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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