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Characterization of full-sized pore structure of coalbearing shales and its effect on shale gas content Jizhen Zhang, Xianqing Li, Xiaoyan Zou, Guangjie Zhao, Baogang Zhou, Jian Li, Zengye Xie, and Feiyu Wang Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.8b04135 • Publication Date (Web): 27 Feb 2019 Downloaded from http://pubs.acs.org on March 3, 2019
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Energy & Fuels
1
Characterization of full-sized pore structure of coal-bearing shales and
2
its effect on shale gas content
3
Jizhen Zhang a,b, Xianqing Li a,b,*, Zou Xiaoyan a,b , Guangjie Zhao a,b, Baogang Zhou a,b, Jian Li c,
4
Zengye Xie c, Feiyu Wang d,e
5
a State
6
Beijing 100083, P. R. China
7
b College
8
P. R. China
9
c Langfang
Key Laboratory of Coal Resources and Safe Mining, China University of Mining and Technology (Beijing),
of Geosciences and Surveying Engineering, China University of Technology (Beijing), Beijing 100083,
Branch, Research Institute of Petroleum Exploration & Development, PetroChina, Langfang 065007,
10
P. R. China
11
d State
12
102249, P. R. China
13
e College
14
* Corresponding author at: China University of Mining and Technology (Beijing), D11, Xueyuan Road, Haidian
15
District, Beijing, P. R. China.
16
Tel.: +8610 62331854-8131, +8610 1355287755; fax: +8610 62339208.
17
E-mail address:
[email protected] (X. Li)
18
ABBREVIATIONS
19
EIA=Energy Information Administration
20
SEM=scanning electron microscopy
21
FIB-SEM=focused ion beam-SEM
22
FE-SEM=field emission-SEM
23
NMR=nuclear magnetic resonance
24
USANS= ultra-small-angle neutron scattering
25
SANS=small-angle neutron scattering
26
IUPAC=International Union of Pure and Applied Chemistry
27
OM=organic matter
28
Fm= Formation
Key Laboratory of Petroleum Resource and Prospecting, China University of Petroleum (Beijing), Beijing
of Geosciences, China University of Petroleum (Beijing), Beijing 102249, P. R. China
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BET= Brunauer–Emmett–Teller
30
BJH= Barrett–Joyner–Halenda
31
D-R= Dubinin–Radush
32
DFT= density functional theory
33
TOC=total organic carbon content
34
Ro=vitrinite reflectance
35
XRD= X-ray diffraction
36
interP=interparticle
37
intraP=intraparticle
38
SLD=simplified local density
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ABSTACT: The characterization of pore structure and shale gas content provides useful information for shale
40
gas reservoir assessment and evaluation and guides the exploration and development of shale gas. Fresh core
41
samples obtained from three different basin formations in China were analyzed by field emission scanning electron
42
microscopy, low-pressure CO2 and N2 gas adsorption–desorption, high-pressure mercury intrusion, and methane
43
adsorption experiments to clarify the pore structure characteristics of coal-bearing shales and their effects on shale
44
gas content. The inter and intraparticle pores, organic matter pores, and microfractures were well-developed in
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coal-bearing shales. These pores had differentiated geneses, morphologies, and sizes with main diameters of 50, 2–50, and < 2 nm,
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respectively.45,46 Microscopic pore structures and coal-bearing shale gas content are thus far rarely
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reported, and the controlling factors for multiscale pore development and shale gas content are still
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confined and require further improvement. The measurement of pore structures and gas adsorption
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capacities that combine high- and low-pressure adsorption techniques is necessary to completely
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understand full-sized pore structures and shale gas adsorption capacity.
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Fresh core coal-bearing shale samples were collected from three typical formations in three
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famous shale gas-bearing basins to clarify the multiscale pore structure therein, the gas occurrence
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characteristics of different types of coal-bearing shale, and the differences between coal-bearing
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shales and marine and terrestrial shales in this work. Longtan shale in Sichuan Basin is a typical
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marine–continental shale, whereas Shanxi shale in Ordos and Junggar Basins is a typical terrestrial
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shale.6,26.30 These two types of shale greatly vary with the parent material sources and thermal
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evolution stages. Moreover, both types are located within stable intracratonic basins. In addition,
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although Shanxi and Badaowan shales in Junggar Basin are terrestrial shale, the latter deposits in
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the representative inland superimposed basin.6,26.30 In this study, high-pressure mercury intrusion
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and low-pressure N2 and CO2 adsorption–desorption experiments were combined to clarify the
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full-sized pore structures. Methane adsorption experiments were used to investigate the gas
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adsorption characteristics in the coal-bearing shales. The relationships among the abundance and
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maturation of organic matter (OM), mineralogical composition, porosity, multiscale of pore
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structures, and methane adsorption volume were also discussed. The research results are important
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to understand the pore networks and their influencing factors within coal-bearing shales and
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deeply investigate shale gas enrichment and storage mechanism in the future.
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2. GEOLOGICAL SETTING
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Coal-bearing shales are widely distributed from the Carboniferous to the Neogene layer in the
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coal-bearing basin of the South, North, Northwest, and Northeast China (Figures 1 and
114
2).31–33,41,47,48 The buried depth of coal-bearing shales is usually shallower than that of marine
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shales, which is generally shallower than 3000 m. At present, except for the northeast region, the
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estimated reserves of coal-bearing shale resources in China are 32×1012 m3.50 The
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Carboniferous–Permian periods are important geological periods of sedimentary facie
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transformation (Fm) from marine to continental facies in China.47–51 During these periods,
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marine–continental shales are widely distributed in the coal-bearing strata in craton tectonic basin,
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and the typical shales in these periods include Carboniferous Shuiquan–Bashan Fms in Junggar
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Basin, Permian Liangshan–Longtan Fms in South China, and Carboniferous Taiyuan Fms and
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Permian Benxi and Shanxi Fms in North China.47–51 To the late Triassic of the Mesozoic, nearly
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all the inland areas, except the Qinghai–Tibet Plateau in China, have entered the
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inland lake development stage, and the continental coal-bearing shales were widely deposited in
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large down-warped basins (e.g., Junggar, Ordos, and Sichuan Basins) and coal-bearing fault basins
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(e.g., Songliao Basin) in Northeastern China during the Upper Triassic to the Middle and Lower
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Jurassic periods. 47–51
128 129
Figure 1. The distribution of major coal-bearing shale of China and the location of sampling wells. 31–33,41,47,48
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Figure 2. Stratigraphic age distribution of major coal-bearing shale in China and target strata in this study.
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3. SAMPLE AND EXPERIMENTS
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3.1 Sample collection and preparation
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In the present study, fresh core coal-bearing shale samples, which were obtained from the
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Lower Jurassic Badaowan Fm of the Junggar Basin, the Upper Permian Longtan Fm of the
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Sichuan Basin, and the Lower Permian Shanxi Fm of the Ordos Basin (Figures 1 and 2), were
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immediately taken and meticulously investigated. The total organic carbon (TOC) content,
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vitrinite reflectance (Ro), and mineralogical components of each sample were tested in the
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laboratory prior to the study, and the results are list in Table 1. The pore structure characteristics
140
of each sample were then measured by FE-SEM and fluid injection experiments.
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3.2 FE-SEM observation
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The FE-SEM imaging of the coal-bearing shale samples was conducted using a Quanta 200F
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following the Chinese oil and gas industry standard SY/T 5162–1997. The samples mounted to
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FE-SEM were prepared as chips with the length, width, and height of approximately 1 cm, 1 cm,
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and 2 mm, respectively. Subsequently, all the investigated samples were coated with a
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10-nm-thick gold to obtain good image quality by enhancing electrical conductivity. All the
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experiments were performed at humidity and temperature of 35% and 25 °C, respectively.
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3.3 High pressure mercury intrusion
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High pressure (up to 60000 psia) mercury intrusion experiments were performed using an Auto
150
Pore IV 9520 under the Chinese national standard GB/T 21650.3-2011. The preparation for the
151
experiments is presented as follows. All shale samples were crushed into particles with sizes
152
between 2 mm and 4 mm and then dried in a vacuum oven for approximately 18 h at about 110 °C.
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The measuring pore diameter sizes ranged from 3 nm to 1 mm with the intruded mercury from
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60000 to 1.5 psia. Micropores within the pore networks could hardly be detected via this
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technique.
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3.4 Low pressure N2 and CO2 adsorption
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In accordance with GB/T 19587-2004 and GB/T 21650.2-2008 standards, N2 and CO2
158
adsorption–desorption analyses were performed using a Quantachrome Nova-4200e with pressure
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range of 0–101.3 KPa after all the samples had been cut into approximately 80–250 μm and dried
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at 90 °C for 2 h under vacuum. Thereafter, N2 and CO2 adsorption–desorption curves were drawn
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with a relative partial pressure of 0.010–0.995 and 0.0001–0.032, respectively. On the basis of the
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N2 adsorption data, mesopore surface area was analyzed using the Brunauer–Emmett–Teller
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(BET) model, and mesopore volume was measured by the Barrett–Joyner–Halenda (BJH)
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model.31,33,42,43 On the basis of the CO2 adsorption data, micropore surface area was analyzed by
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the Dubunin–Radushkevich (D–R) model, and micropore volume was measured by the density
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functional theory (DFT) model.31,33,42,43
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3.5 Methane adsorption experiments
168
Methane
adsorption
experiments
were
performed
using
an IS-300 isothermal
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adsorption–desorption analyzer under GB/T 19560-2008 standard, with high pressure of up to 20
170
MPa. Shale samples were powdered into particles with diameters of 50–150 mm and then oven
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dried for 24 h at 60 °C to prepare for the methane sorption procedure. All experiments were
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conducted at humidity and temperature of 35% and 25 °C, respectively. The analytical results
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were fit using the Langmuir adsorption model. At present, the total content of adsorption shale gas
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is commonly based on the evaluation method of methane adsorption gas content in coal bed,41,42
175
which can be calculated as follows:
176
Vab = (VLP)/(PL+P),
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where Vab is the content of adsorption gas, cm3/g; VL is the Langmuir volume, which represents the
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absolute adsorption capacity, cm3/g; P is the equilibrium gas pressure, MPa; and PL is the
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Langmuir pressure, which corresponds to the half of VL.
(1)
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4. RESULTS
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4.1 Mineralogical compositions and geochemical characteristics
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Table 1 shows the results of the properties of the coal-bearing shale samples, including TOC
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content, Ro, and mineralogical compositions. The diagenetic evolution degree of Badaowan
184
shales in Junggar Basin is low, in which the OM is less affected by the hydrocarbon generation
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evolution; thus, these shales develop with high OM content. The Longtan shales in Sichuan Basin
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suffer from strong hydrocarbon generation evolution, thereby causing pyrolysis hydrocarbon
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generation from OMs. Meanwhile, feldspar and other minerals are transformed into a large
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amount of clay minerals during the diagenetic evolution of shale. Thus, clay mineral contents are
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relatively high. The development of Shanxi shales in Ordos Basin is between them. The
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investigated samples developed with abundant OMs has a TOC content of 1.54%–5.78% (mean
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value of 3.31%). Ro values have samples that range from 0.80% to 2.60% (mean value of 1.52%).
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Moreover, coal-bearing samples from various basins are found in different evolution stages, in
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which the range of the Ro values of Badaowan, Shanxi, and Longtan shales is 0.80%–0.91%,
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1.09%–1.85%, and 1.93%–2.60%, respectively. X-ray diffraction analysis shows that clay
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minerals are rich in samples with content range of 55.2–71.8 wt.% (mean value of 62.1 wt.%). The
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illite–smectite mixed layer in clays with content range of 23.4–61.7 wt.% (mean value of 43.6
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wt.%), is the primary mineral. In addition, the brittle mineral contents in the investigated samples
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are lower than those in commercially developed marine shales in South China and North
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America,12,13,23,29,43 with content range of 25.7–41.4 wt.% (mean value of 34.4 wt.%), in which
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quartz is the most common mineral with content range of 23.6 wt.%–38.6 wt.% (mean value of
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30.5 wt.%)y.
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4.2 Pore morphology and distribution
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4.2.1 OM pores
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OM pores are developed within the OMs, which are closely associated with the formation and
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enrichment of shale gas and provide the major space for the occurrence and accumulation of shale
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gas.12,13,23,29,43,46 Previous studies have suggested that OM pores are developed from kerogen in
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shale during hydrocarbon generation.12,13,23,29,43,46 The investigated coal-bearing shale samples are
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abundant in OMs, and the mass of organic particles is relatively regular, which is more likely to
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develop OM pores compared with amorphous OMs. These pores are round, ellipsoid, scallop,
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crescent, and slit (Figures 3d–3h) and are commonly distributed within the mineral matrix
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framework gaps. Moreover, these pores are commonly developed with a size lower than 1 μm, and
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the main size diameter range is 30–650 nm. In contrast with marine shales,12,13,46 the number of
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OM pores in coal shales is relatively lower. The phenomenon is caused by the usual experience of
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multiple stages of sedimentary cycle of coal-bearing shales, thereby damaging, deforming, and
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filling pores due to subsequent extrusion and denudation.
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Table 1
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The properties of the coal-bearing shale samples in different basin of China. Sample
Location
Depth
TOC
Ro
Mineralogy composition (wt.%)
ID
basin
(m)
(%)
(%)
Quartz
Feldspar
Carbonate
Clay
Others
Badaowan
G6-1
Junggar
673.5
3.23
0.80
36.70
0.9
3.1
57.8
1.5
(J1b)
G6-2
Junggar
687.6
3.81
0.81
32.2
1.2
4.8
59.2
2.6
G2-1
Junggar
694.2
3.74
0.85
38.6
2.4
0.2
57.6
1.2
G2-4
Junggar
697.6
5.78
0.87
35.1
1.2
0.6
59.3
3.8
G2-7
Junggar
675.2
2.83
0.87
35.7
0.8
1.5
56.6
5.4
G1-1
Junggar
727.8
4.85
0.91
34.9
1.3
0.5
61.3
2.0
G1-3
Junggar
734.5
2.56
0.87
28.9
0.5
0.4
67.8
2.4
Fm
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G1-6
Junggar
741.4
3.42
0.89
29.7
0.6
0.8
63.7
5.2
Shanxi
S14-1
Ordos
2674.2
3.74
1.09
31.8
2.1
7.0
57.2
2.0
(P1s)
S14-3
Ordos
2682.1
1.54
1.11
35.4
1.6
1.2
56.4
5.4
S14-7
Ordos
2694.8
1.56
1.12
31.8
0.4
3.5
60.0
4.3
S269-1
Ordos
3473.2
2.58
1.46
29.7
1.5
2.5
63.1
3.2
S269-2
Ordos
3481.7
2.96
1.54
23.6
0.6
3.4
70.5
1.9
S269-3
Ordos
3489.1
3.85
1.63
26.5
1.7
8.1
61.7
2.0
S13-1
Ordos
4415.3
2.14
1.58
34.5
1.5
3.8
55.4
4.8
S13-3
Ordos
4418.2
2.75
1.85
36.8
1.5
3.1
55.2
3.4
S13-5
Ordos
4432.6
2.57
1.63
29.7
0.1
0.8
64.3
5.1
S13-8
Ordos
4453.7
3.58
1.68
33.9
1.5
1.3
60.8
2.5
Longtan
Z15-1
Sichuan
738.7
3.34
1.95
24.8
0.7
0.7
68.2
5.6
(P2l)
Z15-2
Sichuan
742.5
2.14
1.93
26.4
0.8
1.5
67.5
3.8
Z15-7
Sichuan
759.1
3.96
2.04
29.4
0.9
7.0
58.3
4.4
Z15-10
Sichuan
774.8
4.25
2.08
25.3
1.2
5.4
63.4
4.7
Z85-4
Sichuan
880.5
2.34
2.13
28.7
0.5
2.6
63.0
5.2
Z85-5
Sichuan
886.1
5.21
2.24
24.5
1.2
3.5
67.4
3.4
Z314-1
Sichuan
1026.5
4.60
2.60
25.5
0.5
3.9
66.8
3.4
Z314-4
Sichuan
1042.4
2.57
2.15
23.7
0.8
1.2
71.8
2.5
Z314-7
Sichuan
1053.8
3.54
2.34
28.9
1.5
3.5
62.5
3.6
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4.2.2 Interparticle (interP) pores
219
Between mineral matrix and OM particles, the interP pores are well-developed in the
220
investigated samples, particularly in shallow strata due to their susceptibility to stress
221
compression and deformation.12,13,23,29,43,46 InterP pores are common in coal-bearing shales with
222
the shape of triangle, quadrangle, polygon equal and subangular, and irregular (Figures 3a–3d).
223
The sizes of these pores are controlled by the degree of compaction, cementation, and particle
224
sizes, with a size commonly lower than 2 μm, and the main size diameter range is 50–800 nm. The
225
self-generated pyrite aggregates are mostly developed in the marine–terrestrial transitional
226
coal-bearing shales; thus, interP pores are generally developed in these shales due to the untight
227
accumulation among crystal particles (Figure 3c). In addition, pyrite particles are usually
228
associated with OMs because they are usually formed under reduction conditions. Hence, most
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inter-crystal pores are subducted by filling with OMs.
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4.2.3 Intraparticle (intraP) pores
231
IntraP pores, which are commonly developed within mineral matrix particle, can be
232
divided into primary and secondary.12,13,23,29,43,46 IntraP pores are commonly observed in
233
coal-bearing shales with high rangeability (main of the 0.05–2 μm; Figure 3b). The
234
morphologies of these pores are irregular, that is, they commonly have gourd, slit, and
235
moniliform shapes. These pores are also easily affected by tectonic and diagenetic
236
stresses; particularly, those with a diameter of more than 100 nm can be reduced by
237
stress.
238
4.2.4 Microfractures
239
As the main parts of pore fracture networks, microfractures are important for gas
240
exploration and development because they provide effective storage space for shale gas
241
and greatly improve the seepage capacity of fluid.12,13,23,29,43,46 The microfractures in
242
coal-bearing shale can be divided into tectonic and non-tectonic. The former is formed by
243
geological tectonic movement, whereas the latter is produced by diagenetic cementation,
244
mineral crystallization, pressure dissolution, dry cracking, and weathering. These
245
microfractures can run through quartz, carbonate, and OMs, with length and width
246
ranging from approximately 500 nm–2 mm and 20 μm are commonly developed under the pressure of 0.1–80
261
psia, whereas few shale pores are developed under the pressure of 80–800 psia. Furthermore, large
262
amount of nanoscale pores is well-developed because the mercury intrusion volumes increase
263
again as the pressure reaches over 800 psia. Only the structural parameters of the macropores
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is measured using this method due to the limitation of mercury intrusion technique. Thus,
265
N2/CO2 adsorption–desorption methods are used to characterize the properties of
266
mesopores and micropores in shale systems. Table 2 lists the results of multiscale pore
267
structures of the investigated coal-bearing shale samples. The microscopic pores were
268
well-established in the developed coal-bearing shale samples, with a porosity range of
269
2.02%–6.57% (average of 4.94%; Table 2). The calculated pore volume range of
270
macropores is 0.005–0.024 cm3/g (average of 0.016 cm3/g; Table 1). The calculated range
271
of the special surface area of macropores is 0.01–1.45 m2/g (mean value of 0.54 m2/g;
272
Table 2).
273 274
Figure 4. Mercury intrusion/extrusion curves of the coal-bearing shale samples.
275
Table 2
276
Pore structure parameters of coal-bearing shale samples. Pore volume (cm3/g)
Pore surface area (m2/g)
Sample
Porosity
Micro-p
Meso-por
Macro-p
Micro-por
Meso-por
Macro-por
ID
(%)
ore
e
ore
e
e
e
(50 nm)
(50 nm)
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Average pore size (nm)
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G6-2
5.62
0.003
0.01
0.01
10.34
5.86
0.01
70.5
G2-1
5.41
0.004
0.01
0.005
13.95
4.91
0.04
130.8
G1-1
4.93
0.007
0.014
0.019
24.81
7.43
0.07
58.7
S14-1
6.53
0.005
0.015
0.017
14.83
9.09
1.04
95.2
S269-3
2.02
0.002
0.009
0.014
5.16
0.26
0.38
135
S13-3
3.63
0.003
0.009
0.011
11.25
3.71
0.46
68.4
Z15-7
4.45
0.004
0.009
0.02
13.19
4.22
0.71
75.3
Z85-5
6.57
0.009
0.018
0.024
31.3
14.13
1.45
20.2
Z314-1
5.34
0.007
0.013
0.023
21.96
7.91
0.74
46.3
277
Note: Pore volume and pore surface area data of micro-, meso- and macro-pores were tested based on CO2
278
adsorption, N2 adsorption, and mercury intrusion measurements, respectively.
279
4.3.2 Mesopores based on N2 adsorption experiments
280
Figure 5 shows the N2 adsorption–desorption isotherms of coal-bearing shales. Differences
281
among the curves of the investigated shale samples are observed within different basins,
282
thereby demonstrating the different pore shapes developed in coal-bearing shales. As
283
illustrated in Figure 5, the N2 adsorption volume increases with the relative pressure (P/P0), and
284
these curves separate with the desorption as P/P0 over approximately 0.4, thereby
285
generating hysteresis loops and reflecting the capillary condensation that occurs in mesopores.51 In
286
addition, N2 adsorption–desorption isotherms are unclosed in some samples (e.g., Z85-5 and
287
Z131-8) under P/P0 of
