Study of heat transfer characteristics and economic analysis of a closed deep coaxial geothermal heat exchanger retrofitted from an abandoned oil well

Abstract

It is economical to transform abandoned oil/geothermal wells into closed deep geothermal heat exchangers with coaxial tubes. A numerical model of a coaxial geothermal heat exchanger (CGHE) with varying borehole diameters is established according to an abandoned well in Northern China. The finite difference method is adopted to solve the temperature distribution, and the accuracy of the model is validated with experimental data. Based on the existing structure of the abandoned well with different depths, the feasibility of its conversion into a deep CGHE is discussed, and this study uses the orthogonal experimental method to analyze the influence of four main factors and their significance level on the average heat extraction rate, with the heat extraction rate up to 422.18 kW in the optimal combination. This study also integrates with actual project considerations and conducts an economic analysis to determine the most appropriate circulation fluid flow rate. The results highlight the key factors on the heat transfer performance of the CGHE, with the inlet water temperature to the CGHE being the most significant, followed by the configuration of the CGHE retrofitted from abandoned. From the economic perspective, given that the CGHE in this study is retrofitted from the abandoned oil Wells, the drilling cost can be reduced by up to CNY 1800 thousand, and the flow rate design of 35 m3/h is the optimal choice, ensuring a cost-effective system operation while meeting the operational requirements of the deep CGHE.