A revised conceptual model and anlysis of production data for the Ahuachapán-Chipilapa geothermal field in El Salvador

Abstract

The Ahuachapán reservoir has been under exploitation for almost 20 years. Temperature and pressure logs from the 32 wells that have been drilled in Ahuachapán and the 8 wells drilled in Chipilapa have been analyzed in order to establish initial formation temperatures and reservoir pressures. The fieldwide distributions of these parameters show that both Ahuachapán and Chipilapa belong to the same geothermal anomaly and suggest an up-flow zone to the south of the present wellfields and a north-northeast trending lateral flow to an outflow area at El Salitre, 7 km north of Ahuachapán. The pressure history of wells in the area shows that the reservoir pressure drawdown caused by fluid production in Ahuachapán is about 17 bars in the production field but extends to the outer part of the geothermal field as far as Chipilapa where the drawdown is of the order 6-7 bars. Ahuachapán and Chipilapa can, therefore, be considered as sub-areas of the same geothermal field. Simple lumped model simulation of the drawdown history shows that the response of Ahuachapán to production during 1975-1985 is that of a liquid-dominated system whereas the pressure history for the last ten years can only be explained by an expanding boiling zone in the reservoir. The reservoir area and permeability estimated from the lumped model are 10-30 km2 and 30-80 mD, respectively.