Research on the Thermal Conductivity of Ultrafine Graphite and Carbon Fiber Reinforced Cement for Well Cementing
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摘要: 为降低地热井井筒与岩层之间的热阻，提高地热井取热能力，选用具有优异导热性能的石墨（SG）作为导热材料，并使用表面活性剂将其制成石墨分散液，并引入具有高长径比的碳纤维（CF）构建导热网络，协同提高水泥石导热性能。评价了石墨分散液-水泥浆体性能以及水泥石的力学性能和导热性能，采用X射线衍射分析（XRD）、热重分析（TG/DTG）、压汞法（MIP）和扫描电子显微镜（SEM）对水泥石的物相组成、孔结构、微观形貌进行了表征，探究其导热机理。结果表明，当水固比为0.51，掺入石墨和碳纤维后制备的水泥浆体性能满足工程性能，水泥石24 h抗压强度不低于17.0 MPa、7 d抗压强度高于25.0 MPa，且其导热系数能达到2.86 W/(m·K)。石墨的促进水化与碳纤维的抑制水化共同作用下高导热水泥石的C—S—H与CH的失重量为10.91%，略低于纯水泥石的11.04%。适量的石墨能细化水泥石孔径，降低水泥石孔隙率，而碳纤维会显著增大水泥石孔隙度，增多水泥石大孔数量，将二者混掺，水泥石孔隙率为36.95%，高于纯水泥石，但高导热水泥石中孔径大于70 nm的孔数量与纯水泥石相差较小，在水泥浆中掺入石墨与碳纤维能形成导热网络。Abstract: To reduce the thermal resistance between the wellbore of a geothermal well and the rock formation and enhance the heat extraction capacity of the geothermal well, this paper selects graphite (SG) with excellent thermal conductivity as the heat-conducting material, and uses a surfactant to prepare a graphite dispersion. High aspect ratio carbon fibers (CF) are introduced to construct a heat-conducting network, which cooperatively improves the thermal conductivity of the cement paste. The performance of the graphite dispersion-cement slurry, as well as the mechanical and thermal conductivity properties of the cement paste, were evaluated. The phase composition, pore structure and microstructure of the cement paste were characterized by X-ray diffraction (XRD), thermogravimetric analysis (TG/DTG), mercury intrusion porosimetry (MIP) and scanning electron microscopy (SEM), and the thermal conduction mechanism was explored. The results show that when the W/S ratio is 0.51, the performance of the cement paste prepared by adding SG and CF meets the engineering requirements. The 24-hour compressive strength of the cement paste is not less than 17.0 MPa, the 7-day compressive strength is higher than 25.0 MPa, and its thermal conductivity can reach 2.86 W/(m·K). Under the combined effect of SG promoting hydration and CF inhibiting hydration, the weight loss of C—S—H and CH in the high-conductivity cement Ppaste was 10.91%, which was slightly lower than 11.04% of the pure cement paste. Appropriate SG can refine the pore size of the cement paste and reduce the porosity of the cement paste, while CF will significantly increase the porosity of the cement paste and increase the number of large pores. When the two are mixed, the porosity of the cement paste is 36.95%, which is higher than that of the pure cement paste. However, the number of pores larger than 70 nm in the high-conductivity cement paste is not much different from that of the pure cement paste. Adding SG and CF to the cement slurry can form a thermal conductivity network.
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Key words:
- Geothermal well /
- Cementing paste /
- Thermal conductivity /
- Action mechanism
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表 1 水泥浆配方
水泥浆 水泥/
%USZ/
%G33S/
%XP-1/
%SG/
%CF/
%W/S M1 100 0.5 1.5 0.2 0 0 0.51 M2 100 0.5 1.5 0.2 11.5 0 0.51 M3 100 0.5 1.5 0.2 0 1.5 0.51 M4 100 0.5 1.5 0.2 11.5 1.5 0.51 
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表 2 水泥石导热系数及抗压强度测试结果
水泥石 导热系数/
W·(m·K)-1抗压强度/
MPaM1 1.110 25.83 M2 2.144 18.96 M3 1.399 33.61 M4 2.923 26.39
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表 3 水泥试样在不同温度段的质量损失
水泥石 不同温度（℃）下的质量损失/% 105～350 350～500 500～720 720～1000 105～1000 M1 5.70 5.34 2.96 6.72 20.73 M2 5.92 5.82 4.42 8.07 24.23 M3 5.23 4.99 3.56 7.61 21.39 M4 5.55 5.36 3.96 8.55 23.42
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