The Formation of Permeability Contrasts in Geothermal Reservoirs

We present a model of a geothermal system supplied at depth by saturated steam. It is shown that as steam rises from depth to lower pressure regions, liquid forms for pressures greater than 3.1 MPa. Once the pressure reaches 3.1MPa and the steam continues to rise into even lower pressure regions, it becomes superheated. Condensate does not form in the zone where pressures are less than 3.1MPa. The formation of condensate in regions where the pressure is in excess of 3.1MPa may lead to dissolution of the host rock. This in turn may lead to the formation of a higher permeability zone under a lower permeability ‘cap rock’. The depth of the transition point at which condensate no longer forms appears to be primarily controlled by the pressure at the base of the reservoir. The influence of the heat flux as an independent parameter is shown to be relatively small. The depth of the transition point appears to be constrained to ~350-400m. This is an interesting result since the permeability in many vapour-dominated geothermal reservoirs is believed to increase significantly at around this depth.