Numerical modelling of Ahuachapán geothermal field, El Salvador

Abstract

A numerical model was developed for the Ahuachapán geothermal field, based on the last update of the conceptual model. The area under study is a volcanic geothermal system located in El Salvador that has been producing energy since 1975, with a current installed capacity of 95 MWe and a total extraction flow of 890 kg/s. The main features of the conceptual model, such as upflow and outflow zones, barriers, depth of the reservoir as well as the lithologic column, were introduced into the model through a distribution of different rock properties and boundary conditions. For this study the TOUGH2 simulator was used for core calculations, along with python scripts to manage the data, create input files, and process the output. The mesh limits were established based on previous studies. The model is composed of 15 layers, each consisting of 3,020 elements. Pressure and temperature profiles were used to calibrate the natural state while 44 years of mass extraction and reinjection were introduced into the model to recreate how the reservoir conditions change throughout the utilization history. Flowing enthalpy of production wells and drawdown in monitoring wells were used as calibration data. An inverse modelling approach using iTOUGH2 was used to calibrate the permeability distribution and heat sources. Based on the good match of the natural state and the production history, a 20 years forecast was calculated assuming that the current conditions of exploitation will be maintained. The results show a drawdown of 1.5 bar in reservoir pressure, confirming the categorization of the field as an open system. The rock type distribution suggests that the horizontal permeability ranges from 15 to 160 mD and the vertical permeability ranges from 13 to 80 mD in the rocks below the wellfield.