Abstract
To more quantitatively and subtly analyze effects of carbonation on the pore structure of well cement by supercritical CO2 under carbon capture and storage (CCS) conditions, a digital scanning electron microscopy-backscattered electron (SEM-BSE) image analysis with a combination of nontoxic low-melting point metal intrusion is used to characterize the exposed cements to humid supercritical CO2 for 10 and 20 days. The porous area fraction (PAF) and pore size distribution (PSD) profiles obtained by slicing operation are used to describe the pore structure variation along the corrosion direction in a two-dimensional (2D) plane. The results show that the image-based method with the combination of metal intrusion is an effective method for characterizing the layer structure of exposed cement and getting quantitative information about the pore structure. From the surface to the core, the main altered layers in exposed cement for 10 days include the partially leached layer, the carbonated layer, and the calcium hydroxide (CH)-dissolved layer. For the exposed cement for 20 days, the main altered layers include the porous leached layer, the partially leached layer, the carbonated layer, and the carbonated transition layer. The nonporous carbonated layer can effectively block the flow parallel to the corrosion direction, while the porous leached layer can facilitate the flow perpendicular to the corrosion direction. Findings from this study will provide valuable information for understanding the effects of carbonation on the pore structure of well cement under CCS conditions.
Highlights
IntroductionFor a geologic carbon storage project, the wellbore is the key channel for both storing CO2 and monitoring its reservoir migration
Climate change is one of the biggest urgent challenges that human beings are facing today, which is mainly attributed to the increase in carbon dioxide (CO2) emission from fossil fuels.[1,2] Carbon capture and storage (CCS) remains the only technology solution atmosphere.[1−5] The to mitigate the core of CCS release of CO2 into the is to capture CO2 from power generation and industrial processes and to inject CO2 in geological formations for storage over centuries and thousands of years in a supercritical state.[1,6−8]For a geologic carbon storage project, the wellbore is the key channel for both storing CO2 and monitoring its reservoir migration
The relatively high brightness of unhydrated grains contributes to analysis of the mineral phase composition, hydration degree, and hydration process of cement by image analysis on the basis of the gray level.[41,42]
Summary
For a geologic carbon storage project, the wellbore is the key channel for both storing CO2 and monitoring its reservoir migration. The long-term integrity of wellbore is a critical issue for the success of using CCS.[2,9,10] Portland-based well cement, the main sealing material preventing fluid migration and providing mechanical support, is known to be susceptible to CO2 attack as illustrated in the following reaction steps, leading to degradation of wellbore integrity and potentially providing leakage pathways for CO2.11,12. Step 2: Carbonation of the hydration products Ca(OH)2(s) + 2H(+aq) + CO32(−aq) ↔ CaCO3(s) + 2H2O (2).
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