Miravalles geothermal field, Costa Rica : evidence of thermal evolution and a comparison of the mineralogy of an acid well and a neutral well

Abstract

The alteration mineralogy and the measured temperature data were used to find thermal fluctuations in the Miravalles geothermal field. Zones of relative heating were defined to the north and centre of the field; these correspond to zones where the measured temperature at the top of the epidote zone is at least 20°C higher than the temperature assigned to the epidote. A large north-south trending zone of apparent thermal equilibrium was defined along the main production wells in the field. In this zone, the measured temperature corresponds to the epidote temperature (200±20°C). Cooling zones indicate measured temperatures at least 20°C lower than the temperature corresponding to the epidote; these are located to the west and east sides of the main borehole field. The cooling zones may relate to the inflow of relatively cold water along NW-SE fracture systems, as indicated by the alignment of the zones. As part of the characterization of the acid fluid in geothermal wells, a literature survey was conducted and the main types, occurrences and characteristics are summarized. Similar literature review was done for the Miravalles geothermal field. The comparison between the alteration in a neutral and an acid well indicates that there are no major differences between those wells, which suggests that the acid fluids are likely to be restricted to very narrow zones that make identification difficult. This could also indicate that the acid fluids are not in equilibrium with the alteration mineralogy, probably due to the short time of residence.