Two Pennsylvanian coal samples (Spr326 and Spr879-IN1) and two Upper Devonian-Mississippian shale samples (MM1 and MM3) from the Illinois Basin were studied with regard to their porosity and pore accessibility. Shale samples are early mature stage as indicated by vitrinite reflectance (R o ) values of 0.55% for MM1 and 0.62% for MM3. The coal samples studied are of comparable maturity to the shale samples, having vitrinite reflectance of 0.52% (Spr326) and 0.62% (Spr879-IN1). Gas (N 2 and CO 2 ) adsorption and small-angle and ultrasmall-angle neutron scattering techniques (SANS/USANS) were used to understand differences in the porosity characteristics of the samples. The results demonstrate that there is a major difference in mesopore (2−50 nm) size distribution between the coal and shale samples, while there was a close similarity in micropore (<2 nm) size distribution. Micropore and mesopore volumes correlate with organic matter content in the samples. Accessibility of pores in coal is pore-size specific and can vary significantly between coal samples; also, higher accessibility corresponds to higher adsorption capacity. Accessibility of pores in shale samples is low.
Shale is an increasingly important source of natural
gas in the
United States. The gas is held in fine pores that need to be accessed
by horizontal drilling and hydrofracturing techniques. Understanding
the nature of the pores may provide clues to making gas extraction
more efficient. We have investigated two Mississippian Barnett Shale
samples, combining small-angle neutron scattering (SANS) and ultrasmall-angle
neutron scattering (USANS) to determine the pore size distribution
of the shale over the size range 10 nm to 10 μm. By adding deuterated
methane (CD4) and, separately, deuterated water (D2O) to the shale, we have identified the fraction of pores
that are accessible to these compounds over this size range. The total
pore size distribution is essentially identical for the two samples.
At pore sizes >250 nm, >85% of the pores in both samples are
accessible
to both CD4 and D2O. However, differences in
accessibility to CD4 are observed in the smaller pore sizes
(∼25 nm). In one sample, CD4 penetrated the smallest
pores as effectively as it did the larger ones. In the other sample,
less than 70% of the smallest pores (<25 nm) were accessible to
CD4, but they were still largely penetrable by water, suggesting
that small-scale heterogeneities in methane accessibility occur in
the shale samples even though the total porosity does not differ.
An additional study investigating the dependence of scattered intensity
with pressure of CD4 allows for an accurate estimation
of the pressure at which the scattered intensity is at a minimum.
This study provides information about the composition of the material
immediately surrounding the pores. Most of the accessible (open) pores
in the 25 nm size range can be associated with either mineral matter
or high reflectance organic material. However, a complementary scanning
electron microscopy investigation shows that most of the pores in
these shale samples are contained in the organic components. The neutron
scattering results indicate that the pores are not equally proportioned
in the different constituents within the shale. There is some indication
from the SANS results that the composition of the pore-containing
material varies with pore size; the pore size distribution associated
with mineral matter is different from that associated with organic
phases.
1 This article will form part of a virtual special issue on advanced neutron scattering instrumentation, marking the 50th anniversary of the journal.Oak Ridge National Laboratory is home to the High Flux Isotope Reactor (HFIR), a high-flux research reactor, and the Spallation Neutron Source (SNS), the world's most intense source of pulsed neutron beams. The unique colocalization of these two sources provided an opportunity to develop a suite of complementary small-angle neutron scattering instruments for studies of largescale structures: the GP-SANS and Bio-SANS instruments at the HFIR and the EQ-SANS and TOF-USANS instruments at the SNS. This article provides an overview of the capabilities of the suite of instruments, with specific emphasis on how they complement each other. A description of the plans for future developments including greater integration of the suite into a single point of entry for neutron scattering studies of large-scale structures is also provided.
The effect of nanoparticles (NP) on chain dimensions in polymer melts has been the source of considerable theoretical and experimental controversy. We exploit our ability to ensure a spatially uniform dispersion of 13 nm silica NPs miscible in polystyrene melts, together with neutron scattering, x-ray scattering, and transmission electron microscopy, to show that there is no measurable change in the polymer size in miscible mixtures, regardless of the relative sizes of the chains and the nanoparticles, and for NP loadings as high as 32.7 vol%. Our results provide a firm basis from which to understand the properties of polymer nanocomposites.
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