Initial Thermal State of the Los Azufres Geothermal Reservoir

Abstract

The unperturbed initial thermal state of the Los Azufres geothermal field (LAGF) was determined based on multidisciplinary data which included thermal, geological and drilling data. Temperature logs from 42 wells were employed to estimate stabilized formation temperatures SFT using the Horner and the Sphericalradial Flow Method, SRF, (Ascencio et al, 1994). Then for each well, a graphical analysis was performed on composite plots that included the temperature logs, the static formation temperatures obtained by both methods, the lithology, the well termination, the circulation losses and the heating rates estimated between two successive temperature logs. These graphs allowed correlation of the main inflow zone of each well with the reservoir temperature in direct contact with the well. Subsequently, the reservoir temperature of each well was plotted as function of elevation. This plot revealed the existence of compressed liquid at depth (with a temperature of about 308°C) which rises and eventually starts boiling at 1300 m.a.s.l. This two-phase region extends to approximately 1700 m.a.s.l. (with a temperature of about 280°C) where saturated vapor appears to be the fluid phase that controls the behavior of the system. The temperature of the vapor dominated region varies little with elevation and reaches 272°C at 2500 m.a.s.l. Finally, cross sections of the reservoir were made that show the temperature distribution of the geothermal reservoir by means of isothermal curves, obtained with the Horner and SRF methods, which indicate the zones where convective heat flow exists and its relation with the fractures of the field.