Most low-enthalpy geothermal waters contain dissolved gases (e.g., CO2, H2S, and CH4)- In artesian geothermal wells, the absolute pressure of the water flowing towards the surface may drop below the bubble point of the dissolved gases, resulting in their gradual release and the appearance of two-phase flow. To optimize flow conditions we must keep frictional losses to a minimum and prevent undesirable flow regimes from occurring in the well. A mechanistic model has been developed for upward two-phase flow in vertical wells, based on existing correlations for the various flow regimes. Computations have been performed using data measured in wells at the Therma-Nigrita geothermal field, Greece. The methodology presented here allows us to study the effects of changes in well casing diameter on fluid production rate and flow stability within the well, parameters that have to be considered when designing geothermal wells for further exploitation and field development. (c) 2007 CNR. Published by Elsevier Ltd. All rights reserved.