This study aims to guide reservoir engineers/managers in the selection of a combination of parameters from amongst various possible alternatives in developing deep geothermal reservoirs which can meet the desired temperature at the production wellhead for sustainable energy production. The work presents an approach for predicting the long-term performance of a deep geothermal reservoir using multiple combinations of various reservoir parameters. The finite element method and factorial experimental design are applied to forecast which of the parameters has the most influence on long-term reservoir productivity. The solver employed is validated using known analytical solution and experimental measurements with good agreement. After the validation, an investigation is then performed based on the Soultz lower geothermal reservoir. The results showed that fluid injection temperature is the parameter that influences the experiment the most during exploitation involving production temperature, whereas injection pressure rate happens to have a more significant impact on reservoir cooling. (C) 2017 Elsevier Ltd. All rights reserved.