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Fluid-Solid Coupling Characteristics of Gas-Bearing Coal Subject to Hydraulic Slotting: An Experimental Investigation Quanle Zou, and Baiquan Lin Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.7b02358 • Publication Date (Web): 09 Jan 2018 Downloaded from http://pubs.acs.org on January 10, 2018
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Energy & Fuels
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Fluid-Solid Coupling Characteristics of Gas-Bearing Coal Subject to Hydraulic Slotting: An Experimental Investigation
3
Quanle Zou1,2*, Baiquan Lin3
4
1
State Key Laboratory of Coal Mine Disaster Dynamics and Control, Chongqing University, Chongqing 400044, China
5
2
College of Resources and Environmental Science, Chongqing University, Chongqing 400044, China
6
3
School of Safety Engineering, China University of Mining & Technology, Xuzhou 221116, China
1
7 8
ABSTRACT: Chinese coal seams are characterized by high gas content and low permeability. The permeability of coal
9
seams should be improved to achieve maximum extraction of coalbed methane. This study explores how coal and gas
10
behave when subjected to hydraulic slotting. A fluid–solid coupling experimental system of gas-bearing coal subjected to
11
hydraulic slotting was first established. Then, the fluid–solid coupling property of gas-bearing coal subjected to hydraulic
12
slotting was revealed using the established experimental system. Meanwhile, indicators used to describe the process of
13
hydraulic slotting were derived, and the influencing factors affecting the process of hydraulic slotting were analyzed
14
using the aforementioned indicators. The research achievements indicate that the gas pressure response of the monitoring
15
points in the coal sample shows different characteristics at different stages. Corresponding to the change trend of the gas
16
pressure, the vertical and parallel strains demonstrate the five-stage change characteristics. With the increase of gas
17
pressure, the final deformation amount before slotting gradually increases, and the gas diffusion parameter increases
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exponentially. With the increase of the slot radius, the gas diffusion parameter shows a similar change tendency with the
19
ultimate deformation amount, i.e., it tends to become flat after a rapid increase. The research achievements can provide
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certain theoretical and practical references for the reveal of the enhanced coalbed methane recovery mechanism through
21
hydraulic slotting and the rational selection of the key parameters in the field test, respectively.
22
23
1. INTRODUCTION
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Coalbed gas, whose main component is methane, is generated from organic matter during the process of coalification1-4.
25
Its calorific value is more than 8000 kilocalories per cubic meter, which is almost equivalent to that of conventional
26
natural gas5-6. Thus, the exploitation and utilization of coalbed gas can not only make up for the lack of conventional
27
natural gas but also effectively reduce the greenhouse gas effect and the occurrence of coal mine gas disasters7-11.
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Coalbed gas is mainly in an adsorbed state in coal mass, whose exploitation involves a complicated
29
desorption–diffusion–seepage process. As the coal mining depth extends, most coal seams are characterized by low
30
permeability, so permeability-enhancing measures are needed to achieve better recovery efficiency. One of the core ideas
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of enhanced coalbed methane recovery methods is to perform artificial disturbance and positive stimulation on the
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physical and chemical properties of the coal seam12-15. At present, several stimulated measures have been developed for
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the abovementioned purpose, such as hydraulic slotting, hydraulic fracturing, liquid nitrogen fracturing and microwave
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irradiation stimulation16-20.
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Hydraulic slotting has proved to be an effective measure to achieve better enhanced coalbed methane (ECBM)
36
recovery efficiency through the impact of the high-pressure waterjet on the borehole wall to generate numerous artificial
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fractures.
Currently,
hydraulic
slotting
has
achieved
rapid
development
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including
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permeability-enhancing mechanism exploration, technology and equipment development, and field investigation21-25. In
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regards to permeability-enhancing mechanism exploration, the theory of “pressure relief and permeability enhancement”
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has been proposed, and a relatively broad consensus has been reached regarding this theory. As shown in Figure 1, the
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stress concentration exists near the borehole, which blocks the flow of the gas into the borehole. After the implementation
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of hydraulic slotting, the stress concentration is broken, pressure relief is achieved, and plenty of gas flows into the
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borehole through the fracture network and slots. Concerning the technology and equipment development, as shown in
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Figure 2(a), most of the hydraulic slotting systems include emulsion pump, water pressure controller, rotator, drilling rig,
45
drill pipe, drill and nozzle. The emulation pump is used to generate high-pressure water. The water pressure controller is
46
used to adjust the flow of the water according to the actual condition. The rotator is used to connect the high-pressure
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water pipe and the drill pipe. The drilling rig rotates the drill pipe and the drill to break the rock or coal. The nozzle is
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used to generate the high-pressure waterjet. Figure 2(b) displays the procedures of the hydraulic slotting, which can be
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summarized as follows: a borehole is drilled, the drill pipe is rotated, and the high-pressure water impacts the borehole
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wall to generate slots inside the coal seam. Numerous field tests have been conducted, and they have indicated that the
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gas drainage efficiency significantly improves after the implementation of hydraulic slotting. Above all, although
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numerous achievements have been made, the basic theoretical research of hydraulic slotting is still weak, which is
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embodied in the lack of systemic and in-depth studies on the coal rupture and gas seepage property under multifield
54
conditions. These shortcomings result in the irrational choice of the key parameters and relatively low ECBM efficiency. a
b Stress concentration Borehole
Borehole
Pressure relief
Slot
55 c Borehole
Pressure relief
Slot Fracture network
56 57
Figure 1. Permeability-enhancing mechanism of hydraulic slotting. (a) Stress concentration around the borehole. (b) Pressure relief around the borehole
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after hydraulic slotting. (c) A fracture network forms around the borehole, and plenty of gas flows into the borehole. a
Rotator 抗高压密封水辫
Drill pipe 抗高压密封钻杆
Drill and nozzle 水射流割缝器
b Drill bit
Slot 乳化液泵站 Emulsion pump
Borehole Drilling 钻机 rig
59 60
Fractures
Waterjet
Drill pipe
水压控制器 Water pressure controller
Figure 2. Hydraulic slotting system and its procedures. (a) Hydraulic slotting system. (b) The procedures of hydraulic slotting.
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In this work, a fluid–solid coupling experimental system of gas-bearing coal subjected to hydraulic slotting was first
62
established. Second, the fluid–solid coupling property of gas-bearing coal subjected to hydraulic slotting was revealed
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using the established experimental system. Meanwhile, the indicators used to describe the process of hydraulic slotting
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were derived, and the influencing factors affecting the process of hydraulic slotting were analyzed using the derived
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indicators. The research achievements can provide certain theoretical and practical references for the reveal of the
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hydraulic slotting ECBM (HS-ECBM) mechanism and the rational selection of the key parameters in the field test,
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respectively.
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2. EXPERIMENTAL
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2.1. Experimental System. The purpose of this paper is (a) to simulate the entire process of HS-ECBM and
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summarize its characteristics; (b) to reveal the effect of initial gas pressure on the dynamic response characteristics of
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coal during HS-ECBM; and (c) to reveal the effect of the slot radius on the dynamic response characteristics of coal
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during HS-ECBM. Based on these objectives, the corresponding experimental system was established. As shown in
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Figure 3(a～b), the experimental system mainly includes six subsystems, i.e., the triaxial loading subsystem, temperature
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control subsystem, adsorption–desorption subsystem, waterjet generation subsystem, experimental chamber, and
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data-acquisition subsystem. (a)
(b)
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4
2
1
5
5
6
3
2
6
7
3
8
9
10 11
11 13
1
13
0 20 40
12
10
9
60 80 100
620
12
7
640 660 680 700
8
(c)
(d) 17-1 14
15
16
21-2 21-3
21-1
17
21 21-1
17 16 14
21-2
20-1
14 15
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18
19
21-3
20
22
19
18 23
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Figure 3. Experimental system. (a) The entire experimental system: 1. Computer; 2. Gas pressure acquisition device; 3. Strain acquisition device; 4.
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Triaxial loading device; 5. Triaxial stress loading control device; 6. Experimental chamber; 7. Temperature control device; 8. Vacuum pump; 9. Gas
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cylinder; 10. Pressure-reducing valve; 11. Gas volume measurement instrument; 12. Hydrochloric acid solution; 13. Metering pump. (b) The physical
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map of the entire experimental system. (c) Design scheme of the experimental chamber: 14. Press plate A; 15. Pushrod A; 16. Press plate B; 17.
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Pushrod B; 17-1. Inlet channel; 18. Upper cover; 19. Chamber; 20. Bottom cover; 20-1. Outlet channel; 21. Space diagram; 21-1. Through-holes for
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pressure sensors; 21-2. Through-hole for enameled wire bundle; 21-3. Through-hole for temperature sensor; 22. The physical map of the chamber and
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the bottom cover; 23. The physical map of the upper cover.
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(a) Triaxial loading subsystem. The triaxial loading subsystem mainly includes the triaxial loading platform,
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hydraulic pump station, and control panel. This subsystem can realize the separate loading of the vertical and horizontal
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directions. The maximum loading force is 2000 KN. The size of the loading platform is 720 mm × 800 mm. Flexible
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loading of the multiscale samples can be realized through the effective cooperation of the loading platform and square
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thin iron block. Automatic control of the loading can be achieved through the PLC program.
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(b) Temperature control subsystem. Electrical heating is adopted. This subsystem is made up of a temperature
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sensor, electrically heated wire, and control device. The temperature sensor and electrically heated wire are fixed on the
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upper cover of the experimental chamber. When the temperature in the chamber is below the setting temperature, the
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electrically heated wire begins to work until the preset temperature is reached. The temperature control precision is 0.1℃,
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and the temperature variation range is 0～100℃.
94 95
(c) Adsorption–desorption subsystem. This subsystem consists of the vacuum pump, pressure-reducing valve, and gas volume measurement instrument. The maximum vacuum degree of the vacuum pump is –101.325 KPa.
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(d) Waterjet generation subsystem. This subsystem mainly includes the metering pump, connecting lines, and
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waterjet generation device. The rated flow of the metering pump is 8 L/min. The connecting lines are corrosion-resistant.
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The waterjet generation device is made of a stainless steel tube with a closed front end and symmetrical through-holes on
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the sidewall.
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(e) Experimental chamber. This chamber is made up of the pushrod, press plate, chamber, upper board, and bottom
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board. Its design scheme is displayed in Figure 3(c～d). Pushrod A and pushrod B connect with press plate A and press
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plate B through the bolts, respectively. Two vertical through-holes are set in pushrod B, which are used to convey the
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liquid. Several through-holes are set in press plate B, which are used to provide the channels for gas pressure
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measurement. The upper board and bottom board connect with the chamber through fourteen bolts, and there are two
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sealing rings between them. The gas outlet channels are arranged in the bottom board. A schematic diagram of the
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assembly of the experimental chamber is displayed in Figure 3(c). It can be seen from Figure 3(c) that there are
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through-holes in the front wall of the chamber, which are used to connect the gas pressure sensor and enameled wire
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bundle. There are also through-holes in the back wall of the chamber, which are used to connect the gas inlet control
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valve, liquid inlet control valve, vacuum pumping valve, and gas outlet control valve. The flow path of the gas is as
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follows: gas cylinder→gas inlet control valve→experimental chamber→coal sample→outlet channel→gas outlet control
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valve→gas volume measurement instrument. The flow path of the liquid is as follows: metering pump→liquid inlet
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control valve→inlet chamber→coal sample→outlet channel→gas outlet control valve→gas volume measurement
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instrument. The size of the chamber is 360 mm × 260 mm × 320 mm, and its design pressure is 6 MPa.
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(f) Data-acquisition subsystem. Two types of data are collected in this paper, i.e., the strain and gas pressure. The
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strain measurement is realized through the strain indicators developed by Jiangsu Donghua Measuring Technology
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Limited Liability Company. The measurement of the gas pressure inside the coal sample is realized through the CY200
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digital pressure sensor.
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2.2. Coal Specimen Preparation. In our experiment, physical simulation of hydraulic slotting is realized
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through the formation of the pressure relief space, which is achieved by removing partial coal inside the loaded coal
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sample. The coal removal method involved arranging sodium hydroxide powder (whose volume is slightly larger than
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that of the pressure relief space) inside the coal sample and then dissolving it using 10% hydrochloric acid. One of the
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purposes of the experiments is the real-time monitoring of the strain and gas pressure inside the coal sample during
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hydraulic slotting. It is necessary to insert the strain gauge and gas channel in the coal sample to achieve this purpose.
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The arrangement of the strain gauge is realized as follows. The coal samples are arranged layer by layer to form a total of
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five layers. The coal in every layer is prepressed to guarantee surface smoothness, and then the strain gauge is placed at
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the preset position. At layer three, a PVC tube is used to dig out and form a cylindrical space at the center of the coal
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sample. Subsequently, sodium hydroxide is poured into the cylindrical space and allowed to compact. The arrangement
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of the gas channel is realized as follows. After the coal samples are arranged as noted previously, an electric portable drill
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is used to drill cylindrical channels at the preset position on the coal samples. A rubber hose is then inserted into the end
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of the channel, and a flexible glue is used to seal the gap between the channel and the rubber hose. The coal samples and
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the arrangement of the strain gauge and gas pressure measuring tube are depicted in Figure 4. 3 测压管
1 应变片 2
4 进液管
出6 液 管
N a5O H Mould
（a）
（b）
（c）
132 133
Figure 4. Coal samples and the arrangement of the strain gauge and tubes. (a) Coal sample molding; (b) Coal samples; (c) The arrangement of the
134
vertical strain gauge (1), parallel strain gauge (2) gas pressure measuring tube (3), liquid inlet tube (4), sodium hydroxide (5) and liquid outlet tube (6).
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2.3. Experimental Design. The experimental design is displayed in Table 1. The variables considered in the
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experiments are slot radius and gas pressure. All of the variables have four levels. The vertical stress, maximum
137
horizontal stress, minimum horizontal stress and gas pressure are determined in accordance with References [26-27],
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which have taken the similarity ratio and the burial depth into account.
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Table 1. Experimental Design Vertical stress
Maximum horizontal stress
Minimum horizontal stress
Temperature
Slot radius
Gas pressure
σ1/MPa
σ2/MPa
σ3/MPa
/℃
/mm
/MPa
No. 1
0.5
2
1 10
3
1.5
4
2 1.38
1.87
1.06
30
5
5
6
7.5
7
10
8
12.5
2
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2.4. Experimental Procedures. The experimental procedures involve the following steps. (a) Preparation. The
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coal specimen is dried in a drying oven (temperature of 60℃) to eliminate the influence of moisture as much as possible.
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The gauze is used to polish the surface of the coal samples to avoid the partial stress concentration. After drying, the
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samples are placed inside the experimental chamber. The strain monitoring subsystem, temperature control subsystem,
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and gas pressure monitoring system are connected to the chamber, and the upper board of the experimental chamber is
145
installed. Finally, all the valves are closed. (b) Vacuum pumping. The valve of the vacuum pump is opened to conduct
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vacuum pumping for two hours. Meanwhile, the strain and gas pressure monitoring subsystems are turned on to record
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the strain and gas pressure, respectively, inside the coal sample. (c) Gas adsorption. The valve of the vacuum pump is
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now closed. The gas cylinder and the gas input valves are used to conduct gas adsorption for 48 h. (d) Triaxial loading.
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The gas cylinder and the gas input valves are closed, and the triaxial loading system is started to impose the preset
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vertical and horizontal stresses on the coal sample. To avoid the rupture of the coal specimen due to the effect of
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concentration stress during confining pressure loading, the loading pattern of confining pressure in this paper is as
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follows: all the loading intervals are set to 0.25 MPa, and the pressures of all three directions are loaded at the same time.
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When one direction reaches the set pressure value, the pressure is kept constant. (e) Hydraulic slotting and desorption.
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When the gas pressure is stable, the metering pump and water input valve are opened to inject hydrochloric acid into the
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coal sample. The hydrochloric acid spurts from the though-holes in the stainless steel tube to react with sodium
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hydroxide, which realizes the simulation of hydraulic slotting. When sodium hydroxide comes into contact with the water
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output channel, the hydraulic slotting is stopped, and the water outlet is connected to the gas volume measurement
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instrument to measure the gas desorption volume. The data are recorded at 30-second intervals. Meanwhile, other
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parameters, such as the surrounding temperature, are recorded to transfer the gas volume in the surrounding area into the
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gas volume in the standard state. (f) Finally, all of the subsystems are put in order, and the next experiment is conducted.
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3. RESULTS
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Because all of the experiments produced similar results, in this section, experiment #1 (gas pressure of 0.5 MPa and
163
slot radius of 10 mm) was chosen as an example to describe the time-varying response of gas-containing coal subjected
164
to hydraulic slotting and then obtain the indices, which can characterize the gas–solid coupling property in the entire
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process of hydraulic slotting. 3.1. Variation in Gas Pressure Inside the Coal Specimen. The real-time variation in gas pressure inside the
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coal specimen of experiment #1 in the entire experimental process is displayed in Figure 5(a), where it can be seen that
168
the entire experimental process can be divided into five stages: vacuum pumping, gas filling, confining pressure loading,
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slotting, and desorption. Detailed description of the variation laws at every stage is presented in the following
170
subsections.
2
3
4
5
0 0.00
-150 -0.15
-0.06 -0.08
2
4 6 4 时间×10 Time ×10/s4/s
8
3
0.56 0.54 0.52
∆= 0.0214 ∆= 0.0269
0.50 5.0
5.2
∆= 0.0133
5.4
∆= 0.0098
∆= 0.0089
∆= ∆= 0.0016 ∆= 0.0060 0.0024
100
0.0
0.1
0.2
0.3
0.4
0.5
5.8
6.0
6.2
6.4
0.6
0.7
1
0.4 0.2
t=0.74
0.0 0.72
2
0.75
0.78 0.81 Time×104/s
3
0.84
4
Time ×104/s
（b）
（c）
5
0.6 4
0.63
∆= 0.0292
0.62 0.61
2 0.6
Time ×103/s
∆=0.0492
0.60 6.74
0.59 5.6
150
0.64
Pressure/MPa
0.58
50 Time/s
（a） 0.60
0
-0.10 -0.12
0
171
172
-0.04
0.6 0.5 0.4 0.3 0.2 0.1 0.0 -0.1
1
Pressure/MPa
1
150 0.15
0.00 -0.02 -0.04 -0.06 -0.08 -0.10 -0.12
Pressure/MPa
0.00 -0.02
Pressure/MPa
450 0.45 300 0.30
6.48
6.54
6.60
6.66
6.72
6.78
Pressure/MPa
750 0.75 600 0.60
Pressure/MPa
Pressure/MPa 压力/M P a
166
Pressure/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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0.5 p=1.72exp(-x/2199.71)+106.94
0.4 0.3 0.2
Gas pressure decay curve Exponential fitting
∆= 0.5003
0.1
6.84
6.9
7.2
7.5
7.8
8.1
Time ×104/s
Time ×104/s
Time ×104/s
（d）
（e）
（f）
8.4
8.7
173
Figure 5. Variation in gas pressure at the monitoring point of the coal sample under a gas pressure of 0.5 MPa. (a) Variation in gas pressure in the
174
entire experiment; (b) Variation in gas pressure at the vacuum-pumping stage; (c) Variation in gas pressure at the gas-filling stage; (d) Variation in gas
175
pressure at the confining pressure loading stage; (e) Variation in gas pressure at the slotting stage; (f) Variation in gas pressure at the desorption stage.
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(a) Vacuum pumping: As shown in Figure 5(b), after opening the vacuum pump, the gas pressure inside the coal
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specimen rapidly decreases. After approximately 40 s, the internal gas pressure of the coal specimen decreases to the
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rated pressure of the vacuum pump (i.e., –0.1 MPa). Subsequently, the internal gas pressure of the coal specimen shows
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no significant change and presents small-range fluctuations.
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(b) Gas filling: After vacuum pumping for two hours, the valve of the gas cylinder is opened, and the experimental
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chamber is filled with methane. The outlet pressure of the gas cylinder is set to 0.5 MPa throughout the process of gas
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filling. As shown in Figure 5(c), at the initial stage of gas filling, the internal gas pressure of the coal sample exhibits a
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rapid linear increasing trend. At 7400 s, the internal gas pressure of the coal sample reaches the maximum value of 0.58
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MPa. Subsequently, the internal gas pressure of the coal sample gradually decays, and the attenuation amplitude is much
185
smaller than the increased amplitude. The final internal gas pressure of the coal sample is approximately 0.5 MPa.
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(c) Confining pressure loading: The period of gas filling is 12 hours. After gas filling, the valve of the gas cylinder
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is closed, and no further gas is filled. As shown in Figure 5(d), the entire process of the confining pressure loading is
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completed through eight loading steps, with the duration of each step being 1800 s. For each loading step, the gas
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pressure inside the coal sample shows a similar trend, and the difference is the increase in amplitude. Figure 5(d) presents
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the difference between the internal gas pressure at the end of each loading step and the pressure value at the beginning of
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the loading step. It can be clearly observed that the variation magnitude of the internal gas pressure of the coal sample is
192
changed from 0.0269 MPa at the first step to 0.0016 MPa at the eighth step. The internal gas pressure at each step of the
193
loading process is significantly low, which is attributed to the fact that, for each loading step, the coal specimen is in a
194
state of expansion and is easily compressed due to the adsorption of methane. In addition, because of the relatively rapid
195
loading rate of the confining pressure, it is relatively impossible to adjust the gas inside the sample, which leads to the
196
increase in internal pressure of the coal sample. When the loading of the confining pressure is completed, the internal gas
197
pressure of the coal sample is larger than that of the experimental chamber. Subsequently, the gas inside the coal sample
198
will gradually enter the experimental chamber, leading to the gradual reduction of the internal pressure, i.e., for each
199
loading step, the internal gas pressure of the coal sample decreases slowly after the first jump28. With the increase of the
200
loading step, the gas inside the coal sample gradually decreases, and its ability to resist external pressure gradually
201
increases. The compression effect of the same confining pressure is gradually weakened, which leads to a decrease of the
202
compressed amplitude of the internal gas, i.e., with the increase of the loading step, the internal gas pressure of the coal
203
sample decreases.
204
(d) Slotting: After the completion of the loading of the confining pressure, the valve is opened, and the metering
205
pump is opened to inject the hydrochloric acid with a certain concentration into the coal specimen. The hydrochloric acid
206
solution impacts the sodium hydroxide embedded in the coal sample. As shown in Figure 5(e), the internal gas pressure
207
of the coal sample rises first. When the experimental time is 6.74 × 104 s, the maximum value of 0.64 MPa is reached; a
208
rapid decline then occurs, and eventually a stable value is maintained. The pressure difference at the rising stage is
209
0.0492 MPa, and the pressure difference at the declining stage is 0.0292 MPa. This change trend can be explained as
210
follows: the impact of the hydrochloric acid solution on sodium hydroxide has a disturbance effect on the coal, which
211
leads to stress redistribution inside the coal sample. At the initial stage, the disturbance effect is weak, and its effect on
212
the internal gas pressure of the coal sample is relatively small. The monitoring points are located in the compressive
213
stress zone around the slot. Therefore, the monitored gas pressure slowly increases. With the passage of time, the slotted
214
radius continuously increases, and the disturbance of slotting on the surrounding coal correspondingly enlarges. The
215
stress concentration degree at the monitoring points constantly increases, and the gas is continuously compressed. When
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the stress concentration reaches the fracture threshold value of the coal, the coal is broken, and the crack is generated,
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which is coalesced with other cracks. As a result, the compressed gas is released. Accordingly, the gas pressure at the
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monitoring points increases to the maximum and then suddenly drops.
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(e) Desorption: When the slotting is completed, open the outlet valve and perform desorption. At this point, the gas
220
pressure inside the coal sample shows an obvious change, i.e., it first significantly decreases and then gradually
221
approaches a fixed value, which has the characteristics of the exponential function. In this paper, the exponential function
222
is used to fit the relationship curve between the internal gas pressure and time at the desorption stage, which is shown in
223
Figure 5(f). According to the fitted exponential function, the limit pressure of methane desorption is 0.1069 MPa, which
224
indicates that the desorption starting pressure of methane under the experimental condition is 0.1069 MPa. It can be seen
225
that the reduction rate of the internal gas pressure through the entire process of desorption is 0.5003 MPa, which
226
indicates that the weakening effect of desorption on the gas pressure inside the coal sample is very significant.
227
3.2. Variation in Deformation Inside the Coal Specimen. In this paper, the variations of the vertical strain
228
and parallel strain at the monitoring points in the entire process were also monitored, which are elaborated in the
229
following sections.
shown in Figure 6(a). Corresponding to the experimental process, the vertical strain at the internal monitoring points of
232
the coal sample also has five stages: contraction deformation due to vacuum pumping, swelling deformation due to
233
methane adsorption, compression deformation due to loading, relaxation due to slotting, and shrinkage deformation due
234
to desorption. The characteristics of the five stages are as follows. 8 6 4 1
2
2 0 2
4
5.0
5.2
0.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
∆= 1.344
-0.8 -1.2
1
2
3
Time ×104/s
（b）
（c）
1 ∆= ∆= 0.9007 1.0973
0.0
-0.4
（a）
3
0
8
0.1
2
0.0
Time ×104/s
2
-1
6
0.2
0.4
Time ×104/s
235 3
5
(3.78,0.36)
1
0.3
∆= 0.8018
5.4
∆= 0.6001
5.6
∆= ∆= -0.0707 ∆= -0.1110 ∆= -0.0474 0.4933
5.8
6.0
6.2
6.4
8 7 6 5 4 3 2
4
∆= 3.6644
∆= 5.4544
6.48
6.54
6.60
6.66
6.72
6.78
6.84
Time ×104/s
Time ×104/s
（d）
（e）
Vertical strain ×103/µε
0
4
Vertical strain ×103/µε
-2
3
0.4
Vertical strain ×103/µε
231
Vertical strain ×103/µε
3.2.1. Variation in Vertical Strain. The overall variation tendency of the vertical strain in the entire process is
Vertical strain ×103/µε
230
Vertical strain ×103/µε
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 8 of 21
5.6 5.4 5.2 5.0 4.8 4.6 4.4
4
5
5 ∆= 1.1076
6.9
7.2
7.5
7.8
8.1
Time ×104/s
8.4
8.7
（f）
236 237
Figure 6. Variation in vertical strain at the monitoring point of the coal sample under the gas pressure of 0.5 MPa. (a) Variation in vertical strain in the
238
entire experiment; (b) Variation in vertical strain at the vacuum-pumping stage; (c) Variation in vertical strain at the gas-filling stage; (d) Variation in
239
vertical strain at the confining pressure loading stage; (e) Variation in vertical strain at the slotting stage; (f) Variation in vertical strain at the desorption
240
stage.
241
(a) Contraction deformation due to vacuum pumping: There are many imperfections (pores, cracks, etc.) inside the
242
coal specimen, which are often filled with impurities, such as free gas and water. The purpose of vacuum pumping is to
243
eliminate the influence of the original impurities on the experimental results. When these impurities are drawn out, the
244
surface tension of the pores significantly decreases, which leads to shrinkage and deformation of the coal sample29-30. It
245
can be seen from Figure 6(b) that, when vacuum pumping is conducted for 3780 s, the vertical strain of the coal
246
specimen reaches a stable value of 0.36 × 103 µε.
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Energy & Fuels
247
(b) Swelling deformation due to methane adsorption: As shown in Figure 6(c), at the initial moment of gas filling,
248
the phenomenon of compression deformation that results from the sudden filling of 0.5-MPa methane cannot be
249
observed. During the entire process of gas filling, the vertical deformation of the coal sample shows a trend of gradual
250
decrease after a rapid decline. The rapid decline may result from the fact that the internal pressure of the coal specimen is
251
much lower than the external pressure, and the large pressure gradient is attributed to the high methane migration rate.
252
After adsorbing gas, the surface tension becomes larger, and then the expansion phenomenon occurs. The more the
253
adsorbed gas, the more obvious the expansion effect31-32. As the gas (methane) continues to enter the pores and cracks of
254
the coal sample, its internal pressure gradually increases, resulting in a decrease of the pressure gradient and capacity of
255
gas migration. Consequently, the increase amount of the expansion and deformation decreases. It can be seen from
256
Figure 6(c) that the amount of expansion deformation due to methane adsorption is approximately 1.344 × 103 µε, which
257
is 3.73 times larger than that caused by vacuum pumping.
258
(c) Compression deformation due to loading: According to the preset loading procedure, the entire loading process
259
is completed through eight loading steps. The duration of each step is 1800 s. For loading step one through step five, the
260
vertical strain inside the coal specimen shows a similar variation trend. The difference is the increase in magnitude. For
261
each step of loading, the vertical strain inside the coal specimen presents a slow increasing trend after a rapid increase.
262
The initial sudden rise results from the loading action, and the subsequent slow increase is due to the interaction between
263
methane and the coal specimen. In addition, the variation magnitude of the vertical strain in every step of the monitoring
264
point is presented in Figure 6(d). It can be observed that the variation magnitude of the vertical strain from loading step 1
265
through loading step 5 presents a tendency of significant decrease, which is due to the fact that, because the decrease of
266
gas results in the enhancement of the capacity to resist the outer pressure of the coal specimen with the increase in the
267
loading step, the deformation degree of the coal specimen gradually decreases under the same load. In contrast, the
268
vertical strain at the internal monitoring points of the coal specimen from loading step 6 and loading step 8 presents an
269
opposite variation, which indicates that the small-magnitude expansion occurs in the vertical direction of the coal
270
specimen. The total expansion degree is 0.2291 × 103 µε, which is far less than the compression degree. This may be
271
because, from loading step 6 through loading step 8, the coal specimen is subjected to unbalanced loading from two
272
directions (i.e., not the balanced loading from three directions) under the condition that one direction is fixed or
273
one-direction loading under the condition that the other two directions are fixed. Such complicated stress conditions lead
274
to the change of the vertical strain of the coal specimen.
275
(d) Relaxation due to slotting: As shown in Figure 6(e), during the entire slotting process, the vertical strain shows
276
the variation characteristic of multiple rapid drops after a winding rise. The increase in the magnitude of the vertical
277
strain at the rising stage is 5.4544 × 103 µε. The decrease in the magnitude of the vertical strain at the declining stage is
278
3.6644 × 103 µε. This variation tendency can be interpreted as follows: during slotting, the disturbance range caused by
279
impact gradually increases. For the monitoring points, the change of stress around the slot with time experiences three
280
stages: original stress, concentrated stress, and residual stress. The process of stress concentration is the process of
281
loading at the monitoring points, which leads to the gradual increase of the deformation degree. The monitoring points
282
may be located in the compressive stress zone. Therefore, the deformation is characterized by compression. In addition,
283
the impact of the hydrochloric acid solution on the sodium hydroxide is very complex, which may be the reason for the
284
complexity of the strain at the increase stage. The loading causes the coal at the monitoring point to exhibit a “tight”
285
state. When the strain reaches a certain threshold value (which can be interpreted as the fracture threshold of the coal
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286
sample), the coal sample has a larger rupture, and then the relaxation phenomenon occurs, which is characterized by a
287
reversal of the curve of the vertical strain33-34. It can also be seen from Figure 6(e) that the relaxation is not done at one
288
particular time.
289
(e) Shrinkage deformation due to desorption: The desorption-induced deformation of the coal sample is the opposite
290
of the adsorption-induced deformation. The desorption of methane in the pores of the coal specimen leads to an increase
291
in surface energy, which induces the shrinkage of the surface layer. In contrast, the desorption of methane decreases the
292
widths of pores and cracks. The combined effects of these two aspects lead to the phenomenon of desorption-induced
293
shrinkage35-36. It can be seen from Figure 6(f) that the entire curve shows a trend of slow variation after a rapid increase,
294
which may result from the fact that, as time goes on, the great resistance causes a decrease of the desorption amount and
295
thus the deformation amount. This corresponds to the gas pressure variation at the internal monitoring points.
297
shown in Figure 7(a). Corresponding to the experimental process, the parallel strain at the internal monitoring points of
298
the coal sample also has five stages: contraction deformation due to vacuum pumping, swelling deformation due to
299
methane adsorption, compression deformation due to loading, relaxation due to slotting, and shrinkage deformation due
300
to desorption. The characteristics of the five stages are described in detail in the following sections . 3 2 1
2
0 3
4
5
-1
2
4
3
0.1
0.2
0.3
0.4
0.5
0.6
0.7
∆= 1.0981
-0.6 -0.8 0
1
2
3
4
（b）
（c）
∆= 0.5512
∆= 0.3122
∆= ∆= 0.0799 ∆= 0.3006 0.4002
0.6715 ∆= 0.7777
5.2
0.0
-0.4
（a）
∆=
5.0
0.0
2
0.0 -0.2
Time ×104/s
1 ∆= 0.9284
0.1
0.2
Time ×104/s
3
0
8
0.2
0.4
Time ×104/s
2
-1
6
(3.99,0.32)
1
5.4
5.6
5.8
6.0
6.2
6.4
3.5 3.0
4
∆= 0.6720
∆= 2.0570
2.5 2.0 1.5 1.0
6.48
6.54
6.60
6.66
6.72
6.78
6.84
Parallel strain ×103/µε
0
0.3
Parallel strain ×103/µε
1
301
302
Parallel strain ×103/µε
4
Parallel strain ×103/µε
3.2.2. Variation in Parallel Strain. The overall variation tendency of the parallel strain in the entire process is
Parallel strain ×103/µε
296
Parallel strain ×103/µε
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 10 of 21
3.0
5
2.9 2.8 2.7
5
∆= 0.5287
2.6 2.5 6.65
7.00
7.35
7.70
8.05
Time ×104/s
Time ×104/s
Time ×104/s
（d）
（e）
（f）
8.40
8.75
303
Figure 7. Variation in parallel strain at the monitoring point of the coal sample under the gas pressure of 0.5 MPa. (a) Variation in parallel strain in the
304
entire experiment; (b) Variation in parallel strain at the vacuum-pumping stage; (c) Variation in parallel strain at the gas-filling stage; (d) Variation in
305
parallel strain at the confining pressure loading stage; (e) Variation in parallel strain at the slotting stage; (f) Variation in parallel strain at the desorption
306
stage.
307
(a) Contraction deformation due to vacuum pumping: In this stage, the change trends of the parallel strain and the
308
vertical strain at the strain monitoring points are consistent. The difference is the change magnitude. From Figure 7(b), it
309
can be seen that when the vacuum pumping was conducted for 3990 s, the parallel strain reached a stable value of 0.32 ×
310
103 µε, which is slightly less than that of the vertical strain.
311
(b) Swelling deformation due to methane adsorption: As shown in Figure 7(c), in this stage, the parallel strain at the
312
internal monitoring point is similar to that of the vertical strain, i.e., it tends to be stable after a rapid decline. The
313
difference is the magnitude of the expansion. To clearly show the variation of the amplitude and the comparison between
314
the parallel strain and the vertical strain at the internal monitoring points in the gas-filling stage, Figure 8 was drawn. As
315
seen from Figure 8, the vertical strain of this stage is 1.2239 times larger than the parallel strain.
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1.5
Variation magnitude of vertical strain Variation magnitude of parallel strain Increase magnitude
Variation magnitude×103/µε
2.0
Variation magnitude
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
1.0
0.5
0.0
Gas-filling
Desorption
1.2
Variation magnitude of vertical strain Variation magnitude of parallel strain
1.0 0.8 0.6 0.4
Shrinkage
0.2 0.0
Swelling
-0.2 1
2
3
（a）
316
4
5
Loading step
6
7
8
（b）
317
Figure 8. Variation magnitude of vertical strain and parallel strain at the monitoring point of the coal sample under the gas pressure of 0.5 MPa. (a)
318
Variation magnitude of vertical strain and parallel strain at the gas-filling and desorption stages; (b) Variation magnitude of vertical strain and parallel
319
strain at the confining pressure loading stage.
320
(c) Compression deformation due to loading: As shown in Figure 7(d), different from the vertical strain, the parallel
321
strain shows a trend of contraction in the entire process. The change trend of each loading step is gentle after the rapid
322
increase. Similarly, the variation amplitude of the parallel strain is also measured (see Figure 8). It can be observed from
323
Figure 8 that a clear decreasing trend of the vertical strain change amplitude is presented from loading step 1 through
324
loading step 5, which is due to the fact that the enhancement of the deformation-resisting capacity of the coal specimen
325
through the decrease of the gas results in the gradual decrease in the deformation of the coal specimen under the same
326
load. , . In contrast, the vertical strain has a sudden jump at loading step 6 and then gradually decreases, which indicates
327
that a larger contraction occurs in the parallel direction, which may be due to the complex stress conditions at loading
328
steps 6~8. It can be seen from Figure 8 that, except for loading step 6, the absolute value of the vertical strain is greater
329
than that of the parallel strain, which shows that there is also a certain anisotropy characteristic of the coal sample37.
330
(d) Relaxation due to slotting: As shown in Figure 7(e), unlike the vertical strain, in the entire slotting process, the
331
vertical strain shows the characteristics of multiple increases after the tortuous decline. The parallel strain exhibits a
332
tendency of contraction after shrinkage. The decline magnitude of parallel strain at the decreasing stage is 2.057 × 103 µε,
333
and the increase magnitude of parallel strain at the increasing stage is 0.6720 × 103 µε. The two variation amplitudes are
334
significantly smaller than those of the vertical strain, which may be related to the lateral pressure. This trend can be
335
explained as follows: at the initial stage of slotting, the coal at the monitoring point is in the state of tension. As the
336
slotting process proceeds, the tension degree increases. When this type of state achieves the fracture threshold value of
337
the coal sample, the sample is broken, and the relaxation phenomenon occurs, which presents multiple reversals in the
338
variation direction of the parallel strain curve.
339
(e) Shrinkage deformation due to desorption: As shown in Figure 7(f), the parallel strain at the internal monitoring
340
point of the coal sample is similar to that of the vertical strain, which rises quickly and then tends to be stable. The
341
difference is the growth magnitude. The variation of the amplitude and the comparison of the parallel strain and the
342
vertical strain at the internal monitoring points of the desorption stage are also clearly shown in Figure 8. It can be seen
343
from Figure 8 that the vertical strain of this stage is 2.0949 times larger than that of the parallel strain.
344
3.3. Desorption Property of Coal after Hydraulic Slotting. In the aforementioned desorption stage, the
345
natural desorption amount of methane after hydraulic slotting was determined through the gas volume measurement
346
instrument. The change trend of the desorption amount per mass with time is obtained. At the same time, the desorption
347
rate derived from the first-order differential of the time-varying desorption amount was obtained (Figure 9). It can be
348
seen from Figure 9 that the cumulative desorption amount after hydraulic slotting significantly increases and that the
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Energy & Fuels
3.0 2.5 2.0 Desorption amount Desorption rate
1.5 1.0 0.5 0.0
∆t=1.35×104s
6.75
350 351 352
7.20 7.65 8.10 8.55 Desorption time ×104/s
1.4 1.2 1.0 0.8 0.6 0.4 0.2 0.0 -0.2
Desorption rate/(mL/(g·s))
corresponding desorption rate gradually decreases. The stable value is reached at 8.1 × 104 s. Desorption amount/(mL/g)
349
Figure 9. Desorption amount and desorption rate of the coal sample after hydraulic slotting.
4. DISCUSSION
353
4.1. Solid–Fluid Coupling Characteristics of Gas-Bearing Coal Subjected to Hydraulic Slotting. Four
354
time-varying curves of internal pressure, parallel strain, vertical strain, and desorption amount are plotted together
355
(Figure 10). The coal–gas interaction rules are obtained and described using the aforementioned curves; in brief, the
356
variation in internal gas pressure of the coal specimen is closely related to its deformation. For example, a good
357
consistency exists between the gas pressure and coal deformation during gas filling and desorption. There is sometimes
358
the hysteresis of coal deformation, such as the vacuum-pumping stage. When the external force (such as loading and
359
slotting) disturbance appears, this close relationship is broken, and the coal presents a disturbance-dominated
360
deformation law. Therefore, it is necessary to perform the appropriate disturbance, such as slotting, as accomplished in
361
this paper, to induce the development of coal deformation in the favorable direction.
2.0 1.5 1.0 0.5 0.0
Desorption amount
2
1
362 363
Gas pressure decrease； Coal shrinkage
Parallel strain
Vetical strain 8 7 6 5 4 3 2 1 0 -1 -2
Gas pressure increase； Coal swelling
4 Time ×104/s
6
10 8 6 4 2
Vetical strain×103/µε
2.5
Gas pressure 0.7 0.6 0.5 0.4 0.3 0.2 0.1 0.0 -0.1 -0.2 0
Parallel strain×103/µε
3.0
Gas pressure/MPa
Desorption amount/（ mL/g）
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 12 of 21
0 -2
8
2
3
4
Gas pressure increase； Coal shrinkage
Gas pressure increase before decrease； Coal shrinkage before swelling
5
Gas pressure decrease； Coal shrinkage
Figure 10. Coal-gas interaction characteristics in the entire experiment.
364
4.2. Descriptive Indictors of Solid–Fluid Coupling Characteristics of Gas-Bearing Coal Subjected to
365
Hydraulic Slotting. Using the aforementioned analysis, the main purpose of the three stages before slotting is to
366
construct the original state of the coal specimen. Stage 2, marked as 2 in Figure 10, can reflect the effect of gas on coal
367
without external disturbance. Stage 3, marked as 3 in Figure 10, mainly shows the effect of external force loading on coal,
368
but there is also gas desorption, and the change is complicated. Based on this, Langmuir fitting was conducted on the
369
expansion deformation of coal at stage 2, and the final deformation amount of the coal specimen after the loading was
370
used to describe the state of coal under different initial conditions. The deformation of the coal specimen during the
371
process of slotting is also complicated, but the deformation of the coal specimen at the desorption stage is relatively
372
regular. Therefore, the state of the coal specimen after the slotting is mainly described through the Langmuir fitting of the
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373
desorption deformation section and the analysis of the desorption amount. The case with the gas pressure of 0.5 MPa is
374
taken as an example to describe in detail the method of obtaining these descriptive indictors. The specific results are
375
displayed as follows.
376 377
(a) Final deformation amount before slotting εbs: The final deformation amount before slotting, εbs, is defined as the sum of the vertical strain εv and the parallel strain εh at the monitoring points at the end of loading.
ε bs = ε v + ε h
378 379 380
（1）
It can be seen from the aforementioned analysis that, when the gas pressure is 0.5 MPa, the final deformation amount before slotting εbs is 5.903 × 103 µε.
381
(b) Ultimate deformation amount εmax and deformation rate vε: It can be found from the adsorption–deformation
382
curves that the change tendencies of vertical strain and parallel strain under the state of adsorption are consistent, that is,
383
they tend to be gentle after a rapid decrease, which is consistent with the characteristics of an exponential Langmuir
384
curve. The exponential Langmuir model can be expressed as follows:
ε=
385
ε max vε t1− c
（2）
1 + vε t 1−c
386
where εmax and vε represent the maximum amount of deformation and deformation rate of coal under the experimental
387
conditions, respectively.
388
Based on Formula 2, the fitting was performed on the deformation amount at the adsorption stage (shown in Figure
389
11a), where εs is the sum of the parallel strain εh and vertical strain εv. It should be noted that the horizontal axis is
390
relative time, so the beginning of the gas filling was labeled 0. The vertical axis represents the relative strain, i.e., the
391
strain at the adsorption stage minus the strain generated by the vacuum pumping. The same method was used to obtain
392
the fitting curves of coal deformation at the desorption stage, which are shown in Figure 11(b). 0.0
εv
Exponential Langmuir fitting of εv
εh
Exponential Langmuir fitting of εh
εs
Exponential Langmuir fitting of εs
Strain×103/µε
-0.5 Strain×103/µε
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
-1.0 -1.5 -2.0 -2.5 0
393 394
1
2 3 Relative time/s
4
1.8 1.6 1.4 1.2 1.0 0.8 0.6 0.4 0.2 0.0
εv εh
Exponential Langmuir fitting of εv Exponential Langmuir fitting of εh
εs
0.0
0.4
Exponential Langmuir fitting of εs
0.8 1.2 1.6 Relative time/s
（a）
2.0
（b）
Figure 11. Deformation amount fitting at the adsorption stage (a) and desorption stage (b).
395
The key parameters of the ultimate deformation amount εmax and deformation rate vε of the fitting curves were
396
derived, and they are displayed in Table 2. It can be seen from Table 2 that the ultimate deformation amount εmax in the
397
parallel direction at the adsorption stage is much larger than that in the parallel direction at the desorption stage, and the
398
ultimate deformation amount εmax in the vertical direction at the adsorption section is almost equivalent to that in the
399
vertical direction at the desorption stage. Whether for the adsorption stage or desorption stage, the ultimate deformation
400
amount εmax is approximately the sum of the ultimate deformation amounts in the vertical and parallel directions. The
401
change of the deformation rate vε is not strong.
402 403
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Energy & Fuels
404
Table 2 Statistics of εmax and vε at the adsorption and desorption stages Adsorption stage
Desorption stage
Key parameters
εmax vε/×10
405 406
-4
εh
εv
εs
εh
εv
εs
–1226.32
–1482.42
–2708.59
685.32
1435.93
2121.28
3.1255
2.0825
2.5024
3.1458
3.3153
3.2600
(c) Gas diffusion parameter KB: Yang38 obtained the time-varying rules of the ratio of the cumulative gas desorption amount Qt to the ultimate desorption amount Q∞ at t through theoretical derivation, namely Q 2 ln 1 − t = KBt + C Q∞
407
（3）
408
According to this equation, the desorption amount under the condition of a gas pressure of 0.5 MPa was fitted, and
409
the result is displayed in Figure 12. It can be seen from Figure 12 that, when the gas pressure is 0.5 MPa, the gas
410
diffusion parameter KB equals 3.0447 × 10-4 s-1.
ln[1-(Qt/Q∞)2]
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 21
411 412
0.5 0.0 -0.5 -1.0 -1.5 -2.0 -2.5 -3.0 -3.5 -4.0
ln[1-(Qt/Q∞)2]=-3.0447t+0.0276
0.0 0.2 0.4 0.6 0.8 1.0 1.2 Time/s
Figure 12. Gas diffusion constant after hydraulic slotting under a gas pressure of 0.5 MPa.
413
4.3. Effect of Gas Pressure on the Solid–Fluid Coupling Characteristics of Gas-Bearing Coal
414
Subjected to Hydraulic Slotting. Four key parameters were obtained from the detailed analysis of the entire
415
experiment: the final deformation amount before slotting εbs, ultimate deformation amount εmax, deformation rate vε, and
416
gas diffusion parameter KB, where the deformation rate vε represents the speed of deformation. The most direct
417
parameter representing the effect of slotting is the ultimate deformation amount εmax. Based on this, only the final
418
deformation amount before slotting εbs, ultimate deformation amount εmax, and gas diffusion parameter KB were
419
considered as the statistical indicators to reveal the change rules of the coal–gas interaction under different initial gas
420
pressures.
421
4.3.1. Effect of Gas Pressure on the Final Deformation Amount Before Slotting. The time-varying curves of
422
vertical strain and parallel strain under the gas pressures of 1 MPa, 1.5 MPa, and 2 MPa are analyzed. The final
423
deformation amount before slotting in the vertical and parallel directions at the aforementioned gas pressures was derived,
424
and then the final deformation amount εbs was obtained and plotted in Figure 13(a). It can be seen from Figure 13(a) that,
425
with the increase of gas pressure, the final deformation amount before slotting gradually increases. The quantitative
426
relationship between the final deformation amount and the gas pressure is obtained by fitting using the power function.
427
The final deformation amount before slotting is mainly composed of three parts: shrinkage deformation due to vacuum
428
pumping, adsorption-induced expansion deformation, and compression deformation due to loading. Shrinkage
429
deformation due to vacuum pumping is not affected by the gas pressure. With the increase of the gas pressure, both the
430
adsorption-induced expansion deformation and compression deformation due to loading increase. Compared with the
431
compression deformation due to loading, the adsorption-induced expansion deformation is relatively small. Therefore,
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432
the final deformation amount before slotting gradually increases.
433
4.3.2. Effect of Gas Pressure on the Ultimate Deformation Amount. The vertical strain and parallel strain at the
434
adsorption and desorption stages were extracted from the time-varying curves of the entire curves of vertical strain and
435
parallel strains under gas pressures of 1 MPa, 1.5 MPa, and 2 MPa. The scatter plot was drawn and fitted with the power
436
function. The results are shown in Figures 13(b-c). It can be seen from Figures 13(b-c) that, for both the adsorption and
437
desorption stages, the absolute value of the ultimate deformation amount in the vertical direction is larger than that in the
438
parallel direction, which indicates that the adsorption-induced expansion and desorption-induced shrinkage have a certain
439
anisotropy. In addition, the absolute values of the parallel and vertical ultimate deformation amounts at the desorption
440
stage are smaller than those at the adsorption stage, which may be because the irreversible property exists during
441
adsorption and desorption. Even if the coal is subjected to the desorption-enhancing measure, the ultimate deformation
442
amount cannot be fully recovered.
444
under the gas pressures of 1 MPa, 1.5 MPa, and 2 MPa were derived, linear fitting between ln[1-(Qt/Q∞)2] and t was
445
performed, and then the gas diffusion parameters under different gas pressures were obtained. The quantitative
446
relationship between the gas pressure and the gas diffusion parameter KB is obtained by fitting and shown in Figure
447
13(d). It can be seen from Figure 13(d) that, with the increase of gas pressure, the gas diffusion parameter increases
448
exponentially. This is mainly because, the greater the gas pressure, the greater the gas pressure gradient, the more
449
powerful the gas diffusion, and the faster the gas diffusion39-40.
max
bs
10.5 10.0 9.5 9.0 8.5 8.0 7.5 7.0 6.5 6.0 5.5
Ultimate deformation amount ε ×103/µε
4.3.3. Effect of Gas Pressure on the Gas Diffusion Parameter. The time-varying curves of gas desorption
Final deformation amount before slotting ε ×103/µε
443
ε =7791.73p0.36 bs
0.5
1.0 1.5 Gas pressure/MPa
2.0
-1
Parallel direction Vertical direction
-2
ε
Parallel direction Vertical direction
ε
2158.00p0.88
=
vmax
ε 0.5
hmax
-2274.29p0.92
-4 ε
vmax
=
-2707.74p1.02
-5 -6
0.5
1.0 1.5 Gas pressure/MPa
2.0
（b）
1098.39p0.92
=
1.0 1.5 Gas pressure/MPa
2.0
Gas diffusion parameter KB/s-1
max
450
4.5 4.0 3.5 3.0 2.5 2.0 1.5 1.0 0.5
=
hmax
-3
（a） Ultimate deformation amount ε ×103/µε
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
12 10
KB=5.74p1.03
8 6 4 2
0.5
1.0 1.5 Gas pressure/MPa
（c）
2.0
（d）
451
Figure 13. Effect of gas pressure on the coal-gas interaction property. (a) Effect of gas pressure on the final deformation amount
452
before slotting; (b) Effect of the gas pressure on the ultimate deformation amount at the gas-filling stage; (c) Effect of gas pressure on
453
the ultimate deformation amount at the desorption stage; (d) Effect of gas pressure on the gas diffusion parameter.
454
4.4. Effect of Slot Radius on the Solid–Fluid Coupling Characteristics of Gas-Bearing Coal
455
Subjected to Hydraulic Slotting. Another important parameter that affects the effect of slotting is the slot radius.
456
Compared with the entire coal sample, the slot radius is significantly small. Therefore, it has only a slight effect on the
457
final deformation amount before slotting. Based on this, only the effect of slot radius on the ultimate deformation amount
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Energy & Fuels
458
εmax and gas diffusion parameter KB was discussed in this paper.
459
4.4.1. Effect of Slot Radius on the Ultimate Deformation Amount. The exponential Langmuir model was
460
adopted to fit the time-varying curves of vertical strain and parallel strain at the desorption stage under the slot radiuses
461
of 5 mm, 7.5 mm, 10 mm, and 12.5 mm, respectively, to obtain the ultimate deformation amount under different slot
462
radiuses. Subsequently, the SLogistic1 function was used to obtain the quantitative relationship between the slot radius
463
and ultimate deformation amount. The results are shown in Figure 14(a). The relationships between the slot radius and
464
the ultimate deformation amount in the vertical and parallel directions are as follows:
4360.16 1 + exp −0.56 ( rs − 6.77 )
（4）
2390.82 1 + exp −0.41( rs − 6.47 )
（5）
465
ε v max =
466
ε h max =
467
It can be seen from Figure 14(a) that, for both vertical and parallel directions, the ultimate deformation amount tends
468
to be flat after a rapid increase with the increase of the slot radius, which indicates that the slot radius has the optimal
469
value. When the slot radius is below the optimal value, the effect of the slot radius on the ultimate deformation amount is
470
significant. However, when the slot radius is above the optimal value, the change of the ultimate deformation amount is
471
small with the increase of the slot radius. 4.4.2. Effect of Slot Radius on the Gas Diffusion Parameter. The time-varying curves of gas desorption under
473
the slot radiuses of 5 mm, 7.5 mm, 10 mm and 12.5 mm were derived, linear fitting between ln[1-(Qt/Q∞)2] and t was
474
performed, and then the gas diffusion parameters under different slot radiuses were obtained. The quantitative
475
relationship between the slot radius and the gas diffusion parameter KB is obtained by the fitting using the SLogistic1
476
function, as shown in Figure 14(b). It can be seen from Figure 14(b) that, with the increase of the slot radius, the gas
477
diffusion parameter shows a similar change tendency with the ultimate deformation amount, which may result from the
478
fact that, with the enlargement of the slot radius, the disturbance action is weakened. Under the same gas pressure, the
479
gas pressure gradient decreases, and the diffusion power is weakened.
max
4.5 4.0
Parallel direction Vertical direction
3.5 3.0 2.5 2.0 1.5 1.0 5.0
7.5 10.0 Slot radius/mm
12.5
Gas diffusion parameter KB/s-1
472
Ultimate deformation amount ε ×103/µε
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 21
12 11 10 9 8 7 6 5 5.0
（a）
7.5 10.0 Slot radius/mm
12.5
（b）
480 481
Figure 14. Effect of slot radius on the coal-gas coupling property. (a) Effect of slot radius on the ultimate deformation amount; (b) Effect of slot radius
482
on the gas diffusion parameter.
483
4.5. Guiding Significance of the Investigation on the Gas Pressure and Slot Radius for the Field
484
Test. Gas pressure is a crucial external factor influencing the efficiency of HS-ECBM recovery. As shown in Figure 15,
485
with the increase in the burial depth, the permeability of the coal seam decreases, and the gas pressure increases41-42.
486
Enlightenment can be gained from the aforementioned variation rules that the hydraulic slotting is necessary for the
487
permeability enhancement of the deep coal seam to significantly improve the CBM recovery efficiency. Furthermore,
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Page 17 of 21
488
deep coal seams are characterized by high gas content, which indicates that their exploitation value is relatively higher.
489
However, according to the analysis in Section 4.3, with the increase in the gas pressure, the deformation of the coal and
490
gas diffusion increase, which may result in two issues: the effect of hydraulic slotting becomes better, but the problem of
491
strong borehole spray may occur23. To solve the above problem, safety protection is necessary when hydraulic slotting is
492
used in the deep coal seams21. 100
8
10
Gas pressure/MPa
Permeability/10-15m2
1 0.1 0.01 0.001
Measured value of gas pressure Linear fitting
6 4 2 0
0
400
800
1200
1600
0
200
Burial depth/m
400
600
800
Burial depth/m
(a)
(b)
493 494
Figure 15. Variation in permeability of the coal seam and gas pressure with the burial depth. (a) Variation in permeability of the coal seam with the
495
burial depth; (b) Variation in gas pressure with the burial depth.
496
Slot radius is a crucial internal factor influencing the efficiency of HS-ECBM recovery. A larger slot radius can
497
produce a larger disturbance range, which can improve the effect of HS-ECBM recovery. However, the efficiency of the
498
construction of the slotted borehole may be the more reasonable indictor for evaluating the efficiency of HS-ECBM
499
recovery. Based on stress evolution of the coal around the slot under different slot radiuses, Yang43 obtained the variation
500
rules of an equivalent borehole radius with the borehole radius under various rates of stress drop (Figure 16). It can be
501
seen from Figure 16 that the slot radius has the optimal value, which is consistent with the conclusion obtained in Section
502
4.4. Enlightenment can be gained from the above conclusion that it is inadvisable to improve the efficiency of HS-ECBM
503
recovery by blindly enlarging the slot radius. A better choice is the formation of the fracture network by drilling
504
multi-boreholes44. 7 Equivalent borehole radius/m
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
5% 10% 20% 30% Rate of 40% stress drop 50% 60% 70%
6 5 4 3 2 1 0 0.1
0.2
0.3 0.4 0.5 Slot radius/m
0.6
0.7
505 506
Figure 16. Variation in the equivalent borehole radius with the borehole radius under various rates of stress drop43.
507
5. CONCLUSIONS
508
In this paper, a fluid–solid coupling experimental system of gas-bearing coal subjected to hydraulic slotting was
509
established. The fluid–solid coupling property of gas-bearing coal subjected to hydraulic slotting was revealed using the
510
established experimental system. Meanwhile, indicators used to describe the process of hydraulic slotting were derived,
511
and the influencing factors affecting the process of hydraulic slotting were analyzed using the aforementioned indicators.
512
During the experiment, the gas pressure response of the monitoring points in the coal sample shows different
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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
513
characteristics at different stages: at the vacuum-pumping stage, the gas pressure rapidly decreases to the stable value. At
514
the gas-filling stage, the gas pressure tends to be flat after an approximatively linear increase. For every loading step at
515
the confining pressure-loading stage, the gas pressure slowly decreases after a sudden rise. With the increase of the
516
loading step, the change magnitude of the gas pressure decreases. At the slotting stage, the gas pressure tortuously
517
increases, rapidly drops, and then tends towards a stable value. At the desorption stage, the gas pressure tends to be stable
518
after the rapid decrease, and it presents the characteristics of the exponential function. Corresponding to the change trend
519
of the gas pressure, the vertical strain and parallel strain show the five-stage change characteristics, i.e., contraction
520
deformation due to vacuum pumping, swelling deformation due to methane adsorption, compression deformation due to
521
loading, relaxation due to slotting, and shrinkage deformation due to desorption.
522
The final deformation amount before slotting, ultimate deformation amount, and gas diffusion parameter were
523
adopted to describe the coal–gas coupling characteristics. With the increase of gas pressure, the final deformation amount
524
before slotting gradually increases. For both adsorption stage and desorption stage, the absolute value of the ultimate
525
deformation amount in the vertical direction is larger than that in the parallel direction. In addition, the absolute values of
526
the parallel and vertical ultimate deformation amounts at the desorption stage are smaller than those at the adsorption
527
stage. With the increase of gas pressure, the gas diffusion parameter increases exponentially. The ultimate deformation
528
amounts in both the vertical and parallel directions tend to be flat after a rapid increase with the increase of the slot radius,
529
which indicates that the slot radius has the optimal value. With the increase of the slot radius, the gas diffusion parameter
530
shows a similar change tendency with the ultimate deformation amount.
531
To solve the problem of strong borehole spray encountered in the process of hydraulic slotting in the deep coal
532
seams, safety protection measures should be adopted. Moreover, the efficiency of HS-ECBM recovery can be improved
533
through the formation of the fracture network by drilling multi-boreholes instead of blindly enlarging the slot radius.
534
535
Corresponding Author
536
*Tel: +8617783372719. E-mail:
[email protected] (Q. Zou).
537
Notes
538
The authors declare no competing financial interest.
539
540
This work is financially supported by the State Key Research Development Program of China (Grant
541
No.2017YFC080206), the National Natural Science Foundation of China (Grant No. 51704046), and the Fundamental
542
Research Funds for the Central Universities (Project No.106112017CDJXY240001), which are gratefully acknowledged.
543
The authors thank K Anand Kumar for editing this paper.
544
545
(1) Moore, T.A. Coalbed methane: A review. Int J Coal Geol. 2012, 101, 36-81.
546
(2) Wang, L.; Liu, S.; Cheng, Y.; Yin, G.; Zhang, D.; Guo, P. Reservoir reconstruction technologies for coalbed methane
547 548 549
AUTHOR INFORMATION
ACKNOWLEDGMENTS
REFERENCES
recovery in deep and multiple seams. Int J Min Sci Tech. 2017, 27, 277-284. (3) Ni, G.; Li, Z.; Xie, H. The mechanism and relief method of the coal seam water blocking effect (WBE) based on the surfactants. Powder Tech. 2018, 323, 60-68.
ACS Paragon Plus Environment
Page 18 of 21
Page 19 of 21 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
550 551 552 553 554 555 556 557 558 559 560 561 562 563 564 565 566 567 568 569 570 571 572 573
Energy & Fuels
(4) Lau, H. C.; Li, H.; Huang, S. Challenges and opportunities of coalbed methane development in China. Energ Fuel. 2017, 31, 4588-4602. (5) Zhai, C.; Xu, Y.; Xiang, X.; Yu, X.; Zou, Q.; Zhong, C. A novel active prevention technology for borehole instability under the influence of mining activities. J Nat Gas Sci Eng. 2015, 27, 1585-1596. (6) Liu, T.; Lin, B.; Zou, Q.; Zhu, C.; Guo, C.; Li, J. Investigation on mechanical properties and damage evolution of coal after hydraulic slotting. J Nat Gas Sci Eng, 2015, 24, 489-499. (7) Karacan, C.Z.; Ruiz, F.A.; Cot, M.; Phipps, S. Coal mine methane: A review of capture and utilization practices with benefits to mining safety and to greenhouse gas reduction. Int J Coal Geol, 2011, 86, 121-156. (8) Ranathunga, A.S.; Perera, M.S.A.; Ranjith, P.G.; Wei, C.H. An experimental investigation of applicability of CO2 enhanced coal bed methane recovery to low rank coal. Fuel. 2017,189, 391-399. (9) Olajossy, A. Some parameters of coal methane system that cause very slow release of methane from virgin coal beds (CBM). Int J Min Sci Tech, 2017, 27, 321-326. (10) Li X.; Wang E., Li Z. Rock burst monitoring by integrated microseismic and electromagnetic radiation methods. Rock Mech Rock Eng, 2016, 49, 4393-4406. (11) Mohanty, M.M.; Pal, B.K. Sorption behavior of coal for implication in coal bed methane: an overview. Int J Min Sci Tech, 2017, 27, 307-314. (12) Huang, B.; Cheng, Q.; Chen, S. Phenomenon of methane driven caused by hydraulic fracturing in methane-bearing coal seams. Int J Min Sci Tech, 2016, 26, 919-927. (13) Zhang, L.; Zhang, H.; Guo, H. A case study of gas drainage to low permeability coal seam. Int J Min Sci Tech, 2017, 27, 687-692. (14) Zou, Q.; Li, Q.; Liu, T.; Li, X.; Liang, Y. Peak strength property of the pre-cracked similar material: Implications for the application of hydraulic slotting in ECBM. J Nat Gas Sci Eng, 2017, 37, 106-115. (15) Day, S.; Fry, R.; Sakurovs, R.; Weir, S. Swelling of coals by supercritical gases and its relationship to sorption. Energ Fuel, 2010, 24, 2777-2783.
574
(16) Zou, Q.; Lin, B.; Liu, T.; Hu, X.; Zheng, C. Variations in coalbed gas content, initial gas desorption property and
575
coal strength after drilling-slotting integration technique and gas drainage: insight into pore characteristics. Int J Oil
576
Gas Coal T, 2017, 15, 235–266
577 578
(17) Xu, J.; Zhai, C.; Qin, L. Mechanism and application of pulse hydraulic fracturing in improving drainage of coalbed methane. J Nat Gas Sci Eng, 2017, 40, 79-90.
579
(18) Zhai, C.; Qin, L.; Liu, S.; Xu, J.; Tang, Z.; Wu, S. Pore structure in coal: Pore evolution after cryogenic freezing
580
with cyclic liquid nitrogen injection and its implication on coalbed methane extraction. Energ Fuel, 2016, 30,
581
6009-6020.
582 583 584 585
(19) Li, H.; Lin, B.; Liu, S.; Chen, Z.; Hong, Y.; Zheng, C. Evolution of coal petrophysical properties under microwave irradiation stimulation for different water saturation conditions. Energ Fuel, 2017, 31, 8852-8864. (20) Zhang, X. G.; Ranjith, P. G.; Perera, M. S. A.; Ranathunga, A.S.; Haque, A. Gas transportation and enhanced coalbed methane recovery processes in deep coal seams: A review. Energ Fuel, 2016, 30, 8832-8849.
586
(21) Zou, Q.; Lin, B.; Zheng, C.; Hao, Z.; Zhai, C.; Liu, T.; Liang, J.; Yan, F.; Yang, W.; Zhu, C. Novel integrated
587
techniques of drilling–slotting–separation-sealing for enhanced coal bed methane recovery in underground coal
588
mines. J Nat Gas Sci Eng, 2015, 26, 960-973.
ACS Paragon Plus Environment
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
589 590 591 592 593 594
(22) Lin, B.; Wu, H.; Zhang, L.; Lu, H. Integrative outburst prevention technique of high-pressure jet of abrasive drilling slotting. Proc Earth Planet Sci, 2009, 1, 27-34. (23) Shen, C.; Lin, B.; Zhang, Q.; Yang, W.; Zhang, L. Induced drill-spray during hydraulic slotting of a coal seam and its influence on gas extraction. Int J Min Sci Tech, 2012, 22, 785-791. (24) Gao, F.; Xue, Y.; Gao, Y.; Zhang, Z.; Teng, T.; Liang, X. Fully coupled thermo-hydro-mechanical model for extraction of coal seam gas with slotted boreholes. J Nat Gas Sci Eng, 2016, 31, 226-235.
595
(25) Liu, T.; Lin, B.; Zou, Q.; Zhu, C.; Yan, F. Mechanical behaviors and failure processes of precracked specimens
596
under uniaxial compression: A perspective from microscopic displacement patterns. Tectonophysics, 2016, 672-673,
597
104-120.
598 599 600 601 602 603 604 605 606 607 608 609 610 611 612 613 614 615 616 617
(26) Xu, J.; Chen, Y.; Chu, T.; Peng, S.; Feng, D., Zhang, C. Temporal and spatial evolutions of permeability during CBM drainage under different initial conditions. Transp Porous Med, 2017, 116, 645-662. (27) Peng, S.; Zhang, C.; Xu, J.; Liu, D.; Liang, Y. Physical simulation experiment on the evolution of temperature during CBM drainage. Chi J Rock Mech Eng, 2015, 34, 1325-1333. (28) Hol, S.; Peach, C. J.; Spiers, C. J. Applied stress reduces the CO2 sorption capacity of coal. Int J Coal Geo, 2011, 85, 128-142. (29) Nie, B.; Hu, S.; Li, X.; Zhai, S.; Meng, J. Experimental study of deformation rules during the process of gas adsorption and desorption in briquette coal. Int J Min Reclam Env, 2014, 28, 277-286. (30) Zeng, Q.; Wang, Z.; McPherson, B. J.; McLennan, J.D. Theoretical approach to model gas adsorption/desorption and the induced coal deformation and permeability change. Energ Fuel, 2017, 31, 7982-7994. (31) Yang, K.; Lu, X.; Lin, Y.; Neimark, A. V. Deformation of coal induced by methane adsorption at geological conditions. Energ Fuel, 2010, 24, 5955-5964. (32) Meng, Y.; Li, Z. Triaxial experiments on adsorption deformation and permeability of different sorbing gases in anthracite coal. J Nat Gas Sci Eng, 2017, 46, 59-70. (33) Cao, W.; Li, X.; Tao, M.; Zhou, Z. Vibrations induced by high initial stress release during underground excavations. Tunn Undergr Sp Tech, 2016, 53, 78-95. (34) Yang, J. H.; Jiang, Q. H.; Zhang, Q. B.; Zhao, J. Dynamic stress adjustment and rock damage during blasting excavation in a deep-buried circular tunnel. Tunn Undergr Sp Tech, 2018, 71, 591-604. (35) Siriwardane, H. J.; Gondle, R. K.; Smith, D. H. Shrinkage and swelling of coal induced by desorption and sorption of fluids: Theoretical model and interpretation of a field project. Int J Coal Geol, 2009, 77, 188-202.
618
(36) Majewska, Z.; Majewski, S.; Zietek, J. Swelling of coal induced by cyclic sorption/desorption of gas: Experimental
619
observations indicating changes in coal structure due to sorption of CO2 and CH4. Int J Coal Geol, 2010, 83,
620
475-483.
621 622 623 624 625 626 627
(37) Baran, P.; Zarebska, K.; Bukowska, M. Expansion of hard coal accompanying the sorption of methane and carbon dioxide in isothermal and non-isothermal processes. Energ Fuel, 2015, 29, 1899-1904. (38) Yang, Q.; Wang, Y. Theory of methane diffusion from coal cuttings and its application. J China Coal Soc, 1986, 3, 87-94. (39) Naveen, P.; Asif, M.; Ojha, K.; Panigrahi, D. C.; Vuthaluru, H. B. Sorption kinetics of CH4 and CO2 diffusion in coal: theoretical and experimental study. Energ Fuel, 2017, 31, 6825-6837. (40) Guo, H.; Cheng, Y.; Yuan, L.; Wang, L.; Zhou, H. Unsteady-state diffusion of gas in coals and its relationship with
ACS Paragon Plus Environment
Page 20 of 21
Page 21 of 21 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
628 629 630
Energy & Fuels
coal pore structure. Energ Fuel, 2016, 30, 7014-7024. (41) Kang, Y.; Sun, L.; Zhang, B.; Gu, J.; Mao, D. Discussion on classification of coalbed reservoir permeability in China. J China Coal Soc, 2017, 42, 186-194.
631
(42) Yu, Q. Coal mine gas disaster prevention and control. China Univer Min Tech Press, 1992.
632
(43) Yang, W.; Lin, B.; Gao, Y.; Lv, Y.; Wang, Y. K.; Mao, X.; Wang, N.; Wang, D.; Wang, Y.J. Optimal coal discharge
633
of hydraulic cutting inside coal seams for stimulating gas production: A case study in Pingmei coalfield. J Nat Gas
634
Sci Eng, 2016, 28, 379-388.
635 636
(44) Liu, T.; Lin, B.; Yang, W. Mechanical behavior and failure mechanism of pre-cracked specimen under uniaxial compression. Tectonophysics, 2017, 712-713, 330-343.
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