Failure Mechanism of Coal after Cryogenic Freezing with Cyclic Liquid

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Failure Mechanism of Coal after Cryogenic Freezing with Cyclic Liquid Nitrogen and Its Influences on Coalbed Methane Exploitation Lei Qin, Cheng Zhai, Shimin Liu, Jizhao Xu, Zongqing Tang, and Guoqing Yu Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.6b01576 • Publication Date (Web): 07 Sep 2016 Downloaded from http://pubs.acs.org on September 10, 2016

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Failure Mechanism of Coal after Cryogenic Freezing with Cyclic Liquid Nitrogen and Its Influences on Coalbed Methane Exploitation Lei Qin a,b, Cheng Zhai a,b,c *, Shimin Liu c, Jizhao Xu a,b, Zongqing Tang a,b, Guoqing Yu a,b a

Key Laboratory of Coal Methane and Fire Control，Ministry of Education， China University of Mining and Technology, Xuzhou,

Jiangsu 221116, China b

State Key Laboratory of Coal Resources and Safe Mining . Xuzhou, Jiangsu 221116, China

c

Department of Energy and Mineral Engineering, G3 Center and Energy Institute, Pennsylvania State University, University Park,

Pennsylvania 16802, United States

ABSTRACT: This study explores how liquid nitrogen (LN2) freezing affects the physical pore and fracture structure of coal. In lab-controlled condition, coal specimens were frozen with LN2 under different conditions, thawed, and then the uniaxial compressive strengths, acoustic emissions, and ultrasonic wave velocities of the different specimens were compared. After 60 min of freezing for one set of specimens and 30 freeze−thaw cycles for another set, the elastic moduli of the coal specimens decreased by 47.8% for the 60-min freezes and 76.2% for the 30 cycles. For the tested two sets of the same specimens, the uniaxial compressive strengths and longitudinal wave velocities dropped by 13.4% and 40.2%, 47.8% and 76.2%, respectively. At the same time, the coal porosities and Poisson’s ratios increased by 17.5% and 68.1%, 7.14% and 28.6%, respectively. Owing to the reduction of the coal’s mechanical strength, the elastically straining stage was shortened and the peak yield point and the plastic deformation were accelerated. By establishing a relational model for an elastic modulus based damage variable D and the LN2 freezing conditions, it was found that variable D increased to and stabilized at 0.12 with the single freezing experiments. However, the damage to the coal caused by cyclic freezing and thawing was continuous and damage accelerated after 20 freeze−thaw cycles. By modeling the state of stress in fractures of LN2 treated coal, the theoretical governing equations for the tension in a single fracture were derived. In addition, the expression regarding the volumetric strain of ice under the effect of tension for a single fracture was obtained. The results showed that the proposed model and expressions were well agree with the experimental obtained data. KEYWORDS: freeze−thaw; mechanical parameters; scanning electron microscope; acoustic emission; coal-bed methane

*

Corresponding author: Tel: +8613585391210. Email address: [email protected] (C. Zhai) 1

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1. INTRODUCTION The exploitation and utilization of coal-bed methane (CBM) can not only increase the world’s energy supply but also protect the environment and prevent gas disasters in coal mines.1-3 CBM accumulates in coal seams in a free or an adsorbed state. Its extraction is commonly accomplished by migrating desorbed gas to extraction wells through natural or stimulated fracture networks. Because coal generally has low fracture/cleat permeability,4 measures must be taken to improve its permeability which allows economical CBM extraction.5 Conventional hydraulic fracturing requires large amounts of water and can potentially result in water related environmental concerns since the coal seam is normally well connect with local ground water aquifers. Additionally, “fracking” can damage the coal reservoir and partially nullify the fracturing effect.6-9 In the past few years, intensive studies on the extraction of unconventional gases, including CBM, have turned their attention to waterless fracturing techniques using liquid nitrogen (LN2), supercritical carbon dioxide, or other non-aqueous media as the fracturing fluid.10-13 Compared with fracturing coal using gaseous nitrogen, fracturing with LN2 has many advantages. At atmospheric pressure, the temperature of LN2 is −196 °C and the latent heat of vaporization is 5.56 kJ/mol. At 21 °C and atmospheric pressure, 1 m3 of LN2 will expand to a volume of 696 m3, thus generating enormous expansive force for the fracture tip propagation. In addition, because most cleats in coal seams contain water, where coal is in contact with LN2, the water quickly freezes. After this phase change, water increases in volume by about 9%. Theoretically, this can generate a frost heave pressure of around 207 MPa.14 In the 1990s, McDaniel et al.15 used LN2 as the fracturing fluid to successfully stimulate five wells in an oil and gas field. Grundmann et al.16 froze a low permeability Devonian shale using LN2 and found that the gas production rate was improved by 8% compared with production after fracturing using traditional methods. By using LN2 as the fracturing fluid, Coetzee et al.17 effectively accelerated the development of micro-pores and fractures. In addition, Li et al.13 proposed a fracturing technology based on the vaporization of LN2 and designed a corresponding production scheme applicable to the exploitation of shale gas. By studying the variations of compressive strength and elastic energy density of coal before and after a one-time freeze using LN2, Cai et al.11, 18

found that LN2 freezing can cause considerable damage to coal and promote the development of fractures. In the past, studies of LN2 induced fracturing have mainly focused on fracturing using a single injection of

LN2. The author has proposed a technique using cyclic fracturing of coal reservoirs using LN2. This method increases the permeability of coal reservoirs based on cyclic freeze−thaw effects and the vaporization-induced 2


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fracturing by LN2 at low temperatures. By using this cyclic injection technique, freeze−thaw cycles are generated in the coal by the freezing and thawing of the liquid. This results in a change of the physical structure of the coal and the cleat/fractures networks were modified and new fractures were generated. The increase of fracture density automatically decreases the coal strength and enhance the permeability significantly. Cyclic freeze−thaw has long been regarded as a kind of low-temperature disaster. Natural freeze−thaw causes significant damage to buildings and their foundations, damages bridges, and disaggregates natural rock outcrops. Many researchers have studied the mechanical properties of frozen rocks in alpine areas.19, 20 For example, Hale and Shakoor21 investigated the failure modes and compressive strengths of six sandstones after numerous freeze−thaw cycles. They found that multiple freeze−thaw cycles reduced the rock’s compressive strength and changed the porosity. Yavuz et al.22 subjected 12 different kinds of carbonate rocks to 20 freeze−thaw cycles. After analyzing the physical properties of the rocks, including porosity and acoustic wave velocities before and after the freeze−thaw cycles, they established a model for the freeze−thaw weathering of carbonates. Matsuoka23 carried out cyclic freeze−thaw experiments on 47 kinds of rocks and found that the compressive strength and porosity of rocks exerted a significant influence on the frost heave deterioration of the rocks studied. He reported that fractures in rocks severely affect the freeze−thaw and the frost heave damage to rocks. This damage is mainly caused by increasing preexisting damage and by generating new micro-fractures. Altindag et al.24 investigated the mechanical properties of limestone after freeze−thaw cycles and constructed a relational model for the attenuation rate of mechanical properties and freeze−thaw cycles. Currently, no systematic and intensive investigations on the failure mechanisms that take place in coal by cyclic LN2 cryogenic treatments. To fill that gap, this study treated coal with LN2 under different freezing conditions. Then, the uniaxial compressive strength, acoustic emissions, and ultrasonic wave velocities of the coal specimens were measured, analyzed and compared. In addition, scanning electron microscope (SEM) images were acquired and used to observe the microstructures in the fractured coal specimens. The aim was to document the changes in physical properties and the development of fractures in the coal. The research analyzed the effect, mechanisms, and factors influencing the freeze−thaw fracturing of coal to provide an experimental basis for the study of cyclic fracturing technology for CBM reservoirs.

2. SAMPLE MATERIAL AND EXPERIMENTAL PROCEDURES 2.1. Coal Sampling and Preparation. The coal specimens used for this study were lignite taken from the Shengli coal field in Inner Mongolia, China. The coal specimens were cores with 50 mm in diameter and 100 3


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mm in length and to ensure that the specimens were as similar as possible, there were all drilled from the same block of coal (Figure 1). Cores were drilled with the axis of the cylinders perpendicular to bedding. Specimens with a regular structure and complete surfaces were selected and subjected to density and wave-velocity tests to obtain specimens with similar physical properties. The group of specimens with similar properties were then numbered and sent for maceral and proximate analysis. The results of these measurements and analyses are listed in Tables 1 and 2. Table 1. Sample Parameters and Numbers. sample

height (mm)

saturated water content (wt %)

sample

height (mm)

saturated water content (wt %)

T−1

101.6

12.5

C−5

100.1

13.6

T−5

100

13

C−10

101

11.5

T−10

100.5

13.7

C−15

100

14.6

T−20

100

14.1

C−20

100.7

12.8

T−30

100.5

9.8

C−25

100

13.2

T−40

102

13.0

C−30

100.6

9.8

T−50

101

12.3

−

−

−

T−60

101

12.0

−

−

−

Notes: T−1 represents a coal sample that was frozen in LN2 for 1 min then thawed at room temperature for 1 min (one freeze−thaw cycle); C−10 represents a coal sample that was frozen in LN2 for 5 min then thawed at room temperature for 5 min (10 freeze−thaw cycles); wt., weight percentage. Table 2. Maceral and Proximate Analyses for the Sample Listed in Table 1. maceral analysis (vol %)

proximate (wt %) Ro,max (%)

V

I

E

M

80.5

14.5

3.7

1.3

0.331

Mad

Aad

Vdaf

FCad

10.67

14.53

43.5

68.7

Notes: V, vitrinite; I, inertinite; E, exinite; M, minerals; Mad, moisture, air-drying basis; Aad, ash yield, air-drying basis; Vdaf, volatile matter dry ash-free basis; FCad, fixed carbon content, air-drying basis.

2.2. Experimental Procedures. The tested coal specimens were saturated with water in a vacuum water saturation device at a vacuum pressure of −0.1 MPa for 12 hours, and then frozen in the LN2 freeze−thaw system under different freeze−thaw conditions. They were frozen for 1, 5, 10, 20, 30, 40, 50, 60 min every time, while the freeze−thaw cycles, each of which for 5 min, were carried out for 1, 5, 10, 15, 20, 25, 30 cycles. After the freezing, the specimens were thawed at room temperature to achieve one freeze−thaw cycle. Then, the SEM and longitudinal wave velocities of ultrasonic waves and the porosity of coal specimens before and after the freezing 4


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under different conditions were tested. In the experiment, an acoustic parameter tester (HS-YS4A) and MR-60 magnetic resonance imaging (MRI) analysis system were used to measure P-wave velocities of ultrasonic waves and porosity in the coal respectively. Based on the number of specimens, uniaxial compression test was carried out to the specimens in each group, and collect the acoustic emission signals emitted by the specimens during compression. The experimental equipment and procedures are shown in Figure 1.

Coal

Coal Samples

Freeze-thaw Test Chamber

After Freeze-thaw Axial Loading Subsystem

Acoustic Emission Monitoring Subsystem

域控制器

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Computer

Coal Sample

Servo Control System

Axial Loading Subsystem

Broken Samples

Strain Monitoring Subsystem

Figure 1. Experimental equipment and procedures.

3. RESULTS AND ANALYSIS 3.1. Principles of Uniaxial Compression and Acoustic Emissions; Frozen Coal Crack Evolution. The uniaxial compressive strength of a coal specimen refers to the maximum stress when the specimen fails with no confining pressure. When the coal specimen is frozen, its mechanical properties change. By comparing the compressive strength and elastic modulus of coal specimens after different freezing times and after a different number of freeze−thaw cycles, the different amounts of damage after the specimens have undergone different freezing experiments can be determined.25, 26 When coal is deformed, stress concentration was generated at the fracture zones within the specimen and thus results in an increase of the strain energy. When the stress increases to a certain value, microscopic yield and deformation occur in the vicinity of the tips of preexisting cracks. As a consequence, cracks grow and the energy stored in the coal is released in the form of elastic waves. These waves are acoustic emissions.27, 28 By 5


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collecting and analyzing the acoustic emissions emitted during the deformation of the coal, the generation, development, and connection of the cracks and the damage to the coal can be deduced.29 Based on these deductions, a description of how the coal fails can be devised.30-33 The results of the uniaxial compression experiments showed that the failure of the coal specimens frozen by LN2 could be divided into four stages: compaction, elasticity, yield, and failure. As an example of how failure occurs, Figure. 2 shows test results for a coal specimen frozen by LN2 for 60 min. LN2 (-196℃ ℃)

Frozen-thawed fractures

10

Rupture

Stress peak

C

8

Yield point strain

B

6

Initial fissures

Frozen-thawed fractures closure stage

2

O 0

1

2

σ1

3

4

σ1

σ1

Compressive fractures

Strain (%)

(a) Load-deformation curve

Frozen-thawed fractures

Rupture

4

Compressive fractures

Crack closure strain A

Elastic stage

Stress (MPa)

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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σ1

Compressive rupture

(b) Evolution process of coal crack

Figure 2. Uniaxial compression curves and evolution of fractures in a coal specimen frozen by LN2 for 60 min.

(1) In compaction stage OA, the fractures generated in the coal specimen by freeze−thaw gradually close. The coal specimen is compacted and the axial strain is reduced nonlinearly with increase of the compression stress. This is demonstrated by the slow rate of growth in the stress−strain curves. Before this stage, load damage was small so that the acoustic emission signals were mainly generated by sliding friction between grains caused by closure of freeze−thaw-generated cracks in the coal. The longer the OA stage, the higher fractures density in the frozen−thawed coal specimen is. (2) When the stressed specimen begins to deform, in the elastic deformation stage, AB, the original cracks in the coal have been compacted in the previous OA stage, but no new cracks have yet developed. Therefore, the volume of the specimen drops linearly and the stress−strain curve becomes almost a straight line. At this point in the experiment, the strains at points A and B corresponded to those of a fracture close and yield point and the AB section of the curve represents the elastic deformation stage for the coal. At this stage, acoustic emissions are rare and those few emissions that do occur have stabilized at a low value. The shorter AB section of the curve is, 6


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the smaller the elastic deformation of the specimen, and this means that the strength of the coal specimen is lower. (3) Within the yield stage, segment BC in Figure 2(a), cracks began to be generated and extend so the total volume of fractures gradually increases along with a decline in the rate of volume change. As the load grows, small fractures connect to form macro-fractures. This causes the number of acoustic emission events to increase abruptly. (4) The failure stage begins at point C. After peak stress at point C, fractures develop rapidly and connect to generate large fractures. Blocks slip along macroscopic fracture surfaces, and stress rapidly decreases as the strain increases. As the stress−strain curve suddenly drops, the acoustic emission counts rise sharply. After the specimen has failed, the number of acoustic emission events rapidly decreases and goes to zero with the release of stress in the coal. Before LN2 freezing, there were few preexisting micro-fractures in the coal specimen. After the freeze−thaw treatment using LN2, freeze-thaw fractures of varied dimensions were generated in the coal by frost heaving and low-temperature damage to the macerals. These freeze−thaw fractures were compacted and closed in the uniaxial compression stage. Then, small compressive fractures were formed under the compression loads and soon these fractures connected to become macro-fractures. This continued until the specimens failed, as illustrated in Figure 2. By analyzing the acoustic emissions and strength of the specimens under uniaxial compression, the amount of damage generated in the coal specimens and the fracture development under different freeze−thaw conditions was ascertained. 3.2. Results of Uniaxial Compression Tests and Acoustic Emission Measurements. Stress, strain, and acoustic emission signals during uniaxial compression were monitored for coal specimens subjected to different LN2 freezing conditions. Figure 3 shows the stress−strain curves and acoustic emission data for coal specimens before and after LN2 freeze−thaw treatments. The specimens were frozen for 30 or 60 min and were put through 10, 20, or 30 freeze−thaw cycles. According to the experimental results, the uniaxial compressive strength of the coal specimens was reduced by longer freezing times and a larger number of freeze−thaw cycles. The compressive strength of the coal specimens was 11.2 MPa before freezing but after 30 min of being frozen by LN2, the specimen’s compressive strength was 10.1 MPa and after 60 min it was only 9.7 MPa. These are reductions of 9.8% and 13.4% after 30 and 60 min, respectively. After 10, 20, and 30 freeze−thaw cycles, the compressive strengths for the coal specimens were 10.28, 10.16, and 6.7 MPa, which are 8.2%, 9.3%, and 40.2% 7


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lower than the strength for a correlative coal specimen that was never frozen.

A 400

200

100000

8 75000

6 50000

4 25000

Ring-down counts

600

800

A

600

400

200

2

O 600

0

0

800

0

100

200

Time (s)

180000

120000

6

4

60000

2

0

0

600

A 400

200

8

6 100000 4

600

400

50000 200

2

0 200

300

400

500

600

700

0

0

O

8 240000 6 160000 4

80000

0

0

800

200

400

600

10

2

0

800

Time (s)

Time (s)

(b-2) AE counts, cumulative counts and stress after F-T 20 cycles

(a-2) AE counts, cumulative counts and stress after F-T 30 min 200000

B

600

A 400

200

1000

6 100000 4

800000

C

Ring-down counts Cumulative ring-down counts Axial stress

800

8 150000

Ring-down counts

Cumulative ring-down counts

C Ring-down counts Cumulative ring-down counts Axial stress

10

Stress (MPa)

1000

B A

600

400

50000 2

200

0

0

6 600000

4 400000

200000

Stress (MPa)

100

320000

B

A

O 0

C


800

150000

400

1000

10

Ring-down counts

B


800

300


200000

C


Stress (MPa)

1000

Ring-down counts

8

Time (s)

(a-1) AE counts, cumulative counts and stress before freeze-thaw (F-T)

2

O

O 0

0 0

240000

Stress (MPa)

400


200


0

0

800

10

O

0

0

300000

C

B


10

Stress (MPa)


Ring-down counts

800

B

1000

125000

C

Stress (MPa)

12



1000

Ring-down counts

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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200

400

600

0 0

800

100

200

Time (s)

300

400

500

0

600

Time (s)


(a-3) AE counts, cumulative counts and stress after F-T 60 min

Figure 3. Uniaxial compression curves and acoustic emissions for coal specimens frozen by LN2 under different conditions.

In fracture compaction stage OA (Figure 3-a-1), few acoustic emission signals were observed from the non-frozen coal specimens. With increasing freezing times and number of freeze−thaw cycles, the frozen coal specimens emitted a large number of intense acoustic signals in the OA stage for a prolonged time. After 30 min and 60 min of freezing using LN2, the number and amplitude of acoustic emission events from the coal specimens showed an obvious increase (Figure 3-a-2 and -3). Moreover, the coal specimens that underwent 20 and 30 freeze−thaw cycles showed acoustic emission signals with larger amplitudes than emissions from specimens that had undergone only a single freezing (Figure 3-b-2 and -3). This is because a larger number of 8


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freeze−thaw induced fractures were generated within the specimens after 20 or 30 freeze−thaw cycles. Under uniaxial compression, the sliding friction along these fractures induced more acoustic emissions with higher amplitudes. In the yield period, line BC, the acoustic emission counts and duration of emission events gradually grew with increasing freezing time and the number of freeze−thaw cycles. The number of events occurring in the frozen coal specimens was three to seven times that in the non-frozen coal specimen, and the event amplitudes were around two to eight times higher. After point C in Figure 3, the failure stage, small fractures were gradually developing into larger fractures or macro-fractures, thus giving rise to brittle fracture in the coal specimens. The coal specimens frozen by LN2 only once (Figure 3-a-2 and -3) showed an abrupt increase (indicated by the ellipse in Figure 3-a) in the count of acoustic emission events. In contrast, the coal specimens treated with many freeze−thaw cycles (Figure 3-b) showed two or more abrupt increases. This is because repeated freeze−thaw cycles cause greater damage to the coal than does a single freezing treatment. The mechanical strength of the coal was reduced by the generation of many freezing-induced micro-fractures that released some stress energy before the coal was ruptured. This led to the occurrence of several abrupt increases. Looking at the total number of acoustic emissions during the entire set of compression experiments, the number of acoustic emission events in the frozen coal was two to eight times higher than the number of emissions in the non-frozen coal. Similarly, the total number of events counted in the coal specimens treated with many freeze−thaw cycles was larger than that for the coal specimens frozen only once. Turning to crack development, after the coal was frozen using LN2, the water in the pores and preexisting fractures in the coal froze and caused the coal to expand. As a result, the fractures in the coal became wider, developed further, and extended thus forming many larger fractures. For this reason, the mechanical strength of the coal was reduced and therefore the stress−strain energy in the coal bulk was partially released in the uniaxial compression process. This was demonstrated by the growth in the number of acoustic emission events in the compaction stage and the occurrence of abrupt increases at the yield and failure stages. 3.3. Coal Micro-morphologies before and after Freeze−thawing. Gas migration in coal can be briefly summarized as follows: in the pore-fracture system of a coal reservoir, after the gas from the coal matrix is desorbed, it first diffuses into pores, including primary pores and gas-bearing pores, and micro-fractures. It then percolates through gas-bearing pores to local fractures, such as bedding fractures and interlayer fractures. Finally, 9


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it migrates out of the coal seam through connected fractures including pre-existing cleans, tensional fractures and opening-mode shear fractures. To study the influence of LN2 freezing on the microstructure of coal seams, specimens of the coal were examined using a SEM before the specimens had ever been frozen, after a specimen had been frozen once, and after specimens had been through a number of freeze−thaw cycles (cyclic freezing). Figure 4 shows examples of raw coal (Figure 4-a), a specimen frozen once for 10 min using LN2 (Figure 4-b) and a specimen that underwent 30 freeze−thaw cycles (Figure 4-c).

a

b

c

Main fractures Connected fractures network

200× ×

Initial fractures

a

Secondary fractures

200× ×

200× ×

b

c Secondary fractures

Connected fractures network

Main fractures Initial fractures

500× ×

500× ×

500× ×

a

b

c Coal matrix frozen shrinkage

Water-ice frozen expansion Coal particle

Initial fractures Coal matrix frozen shrinkage

Water-ice frozen expansion

2000× ×

Coal particle

(a) Before LN2 freeze-thaw

2000× ×

2000× ×

(b) After freezing 10min by LN2 (1cycle)

Delamination

(c) After 30 freeze-thaw cycles

Figure 4. SEM images of coal specimens before and after freeze−thaw treatments.

Using a SEM, most of the features observed with widths less than 2 µm are primary pores, gas-bearing pores, mineral pores, and micro-fractures in coal matrix blocks. There are several types of pores in matrix blocks, 10


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most showing poor connectivity. In these pores, gas diffuses and percolates slowly. Only when the pores are connected with fractures and the overall gas deliverability can be accelerated. The fractures that are most useful for the migration and production of fluids from a coal reservoir are fractures that are part of the effective porosity and permeability of the coal seams. In terms of the types of fractures, local fractures (interlayer fractures and layer fractures) are fractures in the coal matrix, the generation of which increases the channels for gas migration and connects some closed pores and semi-closed pores in coal matrix blocks. Penetrating fractures, including tensional fractures and opening-mode shear fractures, can be relatively wide and develop on a large scale and penetrate several. They are the fractures between matrix blocks and can connect pores. They can also develop offshoots to form a fracture network. This kind of fracture can run through entire coal reservoirs and contribute substantially to permeability; they are the main channels for gas flow in coal seams. When the whole primary structure of the coal seam is not damaged, the existence of well-developed penetrating fractures helps to improve the permeability of coal reservoirs. As can be seen in Figure 4, the coal specimen that was never frozen presents flat and smooth surfaces with only a few preexisting micro-fractures (Figure 4-a). After 10 minutes of immersion in LN2, some large cracks and secondary crossing cracks formed owing to the freezing. These cracks developed gradually along cleats on the surface of the coal specimen (Figure 4-b). The maximum width of any of these cracks was 13.3 µm. After 30 freeze−thaw cycles, penetrating fractures, including tensional fractures and opening-mode shear fractures, were developed along the cleats (Figure 4-c) with a maximum crack width of 60.5 µm. The widths and lengths of the cracks in this coal specimen were obviously greater than the cracks in the coal frozen once for 10 minutes. Looking at the mechanics of crack formation, it is clear that on one hand, when the matrix grains in the coal came into contact with the LN2, they contracted because of the low temperature. This contraction caused tensile stress, and as a result, local fractures were formed. Examples of these contraction fractures can be seen in the areas outlined with rectangular boxes in Figure 4. On the other hand, when the water in the coal froze, the entire mass of coal expanded. The frost heave pressure caused the fractures in the coal bulk to extend along the cleats and gradually extend under the dual effects of tensile stress and frost heave pressure. After several freeze−thaw cycles, the fractures in the coal developed into a fracture network (tensional fractures and opening-mode shear fractures) penetrating several layers. As a result, macroscopic cracks appeared on the surface of the coal (note the frozen−thawed cracks in Figure 2-a). The development of this fracture network increased permeability in the coal seam and provided good migration channels for gas seepage. 11


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After the different LN2 treatments, a large number of micro-fractures had been generated in the coal, thus increasing fracture density of coal which will ultimately increase the permeability of coal. Meanwhile, because of the fracture density increase the bulk strength of coal was reduced.

4. DISCUSSION 4.1. Variations in Mechanical Properties and Freeze−thaw Induced Damage. The experimental results showed that the LN2 freezing changed the mechanical properties of the coal including the compressive strength, the elastic modulus, and Poisson’s ratio. For rocks, the elastic modulus quantifies the difficulty of deforming the rocks during the elastic stage. The smaller the elastic modulus, the easier the rock can be deformed. The uniaxial compressive strength quantifies the anti-loading capacity of the coal. The smaller the compressive strength, the easier the coal can be failed. Figure 5 shows the variations in the coal’s elastic modulus and uniaxial compressive strength with the lengths of freezing times and number of freeze−thaw cycles. Freeze-thaw cycles 0

5

10

15

Freeze-thaw cycles

20

25

30

0

5

10

15

20

25

30

Decrease 31.4%

R 2 = 0.9033

y = 0.516-0.009e 0.1008x

0.40

R 2 = 0.9540 Freezing time Freeze-thaw cycles

0.35 0

10

20

30

40

50

R 2 = 0.9882

11

10

60

9

8

y = 10.62-0.005e0.2215x

Freezing time Freeze-thaw cycles

7 0

LN2 Freezing time (min)

(a) Elastic modulus under different F-T cycles and time

Decrease 13.4%

y = 0.446+0.056e −0.0284x 0.45

y = 9.49+1.641e

-0.0326x

10

20

Decrease 40.2%

0.50

Uniaxial compressive strength (MPa)

Decrease 10.6%

12

Elastic modulus (GPa)

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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R 2 = 0.9367

30

40

50

60


(b) Uniaxial compressive strength under different F-T cycles and time

Figure 5. Changes in the elastic modulus and compressive strength after freeze−thaw cycling.

As shown in Figure 5, the coal’s elastic modulus fell as the freezing time and the number of freeze−thaw cycles increased. After 60 min of freezing and 30 freeze−thaw cycles, the elastic moduli dropped by 10.6% and 31.4%, respectively. Similar reductions, 13.4% and 40.2%, were found in the uniaxial compressive strength. The rate at which the elastic modulus and uniaxial compressive strength decreased gradually became slower as the freezing time increased. The rates diminished more quickly with an increase in the number of freeze−thaw cycles. The experiments showed that when the freezing time reached to a certain length, increasing the freezing time caused little additional damage to the coal. However, increasing the number of freeze−thaw cycles always caused more coal damage. After about 20 freeze−thaw cycles, the coal’s mechanical strength had declined 12


Page 13 of 25

dramatically that the strength of the coal was very low as indicated in Figure 5(a). After freezing with LN2, micro-fractures in the coal specimens extended owing to the frost heave action caused by the water-ice phase change and uneven contraction of maceral particles in the coal. According to Nemat-Nasser and Taya’s34 theory of damage mechanics, the elastic modulus freeze−thaw based damage variable D can be expressed as: D = 1−

En E0

(1)

where E0 and En are the elastic moduli of the coal in its initial state and after being freeze−thawed, respectively. Figure 6 show the variation of damage variable D with freezing time and the number of freeze−thaw cycles. The curves in Figure 6 can be approximated by the two exponential equations: DT = 0.126-0.1103e-0.0284t

(R 2 = 0.9032)

(2)

,

and DC = -0.0134+0.0168e0.1008C (R 2 = 0.954)

(3)

where DT and DC denote the damage variables for the coal with the freezing time and number of freeze−thaw cycles, respectively; t and C stand for the freezing time and the freeze−thaw cycles. It is obvious from the curves that the time freezing damage variable DT basically grew to and maintained a value of 0.12. However, cyclic freeze−thaw action continuously damaged the coal and the damage was more significant after 20 freeze−thaw cycles. Freeze-thaw cycles

0.32

5

10

15


20

25

30

y = -0.0134+0.0168e0.1008x Accelerated damage

0

Damage variable / D

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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R 2 = 0.954

0.24

0.16

0.08

y = 0.126-0.1103e -0.0284x

Slowed damage

R 2 = 0.9032

0.00 0

10

20

30

40

50

60

LN2 Freezing time (min) Figure 6. Damage with different freezing times and number of freeze−thaw cycles. 13


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Poisson’s ratio, also called lateral deformation coefficient, refers to the ratio of lateral normal strain to axial normal strain. The bigger the Poisson’s ratio, the easier it is to deform the materials laterally with axial loading. Figure 7 illustrates the strain of the coal under uniaxial compression before and after freeze−thawing and the variations in Poisson’s ratio for frozen coal after different freezing treatments. As can be seen, the frozen coal’s Poisson’s ratio increased with both simple freezing and repeated freeze−thaw cycles. After 60 min of freezing and 30 freeze−thaw cycles, the Poisson’s ratios were increased by 7.14% and 28.6%, respectively. It is clear that compared with a solitary freeze−thaw, freeze−thaw cycling influences the coal’s Poisson’s ratio more significantly. Freeze-thaw cycles 0

10

15

20

25

30


0.36

0.0

5

Increase 28.6%

0.2

0.34

-0.2

Axial strain Hoopstrain Volumetric strain

-0.4

0

50

100

y = 0.243 + 0.035e 0.041 x R 2 = 0.977

0.32

0.30

Increase 7.14%

Poisson's ratio

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

Page 14 of 25

y = 0.28 + 4.21x − 1.73 x 2 R 2 = 0.953

0.28

150

0

200

10

20

30

40

50

60


Time (s)

(a) Strain curves under uniaxial compression

(b) Change of Poisson's ratio under different F-T cycles and time

Figure 7. Strain and Poisson’s ratio before and after different freeze−thaw treatments.

The mechanical strength of coal depends mainly on the strength of the bonds between the particles that compose the coal and the number of micro-fractures. The more numerous the fractures in a bulk of coal, the more the coal will deform when subjected to a given force. The coal’s mechanical parameters, including the elastic modulus and the compressive strength, were all reduced to some extent by the LN2 freezing experiments. As expected, the coal specimens subjected to numerous freeze−thaw cycles were damaged more seriously and their mechanical parameters reduced more significantly than was the case for specimens treated with only a single freeze−thaw cycle. On one hand, if water exists in the pores and fractures in coal, the water freezes if LN2 is introduced resulting in a 9% of volume expansion. On the other hand, because LN2 is extremely cold, once coal at its ambient temperature is frozen by LN2, a temperature difference or around 200 °C is generated in a short time. Under such conditions, the volume of the maceral particles in the coal will shrink. Because of these two 14


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opposing volume changes in the coal, strong tensile stresses will form and therefore micro-fractures will be generated, thus reducing the coal’s cohesion. When the coal is deformed and damaged, the internal friction among particles decreases. As a consequence, initial fractures will extend and new fractures are formed, thus causing irreversible damage to the coal and reducing its strength. After cyclic freeze−thawing, more cracks are generated in the coal mass, and this enhances the connectivity of the pores. This improves the permeability of the coal and provides more favorable conditions for CBM extraction. 4.2. Analysis of Closed Cracks and Coal Yield Strains. According to Figure 2-a, the abscissas of closing point A for fractures and yield point B for coal on the stress−strain curve correspond to the axial strains of closed fractures and specimen yield during uniaxial compression. The section between points A and B indicates the strain caused by the elastic deformation during compression. The variations of the strain and yield strain for the coal specimens during compression with different freezing times and number of freeze−thaw cycles when the freeze−thaw induced cracks were closed is shown in Figure 8. It was noted that the strain of closed cracks increased with longer freezing times and a larger number of freeze−thaw cycles. Conversely, the yield strain and the elastic deformation induced strain, decrease with increase of freezing duration and number of freeze-thaw cycles. This implies that with longer freezing duration and more freeze−thaw cycles, more cracks were generated in the coal, thus increasing the strain on closed cracks. In addition, the generation and connection of cracks decreased the mechanical strength of the coal, and therefore the elastic stage of compression was shorten. This accelerated the process of yield in the coal specimens. Moreover, as can be seen from Figure 8, after 60 min of freezing and 30 freeze−thaw cycles, the strain of closed cracks and the yield strain in the coal mass were increased. After 60 min of freezing, closed crack strain increased by 76.1%, but it increased by 96.9% after 30 freeze−thaw cycles. Those same two freezing techniques produced decreases in yield strain of 8.4% and 15.7%, respectively. Freeze−thaw cycles cause more damage to the coal than the single freezing does.

15


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Freeze-thaw cycles 0

5

15

Elastic stage with time

3.5

10

3.0

20

25

30

Elastic stage with cycles

4.0

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

Page 16 of 25

2.5

Crack closure strain with freezing time Yield point strain with freezing time Crack closure strain with freeze-thaw cycles Yield point strain with freeze-thaw cycles

2.0

1.5

0

10

20

30

40

50

60

LN2 Freezing time (min) Figure 8. Strain of closed fractures and yield strain for coal specimens frozen by LN2 under different conditions.

Compared with single freezing, freeze−thaw cycles exerted several times more alternating stress including compression, expansion, and compression on fractures in the coal, thus damaging the coal more severely. The initiation pressure on the tips of the fractures is significantly reduced, and this helps a cross-connected fracture network to form, enabling gas migration. 4.3. Changes in Coal Porosity and Longitudinal Wave Velocity. Ultrasonic testing is an important tool for detecting and evaluating coal damage. For coal, the velocity of sound waves is mainly affected by the distribution of cracks.24, 35 Sound waves propagate at different velocities in different media and the wave travel velocity follows the order of solid>liquid>gas. For coal, when the velocity of sound waves reduces, it indicates that the number of fractures is increasing. Either existing fractures are being extended or new fractures are forming.22,

36

The decrement of the propagation velocity of sound waves can be used to monitor fracture

development. Freezing coal with LN2 causes preexisting fractures in the coal to extend and new secondary micro-fractures to form inside the coal and on the surface of the specimen. Wyllie et al.37, 38 proposed a time-averaging model for calculating the propagation velocity of sound waves in porous media. This model can be used to describe the relationship between the propagation velocity, vp, of sound waves and porosity, φ. Their equation is:

1 (1 - ϕ ) ϕ = + vp vm vf

(4)

where vp is the wave velocity of the equivalent of the coal; vm and vf show the wave velocities of the solid 16


Page 17 of 25

framework of the coal and the fluid in pores, respectively. The equation for computing the porosity is therefore:

(vm - v p )v f

ϕ=

(vm - v f )v p

(5) .

In this equation, vm and vf are fixed values in the same coal mass and fluid and when vm and vf are fixed, φ varies linearly with 1/vp. Therefore, the smaller the propagation velocity of sound waves in the specimens, the greater the porosity of the coal. To present the changes in porosity of the coal mass quantitatively, the porosity increment rate ∆φ% is used to define the rate of change of the porosity in the coal under different freeze−thaw conditions. Thus:

∆ϕ %=

ϕ post − ϕ pre ϕ pre

(6)

where ∆φ% is the increment rate of porosity in the coal; φpost represents the porosity of the frozen−thawed coal; and φpre is the porosity of the original, non-frozen coal. (a) LN2 freezing time

(b) Freeze-thaw cycles

80

Porosity increase rate with freeezing time Fit of ∆φN%

80

2000

Porosity increase rate with F-T cycles Fit of ∆φN%

P-wave velocity 60

y = −1.15 + 4.42e0.09 x

y = 3.49 + 0.23x

1500

R 2 = 0.988 40 1000

Vp (m/s)

1000

∆φ% (%)

40

Vp (m/s)

1500

20

2000

P-wave velocity

60

∆φ% (%)

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

20

R 2 = 0.996

500

500

0

0

0

10

20

30

40

50

60

0

5

10

15

20

25

30

Cycles

Time (min)

Figure 9. Variations in the increment rate of the porosity and changes in the velocities of the longitudinal ultrasonic waves velocities for coal frozen by LN2 under different conditions.

The non-destructive nuclear magnetic resonance (NMR) was used for measuring porosity before and after the freezing under different conditions. Figure 9 shows variations in the increment rate of the porosity and the longitudinal velocities of ultrasonic waves in coal specimens frozen for different freezing times and with different numbers of freeze−thaw cycles. It indicated that the porosity increase with both increase of freezing duration and number of freeze-thaw cycles as shown in Figure 9. After 60 min of freezing and 30 freeze−thaw cycles, the porosities of the coal mass increased by 17.5% and 68.1% respectively but the longitudinal wave velocities slowed by 47.8% and 76.2%. Comparing these data indicates that, again, freeze−thaw cycling cause 17


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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

great damages to coal than does a single freezing event. These tests show, fairly directly, that a large number of fractures are generated in the coal when it is frozen by LN2. The fractures both reduce the longitudinal wave velocities and increasing the porosity. These experimental results are consistent with the results predicted by Eq. (5). With the increase of porosity, the permeability of coal will ultimately increase through the cubic law of porosity and permeability. The cubic law of permeability with porosity has been widely accepted for the CBM reservior permeability quantification and prediction.39, 40 This indicates that the fracture network in coal extended with longer freezing times and a greater number of freeze−thaw cycles and the increased pore space allows gas migration effectively through the pore networks. 4.4. Stress and Fractures in Coal Seams. The water-ice phase change is an essential condition for producing frost heave pressure in fractures. The volume expansion caused by the phase change is constrained by the sides of the fracture, thus generating frost heave pressure. For the mechanical model of single fractures, the following simplifying assumptions were made. (1) The fractures in coal is saturated. Under these conditions, the model only considers water migration in the fractures without taking the permeability of the coal into account; (2) Coal and ice are regarded as homogeneous isotropic elastic media and water in fractures cannot be compressed; (3) The cross sections of fractures are always oval, and the width and the thickness of fractures are equal to the major (a) and minor (b) axes of the ellipse; (4) During volume expansion, frost heave ice and gaseous nitrogen are uniformly distributed along the fracture face. The LN2 phase change satisfies the equation of state for an ideal gas. The mechanical behavior of single fractures in coal seams was analyzed with the simplified mechanical model. The factors influencing the expansion of fractures can be condensed to the following four causes. These causes are the frost heave pressure Pi when water in the fracture is changed to ice, the expansive force Pn when LN2 is vaporized to gaseous nitrogen, the low-temperature damage DLN2 that LN2 causes in coal, and the crustal stress σ . These four forces and features are shown in Figure 10.

18


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σ1

DLN2 (-196℃ ℃)

Pores

Pn

σ3

σ3

Pi

σr τrθ

y Pn

r

Micropores

Fracture Water/ice

b

a

B

Water/ice

o

θ Pi

Coal

σθ

A

x

σ1 (a) Stressing analysis of the fractures in in-situ coal seams

(b) Stressing analysis of crack tip

Figure 10. Stress state of frozen fractures in a coal seam.

Based on the plane problem of elastic mechanics, the stress field on the top of single fractures can be calculated from Eqs. (7) − (9). σr =

K1 θ θ 3θ  cos  1 + sin sin  2 2 2  2π r

(7)

σθ =

K1 θ θ 3θ  cos  1 − sin sin  2 2 2  2π r 

(8)

τ rθ =

K1 θ θ 3θ cos sin cos 2 2 2 2π r

(9)

where σ r , σθ , and τ rθ are the radial normal stress, the tangential normal stress, and the circumferential stress;

r and

θ

show the polar coordinates of the top of the crack; K1 denotes the stress intensity factor of the

Griffith opening-mode crack. By using the coupled equations for volume expansion, Liu et al.41 obtained Eq. (10) concerning the relation of the frost heave pressure Pi in single fractures in rocks to the mechanical parameters of the rock and ice as well as to the geometrical parameters of the fractures. Following the equation of state for an ideal gas, one can write Eq. (11) regarding the expansive force Pn generated by the phase change in fractures can be written. Inserting the frost heave pressure caused by the water−ice phase change and the expansive force induced by the vaporization of LN2, into Eq. (11) results in Eq. (12). Equation 12 solves for the tensile force P of single fractures generated by the freeze−thaw action from LN2. These equations are:

19


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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

Pi =

ki − 1 ki  a 1 − v T  1 + − K T  b 2  G T (1 + v T ) Pn =

P=

nRT V

ki − 1 nRT + T V ki  a 1 − v  1 + − 2  G T (1 + v T ) K T  b

Page 20 of 25

(10)

(11) (12)

where ki and K T are the volume expansion coefficient and the volume modulus of water and ice in fracture;

vT and GT denote the Poisson’s ratio and shear modulus of coal at temperature T; a and b indicate the major and minor axes of the ellipse (the fracture cross section); n stands for the quantity of vaporized nitrogen in the fracture; R and V represent the ideal gas constant and the volume of fracture. Considering the constraint of the fracture face, the volumetric strain of ice under the tensile force P in single fractures can be obtained based on the relevant theory of elastic mechanics, as shown in Eq. (13): εv = −

3(1 − 2v T ) P 3(1 − 2v T )( Pi + Pn ) =− T E ET

(13)

where vT and ET are the Poisson’s ratio and the elastic modulus of ice at temperature T. During the fracturing experiments using LN2, the mechanical strength of the coal seams fell and therefore the tensile force P in fractures increased. Consequently, the volumetric strain of expanded ice in the coal rose. Under the joint effects of the tensile force in the fractures and the tensile stress caused by the contraction of the different materials at low temperatures, the fractures in the coal seam developed and connected. Several injections of LN2 into the coal-rock mass would result in three effects: gasification of LN2 would produce expansion cracks, frost heave would cause fractures, and the low temperatures would cause cracks because of contraction. Once fatigue damage failure occurred in the coal, the micro-fractures could extend more easily, thus forming an intertwined, cross-connected fracture network. LN2-based fracturing technology presents many advantages for increasing the porosity and permeability of coal reservoirs, thereby improving CBM extraction efficiency. This technology is expected to become one of the important methods for efficient exploitation of CBM resources. In addition, it can serve as a backup technology for the exploitation of unconventional gases in the future.

5. CONCLUSIONS Coal specimens were frozen with LN2, and the cryogenic treatment not only can trigger the propagation of 20


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pre-existing fractures but also can induce new fractures generate the fractures. SEM images show that in a coal specimen treated by freezing for 10 min, the maximum crack width is 13.3 µm, but for specimens subjected to 30 freeze−thaw cycles, the maximum crack widths are 60.5 µm. The length and width of cracks increased with the freezing time and the number of freeze−thaw cycles which increase the fracture density per volume of coal. This fracture density increase will ultimately transfer to increase of coal permeability through the cubic law of fracture porosity and permeability. Mechanically, the increase of fracture density will decrease the integrate of the coal bulk which will decrease the mechanical strength of the coal and this was proved by our experimental observations through the uniaxial compression tests. LN2 freezing exerts a significant influence on the mechanical strength of coal and on its pore. After 10 min of freezing and 30 freeze−thaw cycles with LN2, the elastic moduli decline by 10.6% and 31.4%, respectively. In those same experiments, the uniaxial compressive strengths drop 13.4% after the 10 min freeze and 40.2% after the 30 cycles freeze−thaw, the longitudinal wave velocities slow by 47.8% after the 10 min experiment and 76.2% after 30 cycles, the Poisson’s ratios increased by 7.14% (10 min) and 28.6% (30 cycles), and coal porosities increase by 17.5% (10 min) and 68.1% (30 cycles). Owing to the reduction of the mechanical strength, the elastic stage during uniaxial compression of the coal is shorter and the yield process of the coal specimens was accelerated. The damage variable D increases to 0.12 and maintains that level with a 10-min freezing time, but the freeze−thaw cycles continuously damage the coal and damage accelerates after 20 freeze−thaw cycles. LN2 freeze-thaw cycling damages coal continuously and to a higher degree than does freezing the coal only LN2 once.

AUTHOR INFORMATION Corresponding Author *Telephone: +8613585391210. E-mail: [email protected]

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENTS This work was financially supported by the National Natural Science Foundation of China (51274195, U1361106), the National Major Scientific Instrument and Equipment Development Project (2013YQ17046309), the Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD), Postgraduate Research and Innovation Plan 21


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

Project in Jiangsu Province (KYLX16_0574), State Key Laboratory of Coal Resources and Safe Mining, CUMT (SKLCRSM14X02).

REFERENCES: (1) Xia, T. Q.; Zhou, F. B.; Liu, J. S.; Gao, F. Evaluation of the pre-drained coal seam gas quality. Fuel 2014, 130, 296-305. (2) Moore, T. A. Coalbed methane: A review. Int. J. Coal Geol. 2012, 101, 36-81. (3) Mark, C.; Gauna, M. Evaluating the risk of coal bursts in underground coal mines. Int. J. Min. Sci. Technol. 2016, 26, (1), 47-52. (4) Li, J. Q.; Liu, D. M.; Yao, Y. B.; Cai, Y. D.; Qiu, Y. K. Evaluation of the reservoir permeability of anthracite coals by geophysical logging data. Int. J. Coal Geol. 2011, 87, (2), 121-127. (5) Ni, X. M.; Li, Q. Z.; Wang, Y. B.; Gao, S. S. Permeability variation characteristics of coal after injecting carbon dioxide into a coal seam. Int. J. Min. Sci. Technol. 2015, 25, (4), 665-670. (6) Aguilera, R. F.; Ripple, R. D.; Aguilera, R. Link between endowments, economics and environment in conventional and unconventional gas reservoirs. Fuel 2014, 126, 224-238. (7) Bahrami, H.; Rezaee, R.; Clennell, B. Water blocking damage in hydraulically fractured tight sand gas reservoirs: An example from Perth Basin, Western Australia. J. Petrol. Sci. Eng. 2012, 88-89, 100-106. (8) Boudet, H.; Clarke, C.; Bugden, D.; Maibach, E.; Roser-Renouf, C.; Leiserowitz, A. “Fracking” controversy and communication: Using national survey data to understand public perceptions of hydraulic fracturing. Energ. Policy. 2014, 65, 57-67. (9) Jackson, R. E.; Gorody, A. W.; Mayer, B.; Roy, J. W.; Ryan, M. C.; Van Stempvoort, D. R. Groundwater Protection and Unconventional Gas Extraction: The Critical Need for Field-Based Hydrogeological Research. Groundwater 2013, 51, (4), 488-510. (10) Al-Abri, A.; Sidiq, H.; Amin, R. Mobility ratio, relative permeability and sweep efficiency of supercritical CO2 and methane injection to enhance natural gas and condensate recovery: Coreflooding experimentation. J. Nat. Gas Sci. Eng. 2012, 9, 166-171. (11) Cai, C. Z.; Li, G. S.; Huang, Z. W.; Tian, S. C.; Shen, Z. H.; Fu, X. Experiment of coal damage due to super-cooling with liquid nitrogen. J. Nat. Gas Sci. Eng. 2015, 22, 42-48. (12) Cha, M. S.; Yin, X. L.; Kneafsey, T.; Johanson, B.; Alqahtani, N.; Miskimins, J.; Patterson, T.; Wu, Y. Cryogenic fracturing for reservoir stimulation − Laboratory studies. J. Petrol. Sci. Eng. 2014, 124, 436-450. 22


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pressure in a single fracture of frozen rock mass under low temperature. Chinese Journal of Geotechnical Engineering 2015, 37, (9), 1572-1580.

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Failure Mechanism of Coal after Cryogenic Freezing with Cyclic Liquid

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