Efficient Determination of Specific Surface Area of Shale Samples

Dec 12, 2016 - State Key Laboratory of Organic Geochemistry, Guangzhou Institute of Geochemistry, Chinese Academy of Sciences, Guangzhou, China. Anal...
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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,

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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

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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

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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

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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

44

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

46

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

48

using the liquid nitrogen (or drikold).

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adsorbed at the equilibrium pressure, the SSA of the samples can be calculated from

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the BET equation.[16] Unlike the MICP method, the BET method doses not pollute or

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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

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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.,

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equilibrium time in oven, headspace equilibrium time, and the sample size during

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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

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located in southwestern of China. The mineral and total organic content (TOC) were

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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

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vials, dried at room temperature, and sealed for storage prior to analysis.

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Apparatus and operations.

All HS-GC measurements were performed with

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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

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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

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was subsequently placed into a headspace sample vial (20 mL). The vial was

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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.

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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

161

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.

164

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.

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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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Fig. 3

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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

188

agrees to with the description of Eq. [26]. By linearly fitting the data in Fig. 5, the

189

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

196

measurement of the SSA of shale samples.

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Applications: predicting TOC content in shale rock by HS-GC coupled

198

with XRD technique.

199

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)

204

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

206

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.

208

However, it was found that such as correlation is very poor (as shown in Fig. 7) and

209

thus cannot be used for quantitatively predicting the TOC content in shale sample. We

210

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

212

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

218

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

220

content predicted by the model based on PSLR analysis.[34] The square of the

221

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

224

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

228

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

233

SSA data of shale samples was also proposed.

234 235

AUTHOR INFORMATION

236

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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Analytical Chemistry

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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

360 361

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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10

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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25

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

ACS Paragon Plus Environment

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

Table of Contents artwork here

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