Heat extraction from a geothermal reservoir using a single U-tube in a reservoir has been applied in the field. The advantages include extracting only heat instead of water. Based on the geological data of Bazhou geothermal field, China, a three-dimensional steady state numerical model, including the U-tube, wellbore and reservoir is established to analyze the entire flow field comprehensively. The performance of the exchanger is studied through investigation on the influences of four important geological parameters of depth, porosity, permeability and heterogeneity of the formation. Simulation values are validated by results obtained from field tests. Results indicate that the overall flow velocity in the geothermal reservoir is relatively low compared with the flow in the wellbore mainly due to the high flow resistance. In addition, the mass flow rate can hardly affect the flow in the reservoir. Under the conditions of this study, the depth of the geothermal field, the reservoir porosity and the heterogeneity of the geothermal formation may hardly affect the performance of the U-tube. However, if it is possible, it is better to install the single U-tube in the homogeneous area of the reservoir, which may obtain better heat extraction effect. The results in this paper could provide implications for further study of geothermal energy exploitation.