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Geological Applications of Helium Ion MicroscopyAnalysis and
Visualization of the Pore Networks within Shale and Coal
Introduction
Geological materials may be charac-terized using a variety of
microscopy-based techniques (e.g., optical micros-copy, scanning
electron microscopy (SEM), atomic force microscopy (AFM),
transmission and scanning transmission electron microscopy
(TEM/STEM), etc.). These tools traditionally provide criti-cal
structure/property relationship in-formation. Currently, shale gas
and coal bed methane are gaining increased in-terest as energy
sources. Mercury injec-tion capillary pressure (MICP) [1, 2], gas
sorption [3], small angle x-ray scatter-ing (SAXS) [4], and small
angle neutron scattering (SANS) [4] data suggest that micro- to
nano-scale pores exist within shale [1, 2] and coal [3, 4]. Thus,
direct visualization of the pore networks lead-ing to gas transport
within shale forma-tions and coal deposits remains rele-vant to
fully evaluating gas storage and recovery issues associated with
these resources.
Transmitted and reflected light mi-croscopy studies allow large
areas of each material to be examined but are resolution limited
with respect to nano-scale features. Secondary and backscat-tered
electron (SE and BE) imaging stud-ies conducted in the SEM provide
wide area views of inhomogeneous surfaces but fail when handling
non-conductive materials. SEM studies on insulating ma-terials
require either the addition of a conductive coating which
obliterates fine surface details or lowering the acceler-ating
voltage which decreases resolu-tion. Thus, SEM-based analyses
provide minimal information in the critical nano-scale regime.
AFM is highly surface sensitive and can readily image
non-conductive mate-rials. AFM, however, requires that a fine tip
be rastered across a surface to pro-duce an image. Thus, the
surface must be relatively flat in order to effectively resolve
nano-scale features. Obtaining such a surface from a geological
mate-rial generally requires microtomy as var-
ious ion polishing and milling techniques yield non-ideal
surfaces for imaging small features (e.g., curtaining effects,
severe sloping, etc.). Microtomy, unfor-tunately, is a tedious and
time-consum-ing sample preparation process which ultimately yields
a surface that is typi-cally only a few square millimeters in size.
The small analysis area and lengthy sample preparation requirements
make AFM less than desirable for characteri-zation of highly
inhomogeneous geologi-cal materials.
TEM/STEM studies are capable of imaging nano-scale features.
However, sample preparation is notoriously slow, requiring that a
specimen be milled from the bulk material. In addition, TEM/STEM
samples are very small and thus are not suited to characterizing
large (centime-ter-scale) surface areas. Consequently, it becomes
important to utilize an analysis technique that has the ability to
image large areas of non-conductive materials at high resolution.
The only instrument meeting all these requirements is the Helium
Ion Microscope (HIM) [59].
Methods and Materials
The HIM is well documented in the litera-ture [59]. Similar to
the SEM, the HIM is designed to easily accommodate samples that are
several centimeters in diameter. Due to a variety of factors (e.g.,
probe characteristics, probe-specimen interac-tions, etc.),
however, the HIM produces SE images that are significantly more
surface sensitive than those generated by the SEM [1-5]. However,
unlike the SEM, the HIM mitigates charging without resorting to
coating the specimen or low-ering the accelerating voltage, steps
that decrease resolution. The HIMs electron flood gun provides
charge neutralization, enabling the direct examination of
non-conductive geological specimens while maintaining high
resolution.
Several samples are presented here to illustrate potential
application of HIM to geological specimens. Upper Devo-nian shales
of the Muskwa (2247.11 m)
and Middle Otter Park (2256.4 m) forma-tions represent strata
with demonstrated gas productivity from the Horn River Ba-sin [10].
The mineralogy of the Muskwa shale is 67% quartz and 19% clay,
while the Middle Otter Park shale contains 47% quartz and 33% clay.
K-feldspar, plagioclase, pyrite, calcite, ankerite and dolomite are
secondary minerals. The Muskwa shale contains 6.45 wt.% total
organic carbon (TOC) and has a total po-rosity of 5.75%. The Middle
Otter Park shale contains 6.16 wt.% total organic carbon (TOC) and
has a total porosity of 5.15%. Both samples are well into the gas
generative window with vitrinite re-flectance values of ~2.0 %Ro.
Pocahon-tas #3, one of the Argonne Premium coal reference samples,
is a low-volatile bitu-minous coal from Buchanan County, Vir-ginia
[11]. This coal is 86.7 wt.% carbon with a low H/C ratio of 0.586,
which is consistent with its measured mean vit-rinite reflectance
of 1.68%Ro. The sand-stone is from Teal Field, North Sea and
represents a formation where porosity has been preserved by the
precipitation of microquartz.
All sample surfaces investigated here are freshly cleaved just
prior to loading into the HIM. No additional sample prep-aration is
required. Secondary electron images (SEIs) are collected at an
accel-erating voltage of 35 keV with a beam current of 0.3 pA. The
use of an electron flood gun to mitigate charging effects is noted
in the figure captions.
Results
Organic Porosity in Shales
The Upper Devonian shales from Horn River Basin are
representative of the type of low permeability strata that
pro-duces gas upon fracturing. HIM readily images the mineral
fabric and, more im-portantly, the organic porosity and mi-gration
pathways that are key factors in determining gas productivity (fig.
13). The samples were sufficiently conductive that charge
neutralization was not nec-
-
essary. Conductive surfaces are bright while insulating areas
are dark. This is in contrast to SEM where insulating areas may be
bright due to charging.
Features that were previously seen in ion-milled SEM and STEM
studies of
other gas producing shales [1, 2, 12-15] are easily identified
in the HIM images. Most of the pores are found in intraparti-cle
organic matter that has a spongy ap-pearance (fig. 1). This
material, almost certainly solid bitumen formed from the thermal
alteration of residual petroleum, contains pores of varying size.
However, the smallest imaged pores are ~2 nm in diameter, which is
approximately dou-ble the best resolution seen in published SEM
images [14, 15]. HIM also provides a greater depth of field than
SEM and a three dimensional view of the pore net-work can be imaged
without the need of ion milling in dual beam SEM systems. HIM has
similar sample size capacity and field of view as SEM and imaging
both large and small pores over a large sur-face area is easily
done.
Large pores (fig. 2) are considered essential for significant
gas storage15, however, connectivity of these pores appears to be
restricted to relatively smaller pore throats (down to 2 nm in
di-ameter). These observations are consist-ent with MICP
measurements [1, 2].
While total pore volume associated with the organic matter
determines the amount of gas in place, the pore connec-tivity and
pore throat geometry deter-mines the permeability of the shale and
is a critical element for successful exploita-tion. Prior studies
have partially imaged the pore network using dual-beam in-struments
that combine focused ion (FIB) beam milling and SEM. A small volume
of rock, typically ~125 m3, is imaged by backscattered electrons
after ion milling the surface. Sequential images, as many as 500,
are then used to reconstruct the network of organic matter or pores
[15]. The technique has provided great insight into the factors
that control permeability but is time consuming, is limited to
pores >5 nm in diameter, and may have arti-facts imposed by the
ion milling.
HIM offers the prospect of rapidly screening for relatively
larger scale mi-gration pathways. For example, a con-tinuous pore
network is imaged in a Muskwa shale that spans ~2 m in length (fig.
3a). HIM demonstrates that this fea-ture is a continuous network of
pores that form a pathway ~20-50 nm in width (fig. 3c, d), embedded
within the spongy organic matter with the characteristic ~2 to 20
nm diameter pores that connects with larger pores (fig. 3d, e).
Based on these geometries, we inter-pret this feature to be a
relic from when the source rock generated and expelled oil. During
the process of thermal matu-ration, not all generated petroleum was
expelled. The residual bitumen filling the expulsion pathway was
converted into solid organic matter that generated gas upon further
thermal alteration. Note that these geometries are not consist-ent
with mechanically induced fracturing during sample preparation.
In places, the pathway and the spongy organic matter is
partially obscured by a thin layer of clay (fig. 3c, d), This clay
layer may impede gas migration. How-ever, small microcracks
observed along the organic-mineral interface may result in
macroscale connectivity that allows for the rocks to produce gas at
economic rates.
Coal Porosity
Pores in coals are characterized as macro (50 nm to ~50 m), meso
(5 to 50 nm) and micro (< 5 nm) [16]. The methods used to
characterize the micropore size distribu-tion are mostly indirect,
such as mercury porosimetry, nitrogen or carbon dioxide adsorption
isotherms or SANS and SAXS [3, 4], as SEM and AFM have difficulty
re-solving micropores < 5 nm in coals [17].
Fig. 1: HIM of Middle Otter Park Shale from Horn River Basin.
Gas is stored primarily in the spongy organic matter. Pores as
small as 2nm are imaged. Electron flood gun off.
Fig. 2: HIM of Muskwa Shale from Horn River Basin. Large pores
associated with spongy or-ganic matter account for most of the pore
volume available for gas storage. Electron flood gun off.
Fig. 3: Muskwa Shale from Horn River Basin. a. A pathway for gas
expulsion and transport (white arrows) can be traced spanning the
width of the image. b. e. The migration feature filled with the
spongy organic matter that forms an apparently connected network of
pores. Electron flood gun off.
c
e
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TEM can resolve coal micropores with difficulty [18, 19].
Micropore capacity and surface area decrease and then increase
with rank, passing through a minimum in the high to low volatile
bituminous coal ranks [20]. HIM analysis of the low volatile
bi-tuminous Pocahontas #3 coal found no evidence of micropores on
most sur-faces suggesting that these structures are mostly < 1nm
in diameter. However, a few domains of meso- and micropores were
observed (fig. 4). The larger pores are unexpected considering the
rank of the Pocahontas #3 coal.
Microquartz
Porosity typically decreases with depth in sedimentary rocks due
to compaction and cementation. Some sandstone res-ervoirs deviate
from these trends as fine coatings of clays or microquartz prevent
the growth of ordinary quartz cements [21-23]. Recently, a
combination of ad-vanced imaging techniques (high-reso-lution
secondary electron, backscattered electron, and transmission
electron mi-croscopy) revealed that quartz cemen-tation is
inhibited when a nanofilm of amorphous silica and a layer of
chalced-ony forms between the surface of detrital quartz grains and
the microquartz coat-ing [24]. The amorphous silica prevents
syntaxial growth of the quartz grain, while the microquartz adapts
the orien-tation of the underlying chalcedony with its c-axis
parallel to the grain surface. HIM imaging of a North Sea sandstone
shows microquartz covering the surfaces of detrital quartz grains
(fig. 5a). This im-aging was conducted without the need for any
sample preparation. The electron flood gun was used to prevent
charge ac-cumulation. In some locations, the inhi-bition of quartz
cementation by micro-
quartz is not complete and some quartz overgrowths are found
(fig. 5b). The pre-ferred growth orientation of the micro-quartz,
with their c-axis parallel to the surface of the underlying quartz
grain, is easily seen.
Conclusions
HIM has successfully characterized ma-ture shales, coals, and
reservoir rocks. Analyses are conducted without the need of ion
milling or application of conduc-tive coatings that may produce
artifacts. The high resolution and depth of field of-fered by HIM
provides detailed images of features that are difficult to see with
other microscopy methods. Nanoporous bitumen-rich regions that may
serve as places for gas storage and as pathways for gas transport
are readily observed throughout the high maturity shales. Pores as
small as 2 nm are observed in a low-volatile coal along with larger
meso-pores. Microquartz overgrowths, which can preserve porosity in
sandstones, are
easily imaged. These findings illustrate that HIM provides a new
tool in the na-noscale characterization of geologic samples that
are important in developing both conventional and unconventional
resources.
Acknowledgements
We would like to thank the management of ExxonMobil Research
& Engineering, Imperial Oil Ltd, and ExxonMobil Up-stream
Research for promoting this re-search and allowing for its
publication and to the entire team at Zeiss for the de-velopment
and commercialization of the Helium Ion Microscope.
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Authors
Chris E. Kliewer and Clifford C. WaltersExxonMobil Research and
Engineering Co., Annandale, NJ 08801, USAChuong Huynh, Larry
Scipioni and Danielle ElswickCarl Zeiss, Applications laboratory,
Peabody, MA 01960, USARene Jonk, Nicholas AustinImperial Oil,
Calgary, Alberta, Canada T2P 3M9Marsha W. FrenchExxonMobil Upstream
Research Co., Houston, Texas 77027, USA
ContactChris E. KliewerExxonMobil Research and Engineering
Co.Annandale, NJ 08801, [email protected]
