Anisotropic Adsorption Swelling and Permeability Characteristics with

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Anisotropic adsorption swelling and permeability characteristics with injecting CO2 in coal Qinghe Niu, Liwen Cao, Shuxun Sang, Xiaozhi Zhou, and Zhenzhi Wang Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.7b03087 • Publication Date (Web): 14 Dec 2017 Downloaded from http://pubs.acs.org on January 11, 2018

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Fig. 1 Graphical representation of sampling location.

ACS Paragon Plus Environment

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Fig. 2 The well prepared cubic coal samples used in the experiments. a and b are the coal samples from Zhaozhuang coalmine and Chengzhuang coalmine respectively.


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Fig. 3 Schematic diagram of anisotropic adsorption-swelling and permeability experiment setup. 1, Gas cylinder; 2, Valve; 3, Booster pump; 4, Reference cell; 5, Pressure regulating valve; 6, Heater; 7, Pressure sensor; 8, Temperature sensor; 9, Confining pressure tracking pump; 10, Mass flowmeter; 11, O-ring; 12, Sample holder; 13, Computer; 14, Adsorption cell; 15, Strain gauge; 16, Vacuum pump; 17, Water; 18; Cubic coal sample.


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Fig. 4 Experimental procedures of this work. Pc is confining pressure and Pi is the injection pressure.


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5 εx

6

Run 1

εx

εy

4

5

εz

Swelling strain (%)

Swelling strain (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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εV

3 2 1 0

Run 2

εy εz

4

εV

3 2 1 0

0

2

4

6

8

10

0

2

Injection pressure (MPa)

4

6

8


Fig. 5 The relationship of swelling strain and injection pressure measured on sample 1.


10

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4

4 εx

εx

Run 1

εy

3

εV

2

1

Run 2

εy

3

εz

Swelling strain (%)

Swelling strain (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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εz εV

2

1

0

0 0

2

4

6

8


10

0

2

4

6

8




10

Page 7 of 49

0.8 0.7 0.6 0.5

AIε

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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0.4 0.3 0.2 0.1 0

2

4

6

8

10

12

Injection pressure (MPa) Fig. 7 The relationship of anisotropy swelling index (AIε) and injection pressure measured on sample 1.


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0.6 0.5 0.4

AIε

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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0.3 0.2 0.1 0

2

4

6

8

10

12

Injection pressure (MPa) Fig. 8 The relationship of anisotropy swelling index (AIε) and injection pressure measured on sample 2.


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Swelling strain (%) Swelling strain (%)

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0.15

εx

0.12

εy

0.09

Run 1

εz

0.06 0.03 0.00 0.15

εx

0.12

εy

0.09

Run 2

εz

0.06 0.03 0.00 35

40

45 50 55 Temperature (℃)

60

65

Fig. 9 The relationship of test temperature and swelling strain measured on sample 1 at atmospheric pressure.


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

Swelling strain (%) Swelling strain (%)

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0.15

εx

0.12

εy

0.09

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Run 1

εz

0.06 0.03 0.00 0.15

εx

0.12

Run 2

εy

0.09

εz

0.06 0.03 0.00 35

40

45

50

55

60

65

Temperature (℃) Fig. 10 The relationship of test temperature and swelling strain measured on sample 2 at atmospheric pressure.


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0.22 Sample 1 Sample 2

0.20 0.18

AIε

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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0.16 0.14 0.12 35

40

45

50

55

60

65

Temperature (℃) Fig. 11 The relationship of test temperature and anisotropy swelling index (AIε) at atmospheric pressure.


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

Permeability (mD) Permeability (mD)

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0.025

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Sample 1, no CO2 adsorption

Kz

y=0.031e-0.256x

0.020

Kx

y=0.028e-0.244x

0.015

Ky

0.010

y=0.012e-0.175x

0.005 0.025

Sample 2, no CO2 adsorption

0.020 -0.269x 0.015 y=0.029e

Kz

y=0.030e-0.245x

Kx Ky

0.010

y=0.014e-0.207x

0.005 0

1

2

3

4

5

6

7

8

Effective stress (MPa) Fig. 12 The relationship of effective stress and permeability with no CO2 adsorption.


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0.015

Sample 1, 6MPa CO2 adsorption

0.012

y=0.016e-0.189x

0.009

Kz

y=0.018e-0.126x

Kx Ky

0.006 0.003

y=0.007e-0.126x

0.015


0.012

y=0.017e

0.009

Kz

-0.199x

Kx

y=0.016e-0.217x

Ky

0.006 y=0.007e-0.145x

0.003 0

1

2

3

4

5

6

7

8

Effective stress (MPa) Fig. 13 The relationship of effective stress and permeability after 6 MPa CO2 adsorption.


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


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0.015

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0.012

y=0.017e

0.009

KK

-0.194x

z

Kx

y=0.015e-0.187x

Model

Ky

0.006

Equation Reduced Chi-Sqr Adj. R-Square

0.003

y=0.006e

-0.126x B D

0.015

F


0.012

Kz Kx

y=0.054e-0.201x

0.009

Ky

y=0.015e-0.226x

0.006 0.003

y=0.006e-0.136x

0

1

2

3

4

5

6

7

8

Effective stress (MPa) Fig. 14 The relationship of effective stress and permeability with 8 MPa CO2 adsorption.


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Fig. 15 The flow process model of CO2 injecting in coal. (a) is bedding plane fracture; (b) is bright coal band; (c) is cleat, including extensive face cleat and discontinuous butt cleat. For CO2 injecting into a coal seam, a part of CO2 flows from macro fractures (e.g. bedding plane fractures), through cleats and is adsorbed on pore surface of coal matrix. However, the other CO2 flows through macro fractures and arrives at CH4 drainage well directly for CO2-ECBM or other coal/rock seams for CGS. The former is effective seepage, contrarily, the latter is ineffective seepage for CO2-ECBM and CGS.


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Table 1 The ultimate and proximate analysis of coal samples used in the experiments. Sample ID

1 2

Sample from

Zhaozhuang coalmine Chengzhuang coalmine

Proximate analysis

Ultimate analysis

Maceral composition

(wt. %)

(wt. %)

(vol. %)

Ro,max (%) Mad

Aad

Vdaf

FCad

Odaf

Cdaf

Hdaf

Ndaf

Vit

Ine

Min

2.44

1.61

12.16

10.46

78.65

2.13

91.60

4.15

2.13

78.26

19.37

2.37

2.96

2.71

12.18

6.94

81.72

3.27

92.84

2.31

3.27

75.80

21.40

2.80

Note: Mad, moisture content of air-dried basis; Aad, ash content of air-dried basis; Vdaf, volatile content of dry ash-free basis; FCad, fixed carbon carbon content of air-dried basis; Odaf, oxygen content of dry ash-free basis; Cdaf, carbon content of dry ash-sfree basis; Hdaf, hydrogen content of dry ash-free basis; Ndaf, nitrogen content of dry ashfree basis; Vit, vitrinite; Ine, inertinite; Min, mineral.


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Table 2 Basic properties of the selected coal samples. Sample ID 1 2

Sample from Zhaozhuang coalmine Chengzhuang coalmine

Weight

Density (g/cm3)

Lx (cm)

Ly (cm)

Lz (cm)

3.030

3.050

2.900

36.271

1.353

3.036

3.008

2.986

40.549

1.487

(g)

Lx represents the length along the face cleat direction; Ly represents the length along the butt cleat direction; Lz represents the length in vertical bedding plane direction.


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Table 3 The porosity and its anisotropic index of cubic coal samples. Sample ID

Orientation φx

1

φy φz

φx

2

φy

φz

Runs

Porosity

Run 1

0.482

Run 2

0.497

Run 1

0.477

Run 2

0.464

Run 1

0.285

Run 2

0.303

Run 1

0.343

Run 2

0.337

Run 1

0.337

Run 2

0.331

Run 1

0.260

Run 2

0.251

Average

Standard

porosity

deviation

0.490

0.0106

0.471

0.0092

0.294

0.0127

0.340

0.0042

0.325

0.0085

0.256

0.0064


AIφ

0.365

0.208

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Table 4 Permeability anisotropy index AIk of sample 1 with no CO2 adsorption, 6 MPa CO2 adsorption and 8 MPa CO2 adsorption. AIk Effective stress No CO2

6 MPa CO2

8 MPa CO2

adsorption

adsorption

adsorption

1

0.528

0.571

0.581

2

0.545

0.563

0.566

3

0.479

0.520

0.529

4

0.421

0.465

0.469

5

0.364

0.402

0.440

6

0.312

0.381

0.392

7

0.230

0.352

0.369

(MPa)


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Table 5 Permeability anisotropy index AIk of sample 2 with no CO2 adsorption, 6 MPa CO2 adsorption and 8 MPa CO2 adsorption. AIk Effective stress No CO2

6 MPa CO2

8 MPa CO2

adsorption

adsorption

adsorption

1

0.435

0.544

0.573

2

0.525

0.540

0.567

3

0.498

0.522

0.529

4

0.483

0.514

0.525

5

0.359

0.394

0.434

6

0.264

0.380

0.392

7

0.144

0.353

0.372

(MPa)


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Table 6 Permeability adsorption sensitive index S of sample 1 after 6 MPa CO2 adsorption and 8 MPa CO2 adsorption. Effective stress

6 MPa CO2 adsorption


(MPa)

Sx

Sy

Sz

Sx

Sy

Sz

1

0.389

0.404

0.438

0.418

0.423

0.473

2

0.347

0.375

0.378

0.404

0.417

0.431

3

0.325

0.367

0.387

0.357

0.364

0.414

4

0.292

0.320

0.350

0.311

0.320

0.366

5

0.259

0.293

0.313

0.281

0.293

0.366

6

0.216

0.221

0.295

0.258

0.269

0.346

7

0.140

0.157

0.284

0.175

0.197

0.332


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Table 7 Permeability adsorption sensitive index S of sample 2 after 6 MPa CO2 adsorption and 8 MPa CO2 adsorption. Effective stress



(MPa)

Sx

Sy

Sz

Sx

Sy

Sz

1

0.403

0.413

0.506

0.454

0.464

0.573

2

0.378

0.411

0.406

0.438

0.446

0.482

3

0.322

0.345

0.354

0.411

0.413

0.442

4

0.317

0.338

0.352

0.389

0.396

0.432

5

0.305

0.318

0.344

0.355

0.358

0.428

6

0.245

0.246

0.366

0.300

0.352

0.425

7

0.125

0.169

0.358

0.189

0.330

0.418


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1

Anisotropic adsorption swelling and permeability

2

characteristics with injecting CO2 in coal

3

Qinghe Niua,b, Liwen Caob∗, Shuxun Sanga,b∗, Xiaozhi Zhoua,b, Zhenzhi Wangc

4

a.

Key Laboratory of Coalbed Methane Resources and Reservoir Formation Process of

5

Education Ministry, School of Resource and Earth Science, China University of Mining and

6

Technology, Xuzhou, 221116, China

7

b. School of Resource and Earth Science, China University of Mining and Technology, Xuzhou

8 9

221008, China c.

School of Resources & Environment, Henan Polytechnic University, Jiaozuo 454000, China

10

Abstract: The changes of anisotropic adsorption-swelling and permeability with injecting CO2 in

11

coal influence the CO2 injectivity during CO2-ECBM or CGS. To strengthen the understanding of

12

this issue, two special-made cubic coal samples were adopted to test the porosity, swelling and

13

permeability in parallel face cleat and bedding plane direction, parallel butt cleat and bedding

14

plane direction and vertical bedding plane direction. To quantitatively characterize the anisotropic

15

porosity, anisotropic swelling and anisotropic permeability, an anisotropy index was introduced in

16

this paper. The results show that porosity anisotropy reflects the pore connectivity in different

17

directions, which fall in the order of parallel face cleat and bedding plane direction > parallel butt

18

cleat and bedding plane direction > vertical bedding plane direction. The porosity varieties can be

19

owed to the compaction effect, thermal evolution effect, banded structure and cleat distribution in

20

coal seams. The maximum swelling ratios of vertical bedding plane direction to parallel bedding

21

plane direction are 2.30 in sample 1 and 1.89 in sample 2. However, the ratios of parallel face cleat

22

to parallel butt cleat are 1.28 in sample 1 and 1.30 in sample 2. The inhomogeneity of matter

23

composition in vertical bedding direction and the difference of cleat distribution in various coal

∗

Corresponding author at: School of Resource and Earth Science, China University of Mining and Technology,

Xuzhou 221008, China. E-mail address: [email protected] ∗

Corresponding author at: Key Laboratory of Coalbed Methane Resources and Reservoir Formation Process of

Education Ministry, School of Resource and Earth Science, China University of Mining and Technology, Xuzhou, 221116, China. E-mail address: [email protected] 1


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Page 24 of 49

24

bands mainly cause the anisotropic swelling. Both injecting CO2 in coal and raising its

25

temperature increase the anisotropy swelling index, but the effect of thermal swelling is quite

26

weak. Adsorbing CO2 especially for super-critical CO2 will enhance the permeability anisotropy of

27

coal. This is because the low-permeability cleat possesses higher permeability adsorption sensitive

28

and the bedding plane fracture with higher permeability instead does not produce a pronounced

29

permeability drop because of its lower permeability adsorption sensitive. Cleats that easily

30

affected by adsorption-swelling always serve as throats between fractures and coal matrix in a

31

high anisotropic coal, which will restrain CO2 flow in coal pores. Accordingly, cleat seepage and

32

corresponding potential enhanced permeability measures are deserved to pay enough attention on

33

in future researches. This work clarifies the understanding and offers some implications for CO2

34

injecting into coal seams from the perspective of anistropic properties of coal.

35

Keywords: anisotropic porosity; anisotropic adsorption swelling; anisotropy index; permeability

36

adsorption sensitive index; cleat seepage

37

1. Introduction

38

Injecting CO2 into coal seam possesses twofold meanings for enhancing coalbed methane

39

recovery (CO2-ECBM) according to its competitive adsorption effect with CH4 and reducing

40

greenhouse gas emissions by CO2 geological sequestration (CGS)

41

been conducted by various countries, e.g., America, Canada, Poland, China and Japan

42

Unfortunately, a technical obstacle that coal matrix swells largely after injecting CO2 in coal seam

43

is presented in CO2-ECBM or CGS 9, which will partially block the fracture network in coal

44

medium, and then degrade CO2 injectivity by reducing reservoir permeability 10, and finally offset

45

the twofold benefits of injecting CO2 into coal seams. Specially, a reduction in injectivity of about

46

60% was initially observed during CO2 injection at the Allison Unit 11.

1-3

. To date, field tests have 4-8

.

47

During the evolutionary history of several hundred-millions-years, the formation of coal

48

mainly experiences two stages: peatification stage and coalification stage. Various sedimentary

49

and burial environments induce the formation of various macerals, e.g., vitrinite, exinite, inertinite

50

and inorganic constituents

51

heterogeneity. Besides, the difference of pore shape and fracture length, width, roughness and

52

orientation in coal skeleton are deemed to be structural inhomogeneity 13. Specially, diverse pores

12

. This otherness can be inducted as material composition

2


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Energy & Fuels

53

in coal are divided into open pores, semi-open pores and closed pores according to pore

54

connectivity 14-17. The fracture network structure generally presented in coalbeds mainly includes

55

exogenous fractures and cleats (face cleat and butt cleat)

56

multi-stage tectonic movements during coal-forming geological periods

57

resulted from the pyrolysis-induce matrix shrinkage and in-situ stress fields

58

the link bridges between adsorption-pores and macro-fractures

59

CH4 output or CO2 injecting. Generally, face and butt cleats are distributed orthogonal and both

60

are perpendicular to the bedding plane

61

avenues for gas flow in coal; butt cleats are interrupted and ended at intersections with face cleats

62

24

63

characteristics, which have been reported in previous studies 25-27.

18,19

22

, the former are induced by 20

, and the latter are 21

. Cleats, acting as

, control the seepage pivots of

23

. Face cleats are laterally extensive and continuous

. Together with the alternately superposed band structure, coal naturally presents anisotropic

64

In the CO2-ECBM process, CO2 is injected in coal seams and flows from macro-fractures, via

65

cleats and adsorbs in pores, and then the squeezed CH4 is expelled to producing wells along the

66

opposite path relative to CO2. The size and distribution of cleat or fracture restrict the seepage

67

ability. Therefore, investigating the anisotropic porosity, cleat and bedding plane fracture and

68

corresponding permeability property are essential and significative for evaluating the feasibility of

69

CO2-ECBM or CGS. Laubacha et al. stated that the aperture, height, length, connectivity, and

70

orientation of cleats are critical to reservoir permeability 21. Especially, understanding the relation

71

of cleat height and spacing will provide accurate prediction for cleat permeability 28. In the study

72

of Wang et al., the aperture, porosity, density and connectivity of coal cleat were compared, and

73

the result that coal reservoir permeability in parallel bedding plane direction is 1.33-12.91 larger

74

than that in vertical direction was concluded 29. Besides, digital core technology (X-ray computed

75

tomography) and homologous three-dimensional modeling method were used in the study of cleat

76

system in coal 30-33. There is no doubt that investigating the anisotropic characteristics of cleats is

77

valuable and more attention should be pay on it. However, current researches about cleats are

78

always focused on their static patterns, so understandings of the varieties in cleats especially for

79

adsorption-swelling and their effects on the anisotropic reservoir permeability in CO2-ECBM or

80

CGS are imminently needed to be strengthened.

81

The anisotropic swelling of coal, which is greater in perpendicular to the bedding plane than 3


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34,35

82

parallel to it, has been observed in previous researches

83

anisotropic swelling phenomenon about coal after adsorbing supercritical CO2

84

gave a further result that the ratio of perpendicular to parallel swelling was around 1.28 and 1.05

85

for banded and non-banded coal samples respectively

86

critical factor to control the CO2 injectivity in CO2-ECBM or CGS. Li et al. conducted the

87

permeability tests on three cylindrical coal specimens representing the three orthogonal axes of the

88

coal seam, and considered that parallel to both the bedding plane and cleat strike shows the

89

highest permeability, contrarily, parallel to the bedding plane but perpendicular to the cleat strike

90

possesses the minimum penetration capacity 38. Nevertheless, anisotropic permeability tests using

91

cylindrical coal specimens is not unimpeachable, namely, though these coal samples were drilled

92

from the same coal block, inaccuracies are also inescapable considering the extreme heterogeneity

93

of coal and man-made fractures generating in sample preparation processes. Then another

94

improved measuring method using a cubic shale sample in a triaxial cell was implemented in the

95

research of shale 39. Simultaneously, some anisotropic permeability models of coal considering the

96

adsorption-induced swelling effects was were established

97

stress variation, thermal swelling and water saturation on coal were also elaborated

98

practical significance of anisotropic permeability on the optimal design of multi-lateral well for

99

coalbed methane production was emphasized 45,46.

37

. Day et al. also reported the similar 36

. Anggara et al.

. The anisotropic permeability is the

40-42

, moreover, the roles of effective 43,44

. And

100

Apparently, the anisotropic characteristics of coal seams increase enormous challenges for

101

primary and enhanced methane recovery. Though the anisotropic swelling and permeability have

102

received a due attention in a certain extent, studies of three-dimensional swelling (face cleat, butt

103

cleat and bedding plane) and interrelated permeability are lack, especially for physical simulation

104

researches because it is challenging to design such experiments. Additionally, cleats exactly link

105

macro-fractures and coal matrix 47, the injected gas will go along the fractures and will not seep

106

into pores if cleats do not exist in coal matrix. We consider that the cleat seepage deserves equally

107

important treatment as macro-fracture seepage. Therefore, in this work, the three-dimensional

108

porosity, swelling and permeability were tested to investigate CO2 injection and its response for

109

anisotropic characteristics in coal considering the seepage process from the beginning to the end.

110

2. Experimental 4


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Energy & Fuels

111

2.1. Sampling location

112

The coal samples used in this work were collected from Zhaozhuang coalmine and

113

Chengzhuang coalmine of Southern Qinshui Basin, one in-situ experiment base of micro-pilot test

114

of CO2-ECBM and CCS

115

China (see Fig.1). Zhaozhuang coalmine and Chengzhuang coalmine are the two main mining

116

areas of Southern Qinshui Basin. They are adjacent to Gaoping City and Jincheng City

117

respectively, with a straight distance of about 46.2 km between each other.

8,48

. Qinshui Basin is located at the southeast of Shanxi Province, North

118 119 120

Fig. 1 Graphical representation of sampling location.

2.2. Sample preparation

121

To provide reliable measurements and ensure the success rate of sample preparation, a

122

sample with diameter of 5 cm and height of > 4 cm was drilled along vertical bedding plane

123

direction from the complete position without obvious fractures in coal block. A diamond wire saw

124

was adopted to cut the cylinder coal sample along face cleat and butt cleat direction, then

125

metallographic abrasive papers of 240, 600 and 1200 meshes were successively used to grind out a

126

smooth cubic coal sample with sides of 3.0 ± 0.1 cm. The two well prepared cubic coal samples

127

were shown in Fig. 2. In this paper, we defined x, y and z as the parallel face cleat and bedding

128

plane directions, parallel butt cleat and bedding plane directions and vertical bedding plane

129

direction respectively. Noteworthily, cares were taken in the whole process to minimize the

130

possibility of devastation or undetected fractures.

131

The petrography analysis of maceral and maximum vitrinite reflectance measurements (Ro,max)

132

were performed according to ASTM D2799-05a and ASTM D2798-06. Carbon, hydrogen and

133

nitrogen content were calculated from the elemental analysis using Chinese National Standard

134

GB/T 31391-2015. And the proximate analysis was also conducted on selected coal samples

135

basing on GBT212-2008. These results were listed as Table 1. The size of cubic coal sample was

136

measured by a vernier caliper and the weight was measured by an electronic balance down to

137

1-mg resolution. Then the density was calculated, which was shown in Table 2.

138 139

Fig. 2 The well prepared cubic coal samples used in the experiments. a and b are the coal samples from 5


Energy & Fuels 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

140 141

Page 28 of 49

Zhaozhuang coalmine and Chengzhuang coalmine respectively. Table 1 The ultimate and proximate analysis of coal samples used in the experiments.

142 143

Table 2 Basic properties of the selected coal samples.

144 145

2.2. Experimental apparatus

146

The anisotropic adsorption-swelling and permeability experiment setup was schematically

147

shown in Fig. 3. The anisotropic permeability device is modified from triaxial permeability rig,

148

which has been widely applied to measure gas flow in coal or other rocks

149

temperature sensor and pressure sensor are built in sample holder. Confining pressure tracking

150

pump is used to provide confining pressure by injecting water into it. The maximum allowable

151

pressure range is 0.01 MPa and the accuracy of temperature controlling system thermocouple is ±

152

0.1 °C. The booster pump is adopted to promote gas pressure and store it in a 5000 ml reference

153

cell. The pressure of injected gas is adjusted by a pressure regulating valve. The outlet flow is

154

monitored by a mass flowmeter, which was calibrated using nitrogen as the working fluid,

155

possessing a measurement accuracy of ±1.5% F.S. A 500 ml adsorption cell was utilized as the

156

swelling strain test platform, which void volume is ideally adjustable according to the sample size.

157

Equally, the heater, temperature sensor and pressure sensor are built in in it. Strain gauges, using

158

to measure the strain capacity, are pasted on cubic coal surfaces in different directions.

49-53

. The heater,

159

The cubic coal sample wrapped by a layer preservative film is place into a disposable mould

160

with size of 50 mm in diameter and 40 mm in height. Then the space between coal sample and

161

mould is poured by silicon rubber with properties of good sealing performance and high

162

temperature/pressure resistance. At least 48 h is needed to freeze the silicon rubber and reach the

163

favorable experiment effect .

164 165

Fig. 3 Schematic diagram of anisotropic adsorption-swelling and permeability experiment setup. 1, Gas

166

cylinder; 2, Valve; 3, Booster pump; 4, Reference cell; 5, Pressure regulating valve; 6, Heater; 7, Pressure

167

sensor; 8, Temperature sensor; 9, Confining pressure tracking pump; 10, Mass flowmeter; 11, O-ring; 12,

168

Sample holder; 13, Computer; 14, Adsorption cell; 15, Strain gauge; 16, Vacuum pump; 17, Water; 18; 6


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Energy & Fuels

169 170

Cubic coal sample.

2.3. Experimental methods

171

Before the adsorption-swelling and permeability experiment, the porosity was determined for

172

each dried cubic coal sample by the non-adsorbing helium. The porosity of the coal can be defined

173

as follows:

174

(1)

175

Where Vp is the pore volume of the cubic coal sample; lx, ly and lz are the length in

176

corresponding directions. According to mass conservation law, i.e., the decreasing helium mass

177

before the pressure regulation valve is equal to the increasing helium mass after the pressure

178

regulation valve. Together with Boyle's law, the equation is given:

179

(2)

180

Where P, V and T are pressure, volume and temperature respectively; the superscript 0 and 1

181

represent initial state and equilibrium state; the subscript r and s represent reference cell and

182

sample holder; R is the ideal gas constant, equals to 8.3144 J/(mol·K); Vv1 and Vv2 are the pipeline

183

volume before and after pressure regulating valve, and Vf is the free space volume of sample

184

holder, which was calibrated before the experiment. Z is the compressibility factor and can be

185

inquired from NIST Reference Fluid Thermodynamic and Transport Properties Database

186

(REFPROP).

187

Generally, the main permeability computing methods are pressure pulse-decay method (PDM)

188

and steady-state flow method (SFM) in previous studies, both of which are commonly applied

189

when analyzing experimental results. PDM is used in mediums with low permeability, e.g., shale

190

54

191

permeability under various effective stresses because the permeability is relatively high and the

192

steady state is not inaccessibility. Thus, assuming steady-state permeation of an ideal compressible

193

gas at a constant temperature, the SFM is continuously calculated by 56-58:

and sandstone

55

. However, for most coals, SFM is used widespreadly to measure the

194 195 196

(3)

Where K is the gas permeability of coal at mean pressure

; Po is the

standard atmospheric pressure; µ is the gas kinetic viscosity; L is the length of cubic coal sample 7


Energy & Fuels 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

Page 30 of 49

197

in gas flow direction; A is the cross-sectional area of cubic coal sample in vertical gas flow

198

direction; Pin and Pout are the inlet and outlet gas pressure respectively.

199

When injecting CO2 along cleat direction, the adsorption swelling will occur in its vertical

200

direction. Thereby, εx, εy and εz represent the swelling strain induced by butt cleat, face cleat and

201

bedding plane respectively. The volume swelling strain εV of cubic coal sample is expressed as

202

following:

203

(4)

204

Considering the adsorption swelling is small enough for our tests, to simplify the calculation,

205

we ignore the high order mini-term in Eq. (4), which then can be converted to:

206 207

(5)

2.4. Experimental procedures

208

To eliminate the influence of moisture on the experimental results, the cubic coal sample was

209

dried in the PCD-C3000 series thermostat at 60 ℃ before every test. The cubic samlpe was

210

weighed periodically by an electronic balance until the weight remained constant during the

211

drying process. The air-leakage test was frequently conducted at the beginning of each experiment,

212

aiming to improve the accuracy of experimental results. The experimental procedures in this paper

213

were shown in Fig. 4. The porosity of coal was measured by helium under 5 MPa confining

214

pressure (Pc) and 3 MPa injection pressure (Pi). The test pemperature is stabilised at 35 ℃. The

215

test procedures are: (1) maintain a constant confining pressure and evacuate all the air in the

216

experimental setup by a vacuum pump, here, the initial pressure of reference cell and sample

217

holder are

218

regulation valve to the target pressure until the pressure reaches a equilibrium value, here, the

219

equilibrium pressure of reference cell and sample holder are

220

silicon rubber model was removed, and re-poured the cubic coal sample by silicon rubber in

221

another direction. The porosity of φx, φy and φz were calculated by Eq. 1 and Eq. 2.

and

; (2) close the outlet valve, open the inlet valve and adjust the pressure

and

; After each experiment,

222

With the same sample preparation method, the helium permeabilities without CO2 adsorption,

223

after 6 MPa CO2 adsorption and after 8 MPa CO2 adsorption for 12 hours under 35 ℃ test 8


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Energy & Fuels

224

temperature were conducted as the following procedures: (1) set the injection pressure and

225

confining pressure as 3 MPa and 4 MPa to obtain an equilibrium permeability; (2) gradually raise

226

the confining pressure from 4 MPa to 10 MPa with 1 MPa increment to measure corresponding

227

permeability. Noteworthily, before or after this test, the confining and injection pressure were

228

increased or decreased alternately and slowly, advoiding unpredictable damage of coal samples.

229

The poroelasticity effect and the adsorption induced swelling effect all contribute to the total

230

swelling strain. Then the adsorption-induced swelling strain was obtained by subtracting the

231

helium induced strain from the CO2 induced total strain under consistent experimental

232

environment. The adsorption swelling experimrnt is implemented at temperature of 35 ℃ and

233

injection pressure ranging from 2 MPa to 10 MPa with 2 MPa extent progressively. The volume of

234

adsorption cell can be adjusted by non-adsorbing metal gaskets to fit coal sample size. All the tests

235

were conducted at least two runs to ensure the repeatability.

236 237 238

Fig. 4 Experimental procedures of this work. Pc is confining pressure and Pi is the injection pressure.

2.5. Anisotropy index

239

The heterogeneity of coal is not only manifested in the difference of material composition but

240

also performed in its directionality of pore-fracture structure. And the latter property is significant

241

because it controls the seepage capacity of coal, which is regarded as the key parameter for CBM

242

recovery, CO2-ECBM or CGS

243

coal, an anisotropy index (AI) was calculated according to:

59-61

. To characterize the degree of the anisotropic characteristic in

244 245

(6)

Where

represents the anisotropic parameter of coal, e.g., porosity, swelling strain and

246

permeability;

247

cleat and vertical bedding plane direction;

248

AI describes the otherness of coal in parallel face cleat, parallel butt cleat and vertical bedding

249

plane direction. Obviously, the greater the AI value, the stronger the anisotropic property of coal.

250

3. Results and discussion

,

and

are corresponding parameters in parallel face cleat, parallel butt represents the average value of

9


,

and

.

Energy & Fuels 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

251

Page 32 of 49

3.1. Anisotropic porosity

252

The porosity test results of cubic coal sample 1 and 2 were shown in Table 3. The porosity

253

differs in the three directions. Each test was implemented for two runs, and the standard deviations

254

were calculated based on the experimental results, which is distributed in the range of 0.0042 to

255

0.0127. This confirms that porosity measurements have a favorable repeatability. φx possesses the

256

maximum porosity of 0.490 and 0.340 for sample 1 and sample 2, followed by which is the

257

porosity φy of 0.471 and 0.325 for sample 1 and sample 2, and the smallest porosity is φz of 0.294

258

and 0.256 for sample 1 and sample 2. Apparently, anisotropic porosity falls in the order of parallel

259

face cleat and bedding plane direction > parallel butt cleat and bedding plane direction > vertical

260

bedding plane direction though values are different of the two coals. The porosity anisotropy index

261

(AIφ) calculated by Eq. (6) shows that the porosity anisotropy of sample 1 is stronger than sample 2.

262

In addition, the porosity measure in this paper is less than that reported in previous researches 62,63.

263

This is because the inaccessible pore that helium cannot be injected in contributes the porosity

264

measured by SANS method. Besides, the pore porosity is also sensitive to the pressure changes

265

during the experiment. Accordingly, stating the applied pressure of a test is essential in the

266

porosity study.

267

The porosity discrepancy of coal can be owed to the pore connectivity in different directions.

268

The formation of coal is accompanied by sedimentation, compaction, thermal evolution effect and

269

geological tectonism. Accompanied by the in-situ stress, the sedimentation and compaction during

270

geological periods changes primary and secondary pore morphology. Especially, the mechanical

271

properties of different coal macerals are various

272

hardness than inertinite

273

pores with major axis in parallel bedding plane direction (PBD) and minor axis in vertical bedding

274

plane direction (VBD). This case increases the connectivity of pores in PBD and reduces it in

275

VBD. Thermal evolution effect induces coal matrix shrinkage, which causes the generation of

276

shrinkage-induced pores (closed pores, interconnected pore, open pores) 16,67. This will improve or

277

degrade the connectivity of coal pores by homogeneous and inhomogeneous shrinkage

278

redistribution of in-situ stress resulting from multistage geological tectonisms will differentially

279

transform the macromolecular structure

65,66

64

, vitrinite has higher brittleness and lower

. Pores in low strength macerals is easily squeezed and forms to slit

68

17

. The

and the pore connectivity as the structural patterns 10


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Energy & Fuels

15,69

280

varying

281

then the inhomogeneity in this direction mainly reflects in the difference of material composition

282

70

283

the barrier strip to prevent helium flowing onwards. However, when helium is injected in PBD, all

284

the bright band and dull band are treated equally. This induces the measured porosity in PBD is

285

larger than VBD. For PBD, the material composition is relatively uniform and its difference can

286

be neglected, herein, the morphology and distribution of face cleat and butt cleat control the

287

connectivity of pores. That is, the continuous and further extended face cleat will link more pores,

288

contrarily, insufficient joins of pores are constructed for the discontinuity and low distensibility of

289

butt cleat. This leads to the porosity difference in face cleat and butt cleat direction. In this work,

290

we specially avoid the influences of structural fractures because tectonism effect is out of the

291

study scope in this paper. Thus, the porosity discrepancies in different directions can be owe to the

292

compaction, thermal evolution effect, banded structure and cleat distribution in coal seams.

293

. Additionally, bright band and dull band of coal are alternately superposed in VBD,

. When helium is injected in VBD, the dull band with poor-connected pore or cleat can server as

Table 3 The porosity and its anisotropic index of cubic coal samples.

294 295

3.2. Anisotropic adsorption swelling

296

The swelling strains obtained from the data collector were recorded by µε, which were

297

converted and shown in Fig. 5 and Fig. 6. εx, εy, εz and εV are the swelling strain in parallel face

298

cleat and PBD, parallel butt cleat and PBD, VBD and volume strain. Sample 1 and sample 2

299

possess different adsorption swelling strains, i.e., volume strain of sample 1 is 1.24-1.89 times

300

larger than that of sample 2 with the increasing of CO2 injection pressure. The approximately

301

linear relation of adsorption capacity and swelling strain was reported in the research of Kelemen

302

et al. 71. Besides, section 3.1 shows that the porosity of sample 1 exceeds that of sample 2 in all the

303

three directions, then sample 1 with high porosity has the ability to accommodate more CO2 and

304

induce forceful reaction with coal matrix. Anyhow, the different porosity of the two coal samples

305

may be one potential cause of their various swelling if we ignore other influencing factors (elastic

306

property, gas type, etc.). In addition, adsorption swelling unfolds an anisotropic property (εz>εx>εy).

307

The swelling strain of VBD is far greater than that of PBD, the maximum swelling ratios of VBD

308

to PBD are 2.30 for sample 1 and 1.89 for sample 2. However, the ratios of parallel face cleat to 11


Energy & Fuels 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

Page 34 of 49

309

parallel butt cleat are 1.28 for sample 1 and 1.30 for sample 2. The different swelling in PBD and

310

VBD was not just reported in this work. Day et al. considered that adsorption swelling is

311

30%-70% higher in the perpendicular bedding plane than the parallel plane of two Australia coal

312

samples

313

bedding plane is generally up to 60% higher than that along the direction parallel to the bedding

314

plane 72.

36

, Espinoza et al. also stated that swelling along the direction perpendicular to the

315

Coal swelling is the comprehensive effect of multiple influencing factors, including coal rank,

316

moisture, mineral matter content and cleat orientation. The swelling rate was found to depend on

317

maceral composition, the lowest carbon content has shown the maximum swelling increase

318

Additionally, the moisture content decreases the contact chances of coal and CO2 and then moist

319

coal shows a lower swelling

320

heterogeneity of coal adsorption swelling 37,75. In this paper, we will focus on the cleat orientation

321

and maceral on anisotropy swelling and ignore other factors because of the similar metamorphic

322

grade, low moisture and mineral matter content of selected coal samples.

74

73

.

. Reaction of CO2 and mineral or maceral increases the

323

Noteworthily, bright coal bands are both presented in the two coals in this study, they play

324

important roles on the adsorption swelling effect with respect to bedding orientation 37. Comparing

325

to dull band, cleat or pore system is widely developed in bright band for its higher vitrinite content.

326

Therefore, it will trap more CO2 molecules and swell more intensely. In PBD, coal cleat is easily

327

compressed by external stress and adsorption swelling in this direction grows toward interior

328

space; contrarily, in VBD, coal matrix can bear the overburden pressure at a certain extent and

329

corresponding swelling expands toward the outside. Additionally, the adsorption swelling in VBD

330

is contributed by all the bright bands and dull bands, however, in PBD, its maximum swelling

331

strain is equal to the swelling of bright coal band. Therefore, the interlaced distributed bands of

332

coal induce more swelling strain of VBD than PBD. This is consistent with the research by

333

Anggara et al., who considers that banded coal samples tend to have anisotropic linear swelling and

334

non-banded samples showed more isotropic behavior without much preference to bedding plane

335

orientation 37.

336 337

Generally, coal structures are viewed as three-dimensional cross-linking networks, and cross-linking mechanisms play an important role in swelling 12


76

. The sedimentation and

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Energy & Fuels

338

compaction effect induce the difference of cross-linking density in various directions, the denser

339

cross-linking structure produces greater resistance in PBD and suppresses the adsorption swelling

340

effect in this direction. Additionally, mechanical properties of coal are both influenced by the

341

roughness and the continuity/discontinuity of the bedding structure 77, as discussed in section 3.1,

342

the material compositions in VBD are highly non-uniform, they are compressed reciprocally and

343

cause the lower elastic modulus in this direction. Hence, the adsorption swelling performance is

344

easier to appear. Besides of the factors metioned above, the lower elastic modulus and

345

cross-linking density result in the swelling effect in VBD is more obvious than PBD.

376 377


378 379 380


3.3. Anisotropy adsorption swelling index

381

There is no doubt that the heterogeneous material composition and cleat property contribute

382

to the anisotropic swelling strain, however, the interaction of coal matrix and CO2 can also change

383

anisotropic swelling feature. According to Eq. 6, the anisotropy swelling index AIε was

384

determined and its evolution with the varieties of injection pressure in the two coals were shown

385

in Fig. 7 and Fig. 8. Two independent contrast experiments were successively conducted, and the

386

similar trends confirmed the repeatability of sample 1 and sample 2 although the maximum

387

standard deviation 0.07 is existed in these tests.

388

Clearly, the increasing injection pressure promotes the anisotropy swelling index of both

389

sample 1 and sample 2. Generally, CO2 adsorption rates are different in various microlithotypes,

390

and vitrain and clarain contribute significantly higher adsorption capacities than other components

391

78

392

more CO2 and the enlarge strain gap with paraellel direction. Additionally, the size and

393

distribution of cleats cause the differential swelling in PBD. Considering the transverse relatively

394

homogeneous constituent, we assume that the swelling along PBD is coincident. In this case, the

395

narrow and discontinuous butt cleats are easier to be blocked and closed, and the coal proceed to

396

swell toward the surroundings. Therefore, the butt cleat swelling exceeds the face cleat swelling in

397

our tests. The increasing adsorption swelling due to the advance of injection pressure broadens the

. Vertical and multilayer bright coal bands produce a superimposed swelling effect by adsorbing

13


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398 399

Page 36 of 49

disparity in various directions, leading to the enlargement of AIε. As an advantageous technology of sequestrating CO2 into deep unminable coalbeds

79

, the

400

influence of high temperature on the anisotropy swelling in coal during CO2 injection cannot be

401

ignored. The temperature promoting coal swelling tests of the two samples were shown in Fig. 9

402

and Fig. 10. As can be seen that increasing temperature from 35 ℃ to 65 ℃ will impel coal

403

swelling. And the cubic coal samples also exhibit the anisotropic swelling features. Morever, after

404

averaging the values of run 1 and run 2, we obtained that the thermal swelling of sample 1 and

405

sample 2 with temperature ranging from 35 ℃ to 65 ℃ are 0.073%-0.261% and 0.085%-0.270%,

406

and corresponding adsorption swelling of sample 1 and sample 2 with injection pressure rising

407

from 2 MPa to 10 MPa are 1.182%-4.714% and 0.956%-3.623%. Apparently, the swelling

408

induced by heating is far less than the swelling induced by adsorption. The temperature anisotropy

409

index changes in heating process was shown in Fig. 11. Thermal conductivity varies between

410

different macerals of coal, and vitrinite possesses the higher thermal conductivity than inertinite 80.

411

Once the ambient temperature is advanced, the vitrinite will swell rapidly and evidently. Similarly

412

to the swelling induced by CO2 adsorption, the thermal swelling in VBD is the sum of all the coal

413

bands for the coal with band structure, however, in PBD, the maximum swelling is equal to that of

414

bright band. Therefore, reservoir temperature enhances the thermal swelling and its anisotropic

415

property, which further induces the decrease of permeability and restrain the CO2 injectivity.

416

Although the temperature induced swelling is minor, its impact on permeability is also

417

nonnegligible because the seepage capacity is extremely low in deep unrecoverable coal seams.

418 419

Fig. 7 The relationship of anisotropy swelling index (AIε) and injection pressure measured on sample 1.

420 421

Fig. 8 The relationship of anisotropy swelling index (AIε) and injection pressure measured on sample 2.

422 423 424

Fig. 9 The relationship of test temperature and swelling strain measured on sample 1 at atmospheric pressure.

425 426 427

Fig. 10 The relationship of test temperature and swelling strain measured on sample 2 at atmospheric pressure. 14


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Energy & Fuels

428 429

Fig. 11 The relationship of test temperature and anisotropy swelling index (AIε) at atmospheric

430 431 432

pressure.

3.4. Anisotropic permeability The injected CO2 will be in supercritical state in deep coal seams

81

. To comprehensively

433

analyze the anisotropic permeability changes after adsorbing CO2 under sub-critical and

434

super-critical states, three independent comparative experiments of cubic coal samples with no

435

CO2 adsorption, 6 MPa CO2 adsorption and 8 MPa CO2 adsorption were conducted. And the

436

relationships of effective stress and permeability of sample 1 and sample 2 were obtained and

437

shown in Fig. 12-14. Kx, Ky and Kz represent the permeability measured in parallel face cleat and

438

PBD, parallel butt cleat and PBD and VBD respectively. All the curves were fitted by:

439

(7)

440

Where K is the measured permeability; K0 is the initial permeability; αk is the stress sensitive

441

coefficient; Pc and Pi are the confining pressure and injecting pressure. From Fig. 12-14 we can

442

see that permeability in cubic coal samples unfolds anisotropic property, namely, Kx>Ky>Kz. This

443

phenomenon can be related to the cleats and bedding plane fractures existed in coal 82. Along with

444

face cleat and butt cleat seepage, the bedding plane fracture seepage is also included in both Kx

445

and Ky. Thus, Kx and Ky are greater than Kz, which only contains the cleat seepage. For our tests in

446

his paper, the permeability in PBD is 1.16-2.24 times than that in VBD.

447

3.4.1. Permeability anisotropy index

448

The permeability anisotropy index AIk was determined according to Eq. 6 and shown in

449

Table 4 and Table 5. For sample 1, initial AIk ranges from 0.230 to 0.545; Ak increases after

450

treating by 6 MPa CO2 adsorption, distributing from 0.352 to 0.571; sample 1 adsorbing 8 MPa

451

CO2 possesses the maximum AIk, which increases from 0.369 to 0.581 with decreasing effective

452

stress. For sample 2, initial AIk ranges from 0.144 to 0.435; AIk increases after treating by 6 MPa

453

CO2 adsorption, distributing from 0.353 to 0.544; sample 2 adsorbing 8 MPa CO2 possesses the

454

maximum AIk, which increases from 0.372 to 0.573 with decreasing effective stress. Obviously,

455

injecting CO2 into coal enhances its permeability anisotropy, which is more prominent for

456

super-critical CO2. To probe the evolutional reasons of anisotropic permeability with injecting 15


Energy & Fuels 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

457

CO2 in coal, the permeability sensitive analysis was conducted in next section.

458

3.4.2. Permeability adsorption sensitive index

Page 38 of 49

459

After 6 MPa and 8 MPa CO2 adsorption, permeabilities of coal samples in all directions

460

decrease remarkably comparing to the initial values. To quantitatively characterize permeability

461

varieties after adsorbing CO2, permeability adsorption sensitive index S was introduced in this

462

research, which is defined as the ratio of permeability decrease by adsorbing CO2. It can be

463

described as:

464

(8)

465

Where K0 and K1 is the permeability before and after CO2 adsorption. Permeability

466

adsorption sensitive index of sample 1 and sample 2 in different effective stresses after 6 MPa

467

CO2 adsorption and 8 MPa CO2 adsorption were shown in Table 6 and Table 7. Sx, Sy and Sz

468

represent the permeability adsorption sensitive index in parallel face cleat and PBD, parallel butt

469

cleat and PBD and VBD respectively. For sample 1, the values of Sx, Sy and Sz with 6 MPa CO2

470

adsorption fall in the range of 0.140-0.389, 0.157-0.404 and 0.284-0.438; the values of Sx, Sy and

471

Sz with 8 MPa CO2 adsorption fall in the range of 0.175-0.418, 0.197-0.423 and 0.332-0.473. For

472

sample 2, the values of Sx, Sy and Sz with 6 MPa CO2 adsorption fall in the range of 0.125-0.403,

473

0.169-0.413 and 0.358-0.506; the values of Sx, Sy and Sz with 8 MPa CO2 adsorption fall in the

474

range of 0.189-0.454, 0.330-0.464 and 0.418-0.573. The fact of Sz> Sy> Sx reveals us that cleat

475

permeability is more susceptible than fracture permeability, additionally, butt cleat permeability

476

possesses greater decreasing amplitude than face cleat permeability. Fractures and cleats are

477

successively regarded the seepage paths in the process of injecting CO2 in coal seams. Wang et al.

478

derived the expression of permeability and fracture width b 29:

479

(9)

480

The viscosity coefficient µ remains fixed in the constant temperature and pressure test

481

environment. Then permeability is only related to fracture porosity and fracture width. Coal matrix

482

swelling induced by adsorbing CO2 will squeeze the space existed in fractures and cleats and

483

promote the diminishment of fracture porosity and fracture width. According to Eq. 9, the

484

permeability will herewith drop in this evolutionary process.

485

As discussed above, the difference of material composition in VBD contributes to its 16


Page 39 of 49 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

Energy & Fuels

486

inhomogeneity structure. Fractures between the vitrinite-rich layers (bright bands) will adsorb CO2

487

rapidly and swell towards fracture space intensely, however, coal matrix in other layers with

488

faintish adsorption capacity will swell faintly and possesses a minor effect on the permeability

489

along bedding plane direction. Once fractures with higher seepage capacity exist in bedding plane,

490

the injected gas will preferentially flow through these paths and the permeability decrease will be

491

inhibited. Contrarily, the material composition in PBD varies little and the adsorption-swelling can

492

be approximately regarded as homogeneous, the butt cleat with smaller size is more easily filled

493

by matrix swelling comparing to face cleat, and equally, narrow cleats are more susceptible than

494

the through bedding plane fractures. Improving the permeability anisotropy by injecting CO2 into

495

coal seams is not hard to be understand if associating with that the low-permeability cleat

496

possesses higher permeability adsorption sensitive and the bedding plane fracture having stronger

497

seepage capacity will not produce a pronounced permeability drop because of its lower

498

permeability adsorption sensitive index. Besides, the super-critical CO2 adsorption can induce

499

more matrix swelling

500

sub-critical CO2

501

matrix will further enhance the anisotropic characteristic of coal.

83

10

and cause significantly larger permeability difference comparing to

. Accordingly, the more fierce reaction between super-critical CO2 and coal

502 503

Fig. 12 The relationship of effective stress and permeability with no CO2 adsorption.

504 505

Fig. 13 The relationship of effective stress and permeability after 6 MPa CO2 adsorption.

506 507

Fig. 14 The relationship of effective stress and permeability with 8 MPa CO2 adsorption.

508 509 510 511 512 513 514 515 516 517

Table 4 Permeability anisotropy index AIk of sample 1 with no CO2 adsorption, 6 MPa CO2 adsorption and 8 MPa CO2 adsorption. Table 5 Permeability anisotropy index AIk of sample 2 with no CO2 adsorption, 6 MPa CO2 adsorption and 8 MPa CO2 adsorption. Table 6 Permeability adsorption sensitive index S of sample 1 after 6 MPa CO2 adsorption and 8 MPa CO2 adsorption.

17


Energy & Fuels 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

518 519 520 521

Page 40 of 49

Table 7 Permeability adsorption sensitive index S of sample 2 after 6 MPa CO2 adsorption and 8 MPa CO2 adsorption.

3.5. Implications of anisotropic properties on CO2 injecting in coal

522

Instead of coalbed methane production path, CO2 flows from injection well to major fractures,

523

and is adsorbed in pores through cleats for CO2-ECBM or CGS. A brief flow model describing

524

this process was shown in Fig. 15. The coal block includes the main structure of coal seams, e.g.,

525

bedding plane fractures, bright coal bands, cleats and coal matrix. The total seepage process is

526

constituted of the bedding plane fracture seepage and cleat seepage. Cleats communicate massive

527

coal matrix block and are more effective at transporting gas than bedding planes

528

anisotropic property reflects the difference between cleats and bedding plane fractures. The lower

529

anisotropy index indicates cleats and bedding plane fractures possess the almost unanimous

530

permeability and is advantageous to CO2 injection; contrarily, the higher anisotropy index means

531

that cleat seepage capacity is much less than fracture seepage capacity, and cleats thus serve as

532

throats between fractures and coal matrix, which will restrain CO2 flow into coal pores. Then cleat

533

seepage is the substantial factor to determinate the injectivity of CO2 in coal. Unfortunately, as

534

reported in above, cleat is susceptible to adsorption-induced swelling in coal.

38

. The

535

One solution is to inject N2, flue gas or mixed gas in coal to improve reservoir permeability

536

because the lower absorbability and adsorption swelling of coal matrix 84-86, however, this effect is

537

temporary and could not fundamentally solve the CO2 injection issue

538

attempting to promote the formation of fractures by hydraulic fracturing, microwave treatment,

539

thermal treatment, ultrasonic wave treatment. Hydraulic fracturing is favorable for advancing the

540

formation of major fractures because of their differential mechanical properties, but shows little

541

benefit for cleats, then it is inefficient to improve the CO2 injectivity. Contrarily, the high-energy

542

microwave exposure in an unconfined bituminous core has been confirmed as an effective

543

approach to induce new fractures and increase the aperture in existing cleats

544

temperature-treated on coal creates massive seepage pores and fissures (or microfractures) has

545

also been reported in the research of Cai et al. 89; additionally, ultrasonic wave is also treated as a

546

way to induce crack propagation along face cleats, butt cleats, and coal bedding planes

547

Therefore, the cleat seepage and corresponding potential enhanced permeability measures should 18


87

. The other way is

88

; and the high

90

.

Page 41 of 49 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

Energy & Fuels

548

be pay more attention on.

549 550

Fig. 15 The flow process model of CO2 injecting in coal. (a) is bedding plane fracture; (b) is bright coal band;

551

(c) is cleat, including extensive face cleat and discontinuous butt cleat. For CO2 injecting into a coal seam, a

552

part of CO2 flows from macro fractures (e.g. bedding plane fractures), through cleats and is adsorbed on

553

pore surface of coal matrix. However, the other CO2 flows through macro fractures and arrives at CH4

554

drainage well directly for CO2-ECBM or other coal/rock seams for CGS. The former is effective seepage,

555

contrarily, the latter is ineffective seepage for CO2-ECBM and CGS.

556

4. Conclusions

557

(1) Anisotropic porosity fall in the order of parallel face cleat and PBD > parallel butt cleat

558

and PBD > VBD was obtained. This reflects the pore connectivity in different directions. The

559

porosity discrepancies in different directions can be owe to the compaction effect, thermal

560

evolution effect and cleat distribution in coal seams.

561

(2) The maximum swelling ratios of VBD to PBD are 2.30 in sample 1 and 1.89 in sample 2.

562

However, the ratios of parallel face cleat to parallel butt cleat are 1.28 in sample 1 and 1.30 in

563

sample 2. The inhomogeneity of matter composition in VBD and the difference of cleat

564

distribution causes the anisotropy swelling. The injection pressure and temperature all increase the

565

anisotropy swelling index, but the thermal expansion effect is fairly weak comparing to adsorption

566

swelling by raising the injection pressure.

567

(3) Adsorbing CO2 will enhance the permeability anisotropy in coal, especially for

568

super-critical CO2. This is because the low-permeability cleat possesses higher permeability

569

adsorption sensitive and the bedding plane fracture having strong seepage capacity will not

570

produce a pronounced permeability drop because of its lower permeability sensitive. Higher

571

permeability anisotropy index means that cleat seepage capacity is much less than fracture

572

seepage capacity. In this case, cleats that easily affected by adsorption-swelling always serve as

573

throats between fractures and coal matrix, which will restrain CO2 flow into coal pores.

574

Acknowledgements

575

This work was sponsored by the National Natural Science Foundation of China (41330638,

576

41372326, 41727801), the Fundamental Research Funds for the Central Universities 19


Energy & Fuels 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

577

(2014ZDPY27), the project funded by the Fundamental Research Funds for the Central

578

Universities (2014ZDPY27) and the Priority Academic Program Development of Jiangsu Higher

579

Education Institutions. The authors are also thankful to the three anonymous reviewers for their

580

valuable advices and comments on the manuscript.

581

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