Characterization of microscale features and mineral distributions in rock samples can be facilitated non-destructively with imaging analysis. Scanning electron microscopy combined with backscattered electron and energy-dispersive spectroscopy is particularly valuable and can be utilized to identify minerals. Mineral segmentation coupled with quantitative image processing can yield mineral volume fractions and accessibility from these images. Prior estimates of mineral accessibility from images have improved the simulations of mineral reaction rates, but it is unclear how pore connectivity should be accounted for. This is further complex in samples with clay minerals where nanopores in clays need to be considered. Here, the impacts of different approaches to assess pore connectivity on quantification of mineral accessibility are considered for seven sandstone samples with varying composition. Mineral accessibilities are calculated by counting the interfacial pixels between the associated minerals and the adjacent pores from the 2D mineral segmented maps. Three types of accessibilities are considered: the first approach accounts for all the macropore space, the second approach considers only the 2D connected macropores, and the third approach includes the 2D connected porosity considering nanopores in clays. The observed variations in accessibility for most mineral phases are within 1 order of magnitude when nanopore connectivity is considered and thus not anticipated to largely impact the simulated reactivity of samples. However, greater variations were observed for clay minerals, which may impact long-term simulations (years). Larger variations in accessibility were also noted for carbonate minerals, but only some samples contained carbonate phases, and additional data is needed to assess the trends.

The presence of fractures in caprocks can pose increased risks in subsurface energy systems and processes like CO2 sequestration by introducing high-permeability leakage paths. Fracture apertures and permeability can be altered through mineral dissolution and precipitation reactions, but the reactive evolution of fractures is not well understood. In fractures, minerals that are otherwise inaccessible to reactive fluids can become exposed, resulting in mineral reactions unpredicted by bulk formation data. This work seeks to understand the relationship between mineralogy and fracture formation to enhance our understanding of reactive fracture evolution and CO2 leakage potential. Here, the mineral compositions of mechanically induced fracture surfaces in samples of the Mancos and Marcellus shales have been quantified and compared to those of the near-fracture matrices using imaging and bulk X-ray diffraction (XRD) data. In the Mancos shale, the concentrations of clay minerals are enhanced along fracture surfaces with respect to the bulk, and the fracture is most likely to form at kaolinite–kaolinite interfaces. Evaluation of the mineralogical spatial variability through cross-correlation analysis of the surrounding matrix in images of samples cut perpendicular to the fracture shows that clay is 16.7 times more likely to be present than carbonate minerals near the fracture surface. The high correlation persists roughly 200 μm into the surrounding matrix for the Mancos sample and implies that the fracture formed within a defined clay-rich lithofacies.

Geological sequestration of CO2 in deep saline formations is a promising means of reducing atmospheric CO2 emissions. Once injected, CO2 dissolves into formation brine, lowering pH and creating conditions favorable for mineral dissolution. Cations released from dissolving minerals may create conditions favorable for secondary mineral precipitation, which can result in the long-term mineralogical trapping of injected CO2. These reactions may alter the natural rock mechanical properties, which can affect the safety and efficiency of geological sequestration. This work aims to investigate the impact of mineral composition and distribution on the mechanical properties of porous media. In this study, the mineralogy, mineral distribution, and mechanical properties of samples from Escambia County, AL, are evaluated. The mechanical properties of the rock samples are evaluated using the unconfined compression and indirect tensile tests in the combination with digital image correlation. The mineral composition and distribution are determined through the analysis of scanning electron microscopy backscattered electron and energy dispersive X-ray spectroscopy images of thin sections. These analyses showed that the mechanical properties vary with composition, which may have significant practical consequences for geological sequestration of CO2.

The mineral reactive surface area is often quantified through a wide range of approaches (e.g., Brunauer−Emmett− Teller adsorption, geometry approximation, and imaging techniques). As such, values vary 1−5 orders of magnitude which can result in large discrepancies when used in reactive transport models to simulate geochemical reaction rates. Simulations carried out using mineral accessible surface areas (ASAs) determined from a coupled 2D and 3D imaging approach have shown better match with reaction rates measured in core-flood experiments. However, such image processing requires large amounts of time and resources. In this work, the possibility of estimating mineral ASAs from easily measured properties like mineral abundance and porosity is explored. Six sandstone samples of varying compositions were studied along with data from three additional samples from the previous literature. Mineral ASAs were quantified using a combined 2D scanning electron microscopy and 3D X-ray nano-computed tomography imaging approach. Sample properties like mineral accessibility, mineral ASAs, connected porosity, and clay content were compared to explore potential correlations between properties. Overall, it was observed that mineral accessibility can be predicted where feldspar mineral accessibility generally increases with increasing abundance and quartz accessibility decreases with increasing clay content. Mineral ASAs vary between samples, depending on the relative abundance of minerals and overall pore connectivity. While the ASA of quartz decreases with abundance, albite and carbonate mineral ASAs increase with abundance. Quantitative observations, including predictive relationships for ASAs from porosity and mineral volume fraction, are developed. Estimations of ASAs and mineral accessibility from more easily quantifiable properties can largely reduce the required extent of image analysis.

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