Adaptive evolution and selection of stress-resistant Saccharomyces cerevisiae for very high-gravity bioethanol fermentation

Article Cite This: Energy Fuels 2019, 33, 1969−1982

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Characterization of the Full-Sized Pore Structure of Coal-Bearing Shales and Its Effect on Shale Gas Content Jizhen Zhang,†,‡ Xianqing Li,*,†,‡ Zou Xiaoyan,†,‡ Guangjie Zhao,†,‡ Baogang Zhou,†,‡ Jian Li,§ Zengye Xie,§ and Feiyu Wang∥,⊥

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State Key Laboratory of Coal Resources and Safe Mining, China University of Mining and Technology (Beijing), Beijing 100083, People’s Republic of China ‡ College of Geosciences and Surveying Engineering, China University of Technology (Beijing), Beijing 100083, People’s Republic of China § Langfang Branch, Research Institute of Petroleum Exploration & Development, PetroChina, Langfang 065007, People’s Republic of China ∥ State Key Laboratory of Petroleum Resource and Prospecting and ⊥College of Geosciences, China University of Petroleum (Beijing), Beijing 102249, People’s Republic of China ABSTRACT: The characterization of the pore structure and shale gas content provides useful information for shale gas reservoir assessment and evaluation and guides the exploration and development of shale gas. Fresh core samples obtained from three different basin formations in China were analyzed by field-emission scanning electron microscopy, low-pressure CO2 and N2 gas adsorption−desorption, high-pressure mercury intrusion, and methane adsorption experiments to clarify the pore structure characteristics of coal-bearing shales and their effects on shale gas content. The inter- and intraparticle pores, organic matter pores, and microfractures were well developed in coal-bearing shales. These pores had different geneses, morphologies, and sizes with main diameters of <6.5 and 80−200 nm and the main shape of slit, taper, and ink bottle. Pores with diameters <10 nm dominated the shale pore networks. Shale gas content was directly influenced by shale pores, and small pores had a large surface area, which resulted in the high adsorption capacity of shale gas. Clay mineral and total organic carbon contents positively controlled the pore structures and shale gas adsorption, whereas brittle minerals were counterproductive. Coalbearing shale gas content was lower than marine shale gas content, with an adsorption gas content percentage of 50−85%. The proportion of adsorbed gas decreased with the increase of pore size diameters, whereas the proportion of free gas increased. When the pore size diameter reached approximately 3.5 nm, the free and adsorption gases reached dynamic balance. The adsorption gas content would be slightly low with pore size ranges of >6.5 nm, whereas the free gas content would be stable and merely increase in the range of 100−300 nm.

1. INTRODUCTION The research and exploration of shale gas have attained considerable progress worldwide following the breakthrough in commercial shale gas exploration and utilization in the United States and Canada.1−6 Energy Information Administration reported that China possesses the most abundant shale gas resource in the world.7 If the shale gas resource in China can be massively developed and extensively used, then it will be greatly remarkable for the global energy security and economic development.8 As a source rock and reservoir, shale is a porous medium with strong heterogeneity.2,3,9−13 In contrast with tight sandstones and coal, shales typically possess lower permeabilities and porosities, more complex pore types, and larger pore scales.14−17 Shale gas is mainly hosted in the microscopic pore-fracture networks of shale as free or adsorption gas.18−22 The microscopic pore system in these networks directly controls shale gas occurrence and accumulation, thereby making the shale pore structure a research hotspot.11,12,23−26 The pore structure is the key factor that influences generation and accumulation mechanism and gas storage capacity.11,12,27,28 Therefore, full-sized pore structure characterization and shale gas investigation are important for © 2019 American Chemical Society

the evaluation of shale gas reservoir capacity and the prediction of favorable exploration regions. Many previous studies have focused on marine and terrestrial shales, and the research on coal-bearing shales is still confined comparatively.29−34 Reports of coal-bearing shale reservoir mainly focus on regional shales; however, comprehensive comparative studies of various types of coal-bearing shale from different basins are still rare.35−39 As a type of shale gas, coal-bearing shale gas is commonly generated from organic-rich shale in coal measure strata resulting from the marine−continental transitional environment or terrestrial environment with a large depositional area and remarkable shale gas resource potential.40−43 Coal-bearing shale has large pore size scales and complex pore structures, thereby making the comparative analysis of the whole-aperture pore structure difficult.40−44 With technological advancements, a range of multidisciplinary approaches has been used for the quantitative investigation of the microscopic pore system, which mainly Received: November 28, 2018 Revised: February 27, 2019 Published: February 27, 2019 1969

DOI: 10.1021/acs.energyfuels.8b04135 Energy Fuels 2019, 33, 1969−1982

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Energy & Fuels includes direct electron microscopy techniques [e.g., environmental focused-ion beam field-emission scanning electron microscopy (FE-SEM) and nano-CT scan], fluid injection techniques (e.g., mercury intrusion and CO 2 and N 2 adsorption), and nonfluid injection techniques (e.g., nuclear magnetic resonance and ultra/small-angle neutron scattering).12,14,20,23,25,30,44 In addition, given the large size ranges of shale pores, the International Union of Pure and Applied Chemistry (IUPAC) proposed a pore category that is used most extensively for the research of shale pores; the pores are classified into macro-, meso-, and micropores with sizes of >50, 2−50, and <2 nm, respectively.45,46 Microscopic pore structures and coal-bearing shale gas content are thus far rarely reported, and the controlling factors for multiscale pore development and shale gas content are still confined and require further improvement. The measurement of pore structures and gas adsorption capacities that combine highand low-pressure adsorption techniques is necessary to completely understand full-sized pore structures and shale gas adsorption capacity. Fresh core coal-bearing shale samples were collected from three typical formations in three famous shale gas-bearing basins to clarify the multiscale pore structure therein, the gas occurrence characteristics of different types of coal-bearing shale, and the differences between coal-bearing shales and marine and terrestrial shales in this work. Longtan shale in the Sichuan Basin is a typical marine−continental shale, whereas Shanxi shale in Ordos and Junggar basins is a typical terrestrial shale.6,26,30 These two types of shale greatly vary with the parent material sources and thermal evolution stages. Moreover, both types are located within stable intracratonic basins. In addition, although Shanxi and Badaowan shales in the Junggar Basin are terrestrial shale, the latter deposits in the representative inland superimposed basin.6,26,30 In this study, high-pressure mercury intrusion and low-pressure N2 and CO2 adsorption−desorption experiments were combined to clarify the full-sized pore structures. Methane adsorption experiments were used to investigate the gas adsorption characteristics in the coal-bearing shales. The relationships among the abundance and maturation of organic matter (OM), mineralogical composition, porosity, multiscale of pore structures, and methane adsorption volume were also discussed. The research results are important to understand the pore networks and their influencing factors within coalbearing shales and deeply investigate shale gas enrichment and storage mechanism in the future.

Figure 1. Distribution of major coal-bearing shale of China and the location of sampling wells.31−33,41,47,48

Figure 2. Stratigraphic age distribution of major coal-bearing shale in China and target strata in this study.

2. GEOLOGICAL SETTING Coal-bearing shales are widely distributed from the Carboniferous to the Neogene layer in the coal-bearing basin of the South, North, Northwest, and Northeast China (Figures 1 and 2).31−33,41,47,48 The buried depth of coal-bearing shales is usually shallower than that of marine shales, which is generally shallower than 3000 m. At present, except for the northeast region, the estimated reserves of coal-bearing shale resources in China are 32 × 1012 m3.50 The Carboniferous−Permian periods are important geological periods of sedimentary facie transformation (Fm) from marine to continental facies in China.47−51 During these periods, marine−continental shales are widely distributed in the coal-bearing strata in the craton tectonic basin, and the typical shales in these periods include Carboniferous Shuiquan−Bashan Fms in the Junggar Basin, Permian Liangshan−Longtan Fms in South China, and

Carboniferous Taiyuan Fms and Permian Benxi and Shanxi Fms in North China.47−51 To the late Triassic of the Mesozoic, nearly all the inland areas, except the Qinghai−Tibet Plateau in China, have entered the inland lake development stage, and the continental coal-bearing shales were widely deposited in large down-warped basins (e.g., Junggar, Ordos, and Sichuan basins) and coal-bearing fault basins (e.g., Songliao Basin) in Northeastern China during the Upper Triassic to the Middle and Lower Jurassic periods.47−51

3. SAMPLE AND EXPERIMENTS 3.1. Sample Collection and Preparation. In the present study, fresh core coal-bearing shale samples, which were obtained from the Lower Jurassic Badaowan Fm of the Junggar Basin, the Upper Permian Longtan Fm of the Sichuan Basin, and the Lower Permian Shanxi Fm of the Ordos Basin (Figures 1 and 2) were immediately 1970

DOI: 10.1021/acs.energyfuels.8b04135 Energy Fuels 2019, 33, 1969−1982

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Energy & Fuels Table 1. Properties of the Coal-Bearing Shale Samples in Different Basins of China mineralogy composition (wt %) Fm

sample ID

location basin

depth (m)

TOC (%)

Ro (%)

quartz

feldspar

carbonate

clay

others

Badaowan (J1b)

G6-1 G6-2 G2-1 G2-4 G2-7 G1-1 G1-3 G1-6 S14-1 S14-3 S14-7 S269-1 S269-2 S269-3 S13-1 S13-3 S13-5 S13-8 Z15-1 Z15-2 Z15-7 Z15-10 Z85-4 Z85-5 Z314-1 Z314-4 Z314-7

Junggar Junggar Junggar Junggar Junggar Junggar Junggar Junggar Ordos Ordos Ordos Ordos Ordos Ordos Ordos Ordos Ordos Ordos Sichuan Sichuan Sichuan Sichuan Sichuan Sichuan Sichuan Sichuan Sichuan

673.5 687.6 694.2 697.6 675.2 727.8 734.5 741.4 2674.2 2682.1 2694.8 3473.2 3481.7 3489.1 4415.3 4418.2 4432.6 4453.7 738.7 742.5 759.1 774.8 880.5 886.1 1026.5 1042.4 1053.8

3.23 3.81 3.74 5.78 2.83 4.85 2.56 3.42 3.74 1.54 1.56 2.58 2.96 3.85 2.14 2.75 2.57 3.58 3.34 2.14 3.96 4.25 2.34 5.21 4.60 2.57 3.54

0.80 0.81 0.85 0.87 0.87 0.91 0.87 0.89 1.09 1.11 1.12 1.46 1.54 1.63 1.58 1.85 1.63 1.68 1.95 1.93 2.04 2.08 2.13 2.24 2.60 2.15 2.34

36.70 32.2 38.6 35.1 35.7 34.9 28.9 29.7 31.8 35.4 31.8 29.7 23.6 26.5 34.5 36.8 29.7 33.9 24.8 26.4 29.4 25.3 28.7 24.5 25.5 23.7 28.9

0.9 1.2 2.4 1.2 0.8 1.3 0.5 0.6 2.1 1.6 0.4 1.5 0.6 1.7 1.5 1.5 0.1 1.5 0.7 0.8 0.9 1.2 0.5 1.2 0.5 0.8 1.5

3.1 4.8 0.2 0.6 1.5 0.5 0.4 0.8 7.0 1.2 3.5 2.5 3.4 8.1 3.8 3.1 0.8 1.3 0.7 1.5 7.0 5.4 2.6 3.5 3.9 1.2 3.5

57.8 59.2 57.6 59.3 56.6 61.3 67.8 63.7 57.2 56.4 60.0 63.1 70.5 61.7 55.4 55.2 64.3 60.8 68.2 67.5 58.3 63.4 63.0 67.4 66.8 71.8 62.5

1.5 2.6 1.2 3.8 5.4 2.0 2.4 5.2 2.0 5.4 4.3 3.2 1.9 2.0 4.8 3.4 5.1 2.5 5.6 3.8 4.4 4.7 5.2 3.4 3.4 2.5 3.6

Shanxi (P1s)

Longtan (P2l)

taken and meticulously investigated. The total organic carbon (TOC) content, vitrinite reflectance (Ro), and mineralogical components of each sample were tested in the laboratory prior to the study, and the results are listed in Table 1. The pore structure characteristics of each sample were then measured by FE-SEM and fluid injection experiments. 3.2. FE-SEM Observation. The FE-SEM imaging of the coalbearing shale samples was conducted using a Quanta 200F following the Chinese oil and gas industry standard SY/T 5162-1997. The samples mounted to FE-SEM were prepared as chips with the length, width, and height of approximately 1 cm, 1 cm, and 2 mm, respectively. Subsequently, all the investigated samples were coated with a 10 nm-thick gold to obtain good image quality by enhancing electrical conductivity. All the experiments were performed at a humidity and temperature of 35% and 25 °C, respectively. 3.3. High-Pressure Mercury Intrusion. High-pressure (up to 60 000 psia) mercury intrusion experiments were performed using an Auto Pore IV 9520 under the Chinese national standard GB/T 21650.3-2011. The preparation for the experiments is presented as follows. All shale samples were crushed into particles with sizes between 2 and 4 mm and then dried in a vacuum oven for approximately 18 h at about 110 °C. The measuring pore diameter sizes ranged from 3 nm to 1 mm with the intruded mercury from 60 000 to 1.5 psia. Micropores within the pore networks could hardly be detected via this technique. 3.4. Low-Pressure N2 and CO2 Adsorption. In accordance with GB/T 19587-2004 and GB/T 21650.2-2008 standards, N2 and CO2 adsorption−desorption analyses were performed using a Quantachrome Nova-4200e with the pressure range of 0−101.3 kPa after all the samples had been cut into approximately 80−250 μm and dried at 90 °C for 2 h under vacuum. Thereafter, N2 and CO2 adsorption− desorption curves were drawn with a relative partial pressure of 0.010−0.995 and 0.0001−0.032, respectively. On the basis of the N2 adsorption data, the mesopore surface area was analyzed using the

Brunauer−Emmett−Teller (BET) model, and the mesopore volume was measured by the Barrett−Joyner−Halenda (BJH) model.31,33,42,43 On the basis of the CO2 adsorption data, the micropore surface area was analyzed by the Dubunin−Radushkevich (D−R) model, and the micropore volume was measured by the density functional theory (DFT) model.31,33,42,43 3.5. Methane Adsorption Experiments. Methane adsorption experiments were performed using an IS-300 isothermal adsorption− desorption analyzer under GB/T 19560-2008 standard, with a high pressure of up to 20 MPa. Shale samples were powdered into particles with diameters of 50−150 mm and then oven-dried for 24 h at 60 °C to prepare for the methane sorption procedure. All experiments were conducted at humidity and temperature of 35% and 25 °C, respectively. The analytical results were fit using the Langmuir adsorption model. At present, the total content of adsorption shale gas is commonly based on the evaluation method of methane adsorption gas content in the coal bed,41,42 which can be calculated as follows

Vab = (VLP)/(PL + P)

(1) 3

where Vab is the content of adsorption gas, cm /g; VL is the Langmuir volume, which represents the absolute adsorption capacity, cm3/g; P is the equilibrium gas pressure, MPa; and PL is the Langmuir pressure, which corresponds to the half of VL.

4. RESULTS 4.1. Mineralogical Compositions and Geochemical Characteristics. Table 1 shows the results of the properties of the coal-bearing shale samples, including TOC content, Ro, and mineralogical compositions. The diagenetic evolution degree of Badaowan shales in the Junggar Basin is low, in which the OM is less affected by the hydrocarbon generation evolution; thus, these shales develop with high OM content. The Longtan shales in the Sichuan Basin suffer from strong 1971

DOI: 10.1021/acs.energyfuels.8b04135 Energy Fuels 2019, 33, 1969−1982

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Figure 3. Typical FE-SEM observations of multitypes of pores in coal-bearing shale samples [(a−d) interP pores; (b) intraP pores; (d−h) OM pores; and (g,h), microfractures].

Figure 4. Mercury intrusion/extrusion curves of the coal-bearing shale samples.

4.2. Pore Morphology and Distribution. 4.2.1. OM Pores. OM pores are developed within the OMs, which are closely associated with the formation and enrichment of shale gas and provide the major space for the occurrence and accumulation of shale gas.12,13,23,29,43,46 Previous studies have suggested that OM pores are developed from kerogen in shale during hydrocarbon generation.12,13,23,29,43,46 The investigated coal-bearing shale samples are abundant in OMs, and the mass of organic particles is relatively regular, which is more likely to develop OM pores compared with amorphous OMs. These pores are round, ellipsoid, scallop, crescent, and slit (Figure 3d−h) and are commonly distributed within the mineral matrix framework gaps. Moreover, these pores are commonly developed with a size lower than 1 μm, and the main size diameter range is 30−650 nm. In contrast with marine shales,12,13,46 the number of OM pores in coal shales is relatively lower. The phenomenon is caused by the usual experience of multiple stages of the sedimentary cycle of coalbearing shales, thereby damaging, deforming, and filling pores because of subsequent extrusion and denudation. 4.2.2. Interparticle Pores. Between mineral matrix and OM particles, the interparticle (interP) pores are well-developed in the investigated samples, particularly in shallow strata because of their susceptibility to stress compression and deforma-

hydrocarbon generation evolution, thereby causing pyrolysis hydrocarbon generation from OMs. Meanwhile, feldspar and other minerals are transformed into a large amount of clay minerals during the diagenetic evolution of shale. Thus, clay mineral contents are relatively high. The development of Shanxi shales in the Ordos Basin is between them. The investigated samples developed with abundant OMs have a TOC content of 1.54−5.78% (mean value of 3.31%). Ro values have samples that range from 0.80 to 2.60% (mean value of 1.52%). Moreover, coal-bearing samples from various basins are found in different evolution stages, in which the ranges of the Ro values of Badaowan, Shanxi, and Longtan shales are 0.80−0.91, 1.09−1.85, and 1.93−2.60%, respectively. X-ray diffraction analysis shows that clay minerals are rich in samples with the content range of 55.2−71.8 wt % (mean value of 62.1 wt %). The illite−smectite mixed layer in clays with the content range of 23.4−61.7 wt % (mean value of 43.6 wt %) is the primary mineral. In addition, the brittle mineral contents in the investigated samples are lower than those in the commercially developed marine shales in South China and North America,12,13,23,29,43 with the content range of 25.7− 41.4 wt % (mean value of 34.4 wt %), in which quartz is the most common mineral with the content range of 23.6−38.6 wt % (mean value of 30.5 wt %). 1972

DOI: 10.1021/acs.energyfuels.8b04135 Energy Fuels 2019, 33, 1969−1982

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Energy & Fuels Table 2. Pore Structure Parameters of Coal-Bearing Shale Samplesa pore volume (cm3/g)

pore surface area (m2/g)

micropore

mesopore

macropore

micropore

mesopore

macropore

sample ID

porosity (%)

(<2 nm)

(2−50 nm)

(>50 nm)

(<2 nm)

(2−50 nm)

(>50 nm)

average pore size (nm)

G6-2 G2-1 G1-1 S14-1 S269-3 S13-3 Z15-7 Z85-5 Z314-1

5.62 5.41 4.93 6.53 2.02 3.63 4.45 6.57 5.34

0.003 0.004 0.007 0.005 0.002 0.003 0.004 0.009 0.007

0.01 0.01 0.014 0.015 0.009 0.009 0.009 0.018 0.013

0.01 0.005 0.019 0.017 0.014 0.011 0.02 0.024 0.023

10.34 13.95 24.81 14.83 5.16 11.25 13.19 31.3 21.96

5.86 4.91 7.43 9.09 0.26 3.71 4.22 14.13 7.91

0.01 0.04 0.07 1.04 0.38 0.46 0.71 1.45 0.74

70.5 130.8 58.7 95.2 135 68.4 75.3 20.2 46.3

a Pore volume and pore surface area data of micro-, meso-, and macropores were tested based on CO2 adsorption, N2 adsorption, and mercury intrusion measurements, respectively.

Figure 5. N2 adsorption/desorption isotherms of coal-bearing shale samples.

tion.12,13,23,29,43,46 InterP pores are common in coal-bearing shales with the shape of triangle, quadrangle, polygon equal and subangular, and irregular (Figure 3a−d). The sizes of these pores are controlled by the degree of compaction, cementation, and particle sizes, with a size commonly lower than 2 μm, and the main size diameter range is 50−800 nm. The self-generated pyrite aggregates are mostly developed in the marine− terrestrial transitional coal-bearing shales; thus, interP pores are generally developed in these shales because of the untight accumulation among crystal particles (Figure 3c). In addition, pyrite particles are usually associated with OMs because they are usually formed under reduction conditions. Hence, most intercrystal pores are subducted by filling with OMs. 4.2.3. Intraparticle Pores. Intraparticle (intraP) pores, which are commonly developed within the mineral matrix particle, can be divided into primary and secondary.12,13,23,29,43,46 IntraP pores are commonly observed in coal-bearing shales with high rangeability (main of the 0.05−2 μm; Figure 3b). The morphologies of these pores are irregular, that is, they commonly have gourd, slit, and moniliform shapes. These pores are also easily affected by tectonic and diagenetic stresses; particularly, those with a diameter of more than 100 nm can be reduced by stress.

4.2.4. Microfractures. As the main part of pore fracture networks, microfractures are important for gas exploration and development because they provide effective storage space for shale gas and greatly improve the seepage capacity of fluid.12,13,23,29,43,46 The microfractures in coal-bearing shale can be divided into tectonic and nontectonic. The former is formed by the geological tectonic movement, whereas the latter is produced by diagenetic cementation, mineral crystallization, pressure dissolution, dry cracking, and weathering. These microfractures can run through quartz, carbonate, and OMs, with length and width ranging from approximately 500 nm to 2 mm and <10−200 nm, respectively, and with linear and polygonal shapes (Figure 3g,h). Moreover, these microfractures have remarkable openness and ductility, which is beneficial for pore connectivity and fluid transmission. 4.3. Pore Structure Characterization. 4.3.1. Macropores Based on Mercury Intrusion Measurements. The macropore structures of the coal-bearing shale samples were quantitatively characterized. Figure 4 presents the mercury intrusion and extrusion curves of the coal-bearing shale samples. The shape of these curves is analogical, thereby demonstrating that the coal-bearing shales in different basins have semblable pore types. The intruded and extruded amounts of mercury increase 1973

DOI: 10.1021/acs.energyfuels.8b04135 Energy Fuels 2019, 33, 1969−1982

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Energy & Fuels

Sample S269-3 has the lowest CO2 gas adsorption volume, with the lowest calculated DFT micropore volume of 0.002 cm3/g and the lowest calculated D−R micropore special surface area of 5.16 m2/g, thereby showing its poor gas adsorption capacity. Therefore, the CO2 gas adsorption capacity of different samples varies greatly because of the different contents of OMs. 4.4. Pore Shapes and Connectivity. The adsorption characteristics of the surface of solid adsorption medium are diversified, thereby causing difference in gas adsorption isotherms.51 The N2 adsorption−desorption curves generally form hysteresis loops, and their morphological characteristics can be used to recognize the pore shapes in shale.31,33,43 Six types of the gas adsorption isotherm (i.e., types I, II, III, IV, V, and VI) are divided by IUPAC,51 and Labani et al. (2013)52 discussed the corresponding relationships between hysteresis loop types and pore shapes by dividing hysteresis loop types into five types (i.e., types A−E). As shown in Figure 5, the morphology of all N2 gas adsorption−desorption curves has the noticeable characteristic of type IV hysteresis loops, which indicates that the coal-bearing shale pores have good connectivity and adsorption characteristics conducive to gas adsorption and seepage. The hysteresis loops in the samples of different basins with distinct thermal maturities vary. The hysteresis loops of samples with an Ro of <1.0% are similar to type C, which indicates that taper-shaped pores are well developed in these samples. However, those of samples with an Ro of 1.0−1.8% are similar to types B and C, which implies that slit- and taper-shaped pores are the main geometric space configurations of pore networks. Moreover, those of samples with an Ro of >1.8% are similar to types B, C, and E, which demonstrates the mixed pore shapes of the slit, taper, and ink bottle. 4.5. Whole-Aperture Pore Size Distribution. As an important pore structural parameter, pore size distribution can directly influence the adsorption and migration mechanism of shale gas.53,54 In this study, comprehensive experiments are conducted in the investigated coal-bearing shale samples by combining mercury intrusion and N2 and CO2 gas adsorption experiments to analyze systematically the size distributions of whole-aperture pores. These experiments provide remarkable formation, including aperture distribution ranges, dominant frequency ranges, and the proportion of pore structural parameters under different pore aperture intervals (Figure 7). The plot in Figure 7 shows that a significant portion of shale pores are distributed in the size range of <6.5 and 80−200 nm. Generally, the proportion of pore structural parameters decreases with the increase of pore size diameters. The percentages decrease at approximately 6.5 nm stabilize until the aperture closes to 100 nm by showing a small peak, gradually decrease after the peak, and finally stabilize. Micropores and mesopores dominate the nanometer-scale pore networks, providing 56.45 and 41.06% of the total pore surface area and 41.30 and 46.48% for the total pore volume, respectively. Particularly, pores with the size range of <10 nm are the major contributors to the pore structures of the nanometer-scale pore system, which account for 84.62 and 96.37%. Coal-bearing shale samples develop with a large amount of nanoscale pores, with the average pore diameters ranging from 20.2 to 135.0 nm (mean value of 77.82 nm). A large amount of micro- and mesopores provide major space for gas adsorption and storage, which shows good potential for shale gas enrichment.

with pressure, and the increasing trend becomes slower as pressure reaches approximately 80 psia. Shale pores with the size of >20 μm are commonly developed under the pressure of 0.1−80 psia, whereas few shale pores are developed under the pressure of 80−800 psia. Furthermore, the large amount of nanoscale pores is well developed because the mercury intrusion volumes increase again as the pressure reaches over 800 psia. Only the structural parameters of the macropores are measured using this method because of the limitation of the mercury intrusion technique. Thus, N2/CO2 adsorption− desorption methods are used to characterize the properties of mesopores and micropores in shale systems. Table 2 lists the results of multiscale pore structures of the investigated coalbearing shale samples. The microscopic pores were wellestablished in the developed coal-bearing shale samples, with a porosity range of 2.02−6.57% (average of 4.94%; Table 2). The calculated pore volume range of macropores is 0.005− 0.024 cm3/g (average of 0.016 cm3/g; Table 1). The calculated range of the special surface area of macropores is 0.01−1.45 m2/g (mean value of 0.54 m2/g; Table 2). 4.3.2. Mesopores Based on N2 Adsorption Experiments. Figure 5 shows the N2 adsorption−desorption isotherms of coal-bearing shales. Differences among the curves of the investigated shale samples are observed within different basins, thereby demonstrating the different pore shapes developed in coal-bearing shales. As illustrated in Figure 5, the N 2 adsorption volume increases with the relative pressure (P/ P0), and these curves separate with the desorption as P/P0 over approximately 0.4, thereby generating hysteresis loops and reflecting the capillary condensation that occurs in mesopores.51 In addition, N2 adsorption−desorption isotherms are unclosed in some samples (e.g., Z85-5 and Z131-8) under P/ P0 of <0.4, which demonstrates the swelling phenomenon that occurs in some samples. On the basis of the BJH model and the data obtained by N2 adsorption−desorption, the calculated range of mesopore volumes is 0.009−0.018 cm3/g (mean value of 0.012 cm3/g; Table 1). The calculated range of BET mesopore special surface areas is 0.26−14.13 m2/g (mean value of 6.39 m2/g; Table 2). 4.3.3. Micropores Based on CO2 Adsorption Experiments. Figure 6 shows the CO2 adsorption−desorption isotherms of

Figure 6. CO2 isothermal adsorption curves in the coal-bearing shale samples.

coal-bearing shales. These isotherms are classified as type I, which corresponds to microporous solids with monolayer adsorption.51 Among all the investigated shale samples, sample Z85-5 has the highest CO2 gas adsorption volume with the largest calculated DFT micropore volume of 0.009 cm3/g and the largest calculated D−R micropore special surface area of 31.30 m2/g, thereby showing its strong gas adsorption capacity. 1974

DOI: 10.1021/acs.energyfuels.8b04135 Energy Fuels 2019, 33, 1969−1982

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Energy & Fuels

Figure 7. Continuous characterization of the nanometer pore size distribution in the coal-bearing shale pore-fracture system.

4.6. Methane Adsorption Capacity. The adsorption gas content is generally higher than free gas in coal-bearing shales, with the percentage of 50−85%.53,54 The adsorption gas content is commonly analyzed using the methane adsorption experiments and Langmuir adsorption model, which is the key factor for shale reservoir assessment and gas resource favorable area evaluation.32,33,46 Figure 8 shows the results of the

5. DISCUSSION 5.1. Controlling Factors for Pore Development. Figure 9 shows the influences of TOC content and mineralogical compositions of multiscale pore structures. TOC contents have strong positive relationships with the pore volume and pore surface area of total pores, and the fitting coefficient (R2) is 0.681 (Figure 9a) and 0.618 (Figure 9b), respectively. This understanding is consistent with the previous point of view.56 TOC contents have strong positive relationships with the pore volume of micropores (R2 = 0.721) and mesopores (R2 = 0.573) (Figure 9a) and have a significant positive linear correlation with the micropore surface area (R2 = 0.668) and a weak positive linear correlation with the mesopore surface area (R2 = 0.446; Figure 9b). These results show that OMs dominate the development of micropores and mesopores and they extremely influence the special surface area of micropores. This phenomenon is caused by the pores within OMs being the main of micropores and mesopores that generally have a large pore volume and surface area, whereas more OM pores are commonly developed in shales that have more OMs. The pore volumes (Figure 9c) and specific surface areas (Figure 9d) of micro-, meso-, and macropores show similar evolution rules with the increase of maturity. The variation trend of the pore volume and specific surface area with maturity, which can be roughly divided into three stages, is consistent. In stage I (0.8% < Ro < 1.0%), the OM pores are well-developed, and the organic acids produced will corrode the easily dissolved minerals to produce organic acids, thereby making the dissolved pores develop in great quantity because of the hydrocarbon generation from OMs. In stage II (1.0% < Ro < 1.7%), the organic hydrocarbon generation is terminated, and the shale pores are damaged under the influence of confining pressure, thereby resulting in reduced pore volume. In stage III (1.7% < Ro < 2.5%), the shale pores considerably develop again under the influence of the secondary cracking of OMs and hydrocarbon generation.

Figure 8. Methane isothermal adsorption curves of the coal-bearing shale samples.

methane adsorption experiments and Langmuir adsorption isotherms of the investigated coal-bearing shale samples. The measured methane sorption capacities of the samples investigated based on eq 1 range from 1.57 to 4.17 m3/t (mean value of 2.53 m3/t). The shale gas content of commercial marine shales in North America is 1.7−9.9 m3/ t;1,20 comparably, that in coal-bearing shales is relatively lower, with the value of 1.6−6.5 m3/t.1,14,34 Although the gas content is over the lower limit of industrial use (empirically 1.1 m3/t), the high content of the adsorption gas is disadvantageous for the later development of coal-bearing shale gas. 1975

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Figure 9. Relationships between the TOC, clay, and brittle minerals and pore structures.

0.419, respectively, which are lower than the influence of TOC contents. Mineralogical composition is another crucial factor that controls the development of coal-bearing shale pores. Good positive linear correlations between clay mineral contents and the pore volume of micropores (R2 of 0.535) and macropores (R2 of 0.503) are observed (Figure 9c). The results illustrate that the pores within clay mineral layers comprise micropores and mesopores, and the clay mineral contents increase with the pore volumes of micropores and mesopores. However, brittle mineral contents show negative correlations with the multiscale pore structures in the investigated samples (Figure 9e,f). The pore diameter of the OM pores is small because these pores are generated because of the hydrocarbon gas emission from the OMs. The highly plastic clay can be greatly influenced by diagenesis, which reduces the size diameter of native pores, thereby leading to the enrichment of mesopores in clay minerals. In addition, compaction and cementation decrease the pore sizes among brittle mineral particles during diagenesis. Above all, TOC content and clay minerals are beneficial to the development of the multiscale pore structures in coal-bearing shales, and OMs and clays dominate the development of micropores and

In addition, weak positive linear correlation is observed between the clay mineral contents and the pore structures of total pores (Figure 9e,f), with R2 of 0.572 and 0.419, respectively, which are lower than the influence of TOC contents. Mineralogical composition is another crucial factor that controls the development of coal-bearing shale pores. Clay mineral contents and pore volume of micropores (R2 of 0.535) and macropores (R2 of 0.503) have good positive linear correlations (Figure 9e). The results illustrate that the pores within clay mineral layers are the micropores and mesopores, and the clay mineral contents increase with the pore volumes of micropores and mesopores. However, brittle mineral contents show negative correlations with the multiscale pore structures in the investigated samples (Figure 9g,h). Therefore, TOC content and clay minerals are beneficial to the development of the multiscale pore structures in coal-bearing shales, and OMs and clays dominate the development of micropores and macropores, whereas the brittle mineral negatively affects them. Furthermore, weak positive linear correlation is observed between the clay mineral contents and the pore structures of the total pores (Figure 9c,d), with R2 values of 0.572 and 1976

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Figure 10. Correlations between the CH4 adsorption volume and porosity (a), average pore size diameter (b), pore volume (c), and pore surface area (d).

Figure 11. Effects from TOC content (a), Ro (b), clay (c), and brittle mineral content (d) on the shale gas sorption capacity.

macropores, respectively, whereas the brittle mineral negatively affects them. 5.2. Influence of the Pore Structure on Adsorption Capacity of Shale Gas. Total porosity shows good positive correlation with the adsorbed shale gas content, with R2 of 0.588 (Figure 10a). Thus, a large pore space is beneficial for shale gas adsorption. Figure 10b shows the relationships between the average pore diameter with adsorption gas content in the investigated shale samples, in which the large average pore diameters decrease the adsorption gas contents. The phenomenon is caused by the small average pore diameters that resulted from the large amount of micropores, thereby developing with a larger pore surface area and leading to higher adsorption gas content (Figure 10c,d). The

relationships between the adsorption gas content and pore volume of multiscale pores are illustrated in Figure 10c. From the figure, significant positive linear correlations are observed between the adsorption gas content and the pore volume of mesopores and micropores, with R2 values of 0.857 and 0.714, respectively. In addition, significant positive linear correlations are observed between the adsorption gas content and the surface areas of the mesopore and micropore, with R2 values of 0.921 and 0.729, respectively (Figure 10d). However, the relationships between the adsorption gas content and macropore structures are weak, with the R2 of 0.348 (Figure 10c) and 0.338 (Figure 10d). These results demonstrate that the major space and surface area in shale are mainly provided by and micro and mesopores. 1977

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Energy & Fuels 5.3. Controlling Factors for the Shale Gas Adsorption Capacity. Figure 11a shows that TOC content has a good positive linear correlation with the methane adsorption capacity, with R2 of 0.524. This result suggests that the adsorption gas content increases with the OM content because the TOC controls the development of small pores (Figure 9a), which have a large adsorption surface area (Figure 9b), thereby resulting in high methane adsorption capacity (Figure 11). The result is consistent with the observations in typical marine shales.12,20,23,43 Figure 11b indicates that the methane excess adsorption volume initially decreases with the increase in maturity because of buried depth and compaction. Subsequently, it increases dramatically after the Ro reaches higher than 1.7%, which is closely associated with the hydrocarbon generation from OMs. Therefore, OM contents can directly affect hydrocarbon generation and control the development of micropores, thereby affecting the adsorption capacity of shale gas. The clay mineral contents can also influence the shale gas adsorption capacity (Figure 11c). The plot shows that the clay mineral content has a weak positive linear relationship with the shale gas adsorption capacity (R2 = 0.403; Figure 11c). Hence, clay minerals positively affect the gas adsorption capacity in shales and have lower gas adsorption content than OMs. This result is caused by the clay minerals containing considerable amounts of large pores, which have smaller adsorption surface area than smaller pores, thereby resulting in the weak shale gas adsorption capacity. However, different from the effects of OMs and clays, the brittle mineral content shows a negative relationship with the methane excess adsorption volume (R2 = 0.388, Figure 11d). Thus, the Fm of brittle minerals will adversely affect the gas adsorption capacity. This phenomenon is caused by the brittle minerals that hinder the development of pores in coal-bearing shales (Figure 9e,f). 5.4. Gas Occurrence Characteristics in Coal-Bearing Shale Pores. Shale gas is hosted in the shale reservoir and is divided into two phase states, namely, adsorption and free gas, whereas dissolved gas content is relatively low.1,55 The adsorption gas is commonly adsorbed on the surfaces of OMs, clay mineral particles, and the inner surfaces of microscopic pore−fracture networks by physicochemical reactions, whereas the free gas mainly exists in shale pores and fractures.20,57 Elucidating the shale gas occurrence characteristic will provide useful formation for shale gas resource assessment and plays a significant role in post-gas production and final exploration and development. At present, the proportion of adsorption and free gas contents is controversial. The adsorption gas contents of the typical commercial marine shale reservoirs in South China and North America commonly range from 20 to 85%, whereas that of the coal-bearing shales generally ranges from 50 to 85%.1,5,9,20,33 As a significant reference parameter for evaluating reservoir resource potential and optimizing shale gas exploration areas, investigation on shale gas content is a hot topic of shale gas reservoir research at present.5,9,20 Adsorption and free gas content are considered important parameters, which are mainly controlled by the pore surface area and volume, respectively. Few studies highlight the content, proportion and occurrence characteristics of adsorption and free gas in different pore sizes (Figure 12). Particularly, no relative field experiment in coalbearing shales has been reported.58 The simplified local density (SLD) model can be used to analyze the methane adsorption density of different adsorption media.59−61 The potential energy parameters of different

Figure 12. Adsorbed gas content proportion in typical coal-bearing and marine shale reservoirs.1,5,9,20,33

components of shale vary in the SLD simulation analysis (Figure 13a).61 The content and proportion of the occurrence characteristics of adsorption and free gas in different pore sizes (Figure 13b)61 could be calculated based on the SLD model (Figure 13c).59−61 On the basis of the data of clay mineral and OM contents by the whole rock analysis of the shale samples listed in Table 1 and the potential energy parameters of different components of shale (Figure 13a), the average potential energy parameters of each sample can be obtained by weighted calculation.59−61 Then, these parameters are considered in the SLD simulation program59−61 to simulate the methane density distribution in different pore size diameters of the investigated coal-bearing shale samples, and the average methane contents in different pore sizes of coalbearing shale are shown in Figure 13c. On the basis of the proportion of shale gas content within different pore size diameters (Figure 13b) and the average methane content distribution in different pore size diameters of the coal-bearing shale samples (Figure 13c), the content of adsorption gas and free gas in different size diameters of shale pores can be calculated, and the results are shown in Figure 14. Thus, the occurrence characteristics of adsorption and free gas in different pore sizes in coal-bearing shale samples can be clarified. The average shale gas content in nanoscale pores of coal-bearing shales is 3.81 m3/t, in which the average content proportions of the adsorption and free gases are 65.9 and 34.1%, with values of 2.51 and 1.29 m3/t, respectively. The adsorption shale gas is converted in micro-, meso-, and macropores with 1.62, 0.89, and 0.01 m3/t, contributing to 2.5, 23.4, and 0.26% of the total gas content in shale, respectively. The free gas content in micro-, meso-, and macropores are 0, 0.86, and 0.43 m3/t, contributing 0, 22.6, and 11.3% of the total shale gas content, respectively (Figure 14). These results show that huge amounts of shale gas are adsorbed in micropores because of their large pore surface. However, the shale gas is hardly stored in micropores restricted by the diameter of methane molecules (Figure 14). Small pores commonly have a large pore-specific surface area, and thus a high amount of shale gas is adsorbed correspondingly (Figure 14). Single- and double-layer adsorptions of methane gas molecules exist in the micropores because of the adsorption potential energy, which will fill the space of micropores because the methane gas molecule diameter is 0.45 nm.41,59,60 Thus, little space is available for the free gas occurrence in micropores. When the pore diameter is over 2 nm, free gas starts to exist, and the increase of pore sizes will increase free 1978

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Figure 13. Methane density distribution in the throat of 2 nm diameter (a),61 proportion of shale gas content within different pore size diameters (b),61 and methane content in different pore sizes of coal-bearing shale (c).

Figure 14. Adsorption and free gas occurrence characteristics in different sizes of pores from coal-bearing shale.

(1) Coal-bearing shales have complex pore types and structures. FE-SEM imaging has identified four types of pore, namely, interP, intraP, and OM pores and microfractures. All these pores are commonly developed in coal-bearing shales and have different genetic mechanisms, morphologies, and sizes. (2) The combination of mercury intrusion and N2 and CO2 adsorption−desorption experiments provide the pore structure formation of macro-, meso-, and micropores. Most pores range in the size of <6.5 and 80−200 nm. The mixed pore shapes of slit, taper, and ink bottle slitand taper-shaped pores are the main geometric space configurations of pore networks, which have good connectivity and adsorption characteristics. Micropores and mesopores dominate the nanometer-scale pore system. Particularly, pores with sizes of <10 nm are the major contributors to the pore structures of the nanometer-scale pore system, which account for 84.62 and 96.37%. (3) OMs and clays collectively contribute to the pore surface areas and volumes of macro-, meso-, and micropores and

gas contents. Given the pore size of approximately 3.5 nm, the contents of adsorbed and free gases in nanoscale pores are 0.110 and 0.111 m3/t, respectively. The proportion of both gases is equal (Figure 14). At this point, the free and adsorbed gases reach a dynamic equilibrium state. Given the pore diameter of over 3.5 nm, the free gas content is higher than the adsorption gas content. Given the pore size diameter of approximately 6.5 nm, the content of adsorption gas is extremely low, whereas the free gas content remains in a stable state until it reaches the size of 100 nm (Figure 14). Given the size ranges of 100−300 nm, the free gas content increases because of the proliferation of pores and then decreases to a steady state after 300 nm (Figure 14).

6. CONCLUSION In this study, the pore structure of the coal-bearing shale samples from different basins in China is characterized qualitatively and quantitatively using FE-SEM and high- and low-pressure fluid injection experiments. The main conclusions in this work can be drawn as follows. 1979

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adsorption gas content in coal-bearing shales, whereas brittle mineral contents show a negative effect on them. Shale pores are positively related to the adsorption gas content, especially small pores with high gas adsorption capacity.

REFERENCES

(1) Curtis, J. B. Fractured shale−gas systems. AAPG Bull. 2002, 86, 1921−1938. (2) Wang, H.; Li, J.; Zhao, Q.; Lin, Y. Resources and development of new energy in China. Acta Pet. Sin. 2009, 30, 469−474. (3) Li, X. J.; Hu, S. Y.; Cheng, K. M. Suggestions from the development of fractured shale gas in North America. Pet. Explor. Dev. 2007, 34, 392−400. (4) Johnson, C.; Boersma, T. Energy (in)security in Poland the case of shale gas. Energy Policy 2007, 53, 389−399. (5) Selley, R. C. UK shale gas: The story so far. Mar. Pet. Geol. 2012, 31, 100−109. (6) Ma, Y.; Cai, X.; Zhao, P. China’s shale gas exploration and development: Understanding and practice. Pet. Explor. Dev. 2018, 45, 589−603. (7) EIA. Technically Recoverable Shale Oil and Shale Gas Resources: An assessment of 137 Shale Formations in 41 Countries outside the United State. http://www.eia.gov/analysis/studies/ worldshalegas/ (accessed June 17, 2013). (8) Fu, C. China’s shale gas and shale oil resources: Opportunities and challenges. Energy Explor. Exploit. 2014, 32, 759−769. (9) Jarvie, D. M. Shale Resource Systems for Oil and Gas: Part 1 Shale−gas Resource Systems. AAPG Mem. 2012, 97, 89−119. (10) Furmann, A.; Mastalerz, M.; Bish, D.; Schimmelmann, A.; Pedersen, P. K. Porosity and pore size distribution in mudrocks from the Belle Fourche and Second White Specks Formations in Alberta, Canada. AAPG Bull. 2016, 100, 1265−1288. (11) Chalmers, G. R. L.; Bustin, R. M. The organic matter distribution and methane capacity of the Lower Cretaceous strata of Northeastern British Columbia, Canada. Int. J. Coal Geol. 2007, 70, 223−239. (12) Chalmers, G. R.; Bustin, R. M.; Power, I. M. Characterization of gas shale pore systems by porosimetry, pycnometry, surface area, and field emission scanning electron microscopy/transmission electron microscopy image analyses: Examples from the Barnett, Woodford, Haynesville, Marcellus, and Doig units. AAPG Bull. 2012, 96, 1099− 1119. (13) Chalmers, G. R. L.; Ross, D. J. K.; Bustin, R. M. Geological controls on matrix permeability of Devonian Gas Shales in the Horn River and Liard basins, northeastern British Columbia, Canada. Int. J. Coal Geol. 2012, 103, 120−131. (14) Loucks, R. G.; Reed, R. M.; Ruppel, S. C.; Jarvie, D. M. Morphology, Genesis, and Distribution of Nanometer-Scale Pores in Siliceous Mudstones of the Mississippian Barnett Shale. J. Sediment. Res. 2009, 79, 848−861. (15) Nelson, P. H. Pore-throat sizes in sandstones, tight sandstones, and shales. AAPG Bull. 2009, 93, 329−340. (16) Leu, L.; Georgiadis, A.; Blunt, M. J.; Busch, A.; Bertier, P.; Schweinar, K.; Liebi, M.; Menzel, A.; Ott, H. Multiscale description of shale pore systems by scanning SAXS and WAXS microscopy. Energy Fuels 2016, 30, 10282−10297. (17) Xi, Z.; Tang, S.; Wang, J.; Yi, J.; Guo, Y.; Wang, K. Pore Structure and Fractal Characteristics of Niutitang Shale from China. Minerals 2018, 8, 163. (18) Bowker, K. A. Barnett Shale gas production, Fort Worth Basin: Issues and discussion. AAPG Bull. 2007, 91, 523−533. (19) Xi, Z.; Wang, J.; Hu, J.; Tang, S.; Xiao, H.; Zhang, Z.; Xing, Y. Experimental Investigation of Evolution of Pore Structure in Longmaxi Marine Shale Using an Anhydrous Pyrolysis Technique. Minerals 2018, 8, 226. (20) Jarvie, D. M.; Hill, R. J.; Ruble, T. E.; Pollastro, R. M. Unconventional shale-gas systems: The Mississippian Barnett Shale of north-central Texas as one model for thermogenic shale-gas assessment. AAPG Bull. 2007, 91, 475−499. (21) Montgomery, S. L.; Jarvie, D. M.; Bowker, K. A.; Pollastro, R. M. Mississippian Barnett Shale, Fort Worth basin, north−central Texas: Gas−shale play with multi−trillion cubic foot potential. AAPG Bull. 2006, 90, 963−966.

(4) Adsorption gas provides major contribution for total shale gas content, with proportions of 50−85%. The occurrence characteristics of adsorption and free gases in coal-bearing shale pores are relatively different. Large amounts of adsorption gas are available, whereas no free gas is stored in micropores. Adsorption and free gas contents coexist in the mesopore. The small pores commonly adsorb a large amount of shale gas. At the size of approximately 3.5 nm, both phases of gas reach a dynamic balance. The adsorption gas content in the pores with a diameter of >6.5 nm is low (<0.01 m3/t). The free gas content is relatively stable with sizes of 6.5− 100 nm and shows a slight increase with pore sizes of 100−300 nm.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Phone: +8610 62331854-8131, +8610 1355287755. Fax: +8610 62339208. ORCID

Xianqing Li: 0000-0002-5690-4012 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors thank various organizations for the financial support, including the National Natural Science Foundation of China (41572125 and U1810201), the National Science and Technology Major Project of China (2016ZX05007-003), the N a t i o na l K e y R es e a r c h P r o j e c t ( 9 7 3 P r o g r a m ) (2012CB214702), and the Fundamental Research Funds for the Central Universities (2010YM01).

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NOMENCLATURE EIA = Energy Information Administration SEM = scanning electron microscopy FIB-SEM = focused-ion beam SEM FE-SEM = field-emission SEM NMR = nuclear magnetic resonance USANS = ultrasmall-angle neutron scattering SANS = small-angle neutron scattering IUPAC = International Union of Pure and Applied Chemistry OM = organic matter Fm = formation BET = Brunauer−Emmett−Teller BJH = Barrett−Joyner−Halenda D−R = Dubinin−Radush DFT = density functional theory TOC = total organic carbon Ro = vitrinite reflectance XRD = X-ray diffraction interP = interparticle intraP = intraparticle SLD = simplified local density 1980

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reservoirs under different sedimentary environments. Mar. Pet. Geol. 2019, 102, 426−438. (40) Chen, Q.; Zhang, J.; Tang, X.; Li, W.; Li, Z. Relationship between pore type and pore size of marine shale: An example from the Sinian-Cambrian formation, upper Yangtze region, South China. Int. J. Coal Geol. 2016, 158, 13−28. (41) Wang, Y.; Zhu, Y.; Liu, S.; Zhang, R. Methane adsorption measurements and modeling for organic-rich marine shale samples. Fuel 2016, 172, 301−309. (42) Wang, Y.; Zhu, Y.; Chen, S.; Li, W. Characteristics of the Nanoscale Pore Structure in Northwestern Hunan Shale Gas Reservoirs Using Field Emission Scanning Electron Microscopy, High-Pressure Mercury Intrusion, and Gas Adsorption. Energy Fuels 2014, 28, 945−955. (43) Yan, G.; Wei, C.; Song, Y.; Zhang, J. Pore characteristics of organic-rich shale in the Carboniferous-Permian coal-bearing strata in Qinshui Basin. Energy Explor. Exploit. 2017, 35, 645−662. (44) Zhang, J.; Li, X.; Wei, Q.; Sun, K.; Zhang, G.; Wang, F. Characterization of Full-Sized Pore Structure and Fractal Characteristics of Marine-Continental Transitional Longtan Formation Shale of Sichuan Basin, South China. Energy Fuels 2017, 31, 10490−10504. (45) Rouquerol, J.; Avnir, D.; Fairbridge, C. W.; Everett, D. H.; Haynes, J. M.; Pernicone, N.; Ramsay, J. D. F.; Sing, K. S. W.; Unger, K. K. Recommendations for the characterization of porous solids (Technical Report). Pure Appl. Chem. 1994, 66, 1739−1758. (46) Loucks, R. G.; Reed, R. M.; Ruppel, S. C.; Hammes, U. Spectrum of pore types and networks in mudrocks and a descriptive classification for matrix-related mudrock pores. AAPG Bull. 2012, 96, 1071−1098. (47) Cao, D.; Wang, C.; Jing, L. I.; Qin, R.; Yang, G.; Zhou, J. Basic characteristics and accumulation rules of shale gas in coal measures. Coal Geol. Explor. 2014, 42, 25−30. (48) Sun, P.; Sachsenhofer, R. F.; Liu, Z.; Strobl, S. A. I.; Meng, Q.; Liu, R.; Zhen, Z. Organic matter accumulation in the oil shale- and coal-bearing Huadian Basin (Eocene; NE China). Int. J. Coal Geol. 2013, 105, 1−15. (49) Chen, M.; Cheng, Y.; Li, W. Exploitation and utilization of oil shale in the coal measure strata of the haishiwan mine, yaojie coalfield, China. Oil Shale 2015, 32, 335. (50) Zou, C.; Yang, Z.; Dai, J.; Dong, D.; Zhang, B.; Wang, Y.; Deng, S.; Huang, J.; Liu, K.; Yang, C.; Wei, G.; Pan, S. The characteristics and significance of conventional and unconventional Sinian-Silurian gas systems in the Sichuan Basin, central China. Mar. Pet. Geol. 2015, 64, 386−402. (51) Sing, K. S. W.; Everett, D. H.; Haul, R. A. W.; Moscou, L.; Pierotti, R. A.; Rouquerol, J.; Siemieniewska, T. Reporting physisorption data for gas/solid systems with special reference to the determination of surface area and porosity. Pure Appl. Chem. 1985, 57, 603−619. (52) Labani, M. M.; Rezaee, R.; Saeedi, A.; Hinai, A. A. Evaluation of pore size spectrum of gas shale reservoirs using low pressure nitrogen adsorption, gas expansion and mercury porosimetry: a case study from the Perth and Canning Basins, Western Australia. J. Pet. Sci. Eng. 2013, 112, 7−16. (53) Furmann, A.; Mastalerz, M.; Bish, D.; Schimmelmann, A.; Pedersen, P. K. Porosity and pore size distribution in mudrocks from the Belle Fourche and Second White Specks formations in Alberta, Canada. AAPG Bull. 2016, 100, 1265−1288. (54) Furmann, A.; Mastalerz, M.; Schimmelmann, A.; Pedersen, P. K.; Bish, D. Relationships between porosity, organic matter, and mineral matter in mature organic-rich marine mudstones of the Belle Fourche and Second White Specks formations in Alberta, Canada. Mar. Pet. Geol. 2014, 54, 65−81. (55) Curtis, M. E.; Cardott, B. J.; Sondergeld, C. H.; Rai, C. S. Development of organic porosity in the Woodford shale with increasing thermal maturity. Int. J. Coal Geol. 2012, 103, 26−31. (56) Liu, X.; Xiong, J.; Liang, L. Investigation of pore structure and fractal characteristics of organic-rich Yanchang formation shale in

(22) Ross, D. J. K.; Marc Bustin, R. The importance of shale composition and pore structure upon gas storage potential of shale gas reservoirs. Mar. Pet. Geol. 2009, 26, 916−927. (23) Klaver, J.; Hemes, S.; Houben, M.; Desbois, G.; Radi, Z.; Urai, J. L. The connectivity of pore space in mudstones: insights from highpressure Wood’s metal injection, BIB-SEM imaging, and mercury intrusion porosimetry. Geofluids 2015, 15, 577−591. (24) Jiao, K.; Yao, S.; Liu, C.; Gao, Y.; Wu, H.; Li, M.; Tang, Z. The characterization and quantitative analysis of nanopores in unconventional gas reservoirs utilizing FESEM−FIB and image processing: An example from the lower Silurian Longmaxi Shale, upper Yangtze region, China. Int. J. Coal Geol. 2014, 128-129, 1−11. (25) Tian, H.; Pan, L.; Xiao, X.; Wilkins, R. W. T.; Meng, Z.; Huang, B. preliminary study on the pore characterization of Lower Silurian black shales in the Chuandong Thrust Fold Belt, southwestern China using low pressure N2 adsorption and FE-SEM methods. Mar. Pet. Geol. 2013, 48, 8−19. (26) Zhang, J.; Li, X.; Xie, Z.; Li, J.; Zhang, X.; Sun, K.; Wang, F. Characterization of microscopic pore types and structures in marine shale: Examples from the Upper Permian Dalong formation, Northern Sichuan Basin, South China. J. Nat. Gas Sci. Eng. 2018, 59, 326−342. (27) Wang, Y.; Zhu, Y.; Liu, S.; Zhang, R. Methane adsorption measurements and modeling for organic-rich marine shale samples. Fuel 2016, 172, 301−309. (28) Fishman, N. S.; Hackley, P. C.; Lowers, H. A.; Hill, R. J.; Egenhoff, S. O.; Eberl, D. D.; Blum, A. E. The nature of porosity in organic-rich mudstones of the Upper Jurassic Kimmeridge Clay Formation, North Sea, offshore United Kingdom. Int. J. Coal Geol. 2012, 103, 32−50. (29) Li, X.; Wang, Y.; Zhang, J.; Guo, M.; Zhao, P.; Xu, H.; Yang, J.; Wang, F. Pore characteristics of shale gas reservoirs from the Lower Paleozoic in the southern Sichuan Basin, China. J. Nat. Gas Geosci. 2016, 1, 195−202. (30) Li, Z.; Shen, X.; Qi, Z.; Hu, R. Study on the pore structure and fractal characteristics of marine and continental shale based on mercury porosimetry, N 2 adsorption and NMR methods. J. Nat. Gas Sci. Eng. 2018, 53, 12−21. (31) Tiwari, P.; Deo, M.; Lin, C. L.; Miller, J. D. Characterization of oil shale pore structure before and after pyrolysis by using X-ray micro CT. Fuel 2013, 107, 547−554. (32) Tripathy, A.; Srinivasan, V.; Singh, T. N. A comparative study on the pore size distribution of different indian shale gas reservoirs for gas production and potential CO2 sequestration. Energy Fuels 2018, 32, 3322−3334. (33) Han, Y.; Kwak, D.; Choi, S.; Shin, C.; Lee, Y.; Kim, H. Pore Structure Characterization of Shale Using Gas Physisorption: Effect of Chemical Compositions. Minerals 2017, 7, 66. (34) Zhang, Q.; Pang, Z.; Zhang, J.; Lin, W.; Jiang, S. Qualitative and quantitative characterization of a transitional shale reservoir: A case study from the Upper Carboniferous Taiyuan shale in the eastern uplift of Liaohe Depression, China. Mar. Pet. Geol. 2017, 80, 307− 320. (35) Wang, G.; Ju, Y.; Yan, Z.; Li, Q. Pore structure characteristics of coal-bearing shale using fluid invasion methods: A case study in the Huainan-Huaibei Coalfield in China. Mar. Pet. Geol. 2015, 62, 1−13. (36) Han, K.; Ju, Y.; Wang, G. C.; Bao, S. J.; Bu, H. l.; Neupane, B. Shale composition and pore structure variations in the progradation direction: a case study of transitional shales in the Xu-Huai district, southern North China. J. Nat. Gas Sci. Eng. 2016, 36, 1178−1187. (37) Pan, J.; Peng, C.; Wan, X.; Zheng, D.; Lv, R.; Wang, K. Pore structure characteristics of coal-bearing organic shale in Yuzhou coalfield, China using low pressure N2 adsorption and FESEM methods. J. Pet. Sci. Eng. 2017, 153, 234−243. (38) Ju, Y.; Sun, Y.; Tan, J.; Bu, H.; Han, K.; Li, X.; Fang, L. The composition, pore structure characterization and deformation mechanism of coal-bearing shales from tectonically altered coalfields in eastern china. Fuel 2018, 234, 626−642. (39) Yu, K.; Shao, C.; Ju, Y.; Qu, Z. The genesis and controlling factors of micropore volume in transitional coal-bearing shale 1981

DOI: 10.1021/acs.energyfuels.8b04135 Energy Fuels 2019, 33, 1969−1982

Article

Energy & Fuels central China by nitrogen adsorption/desorption analysis. J. Nat. Gas Sci. Eng. 2015, 22, 62−72. (57) Mastalerz, M.; Schimmelmann, A.; Drobniak, A.; Chen, Y. Porosity of Devonian and Mississippian New Albany shale across a maturation gradient: insights from organic petrology, gas adsorption, and mercury intrusion. AAPG Bull. 2013, 97, 1621−1643. (58) Luo, Q.; Zhong, N.; Liu, Y.; Qu, Y.; Ma, L. Organic geochemical characteristics and accumulation of the organic matter in the Jurassic to Cretaceous sediments of the Saihantala Sag, Erlian Basin, China. Mar. Pet. Geol. 2018, 92, 855−867. (59) Clarkson, C. R.; Haghshenas, B. Characterization of multifractured horizontal shale wells using drill cuttings: 1. Fluid-in-place estimation. J. Nat. Gas Sci. Eng. 2016, 32, 574−585. (60) Jiang, Z.; Zhao, L.; Zhang, D. Study of adsorption behavior in shale reservoirs under high pressure. J. Nat. Gas Sci. Eng. 2018, 49, 275−285. (61) Shen, R.; Hu, Z.; Guo, H.; Jiang, B.; Miao, S.; Li, W. Storage space and gas content law of Longmaxi shale in Changning area, Sichuan Basin. Litho. Reser. 2018, 30, 1−8.

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DOI: 10.1021/acs.energyfuels.8b04135 Energy Fuels 2019, 33, 1969−1982