Conclusions:
Our research shows a practical application of the ZTE MRI pulse sequence
in the characterization of sandstone and carbonate samples. The in-house
developed PCA is useful, showing a new method of data comparisonvia simultaneous comparison of porosity and the number of
clusters present at a given threshold. The glass beads sample confirmed
the accuracy of the workflow in an in-house coded Mathematica notebook.
This notebook was then applied to other porous samples such as Berea
Sandstone and various limestone samples. Connectivity patterns
throughout the samples were deduced from the PCA and could correlate
various features in the sample with specific threshold values. This
leads to an in depth comparison between non-fractured and fractured
samples, where each feature could be separately visualized and analyzed.
Unfortunately, resolution limitations do not allow for full connectivity
characterization as many small pores and pore throats are below the
resolution achieved through MRI, as confirmed by TD-NMR NMR
measurements. In general, the PDD obtained from the T2distribution can be considered a complement to MRI in pore structure
characterization. This is because the resolution limitations of MRI do
now allow visualization of the smaller pore sizes. In Figure 3A-3C, the
black dotted line indicates the resolution of the MRI technique
presented herein. From the results, it becomes clear that not all the
pore-size distributions can be resolved by the MRI pulse sequence used
in this work, therefore the connectivity deduced will not always be a
complete representation of the connected pore space. Results also
indicate that some of the Berea Sandstone pores should be visible in the
MRI images, but not the pore throats. Additionally, most of the large
pores in the Indiana Limestone should also be discernable, with a larger
volume of the small pores missing. Lastly, the Madison Limestone sample
shows that the bulk of its porosity lies in the large pores, which
should be resolved with MRI. TD-NMR confirms that the samples are
saturated and have a large range of pore sizes and that MRI is resolving
pores as opposed to noise or artifacts.
In view of the connection between the signal intensity and feature,
threshold choices represent scale sweeps, namely the threshold selection
serves to identify different pore groups in the samples.
Acknowledgements:
This work was supported as part of the Center for Mechanistic Control of
Water-Hydrocarbon-Rock Interactions in Unconventional and Tight Oil
Formations (CMC-UF), an Energy Frontier Research Center funded by the
U.S. Department of Energy, Office of Science under DOE (BES) Award
DE-SC0019165.
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