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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. 

ABSTRACT:This study examines the feasibility of carbon dioxide storage in shale rocks and the reliability of reactive transport models in achieving accurate replication of the chemo-mechanical interactions and transport processes transpiring in these rocks when subjected to CO2 saturated brine. Owing to the heterogeneity of rocks, experimental testing for adequate deductions and findings, could be an expensive and time-intensive process. Therefore, this study proposes utilization of reactive transport modeling to replicate the pore-scale chemo-mechanical reactions and transport processes occurring in silicate-rich shale rocks in the presence of CO2 saturated brine under high pressure and high temperature. For this study, Crunch Tope has been adopted to simulate a one-dimensional reactive transport model of a Permian rock specimen exposed to the acidic brine at a temperature of 100 °C and pressure of 12.40 MPa (1800 psi) for a period of 14 and 28 days. The results demonstrated significant dissolution followed by precipitation of quartz rich phases, precipitation and swelling of clay rich phases, and dissolution of feldspar rich phases closer to the acidic brine-rock interface. Moreover, porosity against reaction depth curve showed nearly 1.00% mineral precipitation occur at 14 and 28 days, which is insufficient to completely fill the pore spaces. 

ABSTRACT:This study examines the chemo-mechanical response of quartz-rich Mancos shale exposed to CO2-rich brine under two pressure conditions, 1000 psi and 1800 psi, at 100°C for 28 days. The objective is to investigate how pressure influences dissolution-precipitation reactions and their impact on mechanical properties. High-speed nanoindentation was utilized to generate spatially resolved mechanical property maps, providing micron-scale resolution of hardness and modulus variations across reacted and unreacted regions. These nanoindentation grids served as the basis for machine learning clustering, which was applied to identify chemo-mechanical transformations by correlating mechanical property variations with volume fraction data obtained from SEM/EDS maps. Results show that at higher pressure, increased dissolution and precipitation led to greater mechanical degradation, reducing hardness and promoting microstructural instability. At lower pressure, salt precipitation was more prominent near the reacted edge, influencing local mechanical properties. The analysis highlights the weakening of Si-O-Si bonds in quartz, altering its mechanical response. These findings enhance understanding of how CO2-induced reactions affect quartz-rich shale integrity, with implications for geomechanical stability. The integration of nanoindentation and machine learning provides a reliable framework for assessing pressure-dependent chemo-mechanical transformations, improving insights into dissolution-precipitation effects on rock stability and mechanical integrity. 

ABSTRACT:This study explores a rapid and precise method for micromechanical characterization and mapping of heterogeneous rocks, utilizing high-speed nanoindentation and mineral volume fractions. Traditional method for determination of mechanical properties of the different phases of a heterogeneous material require the combination of nanoindentation data and chemical analysis of the material. However, this results in increasing the cost, time, and complexity of the process. Hence, the proposed study explores different data mining techniques such as Uniform Manifold Approximation and Projection (UMAP) with k-means clustering, Dirichlet Process Mixture Model (DPMM) clustering, and Density-Based Spatial Clustering of Applications with Noise (DBSCAN), utilizing high speed nanoindentation data for an efficient and accurate evaluation of the micromechanical properties of heterogeneous shale rock. Comparison of the three techniques deduced that UMAP with k-means clustering technique provides appropriate micromechanical characterization and mapping results with a weighted error of about 13.40%. Even DPMM and DBSCAN performed reasonably well with slightly high weighted errors, therefore they can be adopted as a secondary clustering technique for validation of other clustering technique results. The results demonstrate the potential and efficiency of high-speed nanoindentation test in conjunction with data analytics for characterization and mapping of micromechanical properties of heterogeneous material. 

ABSTRACT:This study investigates the distinction between unreacted shale samples and those exposed to CO2-rich brine under elevated temperature (100°C) and pressure (1800 psi) conditions over 28 days. Samples underwent scratch testing under constant loading to ensure independent penetration depth, circumventing variability associated with load-dependent outcomes prevalent in progressive loading methodologies. Vertical hardness profiles revealed significant variations between reacted and unreacted regions, influenced by differential dissolution and precipitation characteristics, while horizontal hardness provided limited insights, particularly in the reacted region where higher tangential forces and deeper scratches indicated greater material compressibility. Distinct scratch path variations were observed, with fractures absent in the ductile reacted region at lower testing forces. The shale samples were sourced from the Eagle Ford Formation, providing insights into the mechanical response of carbonate-rich shale rocks in extreme environments. This research enhances understanding of shale's mechanical properties and material responses under diverse operational conditions, elucidating interactions with influential environmental factors, particularly in CO2-exposed scenarios. Conducted at a microscale level, this study offers detailed insights into material behavior, crucial for predicting long-term stability of geostructures exposed to reactive brine and potential CO2 leakage in subsurface reservoirs. 1. INTRODUCTIONThe investigation into chemical interactions between carbonate rocks and acidic brine is cruical for understanding complex mechanical and microstructural transformations essential for applications like geostructure stability, CO2 storage, and energy exploitation. Under elevated pressure and temperature conditions, the equilibrium between injected fluids and rocks undergoes alterations, leading to geochemical responses, especially with the presence of CO2 as a supercritical phase or in aqueous form (Prakash et al. 2023a; Prakash et al. 2022). In this context, investigating fracture properties becomes essential, aiming to comprehend the development and propagation of fractures within reacted formations to evaluate structural integrity and potential pathways for fluid migration.Prior geochemical investigations have explored the localized repercussions of CO2 attacks on rock permeability, shedding light on alterations attributed to carbonate precipitation sealing fractures and pores or the dissolution of diverse minerals (Burnside et al. 2013; Minardi et al. 2021). Shale rocks exposed to acidic brine predominantly undergo carbonate reactions, particularly carbonates dissolution and precipitation (Prakash et al., 2022; Prakash et al. 2023b). Experimental studies on fracture mechanics and mechanical properties have utilized conventional methods such as single edge notched bend, chevron notched beam, three-point bending, and semi-circular bending tests, acknowledging their inherent limitations (Smith & Chowdary, 1975; Bazant and Kazemi, 1990; Helmer et al. 2014; Dubey et al., 2020). 

ABSTRACT:The chemo-mechanical loading of rocks causes the dissolution and precipitation of multiple phases in the rock. This dissolution and precipitation of load-bearing mineral phases lead to the stress redistribution in neighboring phases, which in turn results in deformational changes of the sample composite. The aim of this study is to investigate the link between microstructural evolution and creep behavior of shale rocks subjected to chemo-mechanical loading through modeling time-dependent deformation induced by the dissolution-precipitation process. The model couples the microstructural evolution of the shale rocks with the stress/strain fields inside the material as a function of time. The modeling effort is supplemented with an experimental study where shale rocks were exposed to CO2-rich brine under high temperature and pressure conditions. 3D snapshots of the sample microstructure were generated using segmented micro-CT images of the shale sample. The time-evolving microstructures were then integrated with the Finite element-based mechanical model to simulate the creep induced by dissolution and precipitation processes independent of the intrinsic viscoelasticity/viscoplasticity of the mineral phases. After computation of the time-dependent viscoelastic properties of the shale composite, the combined microstructure model and finite element model were utilized to predict the time-dependent stress and strain fields in different zones of reacted shale. 1. INTRODUCTIONDetermination of viscous behavior of shale rocks is key in wide range of applications such as stability of reservoirs, stability of geo-structures subjected to environmental forcing, underground storage of hazardous materials and hydraulic fracturing. Short-term creep strains in hydraulic fracturing can change stress fields and in turn can impact the hydraulic fracturing procedures(H. Sone & Zoback, 2010; Hiroki Sone & Zoback, 2013). While long-term creep strains can hamper the reservoir performance due to the reduction in permeability of the reservoir by closing of fractures and fissures(Du, Hu, Meegoda, & Zhang, 2018; Rybacki, Meier, & Dresen, 2016; Sharma, Prakash, & Abedi, 2019; Hiroki Sone & Zoback, 2014). Owing to these significance of creep strain, it is important to understand the viscoelastic/viscoplastic behavior of shales.