Assessment of Thermal Interference in the Northern Part of Los Azufres Geothermal Field, Mexico, by Tracer Test Analysis

Abstract

The Los Azufres geothermal system in Mexico was explored in the mid-1970s and at present its capacity is 188 MWe. Considerable pressure decline has been observed in the reservoir during more than 25 years of production. Therefore an injection strategy has been imple-mented to recharge the reservoir. All the injection is in the west part of the field while zones in the northeast part have also suffered declining pressure. Therefore the best location for an injection well in the northeast part of the field is being evaluated. The aim of a 2006-2007 tracer study was to verify whether well Az-64 may be used as an injector with limited interference in nearby production wells. To assess the danger of temperature decline of wells due to brine injection into the well the data from the tracer test was interpreted by modelling the observed tracer recovery with the ICEBOX computer software. The modelling yielded estimates of flow channel properties, which were consequently used for cooling predictions. The tracer was only detected in two of six monitoring wells, Az-05 and Az-43. The greatest amount was recovered in well Az-05, which is the well closest to the injector, or almost 50%. For well Az-05 the average transport velocity was between 65 and 120 m/day, for two flow channels, while for well Az-43 it was 92 m/day. The cooling forecasts indicate significant cooling in well Az-05, or a temperature decline of about 14°C in 30 years for an injection of 27 t/h. Therefore a lower rate of brine injection into well Az-64 is recommended.