Analysis of influence factors of casing heat transfer in middle-deep geothermal wells in severe cold region

Abstract

The medium and deep coaxial casing heat exchange technology uses the fluid circulation inside the casing to improve the deep geothermal energy. Due to the high outlet temperature at the source side， it is a good choice for the heat source of buildings in severe cold areas in winter. We establish a two-dimensional unsteady numerical heat transfer model of fluid-thermal-structure coupling based on standard <inline-formula><mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" id="M1"><mml:mi>κ</mml:mi></mml:math></inline-formula>-<inline-formula><mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" id="M2"><mml:mi>ε</mml:mi></mml:math></inline-formula> model and Simple pressure velocity solution method by analyzing the heat transfer process between coaxial casing and rock mass at source side. According to the climate and stratum characteristics of Changchun City， we analyze the heat transfer performance of heat exchanger and rock and soil mass in full heating cycle by simulation. The results show that due to the cold winter in Changchun City， during the operation of the heat pump， the shallow formation temperature is lower than the fluid temperature at the inlet of the heat exchanger， and the rock and soil within 3.9 m below the ground is a heat source funnel， which gradually changes into a heat sink funnel with the increase of formation depth. In the first week of operation of the heat pump， the outlet temperature and heat exchange of the heat exchanger decrease rapidly and tend to be stable after one month of operation. In the whole operation cycle， the unit time decline of the outlet temperature of the buried pipe becomes smaller and smaller， and the relationship between the change of the outlet temperature and time decreases in the form of a power function with a slow speed. With the increase of thermal conductivity of cementing cement， the heat exchange first increases， and tends to be stable when the thermal conductivity of cementing cement is greater than that of rock and soil. The thermal conductivity of the inner tube increases， the thermal short circuit is serious， and the outlet water temperature and heat exchange of the heat exchanger decrease. When the heat pump unit 24 hours on and off time （start-stop ratio） is 16 h∶8 h， 12 h∶12 h， 8 h∶16 h， respectively， the heat exchanger outlet temperature and heat transfer are irregular， but the overall trend is gradually reduced. The longer the heat pump stops running， the better the temperature recovery of rock and soil body on the source side， and the greater the water outlet temperature and heat transfer when it is turned on again. Through the numerical model， the variation law of influence factors of casing heat exchanger can be obtained. <fig fig-type="abstract-image" id="F1" orientation="portrait" position="float"><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="SP.J.1249-2022-39-1-42/D20AE2D9-DE77-4b51-BEC5-8F4EB83A9A4A-F001.jpg"/></fig>