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ORGANIC MATTER PORE CHARACTERIZATION IN GAS SHALE BY HIM .pdf


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ORGANIC M ATTER PORE CHARACTERIZATION IN GAS SHALE BY HIM
Jijin Yang1, Sheng Peng2, Bob Loucks2 and Stephen Ruppel2
1-Carl Zeiss Microscopy LLC, Thornwood, NY
2-Bureau of Economic Geology, Jackson School of Geosciences, The University of Texas at Austin, Austin, TX

Problem Statement and Objectives
Understanding the pore system in gas shale is crucial to determine where and how much
gas is stored, and what pathways the gas follows from small pores to fractured channels. SEM
and FIB-SEM imaging techniques enable the pores to be imaged at the nanometer scale
(Loucks et al., 2009, Curtis et al., 2012) and to be characterized as to pore type, size,
distribution, porosity and connectivity.
There remain many critical questions that are not well understood, such as: How big
are the smaller pores and are they connected? Are there more small pores that exist, but the
right tools are not being used to characterize them? Why do different techniques result in
different pore sizes? Is anything missing from the SEM or FIB-SEM characterization
techniques?
The goal for this work is to address some of the above questions by utilizing a new
imaging method on gas shale, particularly focusing on organic matter pores in Barnett shale.

Application
Results from this research on organic matter pores in Barnett shale will aid in
understanding the pore system and gas transport mechanisms in nanopores, evaluating the gas inplace within these pore networks, and eventually determining the reservoir quality of this particular
shale.

Methodology
Pore or pore networks in Barnett shale were imaged by using the Carl Zeiss Orion Plus
Helium ion microscopy (HIM) with spatial resolution of less than 0.5 nm. This technique was
used to image nanometer features even for non-conductive materials without coating (Terpstra et
al, 2013).
The work reported here is an another step of the integrated multi-scale characterization
method for highly heterogeneous gas shale, utilizing a range of techniques including industrial
CT, micro- and nano-CT, SEM, FIB-SEM, and HIM, with a resolution spectrum range from
mm to sub-nanometer.
Three Barnett shale samples from BEG-MSRL were chosen for imaging. The samples
were prepared by different approaches. Sample 1 was polished using Ar-ion milling machine on
one surface. This sample preparation method is routinely used for SEM imaging (e.g., Loucks et
al., 2009, Rine et al., 2013). Sample 2 was cross-sectioned using FIB-SEM (Carl Zeiss Auriga
crossbeam) to produce a smooth surface. This method has been used recently to get 2D and 3D
images of shale (e.g., Curtis et al., 2012, Driskill et al., 2013). Sample 3 was freshly broken either
along the bedding plane or perpendicular to the bedding plane. Slatt and O’Brien (2011)
prepared shale samples using freshly broken samples for their SEM imaging.
Sample 1 was sputter coated with 5 nm thick Pt on one third area of the Ar polished
surface, with two-thirds of the area of polished surface uncoated. The imaging areas from
both sample 2 and sample 3 were not coated.

Results and Discussion of Organic Matter Pore Imaging
The coated region from sample 1 was first imaged with HIM. Figure 1 shows a HIM
image from a coated area. At a 5 µm field of view, the pores can be easily recognized, and the
size of the pores range from tens to hundreds of nanometers. The shape of the pores suggests that
pores surrounded by organic matter are dominant, while a few pores with sharp corners might
form the space between mineral grains.
In order to determine if some smaller pores were missed at low magnification, some areas
were imaged at higher magnification. Figure 2 shows one image from an area with a 400 nm
field of view, showing a few cracks present on the imaging area. In addition, grainy structure
makes up the remainder of the surface. Measurement of the small grains indicated that the size is
about 5 nm, which suggests that the grain structure is from Pt coating. Further, there is one area
in Fig. 2 showing a crack from the Pt coating layer passing through a nanopore with a dimension
of about 5 nm in the horizontal direction and about 15 nm in the vertical direction. Since the
smooth sample was coated with 5 nm Pt, it is highly possible that most small pores 10 nm or less
in diameter have been covered by 5 nm Pt coating, and the size of the imaged pores in the coated
sample surface appear smaller than the real size. Therefore, working on coated sample can lead
to a calculation of a smaller porosity than reality, and an underestimate of the gas in-place for the
sample.
After realizing the potential for artifacts on the coated sample, the surface of samples 1
and 2 were ion milled (Ar ion milling machine or Ga FIB) with no conductive coating materials
applied after ion milling. Figure 3 shows a HIM image at 5 µm field of view of an organic area
with pore sizes of 20 nm and above. To see if there were smaller pores, higher magnification
images were taken. An image with 500 nm field of view (Fig.4) shows that few pores are in the
5-10 nm diameter range on the milled smooth surface. It is also noted from Fig. 4 that the inner
walls of the bigger pores are not smooth, but appear to be lined with grainy or fibrous structures.
Small pores in the 5-20 nm diameter range exist between those grainy structures.
A very high magnification with corresponding resolution is needed to reveal or count
the small pores. Figure 5 shows no visible pores in the two block areas of organic matter. The
field of view is 7 µm and is in the range for FIB-SEM work. However, if those areas are
viewed with the HIM at a higher magnification, there are many small pores, 5-15 nm in
diameter (Fig. 6). Those small pores can account for about 10% of the imaged area. Neglecting
those small pores could have impact on accurately evaluating the gas in place and transport
capability, especially on absorbed gas. It is very interesting to know if those small pores are
interconnected in three dimensions, since the small pores could play a significant role in a gas
transport mechanism.
Sample 3, the freshly broken sample, was imaged without conductive coating to explore
the pore structure in organic matter, and try to determine if there is interconnectivity. The pores
in the 50-200 nm diameter range are easy to distinguish. Figure 7 is an image showing organic
matter in the lower half of the sample, adjacent to the clay materials. A few larger nano- pores
are visible. At higher magnification (Fig. 8), many pores in the 3-10 nm size range become
visible. Though the image is in 2D, the extended depth of field of HIM as compared to SEM
allows for some 3D information, and the small pores seem interconnected. Pore Images from
other organic matter areas also indicate connectivity among small pores. Because of the large
amount of smaller pores in the organic matrix, the amount of absorbed gas there could make a
large contribution to the total gas in place evaluation.

It is interesting to note that pore size distribution can be different when determined with
different measurement techniques. Average pore throat diameter is 10 nm from mercuryporosimetery in Barnett (Bowker, 2009). The SEM method gives 20-100 nm, while the
capillary-pressure method gives 4-15 nm pore throat diameter. SEM based porosity is less than
that based on helium porosimetery (Loucks et al., 2009, Chalmers et al., 2012).
Though SEM and FIB-SEM have been very successful in characterizing the pore system
in shale under low kV or VP mode on uncoated shale samples, their capability to characterize
smaller pores down to the 3-5 nm size is still very challenging. The application of HIM imaging
technique to gas shale to directly reveal the small pores with 3-10 nm in diameter is another step
to further understand the structure of pore system. This new technique could potentially solve
the inconsistency of measurement related to the pore system in shale.

Reference
Bowker, K. A., 2009, Barnett shale gas production, Fort Worth basin: Issues and discussion.
AAPG Bulletin, v.91, p.523-533.
Chalmers, G. R., Bustin, R.M. and Power, I. M., 2012, Characterization of gas shale pore
systems by porosimetry, pycnometry, surface area, and field emission SEM/TEM image
analysis: Examples from the Barnett, Woodford, Haynesville, Marcellus and Doig units. AAPG
Bulletin, v.96, p.1099-1119.
Curtis, M. E., Sondergeld, C. H., Ambrose, R. J., Rai, C. S., 2012, Microstructural investigation
of gas shales in two and three dimensions using nanometer-scale resolution imaging. AAPG
Bulletin, v.96, p.665-677.
Driskill, B., Walls, J., DeVito, J. and Sinclair, S. W., 2013, Application of SEM imaging to
reservoir characterization of Eagle Ford shale. In Camp W., Diaz E., and Wawak B. eds.,
Electron microscopy of shale hydrocarbon reservoirs: AAPG Memoir 102, p.115-136.
Loucks, R. G., Reed, R. M., Ruppel, S. C., and Jarvie, D. M., 2009, Morphology, genesis, and
distribution of nanometer-scale pores in siliceous mudstones of the Mississippian Barnett Shale:
Journal of Sedimentary Research, v. 79, p. 848–861.
Rine J. M., Samrt, E., Dorsey, W., Hooghan, K., and Dixon, M., 2013, Comparison of porosity
distribution within selected North American shale units by SEM examination of Argon-milled
samples. In Camp W., Diaz E., and Wawak B. eds., Electron microscopy of shale hydrocarbon
reservoirs: AAPG Memoir 102, p.137-152.
Slatt, R. M. and O’Brien, N. R., 2011, Pore types in the Barnett and Woodford gas shales:
Contribution to understanding gas storage and migration pathways in fine-grain rocks. AAPG
Bulletin, v.95, p.2017-2030.
Terpstra, A.S., Shopsowitz, K.E., Gregory, C.F., Manning, A.P., Michal, C.A., Hamad, W.Y.,
Yang, J. and MacLachlan, M.J., 2013, Helium Ion Microscopy: A new tool for imaging novel
mesoporous silica and organosilica materials, Chemical Communications, v. 49, p.1645-1647.

Figure 1. HIM image showing the pores of different size on Pt coated surface.
Figure 2. HIM image showing the Pt coating grains and small pore covered almost covered by Pt
coating

Fig. 3. HIM image showing larger pores in organic matter in Ar-milled sample.
Fig. 4. Higher resolution HIM image showing larger pores and smaller pores. The smaller pores
located at milled surface and inner walls of larger pores.

Fig. 5. Low magnification image showing no pores in two blocks of organic matter located at
low half of the image.
Fig. 6. High magnification image showing plenty of smaller pores are visible at the same block
of organic matter in Fig.5.

Fig. 7. Low magnification image showing some pores visible.
Fig. 8. High magnification image of a small area located in the low right part of Fig.7. The small
and interconnected pores are visible in the organic matter matrix.


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