The specific characteristics of the Greek geothermal fields on the volcanic islands of Miles and Nisyros are described, with emphasis on the characteristics affecting well casing. Overpressure, high temperatures and highly saline brines are typical features of the Greek fields. A new treatment of the different load conditions along the casing string is proposed that takes into consideration the possible formation overpressure, the axial thermal stresses and their effect on burst and collapse resistance: This treatment is further applied to some typical wells that have experienced casing failure.