Analysis And Experimental Study On Stratified Heat Transfer Characteristics Of Medium-Deep Geothermal Buried Pipes In Guanzhong Area

The development of green and clean energy and the promotion of the “ dual carbon target ” are important measures and goals for adjusting the energy structure.Geothermal energy,as a clean energy with abundant,stable and wide distribution,has gradually attracted people ’ s attention.Scientific and accurate evaluation of heat transfer efficiency is the premise of efficient development of geothermal energy.Some scholars have studied the heat transfer performance of geothermal buried pipes,but in the process of heat transfer analysis of geothermal buried pipes in the middle and deep layers,there are few stratified studies on the thermal properties of strata.Therefore,this paper relies on the key project of Tiandi Science and Technology ’ Key Technology Research and Experimental Project for the Development and Utilization of Medium and Deep Geothermal Based on Directional Wells ’(2018TDZD017)and the key research and development project of Shaanxi Province ’ Key Technology Research and Demonstration for the Sustainable Development of Medium and Deep Geothermal Heat Transfer in Guanzhong Basin ’(2021ZDLSF05-12).Aiming at the problem of stratified heat transfer characteristics of medium and deep geothermal buried pipes,indoor experiments and numerical simulation are combined to achieve the following results :(1)X-ray diffraction and thermal conductivity test were used to test sandstone and mudstone samples to obtain rock mineral composition,porosity and thermal conductivity.The thermal conductivity of sandstone and mudstone was calculated by parallel model combined with mineral composition and porosity.Through geophysical logging,the strata in Guanzhong area are stratified according to sandstone and mudstone,and the thermal conductivity is calculated.The variation of formation temperature with depth is obtained by using distributed optical fiber temperature measurement equipment,and the thermophysical property parameters of formation stratification are obtained.(2)The heat transfer process of heat exchanger was analyzed.The fluid heat transfer was simplified as one-dimensional problem,and the solid heat conduction was simplified as two-dimensional problem.According to the geothermal well parameters and the thermal physical parameters of stratum stratification,a twodimensional axisymmetric layered heat transfer model was established and solved by COMSOL numerical software.The field test was carried out under the condition of intermittent operation for 5 d with the maximum heat intake,and the model was verified by the measured water temperature data.The differences in the water temperature of internal and external pipes between the stratified and homogeneous models at 5 d of intermittent operation and the ground temperature field distribution at120 d of continuous operation were compared and analyzed.(3)Based on the established stratified heat transfer model,the single factor analysis was carried out on the influence of the thermal conductivity of the inner tube,the circulating water flow,and the thermal conductivity of the cementing material on the heat transfer performance of the heat exchanger.Through the method of orthogonal experiment,the comprehensive analysis of multiple factors is carried out.In the aspect of heat exchanger structure,the well depth and outer tube diameter are selected as two factors.In the aspect of operation condition,the inlet water temperature and circulating water flow are selected as two factors.The orthogonal test table with four factors and three levels is formed to simulate.The importance of each factor is compared and the combination of factors is optimized.In this paper,based on the stratum characteristics in Guanzhong area,the stratification method of stratum parameters is proposed,and the stratified heat transfer model of medium and deep geothermal buried pipes is established.The differences between the homogeneous and stratified models are analyzed,and the influencing factors of heat transfer are analyzed,which can provide certain reference for the practical engineering of medium and deep geothermal energy development.