Downflows during transient geothermal well test analysis

•Internal flows prevent the full characterisation of geothermal reservoirs during well testing.•Downflow causes instant perturbations in transient pressure compared to temperature.•The numerical well-testing framework captures and quantifies the downflow effects. Internal flows (downflow and upflow) occur in geothermal wells because of different factors that affect the pressure and temperature measurements used to characterise a geothermal reservoir. In extreme cases, an internal flow can cool a hot reservoir if a sufficient flow at a low temperature is present. This work examines the internal flows, analysing two geothermal well tests from El Salvador that show downflow. We employed two approaches to model the downflow effect. Although the internal flows affect the pressure and temperature measurements, the model approaches can effectively simulate the downflow, capturing the complex interaction in the well and formation. The pressure response is more perceptible to detect a downflow because it generates instant perturbations. The temperature response experiences less noticeable perturbations because of downflows, being more appreciable by the end of the temperature derivative.