Knowing the long-term performance of geothermal energy extraction is crucial to decision-makers and reservoir engineers for optimal management and sustainable utilisation. This article presents a three-dimensional, numerical model of coupled thermo-hydraulic processes, in a deep heterogeneous geothermal reservoir overlain and underlain by impermeable layers, with discrete fracture. The finite element method is employed in modelling the reservoir, after conducting a verification study to test the capability of the solver and the results obtained are in agreement with the existing models. The model is then used to investigate the responses of human control parameters (injection flow rate, fluid injection temperature, and lateral well spacing) on reservoir productivity, using different operation scenarios. The injection flow rate is found to be more efficient, concerning reservoir productivity, than the other two parameters. To this end, the study concludes that, by varying some parameters in the subsurface, reservoir productivity can be optimised efficiently. The numerical model developed provides in-depth insight to stakeholders and reservoir engineers concerning the essential parameters to control during exploration and exploitation. (C) 2017 Elsevier Ltd. All rights reserved.