Abstract
Effective heat transfer between the working fluid and subterranean rocks is essential for producing green and low-carbon geothermal energy. As the primary thermal conductive medium, cement has low thermal conductivity, leading to high thermal resistance and significantly reducing geothermal wells' efficiency. Therefore, high thermal conductivity cement has emerged as a widely anticipated new research area. The purpose of this research is to address the substantial harm of traditional carbon-based thermal conductive fillers to cement. A novel expanded graphite (EG)/epoxy resin (EP) composite additive (MEG) was designed to increase cement's thermal conductivity while preserving its mechanical strength and pumpability. Firstly, the physicochemical properties of MEG were revealed by FT-IR, UV-Vis, SEM, and TGA. Then, the applicability of MEG cement in adverse geological environments (high-temperature 60-100 ℃, high-mineralization 5-36% NaCl) was evaluated through simulated maintenance experiments. Finally, the hydration products and pore structure of MEG-cement were analyzed by XRD/FT-IR and SEM/MIP, revealing the thermal conductivity enhancing mechanism. The results showed that: 1) MEG uses ZDMA as a bridge to promote the ring opening and curing of EP, and is formed by strong cation -π interaction with EG. 2)After curing at 60-100 ℃, MEG-cement exhibits a significant increase (46.6-182.1%) in thermal conductivity within the optimal dosage range of 5-10%, fully meeting the requirements for compressive strength (10.4-21.7 MPa) and fluidity (19.3-21.2 cm) of cementing. In addition, MEG-cement maintained stable density and significant high thermal conductivity advantage in high-mineralization environments (5-36% NaCl), with an increase in thermal conductivity of 23.8- 54.1%. 3) The mechanism of MEG promoting heat transfer in cement is summarized as the enhancement of the hydration process and the production of C-S-H gels. C-S-H gels filled the gel pores and transition pores in the cement skeleton and formed a dense, high thermal conductivity network, which shortens the heat transfer path and thus greatly improves the thermal conductivity of cement. In summary, this study has successfully developed a MEG geothermal cement with independent intellectual property rights that provides reliable technical support for the efficient development of geothermal resources and has important engineering application value.