Toughness Improvement of Geothermal Well Cement at up to 300<sup>°</sup>C: Using Carbon Microfiber

Abstract

This study aimed at assessing the usefulness of carbon microfiber (CMF) in improving the compressive-toughness of sodium metasilicate-activated calcium aluminate/Class F fly ash foamed cement at hydrothermal temperatures of up to 300˚C. When the CMFs came in contact with a pore solution of cement, their surfaces underwent alkali-caused oxidation, leading to the formation of metal (Na, Ca, Al)-complexed carboxylate groups. The extent of this oxidation was enhanced by the temperature increase, corresponding to the incorporation of more oxidation derivatives at higher temperatures. Although micro-probe examinations did not show any defects in the fibers, the enhanced oxidation engendered shrinkage of the interlayer spacing between the C-basal planes in CMFs, and a decline in their thermal stability. On the other hand, the complexed carboxylate groups present on the surfaces of oxidized fibers played a pivotal role in improving the adherence of fibers to the cement matrix. Such fiber/cement interfacial bonds contributed significantly to the excellent bridging effect of fibers, resistance to the cracks development and propagation, and to improvement of the post-crack material ductility. Consequently, the compressive toughness of the 85˚-, 200˚-, and 300˚C-autoclaved foamed cements reinforced with 10 wt% CMF was 2.4-, 2.9-, and 3.1-fold higher than for cement without the reinforcement.

Bond Durability of Carbon-Microfiber-Reinforced Alkali-Activated High-Temperature Cement Adhering to Carbon Steel

“…Thus, it must possess high resilience and toughness to survive under the harsh environment of geothermal wells. In an attempt to improve the compressive toughness of the TSRC, polyacrylonitrile (PAN) precursor-derived carbon microfibers (CMF), 7 -9 µm in diameter and 100 -200 µm long, was incorporated into it [10]. Consequently, CMF played the following five specific roles in improving cement's properties; 1) minimizing the shrinkage, 2) impeding the development of cracks, 3) arresting the cracks' propagation, and 4) enhancing the post-crack ductile performance.…”