Long-Term Investigation of the Chemo-Mechanical Alterations of Wellbore Cement on Exposure to CO2-Saturated Brine

Abstract This study evaluates the long-term alterations transpiring in wellbore cement on exposure to acidic environmental conditions. To achieve this, the microstructural, mechanical, and chemical processes that occur in wellbore cement subjected to CO2-saturated brine over a 30-year period are investigated, along with their impact on the cement properties. To investigate the evolution of microstructural and chemo-mechanical properties, wellbore field samples are analyzed using nanoindentation, scanning electron microscopy with energy-dispersive X-ray spectroscopy (SEM-EDS), and micro-computed tomography (CT) imaging techniques. Nanoindentation provides spatially resolved maps of elastic modulus and hardness, indicating the sample’s stiffness and strength over the reacted zones. SEM-EDS characterizes chemical composition and mineralogical changes in cement exposed to CO2-saturated brine for 30 years. Micro-CT imaging enables non-destructive characterization of pore structure evolution and reaction-induced alterations in cement phases. The results from this study demonstrate that the major cement phases affected by carbonation are calcium-rich, aluminum-rich, and silicate-rich phases. The mechanical strength and stiffness of the calcium-rich phases are observed to have increased near the interface between wellbore cement and the shale rock, due to carbonation over time. However, following the initial increase in mechanical properties, the mechanical strength and stiffness over distance away from the interface dropped significantly, representing the deterioration in the wellbore cement. Through long-term exposure, this study provides the first detailed analysis of chemo-mechanical alterations at the microscale, highlighting how such conditions lead to phase-specific changes in stiffness and strength. Additionally, micro-CT imaging revealed the dissolution and precipitation trends of cement phases along with the micro-porosity, offering insights into the progressive deterioration of wellbore cement. These findings are crucial for understanding long-term wellbore integrity in CO2 storage and subsurface applications. Leveraging wellbore cement that was exposed to CO2-saturated brine in the field over an extended period, this study presents the first detailed examination of the chemo-mechanical and microstructural evolution of individual cement phases.