Impact of Pore Connectivity on Quantification of Mineral Accessibility in Sandstone Samples

Abstract

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.

Estimation of Mineral Accessible Surface Area from Mineral Abundance and Clay Content

“…The porosity of these samples varies from 13 to 35% (Table 1). 40,53,55,56 Available ASA data were also collected from the literature for the Paluxy 49 and Lower Tuscaloosa 47 samples. Mineral ASAs for the remaining seven samples were determined here, leveraging mineral accessibility analyses from Salek et al 53 and Ma et al 50 For the Baltic Sea sample, the mineral segmented map obtained from Ma et al 50 is analyzed again to determine the connected porosity and mineral accessibility considering pore connectivity.…”

“…Data from nine sandstone samples were selected for analysis in this work. This includes six sandstone samples where mineral abundances and accessibilities were analyzed via SEM imaging in our previous work . In addition, data from three samples from earlier literature are considered.…”

“…49 Mineral ASA reflects the surface area in contact with the reactive fluid. Mineral ASAs were computed by multiplying the average connected surface area determined from the analysis of the processed 3D X-ray nano-CT image for each sample by the mineral accessibility calculated from 2D SEM images for each phase determined in Salek et al 53 Mineral accessibilities were computed from 2D mineral maps based on processed SEM backscattered electron (BSE) images com-plemented with elemental maps from SEM energy-dispersive Xray spectroscopy analysis. In the processed mineral map, each mineral phase was depicted as a unique color.…”

“…Figure 1 shows the 2D processed mineral map for the Lower Tuscaloosa sandstone sample 53 where each color corresponds to a different mineral phase. The mineral abundances and accessibilities, determined from pixel counting, are given in Table 3.…”

“…47 This nanopore connectivity is special because it may make a mineral grain accessible to reactive fluid even if that phase is coated with clay, if the clay mineral has sufficient nanopore connectivity. 45,47,53 This nanopore connectivity can be identified and quantified via nanometer resolution Focused Ion Beam-SEM images of that mineral. 45,47 This information can be incorporated into the image analysis to account for nanopore connectivity when evaluating overall pore connectivity and mineral accessibility.…”