Abstract
The study aims at evaluating the bond durability of a carbon microfiber (CMF)-reinforced alkali-activating calcium aluminate cement (CAC)/fly ash F (FAF) blend cementitious material adhering to carbon steel (CS) under stresses induced by a 350℃ heat-25℃ water cooling cycle. This cementitious material/CS joint sample was originally prepared in an autoclave at 300℃ under a pressure of 8.3 MPa. For comparison, two reference geothermal well cements, Class G modified with silica (G) and calciumaluminum phosphate (CaP), were employed as well reinforced with CMF. In the CAC/FAF blending cement systems, the CAC-derived cementitious reaction products preferentially adhered to CS surfaces, rather than that of FAF-related reaction products. CMF played a pivotal role in creating tough interfacial bond structure of cement layer adhering to CS. The bond toughness also was supported by the crystalline cementitious reaction products including sodalite, brownmillerite, and hedenbergite as major phases, and aragonite, boehmite, and garronite as minor ones. The brownmillerite as an interfacial reaction product between cement and CS promoted the chemical bonding of the cement to CS, while the other phases served in providing the attractive bonding of the cement to CS. The post-stress-test joint samples revealed the formation of additional brown-millerite, aragonite, and garronite, in particular brownmillerite as the major one. The combination of chemical bonding and self-advancing adherence behavior of the cement was essential for creating a better interfacial bond structure. A similar interfacial bond structure was observed with CaP. The crystalline phase composition of the autoclaved cement revealed apatite, zeolite, and ferrowyllieite as major reaction products, and aragonite and al-katoite as the minor ones. Ferrowyllieite was identified as cement/CS interfacial reaction product contributing to the chemical bond of cement, while the other phases aided in providing the attractive bond of cement. After a stress test, two phases, ferrowyllieite and aragonite, promoted the self-advancing adherence of cement to CS. However, the effectiveness of these phases in improving adherence performance of cement was less than that of CAC/FAF blend cement, reflecting the fact that the bond durability of CAC/FAF blend cement was far better than that of the CaP. In contrast, the autoclaved silica-modified G cement consisting of xonotlite, and 0.9 nm-to-bermorite and riversideite, with calcite as the crystalline reaction products, had no significant effect on improving the shear bond strength and the bond’s toughness. No interaction product with CS was found in the cement adhering to CS. After a stress test, the calcite phase acted only to promote the self-advancing adherence of cement, but its extent was minimal compared with that of the other cements, thereby resulting in poor bond durability.
Highlights
One of the principal challenges in ensuring the integrity of geothermal wells is the stability and integrity of the cement sheath surrounding the carbon-steel casings during conditions of thermal shock
The study aims at evaluating the bond durability of a carbon microfiber (CMF)-reinforced alkali-activating calcium aluminate cement (CAC)/fly ash F (FAF) blend cementitious material adhering to carbon steel (CS) under stresses induced by a 350 ̊C heat-25 ̊C water cooling cycle
The highest strength of 3.55 MPa was obtained at the interfaces between the 60/40 ratio and CS. This value declined to 3.04 and 1.2 MPa when the proportion of CAC to FAF was decreased from 60/40 to the 40/60 and 20/80 ratios, suggesting that CAC preferentially served in developing the interfacial bond strength, rather than did FAF. When all these cements were fortified with CMF, their bond strength rose over range of as high as 42% to as low as 14%
Summary
One of the principal challenges in ensuring the integrity of geothermal wells is the stability and integrity of the cement sheath surrounding the carbon-steel casings during conditions of thermal shock. At the hydrothermal temperatures of 200 ̊C and 300 ̊C, this cement formed three major crystalline phases, 1) hydro-garnet including katoite [Ca3Al(OH)6] and hydrogrossular [Ca3Al2Si2(OH)4], 2) fledspar minerals, including dmisteinbergite and anorthite [CaAl2Si2O8], 3) hydro-ceramic including analcime [NaAl(Si2O6)(H2O)] calcium A-type zeolite [Linde A, Ca6(AlSiO4)13∙30H2O], hydroxysodalite [Na4Al3Si3O12(OH)], and Na-P-type zeolite (Na3Al3Si5O16∙6H2O), and 4) hydro-Al oxide including gibbsite [Al(OH)3] and boehmite (γ-AlOOH) These phases were responsible for its good thermal- and hydrothermal-stabilities at >300 ̊C, and the retention of compressive strength of >6.89 MPa after five superheating-cooling cycles (one cycle: 500 ̊C heat for 24 hrs and 25 ̊C waterquenching). Of micro-cracks, conferred resistance to the development of cracks, arrested the cracks’ propagation, as well as enhanced the post-cracks’ ductile performance Based upon this information above, the objective in this study was to assess the adhesive behaviors and bond durability of the 300 ̊C-autoclaved CMF-reinforced TSRC to carbon steel (CS) casing before and after the 350 ̊C heating25 ̊C water cooling cycle test to generate interfacial tensional stress in the cement sheath samples surrounding the CS tubes. Two reference cementitious materials, Class G modified with crystalline silica cements and calcium aluminate phosphate cements, were evaluated in the same manner
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