentary systems with permeable layers at great depth, (d) geopressured systems, (e) hot dry rock or enhanced geothermal systems and (f) shallow resources utilized through ground-source heat pumps. Geothermal systems are also classified on the basis of reservoir temperature, reservoir enthalpy and their physical state. The energy production capacity of hydrothermal systems is predominantly controlled by reservoir pressure decline caused by hot water production, which is in turn determined by the size of a geothermal reservoir, its permeability, reservoir storage capacity, water recharge and geological structure. More generally the capacity of geothermal systems is also controlled by their energy content, dictated by their size and temperature conditions (enthalpy if two-phase).

Hydrothermal systems can in most cases be classified as either closed, with limited or no recharge, or open, where recharge equilibrates with the mass extraction in the long run. Modelling plays a key role in understanding the nature of geothermal systems and is the most powerful tool for predicting their response to future production, which is used to estimate their production capacity. Models are also an indispensable part of geothermal resource management during utilization. In addition to the volumetric assessment method (static modelling) different methods of dynamic modelling are the main techniques used for geothermal reservoir modelling and resource assessment, including simple analytical modelling, lumped parameter modelling or detailed numerical modelling. Thorough understanding of the nature and properties of geothermal resources, via comprehensive interdisciplinary research, as well as reliable and accurate assessment of their production capacity, through modelling, are an absolute prerequisite for sustainable utilization of geothermal resources.