With the increasing development of smart well technology in the oil and gas industry, there is a need for robust and reliable downhole power supply in order to provide stable power for downhole sensors and control tools. Usually, the produced fluid from the reservoir carries abundant thermal energy, which can be converted to electric power with thermoelectric technology. To realize in-situ power generation in the downhole environment, the design of downhole segmented concentric cylindrical thermoelectric generators (CCTEGs) with a cold fluid is proposed. A mathematical model for downhole thermoelectric generation is presented, which considers the heat transfer in the interior of thermoelectric element and from surrounding formation. The power performance in an oil well with 50% of water cut is investigated by using an engineering equation solver program. Numerical results show that about 800 W of power output could be obtained with 200 m length of segmented CCTEG, which is enough to provide power for typical downhole applications. The length of the segmented CCTEG and the injection rate of cold fluid dominantly affect the power output. The power consumption requirement for downhole applications in different operation periods could be met by simply adjust the injection rate of cold fluid. (C) 2019 Elsevier Ltd. All rights reserved.