Heat and Mass Transport in Geothermal Reservoirs: Fid-Bau Portal

Heat and Mass Transport in Geothermal Reservoirs

Bjornsson, Sveinbjorn / Stefansson, Valgardur

Geothermal reservoirs are generally more complex than reservoirs of groundwater or petroleum. Physical states of the hydrothermal fluid fall into four categories: vapor-saturated, two-phase boiling, liquid-saturated and supercritical. Liquid-saturated reservoirs and liquid-dominated or vapor-dominated reservoirs of the two-phase boiling type are the most common types exploited so far. There is growing interest in submarine geothermal systems and heat extraction from hot rock or magma bodies, where the hydrothermal fluid circulates at supercritical temperatures and pressures. Meteoric water dominates in continental systems and ocean water in submarine systems. The contribution of magmatic water is small at upper levels in the crust, but may increase as magma bodies are approached. The larger fumarolic fields have magma as a heat source. The rate of heat transfer required to sustain the intense heat output of such fields remains problematic, unless an intimate contact between circulating fluids and hot boundary rock of the magma is maintained over the lifetime of the activity. Convective downward migration of fluid along existing fractures and water penetration by thermal cracking of hot rock are important processes in this respect. Two-phase convection is of major importance in geothermal reservoirs. The phase change instability mechanism induces convection prior to the onset of ordinary buoyancy-driven thermal convection. Mathematical modelling of geothermal systems has greatly advanced the understanding of the dynamic nature of geothermal reservoirs and their response to exploitation.