Early evaluation of the potential for geothermal breakthrough of reinjected fluids in newly developed geothermal fields can be obtained with the SGP one-dimensional heat sweep model. The model was used to estimate fluid cooldown from wells selected for the first wellhead generating units to be installed at the Los Humeros and La Primavera geothermal fields in Mexico, based on staff-compiled geometric and geologic data, thermal properties of the reservoir rock, and expected production conditions. Geometric considerations were evaluated with respect to known and postulated fault zones and return flow angle of the reinjected fluid. The results show the range of parameter values that affect the rate of thermal breakthrough to an abandonment temperature of 170 oC corresponding to the minimum inlet pressure to the CFE 5-MW wellhead generator units. 9 figs., 4 tabs., 11 refs.