Analysis of quaternary thermal properties and influencing factors on shallow geothermal energy exploitation in Beijing plain

This paper is the result of the geothermal geological survey engineering.
[Objective] Thermal properties are important parameters for the evaluation and utilization of shallow geothermal energy resources. [Methods] In this work, we collected 695 quaternary samples from alluvial and diluvial fans in Beijing plain. Then thermal properties and geotechnical parameters were tested, and the characteristics of thermal properties and their influence on shallow geothermal energy extraction were analyzed. [Results] The results show that the average range of quaternary samples in Beijing plain is 1.465-2.022 W/(m·K) for thermal conductivity, 0.450×10^-6-0.841×10^-6 m²/s for thermal diffusion, and 2.323-3.080 MJ/(m³ · K) for specific heat, respectively. There is a linear relationship between the thermal conductivity (λ) and the thermal diffusivity (κ) with correlation equations λ =1.6973κ + 0.6127. The finer the particles size of quaternary loose sediments, the lower the thermal conductivity. The thermal conductivity increases rapidly with the moisture content within the range of 0-5% and tends to stabilize within the range of 5%-20%. The thermal conductivity decreases with the increase of water content within the range of 20%-40%. Under natural state, the thermal conductivity increases with the increase of the density and decreases with the increase of the pore ratio. When the sample temperature is in the range of 0-40 ℃, the thermal conductivity first decreases and then increases and keeps lowest at 20 ℃. [Conclusions] Thermal properties of the quaternary vary with geological conditions. They are related to the storage, collection and diffusion capacity of shallow geothermal energy. The larger the thermal conductivity, thermal diffusivity and specific heat capacity, the greater the soil heat storage and thermal conductivity.