Heat Extraction Performance Of Downhole Coaxial Heat Exchanger Geothermal System

Geothermal energy is a clean and renewable energy.It has great reserves and wide distribution.Actively exploiting and utilizing geothermal energy has important significance to save energy and reduce emissions.The downhole coaxial heat exchanger system(DCHE)can extract heat by recycling working fluid in closed borehole without taking water.Numerical simulation,lab experiment and field test are used in this paper to study the heat extraction performance of DCHE geothermal system.Firstly,a thermal conductive cement is designed by adding thermal conductive materials in oil cement.The thermal conductivity,compressive strength and porosity are measured,and the influence of thermal conductive materials exerted on cement paste are studied.The results show that the higher the temperature is,the lower the thermal conductivity is.With the increase of graphite,thermal conductivity of cement pastes firstly decreases and then increase.With the increase of ferrum and copper,thermal conductivity of cement pastes increases.With the increase of thermal conductive materials,compressive strength of cement pastes all decrease.Under this paper condition,adding thermal conductive materials can improve thermal conductivity of cement paste by 5%-30%.Secondly,the unsteady flow and heat transfer model considering heat conduction and convection of reservoir is established,the temperature distribution of wellbore and reservoir is analyzed,and the factors of heat extraction performance are studied.The results show that the temperature near the wellbore decreases greatly,and the temperature distribution along the flow direction is asymmetrical;the increase of subsurface water velocity and aquifer thickness will enhance the heat extraction performance;using thermal conductive cement can increase thermal power by 6.5%;With the increase of displacement and thermal difference;With the decrease of thermal conductivity and diameter of tubing,thermal power increases linearly.With the increase of thermal conductivity and temperature gradient,thermal power increases.Finally,field test was carried out in Xiong'an district.The results show that the simulation data and experimental data are in good agreement,and the maximum error is within 12%.During the test,the outlet temperature stays at 15?and the thermal power stays at 160 k W.It has provided heating for residents around 4918 m~2,which verifies the feasibility of DCHE technology.In this study,a thermal conductive cement for geothermal well is designed.An unsteady flow and heat transfer model is established,and factors of heat extraction performance is studied.The reliability of the numerical model and the feasibility of DCHE are verified by field test.This paper can provide theoretical guidance for the development of medium-deep geothermal resources.