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.

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Subsurface porous aquifers are being considered for use as reservoirs for compressed energy storage of renewable energy. In these systems, a gas is injected during times in which production exceeds demand and extracted for energy generation during periods of peak demand or scarcity in production. Current operational subsurface energy facilities use salt caverns for storage and air as the working gas. CO2 is potentially a more favorable choice of working gas where under storage conditions CO2 has high compressibility which can improve operational efficiency. However, the interaction of CO2 and brine at the boundary of the storage zone can produce a chemically active fluid which can result in mineral dissolution and precipitation reactions and alter the properties of the storage zone. This study seeks to understand the geochemical implications of utilization of CO2 as a working gas during injection, storage and extraction flow cycles. Here, reactive transport simulations are developed based on 7 h of injection, 11 h of withdrawal and 6 h of reservoir closure, corresponding to the schedule of the Pittsfield field test, for 15 years of operational life span to assess the geochemical evolution of the reservoir. The evolution in the storage system is compared to a continuously cyclic system of 12 h injection and extraction. The result of the study on operational schedule show that mineral reactivity occurs at the inlet of the domain. Furthermore, the porosity of the inner domain is preserved during the cycling of CO2 acidified brine for both systems.

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Geologic CO 2 sequestration in porous saline aquifers is a promising approach to reduce atmospheric concentrations of CO 2 and provide large scale CO 2 storage. Once injected, CO 2 will dissolve into the brine to create an acidic environment, resulting in dissolution of primary formation minerals. Released ions can reprecipitate as secondary minerals, including carbonate minerals which securely trap injected CO 2 . This mineral trapping is highly desirable as it is the most secure CO 2 trapping mechanism. Reactive transport simulations provide the opportunity to analyze the spectrum of factors that influence geochemical reactivity in the storage aquifer and understand which factors are most important for promoting mineral trapping. In this work, reactive transport simulations are leveraged to enhance understanding of the influence of varying aquifer properties on the overall CO 2 trapping potential. The aquifer properties considered here include porosity, permeability, depth, and carbonate composition. A controlled system of field scale simulations are carried out successively varying aquifer properties to understand the impact of each unique property on CO 2 sequestration. For each simulation, the amount of gaseous, aqueous, and mineralized CO 2 are tracked and compared. Simulations reveal that the considered aquifer properties impact the sequestration efficiency, defined as the rate at which the CO 2 injected into the aquifer is converted to aqueous or mineralized CO 2 . Based on the studied properties, the aquifer carbonate composition has the least impact on sequestration efficiency while the depth of storage has the largest.