Geothermal reservoirs can undergo a transition from single-phase to two-phase or vice versa in their natural setting and during production. Some of these reservoirs can also contain considerable amounts of non-condensable gases, mainly (>90%) CO2. Thus, multi-phase and multi-component effects often complicate the analysis of geothermal well tests as pressure transient analysis (PTA) methods are mainly developed for single-phase flow and slightly compressible fluids. This work aims to investigate the suitability of commonly used PTA methods under two-phase and multi-component conditions. A series of numerical models were developed for a geothermal reservoir containing a mixture of water and CO2 incorporating the production/injection record and the corresponding pressure response. The pressure response of the numerical models was analysed to estimate the reservoir properties, and explore the accuracy of the interpretations. The parameters used in analytical models were also subjected to sensitivity analysis to identify their effect on the characteristic appearance of the pressure derivative plot for realistic interpretations of the reservoir characteristics. This modelling study demonstrates that geothermal reservoirs containing CO2 should be evaluated with great caution as the assumptions that analytical models do not take into consideration may have critical importance when assessing the well test derived properties. (C) 2019 Elsevier Ltd. All rights reserved.