Investigation on the synergy enhancement effect and mechanism of consolidation/thermal conductivity of CNTs-composite cement

Abstract

Geothermal well is completed generally with a cemented casing across the heat source, while the existing casing cement generally suffered from insufficient performance to meet the demand of actual medium-deep geothermal well construction. In view of this, combining theoretical analysis and experimental research, a new type of carbon nanotubes (CNTs) composite cement for geothermal exploitation was successfully developed by introducing thermal conductivity fillers, heat stabilizing material, nanomaterial and additives to synergistically regulate the comprehensive performance of casing cement. The evaluation of the high-temperature rheology and stability of slurry, and the compressive strength and thermal conductivity of cement stone was carried out based on the normative means. And the results showed that cement slurry had excellent high-temperature performance. The 28d compressive strength (With the curing condition of 150 °C & 20.7 MPa) of cement stone reached 8.15 MPa and the thermal conductivity was stabilized at 2.236 W/(m·K), both of which were at a high level in similar studies. Finally, a combination of hydration process analysis (TGA and XRD), microstructure study (MIP and SEM) and heat transfer mode study (Energy spectrum scanning) was used to summarize the nano-composite cement consolidation/thermal conductivity mechanism. Analysis showed that CNTs could play the effect of "pore filling" and "bridging" hydration products, and the additives could participate in the hydration reaction and regulate the hydration process. In addition, the heat transfer mode of the cement matrix was in accordance with the theory of heat conduction path. The study results provide a new idea to improve the cementing quality of cement and optimize the heat extraction efficiency of geothermal wells from the perspective of material development.