Nanoscale Surface Properties of Organic Matter and Clay Minerals in

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Nanoscale Surface Properties of Organic Matter and Clay Minerals in Shale Shouceng Tian, Tianyu Wang, Gensheng Li, Mao Sheng, and Panpan zhang Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.9b00157 • Publication Date (Web): 28 Mar 2019 Downloaded from http://pubs.acs.org on March 29, 2019

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Nanoscale Surface Properties of Organic Matter and Clay Minerals in Shale Shouceng Tian a, b, *, Tianyu Wang a, b, Gensheng Li a, b, Mao Sheng a, b, and Panpan Zhang a, b a

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

Petroleum (Beijing), Beijing 102249, China b

Harvard SEAS-CUPB Joint Laboratory on Petroleum Science, 29 Oxford Street,

Cambridge, MA 02138, USA *Corresponding author: Email: [email protected]

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ABSTRACT Surface properties of shale play an essential role in adsorption, transport, and production of hydrocarbons from shale reservoirs. Nanoscale surface properties of kerogen and minerals of shale were examined by a series of techniques including X-ray diffraction (XRD), scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS), Fourier transform infrared spectroscopy (FTIR) and atomic-force microscopy (AFM). The results show that aluminosilicate is the main component of inorganic matter while kerogen chiefly consists of carbon. FTIR and XPS analysis indicate the chemical bonds of kerogen surface are O-H, C-C, C-O, pyrrolic, etc. In contrast to kerogen, illite’s bonds are mainly Si-O and Al-O. AFM results indicate that the adhesion force of kerogen is higher than that of illite in shale. In addition, at a preloading force of 2500 nN, the adhesion force of kerogen increases from 40.8 to 118.2 nN when retraction velocity increases from 500 to 2500 nm/s. The adhesion force of montmorillonite, calcite and muscovite are 33.7±6.28, 23.8±11.8 and 105.1±9.1 nN, respectively. The chemical composition and bonds have a profound effect on adhesion force of shale, which further reveal the transport and adsorption mechanism of methane in kerogen. Keywords: Shale, Atomic Force Microscope, Kerogen, Illite, Adhesion

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Introduction Commercial shale gas extraction in the United States has transformed the world energy landscape and resulted in environmental and political concerns.1-2 Advanced horizontal drilling and multistage hydraulic fracturing technologies improve the possibility of profitable production in shale gas reservoirs.3-4 Some new hydra-jet fracturing technologies provide new technical options for shale gas development5-7. Shale, as a fine-grained sedimentary rock, is composed of kerogen scattered in several kinds of minerals.8-9 Kerogen is the source of shale gas, and it breaks down into oil and gas under high temperature.10 The remaining kerogen develops a nanoscale pore network, which determines the shale surface properties in shale reservoirs. Additionally, as an important constituent of shale, inorganic matter may affect both the fracturing efficiency and the flow of shale gas to the wellbore11-12. Therefore, investigation of nanoscale surface properties of organic matter and clay minerals in shale is important for efficiently exploiting shale gas and enhancing gas recovery in shale reservoirs. Shale often consists of multiple distinct layers with varying interface properties between layers.13 Kerogen is the insoluble organic matter (OM) from shale and plays a significant role in the storage, transport, and production of shale gas from economically important shale reservoirs.14-16 Scanning electron microscopy （ SEM） has been used to document the spatial heterogeneity of minerals in shale at nanoscale, and bulk spectroscopy measurements have been used to document the large variation in the chemical composition of kerogen in shale.17 SEM images provide

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high-resolution information of shale, whereas they do not provide further information about the chemical composition, chemical bonds, and mechanical properties of shale.18 Atomic-force microscopy (AFM) can obtain topographic surface images and simultaneously identify different materials on the surface.19 Furthermore, X-ray photoelectron spectroscopy (XPS) and Fourier transform infrared spectroscopy (FTIR) were widely used to analyze the chemical composition and chemical bonds of shale.20-21 Since understanding the gas storage and transport is crucial for enhancing the shale gas recovery, many efforts have been made to study the nanoscale surface properties of shale.22-24 Kelemen et al.25 used solid-state

13C

nuclear magnetic

resonance (NMR), XPS and sulfur X-ray absorption near edge structure (S-XANES) techniques to characterize the composition of chemical elements and structural features of kerogen. Yang et al.17 applied the atomic force microscopy-based infrared spectroscopy (AFM-IR) and optical microscopy to characterize OM heterogeneity at the nanoscale. Li et al.26 examined the modulus of organic matter in nanoscale by AFM and found that the elastic properties of OM are heterogeneous at even a few microns apart. On the other hand, the models which describe the interactions between microparticles and substrates include the Hertz model,27 the JKR model28, and the DMT model.29 These models ignore the kinetic effects on adhesion, and only consider the initial surface energies and elastic deformation of particles and substrates.30 However, for hydrocarbons flow in shale media, the detaching forces could be strongly influenced by the dynamic effect, whose mechanism is still unclear. Hence,

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the adhesion forces of kerogen and minerals of shale were measured under different retraction velocities. In our previous work,31 surface properties of organic kerogen in continental and marine shale were investigated by AFM. Nevertheless, identifying components on shale surface at high resolution and measuring the chemical and mechanical heterogeneity of kerogen in shale are stillchallenges. Here, we extracted kerogen from shale by going through a five-fold acid treatment with a mixture of hydrochloric acid and hydrogen fluoride, which can remove the carbonate and silicate.24,

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Minerals of shale were collected and quantified by X-ray diffraction

(XRD). Thus, we can accurately measure the chemical and mechanical properties of kerogen and inorganic matter in shale. Surface properties of shale, particularly adhesion force, play an essential role in adsorption and transport of hydrocarbons in shale reservoirs. Factors deciding adhesion force include the microstructures of the surface, chemical composition of shale, and chemical bonds. Thus, we present and discuss the experimental results from three aspects: shale assessment, chemical composition of shale and adhesion force. In this study, we first quantitatively analyze the mineral contents of shale by XRD and characterize its morphology by SEM and nano CT. Then, XPS and FTIR are used to analyze the chemical bonds of samples. We also use AFM to characterize the adhesion properties of kerogen and inorganic matter. Finally, we discuss the nanoscale surface properties of kerogen and clay minerals in shale. The results can provide theoretical significance for shale gas storage and transport in shale reservoirs.

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Experimental section Sample preparation. Shale samples in this work were collected from an outcrop area in Lower Silurian Longmaxi formation, which is widely distributed in the southeastern Sichuan Basin of China. It is characterized by the shallow depth, large thickness, high thermal evolution, and high OM content. The Lower Silurian Longmaxi shale is organic-rich, with total organic carbon (TOC) content of 5.71 wt. %. The mineral contents of the shale quantified by XRD are presented in Table 1. Note that shale has an extremely complex mineral composition. Clay and quartz are the main mineral constituents, 20.2% for clay minerals and 64.9% for quartz, respectively. Mineral contents of the clay are listed in Table 2. The main component of clay mineral is illite. Permeability, porosity, apparent density, and skeleton density of shale sample are listed in Table 3. In order to extract kerogen from shale, shale samples were crushed and filtered through a metallic sieve (200 mesh), then they went through a five-fold acid treatment with mixture of HCl and HF to get rid of carbonate and silicate. Finally, the remained kerogen powder was pressed into a sheet. We also collected the pure minerals of shale, including illite, montmorillonite, calcite, muscovite and quartz powders smaller than 200 mesh. Purity analysis of the minerals was conducted by XRD following the Chinese Oil and Gas Industry Standard SY/T5163-2010 using D/MAX 2500 X-ray diffractometer.33 The purities of all samples are shown in Table S1. It indicates that the purities of all samples are greater than 95% and muscovite’ purity reaches 100%, which is suitable for our experimental study.

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Table 1. Mineral contents of the shale samples (%). Sample Shale

Quartz 64.9

K-feldspar 0.8

Plagioclase 3.0

Calcite 1.1

Ankerite 0.2

Dolomite 4.6

Pyrite 5.2

Table 2. Mineral contents of the clay (%). Sample S* I/S I K C C/S Shale 41 49 10 *S = Smectite; I/S= Smectite and illite mixed layer; I = Illite; K = Kaolinite; C = Chlorite; C/S = Chlorite and Smectite mixed layer.

Table 3. Permeability, porosity, apparent density and skeleton density of shale.31 Sample Shale

Permeability (103 μm2) 0.01334

Porosity (%) 3.22

Apparent density (g/cm3) 2.514

Skeleton density (g/cm3) 2.5977

Characterization of shale. X-ray nano-CT imaging was performed with Ultra XRM-L200 CT (Xradia). The field of view is 65 μm in the large field of view mode with the spatial resolution of approximately 65 nm. The nanostructure morphology of the shale sample was observed by a field emission scanning electron microscope (SEM, Hitachi SU8010). To reduce the shale surface roughness, a high energy argon ion beam was used to polish the sample. The elemental composition of the samples was analyzed by the energy dispersive X-ray spectrometry (EDS) equipped on the SEM. Moreover, FTIR was carried out with a Bruker VERTEX 70 FTIR to analyze the bond formation of shale surface. Shale and mineral powder of 0.5 mg were mixed with 200 mg potassium bromide separately. The spectra were recorded between 4000 and 400 cm−1 with 50 scans at a resolution of 4 cm−1. XPS was also performed using a Rigaku 7


Clay 20.2

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X-Ray diffractometer (D/max-2400) with an Al Kα X-Ray source. Adhesion force test. Adhesion forces were quantified by Dimension Icon atomic-force microscopy (AFM, Bruker). The schematic of the AFM scanner system is shown in Fig. 1. The SEM imaging of the scanner tip on AFM cantilever is shown in Fig. S1. The AFM cantilever used here is 225 μm in length, 28μm in width and 3 μm in thickness. The average spring constant and average frequency are 20 N/m and 300 kHz, respectively. Microsphere with a diameter of 10 μm was glued at the end of a probe by a slow cure epoxy using an AFM manipulator system. Temperature and humidity were controlled to 27 ± 1 °C and 20% respectively. The cantilever was moved vertically and then the deflected laser beam was detected at the photodiode. Finally a signal was transmitted to the data-processing equipment.19

Figure 1. Schematic of the AFM scanner system.

Results Shale assessment. Nano-CT images of shale sample are shown in Fig. 2. A 65μm in diameter cylindrical shale block was drilled for CT scan. The cube model is obtained through 8


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3D reconstruction technology, whose side length is 38.4 μm. Information about the pore distribution hidden in the model was determined using attribute extraction. It can be seen that the shale is very porous. Thus, the shale samples have large specific surface area.34 Moreover, a wide-area 3D measurement system is utilized to characterize the surface topography and the height distribution. Fig. 3 shows the surface topography of shale sample. The shale sample has mean surface roughness around 0.593 μm. Meanwhile, pore connectivity is also an important factor which affects the transport of methane in shale.35 The microstructures of the surface were observed using SEM and the elemental distribution was determined across the surfaces by EDS. SEM images have shown that the shale is typically highly heterogeneous at the nano/micro scale. SEM images are significantly different between the inorganic rock and kerogen (Fig. 4a). The main chemical elements of shale surface are carbon, oxygen, silicon and aluminum (Fig. 4b). Aluminosilicate is the main mineral component of inorganic matter while kerogen surfaces are rich in carbon.12,

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Fig. 4c and d show the elemental distributions of carbon and silicon,

which were clearly observed at the interface of kerogen and inorganic matter. The similar result was also presented by previous studies that kerogen is rich in carbon comprised of a wide range of chemicals.37

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Figure 2. Nano-CT images of shale sample. a) Digital image of shale samples; b) Pore distribution. The side length of the cube model is 38.4 μm.

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Figure 3. Surface topography of shale sample. a) Topography. b) Height distribution.

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Figure 4. a) SEM images of shale sample; b) Content of each element in shale surface; c) EDX mapping of C element; and d) EDX mapping of Si element. Chemical composition of shale. The chemical bonds of shale were analyzed by FTIR and XPS analysis. FTIR spectra of kerogen and mineral samples are displayed in Fig. 5. For illite, the troughs at 1033.8 cm-1 and 474.4 cm-1 are attributed to Si-O stretching, which is consistent with the previous study that the spectra of silicates are characterized by Si-O stretching and bending vibrations at 1200-800 cm-1 and 600-400 cm-1.38 Partial enlargement FTIR spectra of shale samples and different clay minerals are in Fig. S2. The illite samples also have Si-O-Si stretching at 783.5 cm-1, as shown in Fig. 5a. FTIR spectra of kerogen are more complicated than those of illite. The trough at 1579.8 cm-1 is attributed to C=C stretching and 1124.4 cm-1is attributed to C-O 11


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stretching. The troughs at 605.6 cm-1 and 3358.7 cm-1 are attributed to =C-H stretching and bending vibrations, respectively. Fig. 5b presents the FTIR spectra of muscovite, calcite, montmorillonite, and quartz. FTIR spectra of muscovite and montmorillonite are similar to those of illite as the main constituents of them are aluminosilicates. The trough at 1427.3 cm-1 of calcite is assigned to carbonate. For quartz, the trough at 1087.7 cm-1 is attributed to Si-O stretching.

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Figure 5. FTIR spectra of shale samples and different clay minerals. a) FTIR spectra of kerogen and illite. b) FTIR spectra of muscovite, calcite, montmorillonite, and quartz. XPS was applied to further analyze the chemical bonds of kerogen and illite, as shown in Fig. 6. The elements’ bonding energies of kerogen and illite are presented in Fig. S3. XPS spectra for carbon (1s), nitrogen (1s), oxygen (1s), and sulfur (2p) spectra of kerogen are displayed in Fig. 6a, c, e and g. XPS spectra for aluminum (2p), calcium (2p), oxygen (1s), and silicon (2p) of illite sample are in Fig. 6b, d, f and h. Carbon (1s) kerogen spectra are curve-resolved using three peaks at fixed

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energy positions of 284.8, 286.3, and 287.5 eV, which are attributed to C-C, C-O and C=O, respectively (Fig. 6a). Nitrogen forms in kerogen were defined using XPS, as show in Fig. 6c. Nitrogen (1s) spectra for kerogen indicate that there are three distinct peaks center at 398.6, 400.2, and 401.4 eV. The peak at 398.6 eV is attributed to paridinic nitrogen. The 400.2 eV pyrrolic and the 401.4 eV quaternary nitrogen peaks are present in kerogen XPS spectra. Fig. 6e is illustrative of XPS oxygen (1s) spectra and the curve-resolution into different components for kerogen. The peak at 530.8 and 533.7 eV are attributed to C-C and C=O in kerogen. Sulfur species also were measured by XPS. The XPS sulfur (2p) spectra for kerogen are complex and are usually comprised of a low binding energy signal and a higher binding energy signal,25 as shown in Fig. 6g. The peaks in lower binding energy signal are 163.3, 164.1, and 165.7 eV, which are attributed to aliphatic sulfur, aromatic sulfur, and sulfoxide, respectively. The higher binding energy signal was curve-resolved using two components at 168.0 and 168.5 eV. These signals overlap but are expected to arise predominantly from sulfate. Therefore, only the sum of the curve-resolved intensity for the 168.0 and 168.5 eV components is reported. Illite is the highest among clay minerals in shale according to the XRD analysis. The chemical forms of illite were analyzed by XPS. The XPS spectra Ca2p are comprised of a low binding energy signal and a high binding energy signal (Fig. 6d). The low binding energy signal is curve-resolved using 347.3 eV of Ca2p3/2, and the high binding energy signal is curve-resolved using 351.1 eV of Ca2p1/2. Fig 6f and h depict the XPS

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spectra for oxygen (1s) and silicon (2p) of illite, which indicates that most of the oxygen and silicon exist in silicate and aluminosilicate minerals.

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Figure 6. XPS spectra for kerogen and curve-resolution into different components. a, c, e, g) XPS spectra for carbon (1s), nitrogen (1s), oxygen (1s), and sulfur (2p) spectra in kerogen sample; b, d, f, h) XPS spectra for aluminum (2p), calcium (2p), oxygen (1s), and silicon (2p) in illite sample. 1s and 2p are the electron orbits in element. The bonding energy for the breaking of chemical bonds is from the NIST X-ray Photoelectron Spectroscopy Database39.

Adhesion force. The interactions of the tip on samples were conducted with the AFM system at different retraction velocities. The displacement and force at the tip−samples separation point are significant to understand the dynamic loading effect. Therefore, the displacement and force of the tip were investigated by a high speed data capture (HSDC) technique. Fig. 7a shows typical pull-off curves for AFM tip approaching and retreating from kerogen and illite. The attachment and detachment of the tip from substrates were carried out at pull-off speeds ranging from 500 to 2500 nm/s by the AFM. For each preloading force, three repeated experiments were conducted. The relationship between adhesion force and preloading force of kerogen and illite are shown in Fig. 7b. The adhesion forces of kerogen at all preloading force are higher than that of illite, indicating that the adhesion force of organic matter is higher than that of illite in shale. Specifically, for at the preloading forces of 500, 1000, 1500, 2000, 2500, and 3000 nN, adhesion forces of kerogen are 114.5, 119.0, 111.1, 112.8 124.1 and119.6nN. Meanwhile, the illite’s adhesion forces at a mere are 11.9, 15.8, 16.2, 20.1, 17.5, and 16.4 nN. As shown in Fig. 7c, when retraction velocity increases, the adhesion force of kerogen also increases, which is consistent with the 15


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reports in the literature.30 In particular, at a preloading force of 2500 nN, the adhesion force of kerogen increases from 40.8 to 118.2 nN when retraction velocity increases from 500 to 2500 nm/s. The adhesion forces of illite keep almost constant when the retraction velocity increases from 500 to 3000 nm/s. Compared to kerogen, the effect of retraction velocity on illite’s adhesion forces is not obvious, as estimated in our prior work.31 Moreover, we also measured the adhesion force of montmorillonite, calcite and muscovite at a preloading force of 2500 nN and retraction velocity of 2500 nm/s. It can be concluded from Fig. 7d that the adhesion forces of montmorillonite, calcite and muscovite are 33.7±6.28, 23.8±11.8 and 105.1±9.1 nN, respectively.

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Figure 7. a) Typical pull-off curves for AFM tip approaching and retreating from kerogen and illite; b) The relationship between adhesion force and preloading force of kerogen and illite; c) The relationship between adhesion force and retraction velocity of kerogen and illite; d) Adhesion force of montmorillonite, calcite, and muscovite.

Discussion Shale has been given high expectations for gas exploration and development. However, due to the complex chemical structure and characteristics of kerogen, understanding gas transport and adsorption in shale is a long-standing challenge.40-42 Surface properties of shale, particularly adhesion force, play an essential role in adsorption and transport of hydrocarbons in shale reservoirs.43 Based on the experimental results above, the adhesion force of shale surface is further discussed. There were two kinds of possible contact situations between shale and AFM tips (Fig. 8). Factors including the microstructures of the surface, chemical composition of shale and chemical bonds were taken into consideration. The microstructure of the surface was investigated. SEM images are significantly different between the kerogen and inorganic matter. kerogen is rich in carbon element while the inorganic matter mainly consists of aluminosilicate. Krogen partially controls shale’s geomechanical properties because it is the softest component in shale.44 Additionally, kerogen has a network of nanoscale pores that is responsible for gas storage and transport during gas production and migration.45-46 SEM-EDX also creates an elemental spectrogram to distinguish and identify the elemental distribution on kerogen surface. FTIR analyses further infer that chemical bonds affect the

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adhesion force. From FTIR analysis, Si-O stretching and Si-O-Si stretching in illite samples are confirmed.38 Additionally, the C=C and =C-H stretching in kerogen may be the main source of methane and hydrocarbons.31 The kerogen surface properties analyzed by XPS also agree with the result of FTIR analysis. The carbon (1s) and oxygen (1s) spectra demonstrate that kerogen containes C-C, C-O, and C=O bonds. It is reported that covalent bonds cause an increase of adhesion force shale surface.47 XPS analyses also present quaternary nitrogen, amino nitrogen and pyridinic nitrogen in kerogen, which is one of reasons that the adhesion force of kerogen is higher than that of illite. This is because that kerogen is relatively active with free electron pairs, which is more likely to adsorb other compounds.31 Quantitative analysis shows that the adhesion force of kerogen is higher than that of illite in shale. The mechanism schematic of adhesion force between kerogen and illite is shown in Fig. 8. Kerogen is a kind of complex organic materials which have hydrocarbon structures and organic nitrogen, sulfur, and oxygen. There are different kinds of chemical bonds on kerogen surface, including O-H, C-C, C-O, pyrrolic, et al. In contrast to kerogen, illite is Si-O and Al-O mainly. This is one of the causes of difference between kerogen and illite’s adhesion force. Meanwhile, the differences between adhesion of kerogen and illite cannot be solely explained by surface bonds and component. It can also be explained by the theory of soft contact, which indicates that the viscoelastic or plastic deformation on kerogen increases the contact area, resulting in increasing the adhesion force.30,

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Besides, adhesion force can help identify components on shale

surface at high resolution.19 The present work indicates that the chemical composition

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and bonds have profound effects on adhesion force of shale, further revealing the transport and adsorption of methane in kerogen.

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Figure 8. Schematic figure of comparison of kerogen and illite’s adhesion. a) Macroscopic contact diagram and microcosmic magnifying contact diagram of the AFM tip and kerogen. b) Macroscopic contact diagram and microcosmic magnifying contact diagram of the AFM tip and illite.

Conclusions In conclusion, we demonstrated the nanoscale surface properties of kerogen and illite of shale by AFM. Factors including microstructures of the surface, chemical composition of shale, and chemical bonds were taken into consideration as important parameters affecting the adhesion force of shale. Major findings are summarized as follows. Aluminosilicate is the main mineral component of inorganic matter while kerogen surfaces are rich in carbon. FTIR and XPS analysis indicates there are chemical bands on kerogen surface, including O-H, C-C, C-O, pyrrolic, et al. In

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contrast to kerogen, illite is Si-O and Al-O mainly. Adhesion force of kerogen is higher than that of illite in shale. In addition, at a preloading force of 2500 nN, the adhesion force of kerogen increases from 40.8 to 118.2 nN when retraction velocity increases from 500 to 2500 nm/s. The adhesion force of montmorillonite, calcite and muscovite are 33.7±6.28, 23.8±11.8 and 105.1±9.1 nN, respectively. The chemical composition and bonds have profound effects on adhesion force of shale. The theory of soft contact indicates that the viscoelastic or plastic deformation on kerogen increases the contact area, resulting in the increase of the adhesion force. In further research we will use AFM-based infrared spectroscopy (AFM-IR) to provide in situ and simultaneous characterization of kerogen at nanoscale.

Supporting information The purity analysis of the minerals by XRD can be seen in Table S1. SEM image of the scanner tip on AFM cantilever and Survey binding energy can be seen in Fig. S1 and S3.

Notes The authors declare no competing financial interest.

Acknowledgements This work was financially supported by the National Natural Science Foundation of China (No. 51674275, No. 51490652) and National Science and Technology major project of China (2017ZX05009-003). We thank the Harvard SEAS-CUPB Joint Laboratory. We also thank Dr. Quan Xu for the useful discussion.

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