Figure 8. PCA analysis of Berea Sandstone. (A) Porosity and number of
cluster graph vs. threshold value plot, (B) PCA image, (C)
Histogram of cluster-size distribution, and (D) photograph of sample
(blue line indicates the FOV).
Cluster analysis was also performed on the non-fractured Indiana
limestone sample. Figure 9 shows a porosity and number of clustervs . threshold plot. The valleys are present at the threshold
values of 0.60 and 0.80, indicating where the sample is the most
connected. In this analysis, a porosity of 0.1477 was calculated at a
threshold value of 0.60. This porosity is similar to the measured
gravimetric porosity of 0.173 with a deviation of 14.8% (Table 3). The
cluster valley at the threshold value 0.60 (Figure 10A) is indicative of
the entire pore system, including small pores, and vugs (dissolution
pores). Whereas, the secondary valley (threshold 0.80, Figure 10B)
represents higher proton density areas, such as vugs, which do not
include the smaller pores. As seen in Figure 10B, the vugs have been
isolated, not only from the smaller pores, but also from one another, as
the threshold values for the bulk of the small pores have been put
outside of the range of interest. These vugs are the connection between
groups of smaller pores. Because the vugs have larger water volumes and
tend to be space apart through the sample, compared to the small pores
and throats in the Berea sample, they fall within the threshold limits
and are discernable from one another. Higher connectivity is observed in
spite of some of the pores and pore throats being below the resolution
limits, as seen by Figure 3B, leading to the conclusion that this sample
is dominated by the vug system. Figure 3B also shows that a majority of
the porosity is contained in the larger vugs, therefore confirming that
this sample is dominated by the vugs. Therefore, this sample is reliably
characterized with this workflow, showing relatively higher connectivity
when compared to other samples.