nano-sized pore structure

Subscriber access provided by The University of Texas at El Paso (UTEP)

Article

Precise identification and analysis of micro-/nano-sized pore structure in shale with FeO/Au hybrid nanocomposite 3

4

Na Li, zhong ting, Jialiang Liu, Jun Zheng, Hu-cheng Deng, Wen Zhou, Ming Li, Mingshi Feng, Qijun Liu, and ChongYing Li Anal. Chem., Just Accepted Manuscript • DOI: 10.1021/acs.analchem.8b02992 • Publication Date (Web): 18 Oct 2018 Downloaded from http://pubs.acs.org on October 19, 2018

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 7 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

Precise identification and analysis of micro-/nano-sized pore structure in shale with Fe3O4/Au hybrid nanocomposite Na Li,*,† Ting Zhong,† Jia-liang Liu,‡ Jun Zheng,† Hu-cheng Deng,† Weng zhou,† Ming Li,§ Ming-shi Feng,† Qi-jun Liu,† and Chong-ying Li*‡ State Key Laboratory of Oil and Gas Reservoir Geology and Exploitation, College of Energy Resources, Chengdu University of Technology, Chengdu, Sichuan 610059, China ‡ College of Materials and Chemistry & Chemical Engineering Mineral Resources Chemistry Key Laboratory of Sichuan Higher Education Institutions, Chengdu University of Technology, Chengdu, Sichuan 610059, China § Hefei National Laboratory for Physical Sciences at the Microscale, Structure Research Laboratory, University of Science and Technology of China, Hefei, Anhui 230026, China †

ABSTRACT: Analysis and characterization of micro-/nano-sized pore structure are critical issues in shale geology and engineering. SEM imaging is one of the most widespread methods for the analysis of the micro-/nano-sized pores in shale, but precise identification of the ultra-fine pore structure in shale is still a big challenge because shale is so complex that some components may have overlap with pores based on the simple discrimination of gray scale under SEM microscopy. Here Fe3O4/Au nanocomposite with magnetic properties is synthesized, characterized, and introduced as a novel pore-marker to improve SEM identification and quantitation of micro-/nano-sized pores in shale. Due to the superparamagnetic property, the nanomarker is conveniently controlled by an external magnetic field to fill into pores, and offers a sharp contrast imaging between matrix of shale (various gray) and pores (bright), which makes the identification of micro-/nano-sized pores in shale much more straightforward and reliable. Furthermore, since gold, as a noble metal, is particularly rare in shale, energy-dispersive x-ray spectroscopy mapping of Au is delicately used to precisely calculate area porosity in shale. Combining with the aforementioned merits of the nanomarker, a precise and practical technique is proposed to promote characterization of micro-/nano-sized pores in shale.

Over the last decade, the boom in shale gas and oil has changed the energy landscape worldwide and stimulated both the research branch of unconventional resources as well as political and environmental concerns. Shale, a mixture of solid organic matter (kerogen) and mineral framework, is commonly recognized as a source rock of hydrocarbon, and has been ignored as a reservoir of hydrocarbon due to its low porosity and permeability. However, advances in hydraulic fracturing and modern horizontal drilling have made shale transformed into commercial assets and to be considered as an unconventional reservoir of shale gas1. Different from conventional reservoir, shale reservoir contains a significant portion of porosity in ultra-fine pores (micro-/nano-sized pores), which are embedded into organic matters and mineral matrix. As pore sizes reach to the nanoscale, a large number of micro-/nano-sized pores not only reserve a fair amount of free gas, but preserve a considerable quantity of adsorbed gas with significantly increase of surface area2. By hydraulic fracturing, the micro-/nano-sized pores connect to the macroscopic fracture network, and the hydrocarbon content (both free and adsorbed gas component) in pores is released with the decrease of the pressure in the fracking fluid. Therefore, micro-/nano-sized pore structure in gas shale, which is closely in correlation with the storage and transport of natural gas, is critical to resource assessment and production strategies.

Many petrophysical methods have been utilized to characterize the micro-/nano-sized pore structure in shale, such as low-pressure nitrogen or carbon dioxide adsorption3,4, mercury injection3-5, small angle scattering6,7, nuclear magnetic resonance5,8,9, etc. These methods above, which are rooted on certain mathematical assumptions and models, need complicated explanations of experimental data. However, imaging methods offer the direct approach to characterise the micro-/nano-sized pore structure in shale unambiguously, including electron microscopy2,3,5,10-15, atomic force microscopes16,17, and X-ray computed tomography7,18-20. Among them, scan electron microscopy (SEM) has proven to be a powerful tool for microstructural investigation of shale21. Due to high magnification and wide accessibility, SEM imaging has been speedily applied to characterize micro/nano-sized pore structure in worldwide shale reservoirs, including Bexar Shale (USA)14, Bossier Shale (USA)22, Devonian New Albany Shale (USA)23, Eagle Ford Shale (USA)2, Fayetteville Shale (USA)2, Floyd Shale (USA)2, Haynesville Shale (USA)3, Marcellus Shale (USA)3,23, Woodford Shale (USA)3, Longmaxi Shale (China)13,26, Yanchang Shale (China)12, Horn River Shale (Canada)24,25, Kimmeridge Shale (UK)24,25, Posidonia Shale (Germany)27. Under SEM microscopy, micro-/nano-sized pores are commonly black and the shale matrixes are gray, which can be used to recognize the micro-/nano-sized pores of shale. Meanwhile, in backscatter electron mode, gray levels of shale

ACS Paragon Plus Environment

Analytical Chemistry 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

matrixes decrease with the increase of their atomic weight, providing information about the chemical composition of matrix adjacent to pores. By setting thresholds on the gray scale, the matrix and micro-/nano-sized pore structure are segmented out and area porosity is calculated routinely2,25. However, in SEM imaging, the identification of the micro/nano-sized pore structure depending on the simple strategy of grayscale discrimination is still a big challenge because of the complexes of shale matrix. There are some overlaps in gray value between some shale matrixes and micro-/nano-sized pores in SEM analysis. For example, organic matter appears deep gray and pores appear black, thus the similar gray value between organic matter and pores makes the recognition of micro-/nano-sized pores to be a difficult work, especially when organic matter is embed partly into the pores. Moreover, some organic material appears darker than surrounding organic matter, which exhibit the similar grayscale with the micro-/nano-sized pores dispersed in organic matter and, therefore, may lead to incorrect identification of organic matter pores14. Subsequently, attributed to the raw recognition of the micro-/nano-sized pore structure, digital image automated processing techniques were limited to quantitatively characterize shale microstructure. The automatical segmentation of pore structures based on simple gray level may lead to an incorrect conclusion, including over- or undervaluation of area porosity when some organic matters with deeper grayscale are considered as pores, or some pores without clear boundaries are not involved in. Therefore, an effective strategy to promote the recognition of micro-/nanosized pores in SEM imaging has been desired, and a suitable marker to clearly express micro-/nano-sized pores is likely to be the best approach. Nanomaterials, which are of a commensurate size with micro-/nano-sized pores in shale, will be considered as an excellent pore marker to light up the targeted pores to improve the pore recognition. In fact, due to their excellent properties, nanomaterials are of wide influence on geoscience and it was reported they can participate in geological studies as nanosensors28, tracers29,30, in-site catalysts31, and surfactants32. However, the application of nanomaterials as pore markers for imaging analysis of pore structure in geological materials is rarely reported. Recently, our group introduced a nanoparticle with magnetic and fluorescent properties as a novel marker to improve optical imaging of pore structure in rocks33. By the aid of the nanomarker, cracks in shale are successfully imaged under fluorescent microscope. However, for the analysis of ultra-fine pores, which are of main sizes in the sub-micrometer or nanometer range, the use of optical imaging is very limited due to its low resolution and, therefore, an appropriate nanomarker, which could be used to light up micro-/nanosized pores in SEM imaging, is needed. Herein, Fe3O4/Au nanocomposite is designed and presented as a functional marker for SEM imaging of micro-/nano-sized pores in shale. Due to the superparamagnetic properties of Fe3O4 component, nanocomposite is easily driven into the micro-/nano-sized pores of shale under the control of an external magnetic field. With heavy elements including Au and Fe, the nanomarkers offered a sharp contrast imaging between shale matrix (various gray) and pores (extremely bright) to clearly profile micro/nano-sized pores and

efficiently eliminate fake pores dispersed in organic matters, which makes the identification of micro-/nano-sized pores in shale much more straightforward and reliable. Furthermore, since Au, as a noble metal, is particularly rare in shale, a novel technique for the quantitative analysis of porosity in shale has been developed by using energy-dispersive x-ray spectroscopy (EDS) mapping of Au. With the merits of the nanomarker above, a precise and practical technique was proposed to promote characterization of micro-/nano-sized pore structure in shale. Experimental Section Chemicals: FeCl2·4H2O and FeCl3·6H2O were obtained from Aladdin Inc. (Shanghai, China). Sodium citrate, NH2OH·HCl, NaOH, HAuCl4·4H2O (48 %, w/w), were purchased from Shanghai Reagent (Shanghai, China). All reagents were of analytical grade, and used as received. Ultrapure water obtained by a Direct-Q 3 UV water purification system (Millipore, USA) was used throughout. Preparation of Fe3O4/Au nanocomposite: Fe3O4 nanoparticle were synthesized through the revised coprecipitation of ferrous and ferric chloride salts. In brief, 1.056 g FeCl2·4H2O and 2.574 g FeCl3·6H2O were dissolved in N2-purged H2O (100 mL) to form an orange solution. The temperature was increased to 80 oC and then, NaOH (40 mL, 1.25 mol L-1) was dropwise added. When the solution turned black, sodium citrate (10 mL, 0.187 mol L-1) was added. The reaction solution held on 80 oC for 60 min and then cooled to room temperature. The citrate-coated Fe3O4 nanoparticles were magnetically separated and rinsed with N2-purged H2O by a dispersion/preciptation cycle to obtain a stabilized black ferrofluid. Fe3O4/Au nanocomposite was synthesized by chemical reduction method with the presynthesized Fe3O4 nanoparticle in a solution. ~ 50 mg presynthesized Fe3O4 nanoparticle was dispersed in N2-purged H2O (150 mL) and then, sodium citrate (6.0 mL, 0.01 mol L-1) was added into the system by ultrasonic dispersion about 30 min. Subsequently, HAuCl4 (6.0 mL, 0.00934 mol L-1) was added into the mixture by ultrasonic dispersion about 5 min and then, excessive NH2OH·HCl (0.2 mol L-1) was quickly injected into the mixture. The resulting solution was stirred under 30 min and then, cooled to room temperature. The raw nanocomposite were separated by a permanent magnet and dispersed into hydrochloric acid (~ 30 mL, 0.1 mol L-1) for 24 h to remove partial Fe3O4 without Au. During the acid treatment, there is no significant change in the size of hybrid nanocomposite, neither Fe3O4 nor Au, (figure S1), but Fe3O4 content in the nanocomposite fairly decreases, resulting a slight influence of its magnetism (figure S2). Finally, the nanocomposite were rinsed with N2-purged H2O by three dispersion/preciptation cycles and collected with a permanent magnet. Characterization of Fe3O4/Au nanocomposite: The size and morphology of the synthesized nanocomposite were investigated by high resolution transmission electron microscope (TEM, JEM 2100F, JEOL, Japan). The concentration of Au and iron of the nanocomposite were determined after chemical decomposition in chloroazotic acid medium and quantified by atomic absorption spectrophotometry (Thermo iCE3500, Thermo, USA). The magnetic properties of the nanocomposite were surveyed using a vibrating magnetometer (7400, Lake Shore, USA).


Page 2 of 7

Page 3 of 7 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 The structure of the nanocomposite was examined by using powder X-ray diffraction, which was performed with an X-ray diffractometer (X'Pert PRO, PANalytical, Holland). Geological Materials: Yanchang Shale was used in the study. A shale sample with the porosity 3.2 % (gas porosimetry) and permeability 9.6 × 10-3 m2 was taken as an example to be selected. The details of the sample preparation were described as our previous method33. Impregnation agents and processes: Fe3O4/Au nanocomposite was utilized as impregnation agents. Before the impregnation of nanomarkers, the surface of the shale sample was polished by an Ar-ion-milling. The preparation of thin sections with nanomarked pore structure was based on magnetic control. 10 L the prepared nanocomposite ferrofluid was placed on the top of a sample (thin section). A square neodymium N48 magnet (50 × 50 × 10 mm3), which is significantly bigger than the shale thin sections (10 × 10 × 1 mm3), was placed at the back of the thin section to drive nanomaterials into pore structure of rocks at 4 °C. After the ferrofluid was dried on the surface of the thin section, and the surface pores were impregnated with the nanomarkers, residual nanomaterials on the surface of the thin sections were cleaned with an alcohol-soaked cotton ball. The impregnation process was repeated two times to ensure that pores are full of nanomarkers. Image analysis procedure: Image analysis of micro-/nanosized pores was performed with SEM instruments (Quanta 250, FEG, USA, and G500, Zeiss, Germany), in both the secondary electron mode and backscatter mode, and combining with insite EDS. Threshold techniques based on gray level from the SEM images or RGB color space from the EDS mapping of Au were employed to profile pore structure, and then porosity was calculated. The same threshold values were conducted for all images. Results and discussion Fe3O4/Au hybrid nanocomposite was synthesized by modified chemical reduction method to combine the presynthesized Fe3O4 nanoparticle with Au nanomaterial in a solution34. Figure 1 shows TEM images of the nanocomposite, and the corresponding particle-size histograms. In the nanocomposite, Au nanoparticle is spherical and the particle diameter is in the range between 15.6 nm to 115.0 nm with the average of 31.1 nm, while Fe3O4 nanoparticle is irregular and particle diameter is in the range between 2.8 nm to 11.2 nm with the average of 6.7 nm. Fe3O4 nanoparticles perform to aggregate around Au nanoparticles and Au nanoparticles are isolated by the aggregation of Fe3O4 nanoparticles.

Components of the prepared Fe3O4/Au nanocomposite were investigated by atomic absorption spectrophotometry after chemical decomposition in chloroazotic acid medium. Fe and Au wt % in the nanocomposite are 44.1 % and 24.3 %, respectively. Structure of the prepared Fe3O4/Au nanocomposite was characterized by the powder X-ray diffraction (figure 2). The diffraction peaks due to both the Fe3O4 and Au nanoparticle are present, indicating that the nanomaterial is a composite of Fe3O4 and Au nanoparticles, and well corresponding with the results of TEM images. The magnetic properties of Fe3O4/Au nanocomposite were examined by a vibrating sample magnetometer under the magnetic field cycle between – 6000 and 6000 Oe (figure 3). The saturation magnetization of the nanocomposite is 71.0 emu g-1, which is less than that of pure Fe3O4 nanoparticles (81.7 emu g-1). The decrease of saturation magnetization might be derived from the lower concentration of Fe3O4 in the prepared nanocomposite. The field-dependent magnetization curve suggests the magnetic behavior of the nanoparticles matching to a superparamagnetic manner, which makes the nanocomposite to be conveniently controlled by an external magnetic field.

Figure 2. XRD patterns of Au nanoparticle (a), Fe3O4 nanoparticle (b) and Fe3O4/Au nanocomposite (c)

Figure 3. Field-dependent magnetization curves of Fe3O4 nanoparticles (a) and Fe3O4/Au Figure 1. TEM image (A) and the corresponding size distributions of Au (B) and Fe3O4 (C) in the nanocomposite.

nanocomposite (b). The inset shows the photograph of the Fe3O4/Au nanocomposite before (left) and after (right) magnetic separation by an external magnetic field.



As a pore marker, Fe3O4/Au nanocomposite was driven into shale sections directly by using a permanent magnet placed at the back of shale sections. Attributed to the magnetic injection, our technique is of many advantages, including nondestructive (avoiding the problem of artificial damage from commonly pressure injection), simple (needing none equipment other than a magnet), and available to the irregular shape of shale. After the impregnation of micro-/nano-sized pores with the nanomarker, pores containing Au and Fe components, are extremely bright, but most minerals and organic matter appear dark or deep gray in SEM, which make the identification of pores effortlessly, even under low magnification. An example of a shale section using SEM imaging with Fe3O4/Au nanocomposite as markers of pore structure is shown in figure 4. Under low magnification, most of organic matter and mineral framework appear dark or deep gray, but the bright zones highlighted with the nanomarker are easily identified as where the pores are situated, demonstrating the advantage of the nanomarker in SEM image technique. The bright zones, whose profiles are clear, are isolated with each other, indicating the poor connectivity of pores and corresponding to the low permeability (9.6 × 10-3 m2).

nanomarker (figure 5B), a cleared outline of the organic matter pore is obtained. Under the high resolution of SEM, the filling detail of nanomarkers can also be documented as shown in Figure 5C. Significantly, before impregnation of the nanomarker, some organic matter pores (green arrow area in Figure 5A) are of a bright border, while some zones (yellow arrow area in Figure 5A) without a bright border seem to be another form of organic matter pores as they are of the similar shape and similar gray value with the bright border ones. However, after highlighting the pores with the nanomarker (Figure 5B), all the organic matter pores labeled with green arrow area are full of bright nanomarkers, but the area, which are labeled with yellow arrows, is still dark, revealing that they are not pores. It is likely to be residual oil drying out within bubble-shaped pores of shale14. Due to the similar gray level, the false pore area is difficult to be identified and distinguished from organic matter pores. But, our technique offers a reliable way to eliminate false organic matter pores and makes identification of organic pores in shale more precisely.

Furthermore, under the large magnification by using our technique, the details of pores are visibly observed. Organic matter pores (figure 4B), moldic pores (figure 4C), pores at the edge of rigid grains (figure 4D), intercrystalline pores within pyrite framboids (figure 4E) and ultra-fine fractures (figure 4F), are easily to be classify depending on the reported spectrum of pore types 14,15. Figure 5. SEM image of shale showing typical organic pores before (A) and after (B) impregnation of Fe3O4/Au nanocomposite. Enlarged areas (C) and (D) show filling details of nanomarkers in organic matter pores as well as fake organic pores

Figure 4. Typical SEM images of shale showing pore structures after impregnation of Fe3O4/Au nanocomposite as pore markers (A). Enlarged red zone (B), yellow zone (C) and green zone (D) shows organic pores, moldic pores and pores at the edge of rigid grains with the nanocomposite, respectively. Nanocomposite is also embedded in micro intercrystalline pores within pyrite framboids (E) and ultra-fine fractures (F).

Organic matter pores, which are closely correlation with hydrocarbon production, storage and transport, have been considered as a critical effect to resource assessment and production strategies11. In the present work, typical organic matter pores before and after impregnation of Fe3O4/Au nanocomposite are overlaid in the same visual field for comparative study (figure 5). Before impregnation of the nanomarker, organic matter pores, which appear dark, are embedded in organic matter, which appear deep gray. The similar gray values limit to the identification of organic matter pores, just like the organic pore labeled with red arrow in figure 5A is indistinct. After lighting up the pores with the

Similarly, moldic pores formed by partial or complete dissolution of minerals perform large highlight areas with nanomarkers (Figure 4C). EDS results reveal that the mineral particle holding the moldic pores was calcite (as shown in figure S3). Some small intraparticle pores within calcites or pores at the edge of calcites are also obvious with bright nanomarkers (Figure 4D). Besides, intercrystalline pores within pyrite framboid are locally common, but the recognition of the ultra-fine pores was extremely difficult without pore markers because they are often filled with organic matter, which exhibits the similar gray values with pores. Even in a very large magnification, it was hard to judge whether and where is filled with organic matter. But, after filling with the nanomarker, the intercrystalline pores are obvious (Figure 4E) because they are quite brighter than pyrite framboid. Additionally, nature microcracks, which are another important part of pore structure in shale (Figure 4F) and have a significant effect on oil and gas production, can also be represented easily by using the nanomarker. In order to confirm the smallest filled cracks with the nanomarker in the shale sample, concurrent imaging experiments were conducted on two laboratories with different instruments and researchers to eliminate subjective preference. Although some microcracks, which are slightly smaller than 30 nm, could be observed with the filling of the nanomarker (figure S4), most pore structure below 30 nm are really hard to fill with the nanomarker. Thus, in our work, the smallest filled cracks, imaging in the shale sample, is about ~ 30 nm in width (Figure 4F), which is depending on the size of the nanomarker.


Page 4 of 7

Page 5 of 7 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 In fact, major nanopores, in a large number of shale samples documented by SEM, are larger than 30 nm, including Barnett shale10, Eagle Ford shale35, Haynesville Shale, Bossier Shale in USA22 as well as marine and continental Shales in China12-13, 36. Thus, although the nanomarker technique, which highlights pore structure larger than 30 nm, could not cover the whole range of nanopores in shale, it could hold upon the major nanopore size range documented by SEM in a mount of shale reservoirs, and make the identification of pore structure more straightforward and precise. Except for precise identification of micro-/nano-sized pores in shale, our technique offers a quantitative EDS Mapping analysis of pore structure based on Au component in pore nanomarkers. Since, 2D imaging analysis has been still considered as a statistically invalid method to analysis of pore structure in gas shale reservoirs, representative elementary volume (REV) and statistical representative elementary volume (SREV) are investigated to determine how large a field of view is acceptable to represent the whole core sample for the analysis of area porosity by using EDS mapping. REV and SREV were calculated by using reported method 36. As shown in figure S5, REV and SREV results show that, porosity fluctuates obviously in small field view, and has an outsized standard deviation, but, as the field view increases, the porosity is close to a constant, and the standard deviation decreases substantially. In this case, SREV for macroscopic properties are ~ 500 m2 in the scale, because the standard deviation of porosity decreases to 30 %. Subsequently, a random visual field of the shale sample, giving an area of ~ 500 m2 (in Figure 6), is selected as an example to utilize in the analysis of area porosity. As the pore structure is highlighted with the nanomarker, area porosity is calculated to be 4.3 % based on the grayscale analysis, firstly (Figure 6B). Although our technique offers an improved identification of pore structure in shale, automatical segmentation of pore structure in extremely complex shale by using a simple grayscale analysis may also suffer from some tough problems. For example, the pores with different depths perform different gray levels. With the decrease of the depth, the brightness reduces due to the decrease of the nanomarker. Some heavy minerals may have overlap with some pores under similar gray level in SEM imaging. As shown in Figure 6A, the organic matter pore lighted up with the nanomarker in the red zone appears a comparable gray level with the pyrite in the blue zone. Since some minerals, which are of the similar gray level with some pores, are mistakenly considered as pores, the calculated area porosity (4.3 %) based on grayscale analysis is overvalued. Compared to grayscale recognition, the X-ray map of Au elemental analysis offers a reliable method for the calculation of area porosity because Au is rare in shale. In Figure 6D, the distribution of Au in shale is investigated with EDS mapping, and area porosity is calculated to be 3.0 %, which is obviously lower than the results of gray scale recognition (4.3 %) and prevents the overvaluation from a threshold technique based on simple gray level. Furthermore, gas porosimetry technique was employed to validate the area porosity of the proposed imaging method. The porosity from gas porosimetry was calculated to be 3.2 %, which is well corresponding to the result of Au X-ray map (3.0 %), indicating the potential of EDS mapping method in quantitative analysis of shale porosity.

Figure 6. SEM image of a shale sample highlighted with nanomarkers (A), bright area extracted from the SEM image (B), and X-ray maps showing the distribution of Fe (C) and Au (D) in the same field of vision

Conclusion Fe3O4/Au nanocomposite, which is served as an example of a novel marker, is designed, synthesized, characterized and introduced for SEM imaging of micro-/nano-sized pore structure in shale. With the paramagnetic property, the nanomarker is successfully driven into pores in shale by the control of an external magnetic field. Under SEM imaging, pores of shale are easily identified due to the high brightness of Au and Fe3O4, and micro-/nano-sized pores with various types are clearly imaged. Compared with the previous SEM methods, the nanomarker with heavy element of Au and Fe provides a clearer contrast imaging between shale matrix (gray or deep gray) and porosity (bright) to clearly profile micro/nano-sized pores and efficiently eliminate fake pores dispersed in organic matter, which offers a more reliable strategy to identify micro-/nano-sized pore structure in shale effortlessly. The smallest pores filled with the nanomarker are about ~ 30 nm, which could cover the major nanopore size range in a mount of shale documented by SEM, and make the identification of pore structure more straightforward and precise. Furthermore, with Fe3O4/Au nanomarker, EDS mapping of Au is successfully used to quantitate the porosity reliably and prevent from the overvaluation of a threshold technique based on simple gray level in SEM analysis. To benefit from a great boom of nano-science and nanomaterials with various elements and high designability, more nanoparticles would be desired as pore markers for improving imaging analysis, which is of significant potential to expand SEM imaging and EDS mapping to be a devisable method with analytical objectives-oriented in geological applications.

ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website. TEM and field-dependent magnetization curves of Fe3O4/Au hybrid nanocomposite before and after the acid treatment; EDS results of the mineral particle holding the moldic pores; Some typical SEM images of fine cracks with the impregnation of Fe3O4/Au nanocomposite as pore markers; REV and SREV investigation for the analysis of area porosity. (PDF)



AUTHOR INFORMATION

(15) Slatt, R. M.; O'Brien, N. R. AAPG Bulletin 2011, 95, 2017-2030.

Corresponding Author

(16) Ahmadov, R.; Vanorio, T.; Mavko, G. Lead. Edge 2009, 28, 1823.

* E-mail: [email protected] * E-mail: [email protected]

Page 6 of 7

ORCID

(17) Javadpour, F.; Moravvej Farshi, M.; Amrein, M. J. Can. Pet. Tech. 2012, 51, 236-243.

Na Li: 0000-0002-5787-6332 Notes

(18) Jiang, F.; Chen, J.; Xu, Z.; Wang, Z.; Hu, T.; Chen, D.; Li, Q.; Li, Y. Energy Fuels 2017, 31, 2669-2680.

The authors declare no competing financial interest.

ACKNOWLEDGMENT The financial support of the research by the Open Fund of State Key Laboratory of Oil and Gas Reservoir Geology and Exploitation (PLC201608), National Natural Science Foundation of P. R. China (Grant Nos. 41502139, 41573014), and National Key Technology Research and Development Program of China during the “13th Five-Year Plan” (2016ZX05034-002-006) are gratefully acknowledged.

(19) Cnudde, V.; Boone, M. N. Earth Sci. Rev. 2013, 123, 1-17. (20) Ji, Y. T.; Hall, S. A.; Baud, P.; Wong, T. F. Geophys. J. Int. 2015, 200, 699-717. (21) Bultreys, T.; De Boever, W.; Cnudde, V. Earth Sci. Rev. 2016, 155, 93-128. (22) Klaver, J.; Desbois, G.; Littke, R.; Urai, J. L. Mar. Pet. Geol. 2015, 59, 451-466. (23) Mastalerz, M.; Schimmelmann, A.; Drobniak, A.; Chen, Y. AAPG Bulletin 2013, 97, 1621-1643.

REFERENCES (1) Yang, J.; Hatcherian, J.; Hackley, P. C.; Pomerantz, A. E. Nat. Commun. 2017, 8 (1), 2197-2205. (2) Curtis, M. E.; Sondergeld, C. H.; Ambrose, R. J.; Rai, C. S. AAPG Bulletin 2012, 96, 665-677. (3) Chalmers, G. R.; Bustin, R. M.; Power, I. M. AAPG Bulletin 2012, 96, 1099-1119. (4) Ross, D. J. K.; Bustin, R. M. Mar. Pet. Geol. 2009, 26, 916-927. (5) Zhang, P. F.; Lu, S. F.; Li, J. Q.; Zhang, J.; Xue, H. T.; Chen, C. Energy Fuels 2017, 31, 9232-9239. (6) Yang, R.; He, S.; Hu, Q. H.; Sun, M. D.; Hu, D. F.; Yi, J. Z. Fuel 2017, 197, 91-99. (7) Gu, X.; Cole, D. R.; Rother, G.; Mildner, D. F. R.; Brantley, S. L. Energy Fuels 2015, 29, 1295-1308. (8) Zhang, Q.; Dong, Y. H.; Liu, S. M.; Elsworth, D.; Zhao, Y. X. Energy Fuels 2017, 31, 6951-6959. (9) Laun, F. B.; Kuder, T. A.; Wetscherek, A.; Stieltjes, B.; Semmler, W. Phys. Rev. E 2012, 86 (2), 021906. (10) Loucks, R. G.; Reed, R. M.; Ruppel, S. C.; Jarvie, D. M. J. Sediment. Res. 2009, 79, 848-861. (11) Milliken, K. L.; Rudnicki, M.; Awwiller, D. N.; Zhang, T. AAPG Bulletin 2013, 97, 177-200. (12) Cao, Q.; Zhou, W.; Deng, H.; Chen, W. J. Nat. Gas Sci. Eng. 2015, 27, 1381-1388. (13) Deng, H.; Hu, X.; Li, H. A.; Luo, B.; Wang, W. J. Nat. Gas Sci. Eng. 2016, 35, 309-319. (14) Loucks, R. G.; Reed, R. M.; Ruppel, S. C. AAPG Bulletin 2012, 96, 1071-1098.

(24) Curtis, M. E.; Ambrose, R. J.; Sondergeld, C. H.; Rai, C. S. Soc. Pet. Eng. 2011, 144391. (25) Curtis, M. E.; Ambrose, R. J.; Sondergeld, C. H. Soc. Pet. Eng. 2010, 137693. (26) Liu, S.; Ma, W.; Jansa, L.; Huang, W.; Zeng, X.; Zhang, C. Energy Exploration & Exploitation 2013, 31, 187-219. (27) Klaver, J.; Desbois, G.; Littke, R.; Urai, J. L. Int. J. Coal Geol. 2016, 158, 78-89. (28) Stordal, A. S.; Oliver, D. S. Adv. Water Resour. 2011, 34, 946956. (29) Li, Y. V.; Cathles, L. M.; Archer, L. A. J. Nanopart. Res. 2014, 16, 1-14. (30) Alaskar, M.; Ames, M.; Liu, C.; Li, K.; Horne, R. J. Pet. Sci. Eng. 2015, 127, 212-228. (31) Shokrlu, Y. H.; Babadagli, T. SPE Reservoir Evaluation & Engineering 2013, 16, 333-344. (32) Nazari Moghaddam, R.; Bahramian, A.; Fakhroueian, Z.; Karimi, A.; Arya, S. Energy Fuels 2015, 29(4), 2111-2119. (33) Li, N.; Yi, F.; Liao, P.; Chen, W.; Fu, M.; Zheng, J.; Du, L.; Liu, J.; Li, C. Anal. Chem. 2017, 89(22), 12550-12555. (34) Gao F.; Jiang Y.; Hu M.; Li S.; Zhai Q. Materials and Design, 2016, 111, 414–420. (35) Pommer, M.; Milliken, K. AAPG Bulletin, 2015, 99(9), 1713– 1744. (36) Tianhao, W.; Xiang, L.; Junliang, Z.; Dongxiao, Z. Water Resour. Res. 2017, 53, 5438-5450.


Page 7 of 7 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

figure 4 462x266mm (150 x 150 DPI)


nano-sized pore structure

Recommend Documents