With 2-acrylamido-2-methylpropanesulfonic acid (AMPS) units on polymeric additive, additive showed high effectiveness used for oilwell cement. However, due to chemical absorption and chelation mechanism of AMPS units to Ca2+ hydrating cement particles, adding of AMPS type additives caused delay of cement hydration process. In this research, AMPS type fluid loss additive, named as FLA A additive, was studied for its hydration delay side effect to class G Portland cement. Furthermore, polyvinyl alcohol (PVA) polymer, modified by glyoxal and boric acid, called as PVAGB was used as a synergistic functional additive to AMPS type polymer fluid loss additive to research on hydration delay problem of AMPS type additive to cement and the improvement for the effectiveness of AMPS type fluid loss additive. When AMPS type additive showed functional drawbacks, with more disordered chemical absorption and chelation behaviors to Ca2+ hydrated cement particles rather than constituting a completed and superior fluid loss control system, and this kind of modified PVA polymer was utilized for making up its failure. New compound additive formula, PVAGB/FLA A fluid loss additive formula, was investigated, which showed superior and more stable fluid loss control ability, i.e. about 50 mL at 30°C and 108 mL at 80 °C with just 0.2 % BWOC (weight percentage by weight of cement) PVAGB and 0.5 %BWOC (weight percentage by weight of cement) FLA A addition. In addition, within 28-day curing period, cement samples showed a healthy compressive-strength development with no less than 28MPa after 7-day curing period rather than failure due to cement strength retrogression. With scanning electron microscope (SEM) analysis, PVAGB showed accelerating effect to cement hydration process, in which hexagonal plate Ca(OH)2 crystal and aggregated product of C-S-H gel were formed when compared with pure cement and cement with FLA A additive added.
This paper presents an experimental study to investigate the effects of compressive stress during the CO2 attack on wellbore cement under carbon capture and storage (CCS) conditions. Oil well cement samples were designed to be exposed to humid supercritical CO2 gas and CO2-saturated brine and simultaneously subjected to external compressive stresses with load levels of 0, 25%, 50%, and 75% of the ultimate compressive strength. Morphology changes were determined using phenolphthalein dye testing and scanning electron microscopy. Mineral changes were detected by X-ray diffraction. Relative compressive strength and gas permeability of exposed cement were analyzed. It is shown that the 25% stress level has little effect on degradation of cement while the applied compression load up to 50% increased the compactness of cement and finally slowed down the degradation rate. In contrast, a much higher compressive stress level up to 75% facilitated the generation and propagation of micro-cracks. The stress induced micro-crack finally caused a surge in CO2-rich fluids and then significantly accelerated the degradation rate of oil well cement. Findings from this study expanded the understanding of the integrity of oil well cement for CCS wells.
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