Study on the performance and mechanism of high thermal conductivity and low-density cementing composite for deep geothermal wells

The efficient exploitation and utilization of deep geothermal energy can alleviate the energy crisis in the world today. The high thermal conductivity and low-density cementing composite (HTLDC) is helpful to improve the development efficiency of deep geothermal energy. In this paper, the optimal formulation of HTLDC is obtained by theoretical analysis, orthogonal test, matrix analysis, SEM, XRD, MIP and other methods. Then the basic properties, microstructure and performance changes at different temperatures are studied. The results show that the formula of HTLDC is 0.8 W/S ratio, 60 wt% cement, 9 wt% graphite, 15 wt% quartz powder, 6 wt% silica fume, 3 wt% fly ash and 7 wt% admixture. The density of HTLDC is 1.53 g/cm3. The thermal conductivity is 1.6702 W/(m·K), which is 94.71 % higher than that of conventional low density cementing material (CLDC). The 48 h compressive strength is 7.63 MPa, which is 40.77 % higher than CLDC. The content of Ca(OH)2 is as high as 68.9 %, which is 20 times higher than CLDC. The harmful porosity is only 15.6 %, which is 46.65 % lower than CLDC. The thermal conductivity of HTLDC increases rapidly and then tends to be stable with the increase of temperature. The maximum growth rate of thermal conductivity is 0.0026 W/(m·K2), which is 136 % higher than that of CLDC. The thermal conduction mechanism of HTLDC is consistent with the heat conduction pathway theory. HTLDC has a good high-temperature resistance due to the SiO2 participates in the hydration reaction and generates C5S6H5 and C6S6H with the increase of temperature. The research results can provide strong technical support for the efficient exploitation of deep geothermal energy.