Abstract It is important to determine reservoir characteristics as early in the life of the reservoir as possible. This paper presents a summary of the efforts made by Comision Federal de Electricidad to evaluate Los Azufres field by means of the implementation of standard pressure transient analysis techniques to geothermal reservoirs. Among these techniques, constant injection, falloff, multiple-rate injection, build-up, drawdown, two-rate and variable rate tests have been applied. Discussion is presented on constant injection, falloff, two-rate and variable rate testing. Interference testing is now being conducted on a small portion of the field. Introduction After the discovery of a geothermal field has been confirmed by successfully drilling one or more wells, the reservoir engineer has to obtain as much information out of them as possible. This information is needed in order to perform a proper evaluation of the field, before the utility company can proceed with the construction of an electric power generating facility. Among those geothermal fields undergoing exploration in Mexico, Los Azufres is the one in the most advanced stage of exploration. This geothermal field is located some 300 kilometers southwest of Mexico City, in the state of Michoacan in Central Mexico. As shown in Figure 1, this field is situated on the E-W oriented neo-volcanic axis. It is a highly fractured system, located on neo-quaternary volcanic deposits overlying a basement which is believed to be made of limestone. The lower part of the igneous deposits is constituted part of the igneous deposits is constituted by microgranular and esites which are over-laid at places by ryolites and pyroclastics. There are many thermal manifestations and alteration zones around the field which are related to tectonic activity and are located near faults and fracture zones. To date, 8 wells have been drilled in the field, 6 of them are for production and 2 injectors. Three more wells are on the drilling stage and 6 additional wells are scheduled to be drilled during 1979, Figure 2 illustrates the main faults and the location of the wells. The wells so far drilled in this area are sited near the known major faults. The reason behind this may of selecting locations is that because of the tightness of igneous rocks, the main production is thought to come through fractures. The deepest well is 2450 m and the shallowest is 960 m deep. Los Azufres field is under the last stage of the feasibility study, which has been carried out in order to determine the optimum size of turbines to be installed for electricity generation. This field has been divided into several possible producing areas called "modules" around the producing areas called "modules" around the main fracture zones. This areas will be developed independently from each other. Thus, reference will be made in the text to Laguna Verde Module, Agua Fria Module and Tejamaniles Module, among them the latter is under intensive drilling development (See Fig. 2). Figure 3 shows a typical temperature survey and well completion of this field. Wells are drilled using low density bentonitic mud from surface up to the top of the reservoir and with water from that depth downwards. Los Azufres reservoir is a liquid-dominated system having a steam cap located on the Tejamaniles Module, in the zone of well A-6, whose productivity test showed a 99 percent steam.