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Efficient determination of specific surface area of shale samples by a tracer-based headspace gas chromatographic technique Liang He, Tengfei Li, Xin-Sheng Chai, Hui Tian, Xian-Ming Xiao, and Donald G. Barnes Anal. Chem., Just Accepted Manuscript • DOI: 10.1021/acs.analchem.6b04235 • Publication Date (Web): 12 Dec 2016 Downloaded from http://pubs.acs.org on December 18, 2016
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Analytical Chemistry
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Efficient determination of specific surface area of shale samples by a
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tracer-based headspace gas chromatographic technique
3
Liang He, a Teng-Fei Li,
4
Donald G. Barnes c
b
Xin-Sheng Chai, a,* Hui Tian,
5
a
6
South China University of Technology, Guangzhou, China
7
b
8
Chinese Academy of Sciences, Guangzhou, China
b
Xian-Ming Xiao
b
and
State Key Laboratory of Pulp and Paper Engineering, c School of Environmental and Energy,
State Key Laboratory of Organic Geochemistry, Guangzhou Institute of Geochemistry,
9 10
ABSTRACT: :This paper reports on a novel method for determining the specific
11
surface area (SSA) of shale by headspace gas chromatography (HS-GC). The method
12
is based on the water adsorption on the surface of shale sample after achieving phase
13
equilibrium at an elevated temperature; i.e., heating at 125oC for 48 h. A mathematical
14
model shows that the SSA can be determined from the signal of the vapor water
15
released during HS-GC analysis. The results obtained by this method correlated well
16
(R2 = 0.992) with data obtained by the reference BET method. Because the phase
17
equilibrium step for multiple samples can be conducted simultaneously and because
18
the phase re-equilibrium step is much faster in the HS-GC measurement, the present
19
method is more efficient for batch sample testing.
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With the commercial success in drilling and hydraulic fracturing techniques,
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shale gas has become an important energy resource in North America.[1, 2] It also has
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caught the attention for other countries, such as China, India and Australia, that have
24
potential for such unconventional gas reserves.[3, 4] In order to more accurately
25
estimate the size of these reserves, many predictive mathematical models have been
26
developed, that depend upon a variety of parameters of shale samples,[3, 5, 6]
27
including total organic content (TOC), porosity, and specific surface area (SSA).[7]
28
For many years the SSA of solid samples was often measured by using the
29
mercury injection capillary pressure (MICP) technique,[8, 9] in which the mercury is
30
gradually forced through the porous shale at pressures between 0.2 and 400 MPa. By
31
measuring the decrease of mercury volume at the corresponding injection pressure,
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the SSA can be calculated by the Young-Laplace equation.[10] However, the MICP
33
method is not suitable for measuring the SSA in very small pores, because of the
34
practical limit on the maximum injection pressure. The minimum measurable
35
diameter of pores by the MICP method is ~ 3 nm [8, 9] while pores in shale rocks
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range from 0.7 to 100 nm [11, 12]). Moreover, the mercury toxicity and the
37
concomitant contamination of shale samples during the SSA measurement are also
38
disadvantages of this technique.
39
Recently, based on an advanced high-precision pressure sensing system, the
40
Brunauer-Emmett-Teller adsorption method (BET) can now perform the SSA test on
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samples with very small pore sizes (down to 0.4 nm) and has been widely used in
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shale gas reservoir assessments.[13-15] The method is based upon the physical
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adsorption of gases (usually N2 or CO2, referred to BET-N2 or BET-CO2) on the
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external- and internal-surfaces of a porous material.[15] Before the BET test, it is
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necessary to completely remove the gaseous impurities from the sample by a
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degassing procedure in a vacuum chamber. During the testing, N2 (or CO2)
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adsorption–desorption isotherm measurements must be performed in ultra-cold tubes
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using the liquid nitrogen (or drikold).
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adsorbed at the equilibrium pressure, the SSA of the samples can be calculated from
50
the BET equation.[16] Unlike the MICP method, the BET method doses not pollute or
51
destroy the shale sample during the testing. However, in addition to the sample
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degassing procedure, the BET method suffers from being very time-consuming in
53
order to obtain the adsorption isotherm. For example, it typically takes about 8 ~12 h
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to test only one shale sample by the BET method.
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that can efficiently determine the SSA in the solid sample is highly desired in many
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surface adsorption-related research studies and practical applications.
By measuring the volume of N2 (or CO2)
Therefore, an alternative method
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Headspace gas chromatography (HS-GC) is an effective technique for analyzing
58
the volatile species in complicated matrices and has been successfully applied in
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many situations.[17-19] Recently, we have developed a volatile-tracer based HS-GC
60
technique to explore several new applications; e.g., the determination of the viscosity
61
of a thick liquid and the solubility of inorganic or organic compounds in water.[20-23]
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These techniques are based on the interaction behaviors (e.g., solubility, absorption or
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adsorption) between the spiked volatile-tracer substance and the analyte, which results
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in the tracer content being released into the headspace.
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In this study, we demonstrate the use of HS-GC to determine the SSA in shale
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samples, in which water is used as a tracer species. The main focus of the work was to
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establish the proof-of-concept for the methodology and to optimize conditions (i.e.,
68
equilibrium time in oven, headspace equilibrium time, and the sample size during
69
analysis) for its use. The results of this method correlated well with the results of the
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conventional BET method. Since multiple samples can be equilibrated in the oven in a
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single batch, the HS-GC method results in increased time-efficiency.
72 73 74
EXPERIMENTAL SECTION Samples.
Fourteen shale samples were collected from three shale gas reserves
75
located in southwestern of China. The mineral and total organic content (TOC) were
76
determined by X-ray diffraction analyzer (Bruker D8 Advance, Germany) and
77
Carbon/Sulfur analyzer (LECO CS-200, USA) respectively [24], and the data are
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listed in Table 1. The samples were ground, screened to 40 - 60 meshes, placed in
79
vials, dried at room temperature, and sealed for storage prior to analysis.
80
Apparatus and operations.
All HS-GC measurements were performed with
81
an automatic headspace sampler (DANI HS 86.50, Italy) and a GC system (Agilent
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7890A, USA) equipped with a capillary column (GS-Q, 30 m × 0.53 mm, J&W
83
Scientific, US) operated at a temperature of 105°C with nitrogen carrier gas at the
84
flow rate of 3.1 mL/min and using a thermal conductivity detector (TCD) operated at
85
220 °C.
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sample equilibration at a temperature of 105°C for 12 min, vial pressurization for 0.2
The headspace operating conditions were as follows: gentle shaking for the
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min, and sample loop (3 mL) filling time of 0.2 min.
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A Micromeritics ASAP 2020M apparatus was used for the SSA measurement as the
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reference method. The Brunauer–Emmett–Teller (BET) equation was applied to N2
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adsorption isotherms to derive the total specific surface area [24].
91
Preparations and procedures.
SSA Measurement Using HS-GC.
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0.20±0.01 gram of shale (oven-dried at 150oC for 24 h) with the particle size in the
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range of 40 − 60 mesh and 25 µL of water were placed in a small tube (1 mL) which
94
was subsequently placed into a headspace sample vial (20 mL). The vial was
95
immediately sealed with a PTFE/silicone septum and an aluminum cap.
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heating in an oven for 48 h at 125oC, the vial was transferred to the headspace
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auto-sampler for automated HS-GC analysis.
98
SSA Measurement Using BET-N2 method.
After
In this study, the nitrogen
99
adsorption isotherms were obtained at 77K (-195.8℃) on an accelerated surface area
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and porosimetry system (ASAP 2020M, Micromeritics Instrument). The oven-dried
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samples (40-60 mesh) were degassed under high vacuum ( 100 oC), the pressure created by the water
148
vapor in the headspace vial can hasten its adsorption on the solid surface of the
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nano-scale pores in the shale sample.
150
As noted above, the VSE equilibration of water tracer was conducted at 125oC in
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order to create a high pressure. Based on the ideal gas law, a pressure of 0.05 MPa can
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be achieved from 5 µL of water or a pressure of 0.23 MPa from 25 of µL of water in
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the 20-mL sealed headspace vial at 125 oC. Fig. 1 shows the effect of equilibration
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time on the vapor water mass transfer into the surface of shale pores. The vapor
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content of the water (spiked) in the vial decreases during the process until reaches at a
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plateau (i.e., the VSE) at ~ 30 h, indicating that the adsorption for the water vapor on
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the surface of the nano-scale pores is a very slow process.
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similar to what is observed in the BET test.
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phase equilibrium, we chose 48 h as the equilibration time for the sample preparation.
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This phenomenon is
Therefore, in order to ensure a complete
Effect of the amounts of shale and water.
Fig. 2 shows the changes in the
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GC signal associated with different amounts of water and shale.
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of water, the GC signal for the water vapor decreases linearly with the size of the
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shale sample.
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has a greater surface area, which means that more water can be adsorbed onto the
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sample surface. The figure also indicates that more water spiking is needed for larger
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sample weights.
167
desired in the testing.
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the following testing.
169
For a given amount
This observation reflects that fact that the greater shale sample size
In order to have good sample representative, a larger sample size is Therefore, we use 0.20 g shale spiked with 25 µL of water in
Conditions for HS-GC measurement. Water vapor measurement.
The
170
major interference to the measurement of the water vapor in the HS is the air
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(nitrogen and oxygen) enclosed in the headspace sample vial. Therefore, it is
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important to separate the water vapor signal from that of these two gases.
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shows the chromatogram in the sample measurement. Since nitrogen was used as the
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GC carrier gas, it is no signal in the GC measurement. The Figure shows that the
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water is well-separated from O2 and CO2 under the GC column operating conditions.
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Therefore, we can accurately determine the water vapor content under the GC
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conditions.
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Headspace equilibration.
To improve the method efficiency, the phase
179
equilibrium for the batch sample vials was conducted in an oven. After equilibration,
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the sample vials were transferred to headspace auto-sampler where a re-equilibration
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was required before the HS-GC measurement. As shown in Fig. 4, the re-equilibration
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is much quicker in that the GC signal of water leveled off after 10 min. Therefore, we
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selected 12 min as the re-equilibrium time in the following study.
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Method calibration and evaluation. calibrated
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measurement[13-15]. As shown in Fig. 5, there is the linear relationship between the
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reciprocal of GC signal [26] and the SSA determined by the reference method; i.e., it
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agrees to with the description of Eq. [26]. By linearly fitting the data in Fig. 5, the
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slope (a) and intercept (b) of Eq. (7) can be obtained. Evaluation.
the
SSA
data
determined
by
The present method was
185
190
using
Calibration.
the
reference
BET-N2
A set of shale samples (the calibration set was not included) were
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collected and their SSA were determined by the BET method [27] and the present
192
method, in which the factors in Eq. (6) were obtained by the above calibration. The
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major components of the samples are listed in Table 1. Fig. 6 shows the strong
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correlation (R2 = 0.992) between the data measured by these two different methods,
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indicating that the present HS-GC method can become an alternative method for batch
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measurement of the SSA of shale samples.
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Applications: predicting TOC content in shale rock by HS-GC coupled
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with XRD technique.
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related to the gas content and gas-generating potentiality of shale rocks.[28] It is
200
because the adsorbed-state gas is dominant and mainly stored on the surface of
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organic matter particles in shale rocks.[29] Traditionally, the TOC content in shale is
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determined by the carbon analyzer, in which the sample must be pretreated with 2.4
203
mol/L hydrochloric acid for more than 10 h (to completely remove carbonates)
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followed by washing with deionized distilled water and dried in an oven.[30]
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Obviously, such a test procedure is complicated and time-consuming, especially for
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the batch sample analysis. According to the literatures,[31, 32] the SSA (or porosity)
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of shale samples has a positive correlation to the TOC content in shale rocks.
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However, it was found that such as correlation is very poor (as shown in Fig. 7) and
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thus cannot be used for quantitatively predicting the TOC content in shale sample. We
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believe that the sample nature, e.g., the inorganic compositions, is also important
211
factor that affects the TOC content prediction. Therefore, it is necessary to introduce
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the inorganic composition variables in the predict model establishment.
The content of TOC is an important parameter that directly
213
Since there are multiple inorganic minerals in the shale rock sample, we applied a
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chemometrical software (i.e., SIMCA-P 11.5, Umetrics AB, Sweden) to the
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multivariate calibration. This can reduce the interference of noise and other irrelevant
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information from the variables through appropriate data-processing techniques and
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then establish the TOC prediction model by implicating the relationship between the
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variables and TOC content by partial least square regression (PLSR). [33] In Fig. 8, it
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shows a correlation between the reference measured TOC content and the TOC
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content predicted by the model based on PSLR analysis.[34] The square of the
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regression coefficient for the model is 0.939, indicating that the model can provide a
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good prediction for TOC content in shale sample. Because the proposed SSA test and
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XRD method is relative simple and efficient, the present model can be used for
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providing a quick estimation of the TOC content in shale samples.
225 226
CONCLUSIONS
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We have demonstrated the utility of a new HS-GC technique for determination of
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the SSA of shale samples which compares favorably with results obtained by the
229
BET-N2 method. Because samples can be more easily batched processed for analysis
230
with the new method, it has significant time efficiency advantages over the traditional
231
approach which are particularly important during the development of these resources.
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A PLS model for predicting the TOC content based on the inorganic compositions and
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SSA data of shale samples was also proposed.
234 235
AUTHOR INFORMATION
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Corresponding author
237
*E-mail:
[email protected]; Tel.: +86 20 87113713.
238
Notes
239
The authors declare no competing financial interest.
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ACKNOWLEDGEMENTS
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This study was jointly supported by the National Key Basic Research Program of
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China (973 Program: 2012CB214705) and the Natural Science Foundation of China
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(Project Nos. 21576105 and 51409287) for sponsoring the research.
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Tables and Figures Captions
348
Table 1 Compositions of shale samples
349 350
Fig. 1 Effect of time on water equilibration
351
Fig. 2 Effect of shale sample size on the GC signal of vapor water at equilibrium.
352
Fig. 3 Chromatogram for water detection
353
Fig. 4 Effect of time on the water re-equilibrium
354
Fig. 5 Relationship between the reciprocal of GC signals and the surface area of shale
355
samples measured by BET-N2 method.
356
Fig. 6 Correlation between the SSA data measured by the HS-GC and BET methods
357
Fig. 7 Relationship between the SSA and TOC content in the shale samples
358
Fig. 8 TOC model prediction based on the inorganic compositions and SSA in the
359
shale samples
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Table of Contents
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368 369
Table 1
370
371
Sample
Mineral composition, %
no.
Quartz
Feldspar
Illite
Chlorite
Calcite
Dolomite
1
43.7
9.6
25.7
16.1
5
ND
2
39.4
12.2
31.4
10.4
5.1
3
49.1
10.7
19.3
9.6
4
65.7
8.2
22**
5
66.6
7.9
6
57.6
7
Pyrite
TOC, %
ND
1.35
ND
1.5
4.03
7.1
2.5
1.7
3.72
ND
4.1
ND
ND
8.49
19.8
5.7
ND
ND
ND
1.90
7.2
14
21.1
ND
ND
ND
5.20
26.4
15.4
26.5
23.1
6.2
ND
2.5
1.19
8
31.3
13.2
25.8
24.6
3.2
ND
1.8
0.75
9
28.0
11.9
18.1
26.8
14.6
ND
0.7
0.25
10
34.1
17.4
21.8
16.6
8.1
ND
2.0
2.96
11
47.0
19.9
22.5
ND
ND
6.8
3.8
2.02
12
44.7
22.3
18.7
ND
ND
6.5
7.8
4.53
13
83.2
3.4
6.4
ND
ND
4.3
2.8
7.17
14
83.1
6.1
7.6
ND
ND
0
3.2
8.52
15
54.5
14.1
20.3
ND
ND
6.2
4.8
5.85
*
-- Non-detectable;
**
-- illite/smectite mixture.
372
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Page 17 of 25
373 374 375
3500
VH2O = 5 µL mShale = 100 mg
3000
o
Equil. temp. = 125 C
2500
GC signal
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Analytical Chemistry
2000 1500 1000 500 0 0
376 377
10
20
30
40
50
60
Time, h Fig. 1
378 379 380 381 382 383 384 385 386 387 388
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70
80
Analytical Chemistry
389 390 391
8000 Water, µL 5 15
25
6000
GC signal
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Page 18 of 25
4000
2000 o
0
Equil. temp. = 125 C Equil. time = 48 h
0.0
392 393
0.4
0.8
1.2
Sample size, g Fig. 2
394 395 396 397 398 399 400 401 402 403 404
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Page 19 of 25
405 406 407
500
10000
o
Column temp. = 105 C
y = 293.74x + 50.757 (R2 = 0.9907) 8000
300
Peak area
400
GC signal
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Analytical Chemistry
H2O
6000
4000
2000
0
200
0
O2
5
10
15
20
25
30
The amount of water, µg
100 CO2
0 0
408 409
2
4
6
8
Retention time, min Fig. 3
410 411 412 413 414 415 416 417 418 419 420
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Analytical Chemistry
421 422 423
10000
8000
GC signal
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Page 20 of 25
6000
4000
2000 o
Equil. temp. = 105 C
0 0
424 425
5
10
15
20
Equilibration time, min Fig. 4
426 427 428 429 430 431 432 433 434 435 436
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Page 21 of 25
437 438 439
Reciprocal of GC signal
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Analytical Chemistry
2.5x10
-4
2.0x10
-4
1.5x10
-4
1.0x10
-4
5.0x10
-5
-6
-4
2
y = 1.953*10 x +1.355*10
R = 0.973
VH2O = 25 µL mShale = 200 mg o
Equil. temp. = 125 C Equil. time = 48h
0
5
10
15
20
Surface area, m2/g 440 441
Fig. 5
442 443 444 445 446 447 448 449 450 451 452
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Analytical Chemistry
453 454 455
25
HS-GC method, m2/g
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Page 22 of 25
20 y = 1.083x 2
R = 0.992
15
10
5
0 0
456 457
5
10
15
BET method,
20
m2/g
Fig. 6
458 459 460 461 462 463 464 465 466 467 468
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Page 23 of 25
469 470 471
10 Sample sources #1 well #2 well
8
TOC, wt%
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Analytical Chemistry
6
4
2
0 0
5
10
15
20 2
472 473
Specific surface area, m /g Fig. 7
474 475 476 477 478 479 480 481 482 483 484
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25
Analytical Chemistry
485 486 487
10 Sample sources #1 well #2 well
8
Predicted value
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Page 24 of 25
6
4
y = 0.9998x 2 R = 0.939
2
0 0
488 489
2
4
6
8
Measured value Fig. 8
490 491 492 493 494 495 496 497 498 499 500
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Page 25 of 25
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Analytical Chemistry
501 502
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