Surface Properties of Organic Kerogen in Continental and Marine Shale

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Interfaces: Adsorption, Reactions, Films, Forces, Measurement Techniques, Charge Transfer, Electrochemistry, Electrocatalysis, Energy Production and Storage

Surface Properties of Organic Kerogen in Continental and Marine Shale Shouceng Tian, Xiaoxiao Dong, Tianyu Wang, Rui Zhang, Panpan zhang, Mao Sheng, Shizhong Cheng, Hong Zhao, Ling Fei, Jason Street, Yusheng Chen, and Quan Xu Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.8b03151 • Publication Date (Web): 18 Oct 2018 Downloaded from http://pubs.acs.org on October 19, 2018

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Surface Properties of Organic Kerogen in Continental and Marine Shale Shouceng Tian,a Xiaoxiao Dong,a Tianyu Wang,a Rui Zhang,a Panpan Zhang,a Mao Sheng,a* Shizhong Cheng,a Hong Zhao,a Ling Fei,b Jason Street,c Yusheng Chen,d and Quan Xua* a

State Key Laboratory of Petroleum Resources and Prospecting, China University of

Petroleum-Beijing, 102249, China; Email: [email protected]; [email protected] b

Chemical Engineering Department, University of Louisiana at Lafayette, Lafayette,

LA 70504; c

Department of Sustainable Bioproducts, Mississippi State University, MS 39762,

USA; d

Department of Chemistry, University of Akron, Akron, OH, 44325, USA;

ABSTRACT: The adhesion energy of kerogen in continental and marine shale was innovatively discovered using the colloid probe technique with atomic-force microscopy (AFM). AFM results indicated that the adhesion force of kerogen was higher than the inorganic material in both the continental and marine shale samples. The chemical elements in the two kinds of samples were measured by energy dispersive X-ray analysis (EDX) with scanning electron microscopy (SEM). The chemical compositions of kerogen involved C=C bonding, C=O bonding, pyridine nitrogen and pyrrole nitrogen while the primary constituent involving inorganic matter was Si-O bonding. These results were confirmed by Fourier transform infrared spectroscopy

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(FTIR) and X-ray photoelectron spectroscopy (XPS). The high percentages of C=C and C=O bonding in kerogen are attributed to the large dipole on the kerogen surface which allowed kerogen to contain liquid and gaseous hydrocarbons.

Keywords:

Kerogen, Force Curve, Atomic Force Microscope

1. INTRODUCTION As the oil drilling industry continues to develop, gas exploitation from organic-rich shale has dramatically changed the universal economy and also involves environmental and political concerns.1 Shale is an intrinsically complex system and its gas content is affected by its composition, initial pore structure, and the concentration of the organic composite known as kerogen.2-5 Although kerogen is considered to be a composition of solid organic materials (OM) that control the shale’s geochemical and geomechanical properties, its initial chemical, mechanical, elastic properties and morphology remain unclear.6 Therefore, the ability to determine the gas adsorption characteristics of kerogen is important to understand the gas production aspects in a shale reservoir. On the nanoscale level, the pore size distribution of kerogen ranges from angstrom to micron sizes.6-9 Scientists have developed numerical methods to explore the internal details of kerogen using modeling technology. For example, Curtis et al. found that adjacent micron-sized OM grains differed greatly in porosity by using scanning electron microscopy (SEM),10and the organophilic pores with bubble-like shapes in kerogen dominated the organic-rich sample. Javadpouret al.

11

used atomic-force microscopy

(AFM) to measure the Young’s modulus on a sample, which opened new research


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frontiers for AFM applications in the field of reservoir engineering. The chemical composition of kerogen was first studied by nuclear magnetic resonance (NMR) and X-ray photoelectron spectroscopy (XPS) methods. 12-15 The data revealed that kerogens possessed on average 2- to 5-ring aromatic carbon units that were highly substituted. In addition, Fourier transform infrared spectroscopy (FTIR) provided the chemical composition and structural characteristics of kerogen.5,16-19 Yang et al.5 first used AFMbased infrared spectroscopy (AFM-IR) to measure the chemical and mechanical heterogeneity of OM in shale at the nanoscale, providing a microscopic image of the heterogeneous process of petroleum generation.5,17-18 Although great progress was achieved, the quantitative measurement of adhesion forces between kerogen and different surfaces is still unknown. This may impede the exploration of the absorption mechanism of CH4 and CO2 on kerogen surfaces. In this study, we first utilized colloid probe technology to quantitatively map the morphology and adhesion properties of kerogen on continental and marine shales using AFM. The kerogen adhesion forces were 300% and 130% higher than the continental and marine shale surfaces, respectively. XPS12-13 and FTIR16-19 results revealed C=O and C=C bonding were the main reasons for the ability of kerogen to absorb gas. Si-O stretching is thought to enhance the adhesion force. In addition, this adhesion force trend remained stable over various temperatures, further proving that chemical isolation was the main influence for kerogen’s ability to absorb CO2 and CH4. This aspect is essential for hydrocarbon production.20-22 These findings can help scientists and engineers understand the mechanism and chemical properties of kerogen.


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EXPERIMENTAL SECTION Sample preparation. Continental and marine shale samples were collected from the “Yanchang” formation and the “Longmaxi” formation. The samples were first polished for 4 h with sandpaper ranging from coarse to fine using a high energy argon ion beam (Leica EM RES 102) with a voltage of 4 kV and an angle of 8 o. Then, the sample was continuously polished for 3 h at the voltage of 3 kV and an angle of 6 o while under vacuum. The sample was then cleaned with pressurized air. The temperature of the sample was controlled to 40-50℃ during the process. The speed of specimen stage was 1.5 rpm. A photograph of final samples (8 mm×8 mm×2 mm) is shown as Figure S1. A cylindrical core sample with a diameter of 25 mm and a height of 30 mm underwent permeability and porosity tests. Permeability was measured with a nitrogen based test using a modified version of Darcy's law. The core was placed in the core holder, and the nitrogen gas passed through the core under a pressure difference applied on the two ends. The nitrogen flow rate was measured to obtain the permeability with the modified Darcy's law equation. Porosity of the samples were tested based on the equation of state of an ideal gas. Under high-pressure conditions, gas entered the pores of the rock and the pressure was reduced accordingly. The amount of gas entering the pores can was determined by the equation of state of an ideal gas to calculate the porosity of the rock.

Characterization of Materials. The morphology of the shale samples' surface (Figure S2) was measured using a scanning electron microscope (SEM, SU8010). To capture high-resolution images, the accelerating voltage was 5 kV and the emission


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current was 10 mA. SEM-EDX was used to create an elemental spectrogram to distinguish the elemental distribution in kerogen from continental and marine shale samples in Figure S3. Fourier Transform Infrared spectroscopy was carried out with a Bruker VERTEX 70 FTIR and NICOLET iZ10 module (Thermo Scientific). Fresh continental and marine shale powder samples were mounted on a metallic nub via nonconducting double-sided tape and analyzed with an X-ray photoelectron spectrometer (XPS, Thermo Scientific K-Alpha). The device was equipped with an automatic sample charge neutralization device to ensure uniform charge in the sample space. Chemical bonding forms and functional groups of the continental and marine shale samples were verified by FTIR and XPS. The FTIR analysis was performed on a nanoscale to analyze the texture heterogeneity of mineralogy, porosity and organic matter types in the shale.23 The chemical bonding of kerogen from the marine and continental shale was quantitatively studied by FTIR. The FTIR sample was pressed using a mixture of 10 mg potassium bromide and 0.1 mg of the kerogen. In order to fully understand the temperature effects on the properties of kerogen, the samples were measured at different temperatures from 30 to70℃ at intervals of 10℃. The chemical bonding was measured by XPS using 5 mg of the pressed samples. The adhesion force measurement was found using AFM (Dimension Icon AFM, Bruker Co., Inc.) with the colloid probe technique. A high-density carbon tip (spherical, 20 nm of radius) was used and diagram of the measurement technique is shown in Scheme 1. The AFM cantilever (FMR) used in this experiment was 225 μm in length, 28μm in width and 3 μm in thickness, with an average spring constant of 2.8 N/m and an average frequency of 75


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kHz. The tests were conducted with a retraction velocity of 1 μm∙s-1 and the room temperature and humidity were respectively controlled to 27±1℃and 20% to eliminate the temperature and humidity effect on the samples.

Scheme 1. Adhesion force measurement diagram using AFM.

2. RESULTS AND DISCUSSION Argon ion milling was required before the sample underwent SEM analysis. The change of physical and chemical properties on the surface of samples can be eliminated by controlling the voltage and temperature during the argon ion milling process. 24-27 For example, Sanei et al. suggested that initial heating caused by ion bombardment results in significant thermal alteration of the OM surfaces.24 Park et al. has suggested a simple engineering model for the formation of regular geometric ripples made by the serial cutting sequence of a focused ion beam.25 Bassim et al. has shown that beaminduced heating was not observed to play a critical role in the potential alteration of functional chemistry by controlling the parameters final milling voltage, temperature and ion beam overlap.26 Mastalerz et al. discovered that the temperature effects of the beam can be counteracted by using liquid nitrogen cooling and can be further ameliorated by using a highly conductive sample stage.27 SEM images of the interfaces between the inorganic rock and kerogen are shown in Figure 1. The elemental


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distribution was clearly observed via SEM-EDX mapping, where inorganic areas were rich in silicon and aluminide while kerogen surfaces were rich in carbon and oxygen in both the continental and marine shale samples. The permeability and porosity of the continental and marine shale are further summarized in Table 1. The permeability of marine shale was higher than that of continental shale. The marine shale had a better oil capture ability than the continental shale. The contact angle of the samples was tested and results are shown in Figure S4. These results were in agreement with a previous report that kerogen was rich in carbon28-29 and the kerogen was comprised of a wide range of other chemicals.30

(a)

C

O

Si

Al

C

O

Si

Al

rock kerogen 100 nm (b)

kerogen rock 100 nm Figure 1. SEM images of a) continental shale sample and b) marine shale sample with EDX mapping. Table1. Permeability, porosity, apparent density and skeleton density of continental and marine shale measured by a high pressure gas permeability (HPP) porosimeter. Permeability 2

(10 μm ) 0.00965

3

Continental

Porosity

Apparent density

Skeleton density

(%)

3

(g/cm )

(g/cm3)

2.44

2.689

2.7561


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shale Marine shale

0.01334

3.22

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2.514

2.5977

FTIR spectra of continental and marine shale samples are displayed in Figure 2. The peaks at 1093.6 cm-1 and 1020.3 cm-1 were attributed to Si-O stretching for the marine and the continental shale, respectively. This data is similar to the reported work that the spectra of silicates are characterized by Si-O stretching and bending vibrations at 1200800 cm-1 and 600-400 cm-1.23 Both continental and marine shale samples had Si-O-Si stretching at 798-780 cm-1. The marine shale had a much more pronounced O-H stretching result than that of continental shale at the peak at 3626 cm-1.The peaks at 1645.2 cm-1 in the marine shale sample and 1463.9 cm-1 in the continental shale were assigned to C=O or C=C bonding, corresponding to the conclusion that the kerogen in the shale samples offered a set of absorption bands between 1800-1000 cm-1 (C=O, C=C).31 To minimize the effect of temperature, we retested the FTIR of samples before and after the polishing process and results can be seen in Figure S5. The surface chemical bonding was constant both before and after polishing process, which revealed the surface chemical structure was maintained after the polishing process. It was expected that the polishing process would not introduce significant changes on the sample surface.


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Figure 2. FTIR spectra of marine and continental shale samples. Figure 3 shows the changes of transmittance of the continental and the marine shale samples at temperatures ranging from 30 to 70 ℃. Both the continental and the marine shale samples had Si-O stretching peaks at 979.7 and 995.1 cm-1, respectively. Si-O stretching was mainly contained in inorganic samples, which was confirmed by a previous EDX result (Figure 1). This result also matched well with a previous report 23 showing that Si-O bonding was observed in shale inorganic samples. FTIR spectra also indicated that the Si-O stretching was stable and did not shift as the surface temperature increased from 30 to 70℃.

Figure 3. FTIR spectra of Si-O stretching at temperatures ranging from 30 to70℃ a) continental shale sample; and b) marine shale sample. XPS was applied to further analyze the chemical component of the continental and the marine shale samples, including chemical bonding in the kerogen and the chemical


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element bonding energy (Figure S6). XPS analysis of the carbon (1s) spectra in Figure 4a for the continental shale sample indicated that there were three distinct peaks centered at 283.88, 284.5 and 292.88 eV. These peaks were assigned to C 1s, confirming the presence of C-Si, C-N and C-C. XPS analysis of the carbon (1s) spectra in Figure 4b for the marine shale sample showed that peaks at 283.78, 284.48 and 292.88 eV were attributed to C-Si, C-N and C-C, respectively. Although the major peak of C 1s was from C-Si, results indicated that there were more C-C bonds in the marine shale sample than the continental shale sample, implying the existence of a highly organic substance. The C-C bond in the marine shale also exhibited a narrower distribution than the continental shale sample. Both results indicated that the kerogen played an important role in changing the chemical/physical condition of the shale surface. Nitrogen forms in the continental and the marine shale samples were defined and quantified by XPS and results are shown in Figures 4c and 4d. Figure 4c shows that the continental shale sample peak at 401.18 eV was quaternary nitrogen, and the peak at 399.18 eV contributed to the amino or pyridinic nitrogen of the continental shale sample, which matched a previous report.12, 32 Figure 4d shows the marine shale sample peak at 400.98 eV was quaternary nitrogen while the peak at 399.18 eV corresponded to amino or pyridinic nitrogen.33 There was no substantial difference in the N composition between the continental and marine shale samples. XPS analysis of oxygen (1s) is shown in Figures 4e and 4f. Three peaks at 530.08, 530.98 and 531.78 eV in the continental shale sample represented Ca-O, O=S, and Si-O, respectively. Peaks at 530.18, 530.98 and 531.78eV in the marine sample represented Ca-O, O=S, and Si-O,


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respectively. The marine shale sample exhibited higher concentrations of Ca-O and SiO bonds when compared with the continental shale sample. The EDX mapping result in Figure 1 corresponds with the continental and marine shale samples having a differing elemental content and bond properties, such as Ca-O and Si-O. As previous research has shown,34 kerogen can use calcium hydride (Ca(OH)2) as a hydrogen resource for its metabolism, which can be one of the reasons for the high Ca-O content in the marine shale sample. XPS analysis of the silicon (2p) spectra for the continental and marine shale samples shown in Figures 4g and 4h were usually comprised of a low binding energy signal (101.98eV) in both samples and a higher binding energy signal (102.48 eV in the continental shale sample and 102.78 eV in the marine rock sample). The lower 101.8 eV and 102.8 eV binding energy signals referred to Si-C and Si-O bonds, respectively. There was a greater amount of Si-C bonds in the marine shale sample than in the continental shale sample, implying that the test depth influenced the interface between the inorganic and organic substances.


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Figure 4. (a, c, e, g) XPS spectra for carbon (1s), nitrogen (1s) oxygen (1s), and silicon (2p) spectra in the continental shale sample; (b, d, f, h) XPS spectra for carbon (1s), nitrogen (1s) oxygen (1s), and silicon (2p) in the marine shale sample.


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Figure 5 shows that the adhesion force of the continental and the marine shale samples was measured at a preloading force of 100 nN. The adhesion force of kerogen in both the continental and the marine shale samples was higher than that of the inorganic substance, indicating that the adhesion force of organic matter was higher than that of inorganic matter in shale sample. In the continental shale sample, the adhesion force of kerogen was 100.86±5 μN, which was 300% than the adhesion force of the inorganic material which measured 31.5±1.57 μN. In the marine shale sample, the adhesion force of kerogen was 94.28±4.7 μN, which was 130% higher than the adhesion force of the inorganic sample which measured 73.05±3.65 μN. When the preloading force was increased from 100 to 1000 nN, the adhesion force of the kerogen and inorganic material was unchanged, revealing the test occurred in the elastic range. Xu et al.35-37 reported that retraction speed and contact time affected adhesion force; therefore, adhesion forces at different retraction speeds and contact times at a constant preload at 100 nN were tested. In the marine shale sample, the kerogen’s adhesion force had an obvious upward trend as the retraction velocity increased. This may have occurred because the surface was not able to deform quickly enough when the retraction speed increased, and air molecules were not able to enter the surface space, resulting in an increase of adhesion force. The inorganic rock sample had a slight upward trend concerning adhesion force as retraction velocity increased, but the change was not as obvious when compared to the kerogen. The marine shale sample and the continental shale adhesion forces vs contact times are shown in Figure S7, indicating that there was no major change in the kerogen, inorganic continental or marine shale sample as the


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contact time increased. Thus, the variation in adhesion forces because of the plastic deformation of the marine and continental shale samples can be eliminated. 300 300

Land

Kerogen Rock Rock

100 100

00

-100 -100

Rock

100 100

00 -100 -100

-200 -200

-200 -200

00

100 100

200 200

300 300

400 400

-300 -300

500 500

-100 -100

Adhesion force (nN)

Adhesion force (nN)

00

100 100

200 200

120

100

80

60

100

200

400

600

800

1000

Preloading force (nN)

140

120

100

80

60

40

20

100

200

400

600

800

1000

Preloading force (nN)

140

Adhesion force/nN

Rock Kerogen Rock

Adhesion force/nN

Kerogen

140

300 300

400 400

500 500

Z-piezo displacement/nm (nm) Z-piezo displacement

140 (d) Marine Kerogen Kerogen 140 (e) Land Rock Rock 120 Kerogen Kerogen Rock Rock 120 100 120 80 100 100 60 80 80 40 60 20 60 100 200 Preloading 400 600 800 1000 100 200 400 600 800 1000 0.1 0.2 0.5 1 Preloading force/nN force/nN Preloading force/nN

140 (c) Land Adhesion force/nN

Adhesion force (nN)

Z-piezo displacement/nm(nm) Z-piezo displacement

120

100

80

60

0.1

0.2

0.5

1

Retraction velocity (μm/s)

140 (f) Kerogen Marine 120 Rock Kerogen Rock 100 80 60 40 20 0.1 0.2 0.5 1 Preloading force/nN 140

120

Adhesion force/nN

-100 -100

Adhesion force (nN)

-300 -300

Kerogen

200 200

R

200 200

force (nN) Adhesion Adhesion force/nN

Marine

Kerogen

(nN) Adhesion force/nN Adhesionforce

300 300

100

80

60

40

20

0.1

0.2

0.5

1

Retraction velocity (μm/s)

Figure 5. A typical adhesion force curve of (a) the continental and (b) the marine kerogen and shale, respectively, with preload at 100 nN. Adhesion forces vs preload forces ranging from 100-1000 nN of (c) the continental and (d) the marine shale; and adhesion forces vs retraction velocity of (e) the continental and (f) the marine shale.

O

C=O OH

N

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

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Kerogen

C-OH

O

Si

O

Al

Si

Rock

Figure 6. The mechanism diagram of adhesion force of the continental and the marine shale samples.

Researchers have high expectations for the ability of kerogen to store carbon dioxide and methane as well as hydrocarbon products from source rocks.6, 38-40 However, due to the complex chemical structure and characteristics of kerogen, there has been no


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significant breakthrough concerning the ability of kerogen to store gas.8, 10, 13 In this study, quantitative analysis showed that the adhesion force of kerogen was higher than that of the inorganic substance in shale. Covalent bonds formed in slow chemical reactions and caused an increase of adhesion force in the particle. EDX analysis proved that the inorganic sample contained a large quantity of Si, Al and O elements. FTIR and XPS analysis further inferred that Si-O stretching affected the adhesion force. FTIR analysis at variable temperatures (30-70℃) indicated that Si-O stretching did not change with temperature. Therefore, the stability of Si-O stretching was fully confirmed, which reveals the influence of the hard surface of the inorganic material and a low adhesion force in the samples. XPS analysis of the nitrogen (1s) spectra demonstrated that kerogen contained in the continental and marine shale samples contained organic matter including quaternary nitrogen, amino nitrogen or pyridinic nitrogen. These organic materials were relatively active with free electron pairs, which were more likely to absorb other substances. This may explain why the adhesion force of kerogen was higher than that of the inorganic material. In addition, the C=C and C=O stretching in the kerogen may be the main reason for the kerogen to produce carbon dioxide, methane and hydrocarbons. The mechanism diagram of adhesion force is shown in Figure 6. F. Javadpour10 indicated that the adhesion force had a positive effect to identify components on a composite surface at high resolution. The current research indicated that the adhesion force had a profound and positive effect on analysis the chemical composition of kerogen, further revealing the adsorption of methane and carbon dioxide by kerogen.


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3. CONCLUSIONS The current study showed that the adhesion force of kerogen was higher than that of the inorganic substance found in shale by using AFM analysis for the first time. In the continental shale sample, the adhesion force of kerogen was over three times than that of the inorganic material, while the adhesion force of kerogen was approximately 1.3 times than that of the inorganic material found in the marine shale sample. The adhesion force of kerogen showed no substantial change at various preloading forces. These high percentages of C=C and C=O bonding in kerogen could be attributed the large dipole on the surface and help kerogen capture liquid and gaseous hydrocarbons. The findings in this study have great significance for influencing further studies of kerogen in the fields of rock mechanics and geology.

Acknowledgements We thank the General Projects of the Natural Science Foundation of China (No. 51674275, 51575528, 51505503, 51875577), the State Major Program of the National Science Foundation of China (No. 51490652), the Beijing Nova Program (No. Z171100001117058), and the Science Foundation of China University of PetroleumBeijing (No. 2462018BJC004) for the support. This study was supported by the National Institute of Food and Agriculture, U.S. Department of Agriculture, and McIntire Stennis under accession number 1009735.


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Table of Content


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Surface Properties of Organic Kerogen in Continental and Marine Shale

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