Geothermal potential of Portuguese granitic rock masses: lessons learned from deep boreholes

The northern part of Portugal is made of granitic large outcrops where a huge number of thermal sulphurous water springs can be found, located in the surroundings of extensive fracture systems. Most of these springs are used for Medical Spa purposes. Except for a limited number of cases, important parts of the heat water content are continuously wasted. In the last decades, a considerable number of Medical Spa abstractions have been improved by deepening wells which, in most situations, is accomplished by water temperature increase. Based on the well temperature records, to around 1,000 m borehole depths, the authors were able to calculate the geothermal gradients for six hydrothermal areas with values from 0.0122 to 0.0762 °C/m. Taking into consideration the reservoir temperature inferred by chemical geothermometers and the geothermal gradients, the reservoir depth was estimated in the range of 1,241–6,244 m. The higher depth estimation might be overestimated by not including convection heat transport processes. Based on the minimum reservoir depth estimation, none of the wells reached the reservoir, thus precluding higher flow rate figures. The data obtained in this study along with the described drilling operations can guide and improve future drilling operations in those areas with regard to time and money.