Characterizing
nanopore structure is one of the most important
factors in understanding gas storage and transport in shale reservoirs,
but remains a significant challenge. In this work, we combine the
benefits of Focused Ion Beam Scanning Electron Microscopy (FIB-SEM)
and Nuclear Magnetic Resonance Cryoporometry (NMR-C) to characterize
the nanopore structure of lower Paleozoic shales from Chongqing, southwestern
China. Mineral composition is qualified through X-ray Energy Dispersive
Spectroscopy (EDS) and Field-Emission Scanning Electron Microscopy
(FE-SEM). Voids in 2D micrographs are classified into types of meso-
and/or microfractures, inter particle pores (InterP pores), intraparticle
pores (IntraP pores), and pores in organic matter (OM pores). An Otsu
thresholding algorithm and an edge detection algorithm (in Avizo)
are used to segment OM, InterP, and IntraP pores and to establish
3D pore structure for pore-network modeling (PNM). The pore size distribution
(PSD) is measured via PNM and compared to the PSD recovered from NMR-C.
Results indicate that the small OM pores have favorable potential
for storing adsorbed gas because of their apparent interconnectivity
and higher porosity; conversely, InterP and IntraP pores are mostly
isolated and with low porosity. The peaks of the PSD, recovered from
PNM, are in the ranges 10–20 nm and 60–120 nm and are
in good agreement with the peaks of the PSD recovered from NMR-C.
This suggests that the NMR signals in these discontinuous ranges are
due to OM, InterP, and IntraP pores. This indirectly demonstrates
that the probe liquid in the NMR-C experiments (i.e., water) may indeed
enter the nanoscale pores in the shale during centrifugal saturation.
Comparison of the PSD from PNM and NMR-C also shows that NMR-C has
a higher sensitivity and accuracy in detecting nanopores. Combining
FIB-SEM and NMR-C is a promising technique in detecting and characterizing
the nanoporosity of shales.
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