- Email: [email protected]
Ultrasonic vibrations and coal permeability: Laboratory experimental investigations and numerical simulations
International Journal of Mining Science and Technology 27 (2017) 221–228
Contents lists available at ScienceDirect
International Journal of Mining Science and Technology journal homepage: www.elsevier.com/locate/ijmst
Ultrasonic vibrations and coal permeability: Laboratory experimental investigations and numerical simulations Zhang Junwen a,⇑, Li Yulin a,b a b
Heilongjiang University of Science and Technology, School of Mining Engineering, Harbin 150022, China China University of Mining and Technology, School of Mechanics and Architectural Engineering, Beijing 100083, China
a r t i c l e
i n f o
Article history: Received 2 April 2015 Received in revised form 12 July 2015 Accepted 2 September 2015 Available online 26 January 2017 Keywords: Ultrasonic cavitation Rock-coal Coal fracture Permeability Experimental analysis
a b s t r a c t Ultrasonic vibrations in coal lead to cavitation bubble oscillation, growth, shrinkage, and collapse, and the strong vibration of cavitation bubbles not only makes coal pores break and cracks propagate, but plays an important role in enhancing the permeability of coal. In this paper, the influence of ultrasonic cavitation on coal and the effects of the sonic waves on crack generation, propagation, connection, as well as the effect of cracks on the coal permeability, are studied. The experimental results show that cracks in coal are generated even connected rapidly after ultrasonic cavitation. Under the effect of ultrasonic cavitation, the permeability increases between 30% and 60%, and the number of cracks in coal also significantly increased. Numerical experiments show that the effective sound pressure is beneficial to fracture propagation and connection, and it is closely related to the permeability. Moreover, the numerical simulations and physical experiments provide a guide for the coal permeability improvement. Ó 2017 Published by Elsevier B.V. on behalf of China University of Mining & Technology. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).
1. Introduction Ultrasonic waves can promote mass transfer and accelerate chemical reactions [1–5]. Ultrasonic waves have been applied in hydraulic drilling and industrial cleaning. In coal, these waves cause cavitation bubbles to oscillate, grow, shrink, and collapse, and the strong bubble vibrations cause the pores in the coal to break and cracks to propagate, which play an important role in enhancing the coal’s permeability. Ultrasonic cavitation refers to the activation of the micro-bubble’s nucleus by ultrasonic waves in a liquid [6–10]. This activation is the dynamic oscillation, growth, shrinkage, and collapse of the bubble nucleus. Many investigators have conducted research on ultrasonic cavitation and the research has embraced both different physical research methods and mathematical modeling. Jiang et al. [11] carried out an experiment to study acoustic vibrations to promote methane desorption from coal. Li et al. [12] carried out research on coal-bed methane desorption and emissions under the action of sound waves and obtained coal methane gas desorption and diffusion parameters. Li et al. [13] developed a mechanical coal sorption/desorption test system and used it to study coal adsorption under low frequency vibrations. Klobes et al. [14] determined rock porosity with a combination of X-ray computerized ⇑ Corresponding author. E-mail address: [email protected] (J. Zhang).
tomography and mercury porosimetry. Goodwin et al. [15] investigated particulate interactions within opencast coal mine backfill by the use of X-ray computer tomography. Ren et al. [16] performed an experimental study on the effect of power ultrasound on coal and rock. That study looked at crack generation and development and the change in the stress state and the mechanical properties of the coal and rock. The study also presented a qualitative analysis of the changes in compressive strength and elastic modulus of coal and rock under ultrasonic vibration. This study provided an experimental basis for the changes of coal mechanical properties caused by cracks which are generated by ultrasonic vibration. Jiang et al. [11] studied the desorption of methane gas in coal under ultrasonic waves and the thermal effect of the waves. Li [12] using a selfdeveloped acoustic field coal gas desorption device, established a physical model for coal gas emissions and monitored the emission of total pressure, gas pressure, and gas emission rates. Nie et al. analyzed the influence of changes in the porosity and permeability of coal caused by power ultrasonic waves on coal-bed methane. Yu et al. [17] explored ultrasonic interference for improving the permeability of coal reservoirs and increasing the coal-bed methane desorption rate. Using ultrasonic waves at 20 kHz, Shao et al. [18] studied the effect of ultrasound on rock permeability. A number of other investigations conducted by Li et al. have looked at how to solve the problems of coal bed micro cracks, low permeability, high gas adsorption, and the difficulty of draining methane by using cavitation water jet acoustic shocks to improve methane
http://dx.doi.org/10.1016/j.ijmst.2017.01.001 2095-2686/Ó 2017 Published by Elsevier B.V. on behalf of China University of Mining & Technology. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).
222
J. Zhang, Y. Li / International Journal of Mining Science and Technology 27 (2017) 221–228
desorption and seepage flow [19–23]. Lu et al. [24] determined how temperature affects coal permeability by studying the thermal effects produced at the moment of cavitation bubble breaking. They also studied the cavitation noise sound field produced during tests of coal temperature and tested methane permeability under different cavitation conditions (pump pressure and cavitation pressure). Abramov et al. [25] found that when used on oil wells with a permeability greater than 20 md and a porosity of more than 15%, ultrasonic technology could increase oil production by 30–50% or more. Another experimental study, by Alhomadhi et al. [26] was done to investigate improving oil recovery by using ultrasonic waves during water flooding. Many previous studies have focused on the effects of ultrasonic waves on coal methane desorption and diffusion, while less attention has been paid to research the effects of sound waves on crack propagation and permeability in coal. The way in which strong ultrasonic cavitation enhances permeability in coal is still not very clear. In this paper, the method used combines theoretical analysis with physical experiments to analyze cavitation bubble oscillation, growth, shrinkage, and collapse, and discuss ultrasonic cavitation in coal. The experiments determined the permeability of the coal and used computed tomography (CT) to study coal crack generation, propagation, and connectivity along with the changes in permeability under ultrasonic cavitation. In addition, stress wave loading reproduced ruptures in the coal caused by ultrasonic mechanical effects. The study showed that sound pressure promotes crack propagation and connection. The experimental results can provide a foundation for the study of ultrasonic cavitation rupture to enhance coal permeability.
2. Ultrasonic cavitation experiments on coal rupture and permeability enhancement
0.02–0.90 m thick. The longwall face on the No. 3 bed is about 350 m long. The coal samples used for this study were rectangular blocks 50 mm on each side and 100 mm long. The purpose of the experiment was to study the effect of ultrasonic waves on the evolution of the cracks in the coal and the ultrasound’s effect on the coal permeability. Some of the coal samples used for these experiments are shown in Fig. 1. 2.2. Experimental apparatus The device used for ultrasonic excitation of the coal was a ZJS500N type ultrasonic generator, which will work in an indoor environment if the humidity is less than or equal to 85% relative humidity and the temperature is between 0 °C and 40 °C. The specific components of the ultrasonic device include: (1) an ultrasonic vibration source (drive power): this converts the 50–60 Hz current to a higher power and frequency (15–100 kHz) to supply the transducer; (2) a transducer (controller transducer): this converts the high frequency electric current into mechanical vibration energy; (3) an amplitude transformer: connected to the fixed transducer and the tool head, this device amplifies the vibrations from the transducer and sends them to the tool head; (4) the tool head (guide rod): this transfers the mechanical energy and the pressure to the sample. It also independently increases the amplitude; (5) connecting bolts: these connect the components mentioned above. The three axis penetration apparatus and ultrasonic probe component connect the ultrasonic generator to the experimental equipment. The three axis penetration instrument, the precision digital flow meter, the ZJS-500N ultrasonic generator, and a schematic diagram of experimental assembly used for the experiments are shown in Fig. 2. 2.3. Experimental design
2.1. Sample description Coal samples used in these experiments were collected from the Jincheng Tiandiwangpo No. 3 coal bed, Shanxi Province, China. The coal mines there are high methane mines but the excavated coal is not prone to spontaneous combustion. The Wangpo coal mine is located in the southern Taihang Mountain area, the watershed of the Qin and Long Rivers. The area is moderately mountainous and dissected by gullies. Terrain slopes are more than 20–30°. The highest areas in the Wangpo mine are in the northeast central area, the lowest are in the north, south, and east areas. The highest topographic point is on the northeast ridge at an elevation of 1327.5 m, the low point is in the Long River valley, elevation 877.2 m. The No. 3 coal bed is hosted by the lower Permian Shanxi Formation in the lower part of the mine. The No. 3 coal bed ranges from 4.10 to 6.70 m in thickness (averaging 5.76 m) and it has simple structure only containing 0–2 layers of carbonaceous shale
To analyze the effects of ultrasonic cavitation, confining pressure, and pore pressure on the coal’s permeability, the experiments were run under two sets of experimental conditions. First, the experiments were run both with and without ultrasound with the confining pressure set to 4.0 MPa and pore pressures of 0.2, 0.3, 0.4, 0.5, 0.6, 0.8, and 0.7 MPa [27,28]. Then, the confining pressure was increased to 12 MPa and the experiments were run again at pore pressures of 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8, 1.1, and 1.6 MPa. The experimental scheme is shown in Table 1. 2.4. Ultrasonic coal permeability analysis A triaxial permeability measuring system, based on the principle of a steady state method, was used to determine the permeability of the coal samples to compare the permeability of the samples before and after ultrasonic treatment. According to
Fig. 1. Photographs of some of the coal samples used in the experiments.
223
J. Zhang, Y. Li / International Journal of Mining Science and Technology 27 (2017) 221–228
(a) Three axis penetration instrument and ultrasonic probe
(b) Precision digital flow meter Six way valve
Six way valve
High pressure regulating valve
Pressure gauge Confining pressure inlet
Ultrasonic probe
Three axis penetration instrument
Gas cylinders
(c) Ultrasonic generator
Axial compression inlet Pressure pump
Collection device
Power
Ultrasonic generator
(d) Schematic diagramof experimental assembly
Fig. 2. Ultrasonic equipment and schematic diagram of the experimental apparatus.
Table 1 Experimental conditions. Project
Confining pressure (MPa)
Pore pressure (MPa)
Without ultrasound Ultrasonic Without ultrasound Ultrasonic
4.0 4.0 12.0 12.0
0.2 0.2 0.2 0.2
0.3 0.3 0.3 0.3
0.4 0.4 0.4 0.4
Permeability measured experiment 0.5 0.5 0.5 0.5
0.6 0.6 0.6 0.6
0.7 0.7 0.7 0.7
0.8 0.8 0.8 0.8
1.1 1.1
Comparison of permeability of coal samples before and after ultrasonic test Comparison of permeability of coal samples before and after ultrasonic test
1.6 1.6
0.9
No ultrasonic cavitate
Permeability (µm2 )
Permeability (µm2)
0.26
Ultrasonic cavitate
0.8 0.7 0.6 0.5 0.4 0.3
0.22 0.18 0.14 0.10 0.06
0.2
0.3
0.4
0.5
0.6
0.7
0.8
Pore pressure (MPa) (a) At a confining pressure of 4 MPa
0
0.2
0.4
0.6
0.8
1.0
1.2
1.4
1.6
1.8
Pore pressure (MPa) (b) At a confining pressure of 12 MPa
Fig. 3. Graph showing the relationship between permeability and pore pressure at confining pressure 4 MPa and 12 MPa.
the experiments, the permeability of the coal after ultrasonic cavitation was greatly improved. When the confining pressure was 4 MPa and the pore pressure was increased from 0.2 to 0.8 MPa, the permeability of coal was between 0.68 and 0.29 lm2. However, after ultrasonic cavitation, the permeability of coal for those same pore pressures was between 0.83 and 0.42 lm2 (Fig. 3a). When the confining pressure was 12 MPa and the pore pressure was increased from 0.2 to 1.6 MPa, the permeability of coal before
ultrasonic cavitation was between 0.21 and 0.10 lm2, but after ultrasonic cavitation the permeability ranged from 0.27 to 0.17 lm2 (Fig. 3b). These coal permeability experiments show that, under the same conditions, when confining pressure increases, the permeability decreases significantly. The confining pressure effect on crack pore squeezing is obvious. After ultrasonic cavitation, coal permeability improves significantly and the reason is that the coal inter-crack has been fully developed and broken through [29],
224
J. Zhang, Y. Li / International Journal of Mining Science and Technology 27 (2017) 221–228
resulting in a significant permeability increase. This phenomenon shows that the ultrasonic cavitation effect on the coal favored crack extension and this improved the permeability.
matched those of the coal samples used for the physical experiments. Those mechanical parameters and the associated control conditions are shown in Table 3.
3. Numerical simulations of ultrasonic rupture in coal to enhance permeability
3.3. Analysis of effective sound pressure
3.1. Numerical model and experimental design To analyze the effects of ultrasonic waves on cracks produced during the physical experiments in more detail, numerical simulations were employed to study crack growth related to acoustic energy emissions. The numerical model was the same size (50 mm 50 mm 100 mm) as the physical coal sample, (Fig. 4) and the geometric model was divided into 20,000 cells. The Weibull distribution was used to represent sample in homogeneity and the Moire-Kulun strength theory was chosen to act as a sample failure criterion. The effect of ultrasonic waves on a crack in the coal was represented by the effective sound pressure, the stress wave method was applied to the load, and the waveform chosen was a sine wave. Table 2 presents the conditions used for the numerical experiments. 3.2. Coal mechanical parameters and control conditions To make the results of the numerical experiments applicable, the mechanical parameters chosen for the numerical simulations Ultrasonic probe
The numerical experiments were run to determine the shape of the Y-axis stress curve with an increasing acoustic emission load. The results are shown in Fig. 5. Fig. 5 shows that, at the initial stage of loading, the acoustic emission frequency was very low and, because of the viscous nature of the coal, the Y-axial stress is attenuated. That is because the closely spaced and new generation of crack-fractures and the generation of acoustic emissions all need to absorb energy at this stage, while the energy accumulation remains in local dynamic equilibrium, so that the acoustic emission frequency is not very high. However, when the energy accumulates and the stress concentrations reach a specific threshold, acoustic emissions will be produced as the crack-fractures begin to extend. With the increasing number of acoustic emissions, energy is gradually released and the value of the Y-axis stress also increases gradually from its low initial state. The curves for the acoustic emission frequency and the Y-axis stress are nearly parallel and it can be seen that when the rate of change of stress accelerates, the number of acoustic emissions also increases. The frequency of sound emissions moderates when the stress rate decreases. As the stress increases and decreases and energy emitted by the acoustic emissions changes, the stresses in the crack-fracture area gradually converge, thus promoting the initiation and propagation of crack-fractures.
100 mm
3.4. Analysis of crack extension and coal deformation
50 mm
Fig. 4. Diagram showing the numerical simulation model.
Table 2 Conditions for the numerical simulations. Scheme
Voltage (V)
Electric current (A)
Effective sound pressure (MPa)
I II III IV V
220 220 220 220 220
4.09 5.91 6.82 8.64 10.00
2.64 3.81 4.39 5.56 6.44
Table 3 Mechanical parameters and control conditions for the numerical model. Parameter
Numerical value
Coefficient of homogeneity Mean value of elastic modulus (GPa) Mean value of compressive strength (MPa) Density (kg/m3) Poisson ratio Internal friction angle (°) Time step (s)
5 15 13 1350 0.25 37 2 10
7
The numerical simulations can produce maximum principal stress gradient maps showing the changes in maximum principal stress as the experiment progresses. Data from the simulations can also be used to generate the corresponding maximum principal stress versus time curves and the X-axis displacement versus time curves. These maps and graphs were generated from the simulation results for effective sound pressures of 2.64, 4.39, and 6.44 MPa and the resulting maps and graphs are shown in Figs. 6 and 7. In Fig. 6, blue represent coal; red, orange, and green indicate stress from high to low, and empty is a crack. Fig. 6 shows that with an increase in the effective sound pressure, the crack in the coal is formed in advance and the crack breakthrough increases. Fig. 7a shows that the maximum principal stress increases with an increase in the effective sound pressure and the maximum principal stress fluctuation is ahead of time, that is, the mutation rate of stress changes in advance. In the phase where the maximum principal stress suddenly changes, which is the frequency phase of acoustic emissions, the acoustic emission is produced by a frequency-generated surface crack-fracture which is initiated and gradually expands to breakthrough. In Fig. 7b, the X-axis displacement directly reflects the total width of cracks plus fractures and this sum increases at greater effective sound pressures. The Xaxis displacement curve changes from linear to nonlinear at higher sound pressures and the higher the effective sound pressure, the faster the displacement rate increases. That is, the faster the crack-fractures propagate, the more consistent the data shown in Fig. 7b are with the experimental data from the CT observations. Fig. 7a and b shows that, when the ultrasonic generator frequency has been determined, ultrasonic cavitation is a feasible tool that can be used effectively to improve the permeability of a coal-bed methane reservoir by increasing the ultrasonic power to an effective sound pressure level.
225
1.2
200
0.6
100
20
30
40
50
200 1.2 100
0
60
(a) Effective sound pressure 2.64 MPa
2.4 200
10
20
30
1.2 0.6
40
0
50
0
5 300 4 200
3 2
100 1 0 20
10
30
20
30
40
Calculation time step (c) Effective sound pressure 4.39 MPa
300
6
10
1.8
100
(b) Effective sound pressure 3.81 MPa
400
0
3.0 300
Calculation time step
Calculation time step
Acoustic emission frequency
0.6
Acoustic emission frequency
10
1.8
3.6
0
0 0
2.4
300
400
Stress (MPa)
300
400
3.0
4.8 4.2 3.6
200
3.0 2.4 1.8
100
Stress (MPa)
1.8
3.6
Acoustic emission frequency
Y-axial stress curve
500
Stress (MPa)
2.4
Stress (MPa)
400
Acoustic emission frequency
Stress (MPa)
500
Acoustic emission frequency
Acoustic emission frequency
J. Zhang, Y. Li / International Journal of Mining Science and Technology 27 (2017) 221–228
1.2 0.6 0
10
20
30
0
Calculation time step
Calculation time step
(e) Effective sound pressure 6.44 MPa
(d) Effective sound pressure 5.56 MPa
Fig. 5. Acoustic emission and Y-axial stress curves for sound pressures of 2.64, 3.81, 4.39, 5.56, and 6.44 MPa.
(a) Effective sound pressure 2.64 MPa
(b) Effective sound pressure 4.39 MPa
(c) Effective sound pressure 6.44 MPa
Fig. 6. Stress gradient maps for effective sound pressures at (a) 2.64 MPa, (b) 4.39 MPa, and (c) 6.44 MPa.
3.5. Analysis of crack propagation in coal under confining pressure A numerical analysis of crack generation in coal under the combined influence of sound pressure and the surrounding rock stress was carried out. The influence of wall rock stress on crack initiation and propagation was done as part of the investigation of power ultrasonic cracking of coal to improve coal-bed methane extraction from the actual coal seam. Table 4 presents experimental conditions for the numerical simulations under the effects of different sound pressures and confining pressures.
Fig. 8 shows the acoustic emissions from coal under different confining pressures with an effective sound pressure of 4.39 MPa. The red lines outline the destroyed areas and the white lines outline the areas of pressure damage. In Fig. 9, only the displacements of the surrounding rock under confining pressures of 0.2, 0.4, and 0.8 MPa are shown. It can be shown from the way the coal breaks that, because of the stress in the surrounding rock, the mode of damage in the coal gradually changes from tensile failure to compressive shear failure. The greater the stress surrounding the coal, the smaller the area of influence of the ultrasound cracking will be.
J. Zhang, Y. Li / International Journal of Mining Science and Technology 27 (2017) 221–228
Maximum principal stress (MPa)
20
Displacement curve (6.44MPa)
5
Maximum principal stress curve (6.44 MPa)
Maximum principal stress curve (4.39 MPa)
15
X-axial displacement (μ )
226
10
5
Maximum principal stress curve (2.64 MPa)
0
4 Displacement curve (4.39MPa)
3
Displacement curve (2.64 MPa)
2 1
0 10
20
30
40
10
50
20
30
40
Calculation time step
Calculation time step
(a) Maximum principal stresses versustime
(b) X-axis displacement versustime
50
Fig. 7. Graph showing maximum principal stresses versus time and X-axis displacement versus time for three different effective sound pressures (the sound pressures are shown in the box in the upper right corner of the graph).
Table 4 Numerical simulation experimental conditions. Scheme
Voltage (V)
Electric current (A)
Effective sound pressure (MPa)
Elastic modulus (GPa)
Compressive strength (MPa)
Confining pressure (MPa)
I
220
4.09
4.39
15
15
II III
220 220
5.91 6.82
5.56 6.44
15 15
15 15
0.2 0.4 0.8 3.2 3.2 3.2
(a) Confining pressure 0.2 MPa
(b) Confining pressure 0.4 MPa
(c) Confining pressure 0.8 MPa
(d) Confining pressure 3.2 MPa
Fig. 8. Maps showing the effect of different confining pressures on the acoustic emissions from coal under an effective sound pressure of 4.39 MPa (the red lines outline the destroyed areas; the white lines outline the areas of pressure damage).
8
Confining pressure 0.2 MPa
Y-axial displacement (μ )
X-axial displacement (μ )
0.6
Confining pressure 0.8 MPa
0.4
Confining pressure 0.4 MPa
0.2
6
4
2
0
0 0
50
100
150
200
250
Calculation time step
0
50
100
150
200
250
Calculation time step
Fig. 9. Graphs showing the effect of three different confining pressures on X- and Y-axis displacement during coal rock fracturing under an effective sound pressure of 4.39 MPa.
Therefore, it is necessary to increase the ultrasonic power to enlarge the area of cracked coal enough so that the amount of coal-bed gas collected can be increased. To analyze the inhibiting effect of the confining pressure on ultrasonic cracking, it is necessary to set the confining pressure to 3.2 MPa. The threshold for
effective sound pressure increases gradually but the observed acoustic emission rule of the coal rupture process can provide a basis for the selection of a reasonably effective sound pressure for the gas in a low permeability coal seam. The experimental results are shown in Figs. 10 and 11.
227
J. Zhang, Y. Li / International Journal of Mining Science and Technology 27 (2017) 221–228
(a) Sound pressure 4.39 MPa
(b) Sound pressure 5.56 MPa
(c) Sound pressure 6.44 MPa
(d) Sound pressure 4.39-6.44 MPa
Fig. 10. Maps showing numerically simulated acoustic emissions from coal samples being fractured by different sonic effective pressure at a constant confining pressure.
Sonic effective pressure 5.56 MPa
10
Sonic effective pressure 6.44 MPa
0.6
0.4 Sonic effective pressure 4.39 MPa
0.2
Y-axial displacement (μ )
X-axial displacement (μ )
0.8
8 6 4 2 0
0 0
50
100
150
200
0
50
100
150
200
Calculation time step
Calculation time step
Fig. 11. Graphs showing the effect of three different sonic pressures (4.30 MPa, 5.56 MPa, 6.44 MPa) on X- and Y-axis displacement during coal rock fracturing at a constant confining pressure of 3.2 MPa.
Fig. 10 shows diagrams for numerically simulated acoustic emissions from coal samples subjected to different sound pressures at constant confining pressure. Fig. 11 shows the X- and Y-axis displacement diagrams for the corresponding coal deformation. The major load in coal breakage is the dynamic load applied by the high frequency tapping on the coal body of the ultrasonic probe. Tensile failure can be regarded as the macroscopic manifestation of a crack opening and this can be represented by X-axis displacement. The slope of the Y-axis displacement curve reflects the slip friction of the coal during crack propagation. The more intense the tensile failure, the greater the X displacement; increased slope on the Y displacement curve is mainly caused by shear failure. The experimental results show that the deformation in the coal gradually changes from compressive shear failure to tensile failure after the ultrasonic pressure is increased. When the ultrasonic pressure is increased, a pressure shear failure zone appears in the coal at the end of the probe and the internal damage to the coal is mostly due to tensile failure. The position of the tensile rupture zone in the coal is far away from the probe and the crack area gradually forms continuous cracks. The acoustic emissions from the cracks in the coal can be well reflected by the ultrasonic wave, which can reflect the effect of the ultrasonic wave on the coal and how the acoustic wave propagates. Through numerical simulation of the effect of ultrasonic sound pressure on coal, the effective sound pressure was used to determine the working power of the ultrasonic transmitter. After using several different ultrasonic sound pressure schemes to crack coal, the way in which the cracks were produced and the way they expanded were used to determine the infiltration effect. Fig. 5 showed the mesh produced by crack initiation and continued crack development. Some aspects of acoustic emission variations during
the ultrasonic fracturing of coal led to the definition of the effective sound pressure and other important engineering parameters. These definitions further clarified the mechanisms by which ultrasonic waves crack coal and thus the mechanisms that may allow the permeability of coal to promote the extraction of coal-bed methane. 4. Conclusions The following conclusions can be obtained, based on the analysis of the crack propagation experimental results: (1) The experimental results on coal permeability show rapid crack development and connection after ultrasonic cavitation, and the average permeability increases by between 30% and 60%. Ultrasonic cavitation greatly increases the permeability of coal. (2) There is a close relationship between the acoustic emissions from coal and the stress generated by the passage of ultrasonic waves. The alternating change of the rate of change of the stresses and the acoustic emissions can increase the wave stress and cause the stress to converge at the tip of a crack in the coal, which can promote both fracture formation and crack propagation. (3) Coal rupture is reproduced well by the numerical simulations. The numerical results show that effective sound pressure is closely related to permeability and increasing effective sound pressure is beneficial to initial fracturing, crack production, and crack expansion and connection. (4) Changes in the acoustic emissions during ultrasonic fracturing of coal result in effective sound pressure change, which is an important engineering parameter for the propagation
228
J. Zhang, Y. Li / International Journal of Mining Science and Technology 27 (2017) 221–228
of ultrasonic waves in coal. This plays an important role in guiding the selection of the appropriate engineering parameters for successful coal infiltration.
Acknowledgments This research was supported by the National Natural Science Foundation of China (No. 51574114), and the National Key Research and Development Program (No. 2016YFC0600901). References [1] Liu XY, Zhang SM, Shi MH. Numerical analysis on single bubble dynamics equation of the real gas in ultrasonic cavitation. J Anhui Normal Univ 2010;33 (5):446–50. [2] Lu YG, Wu XH. Computational analysis of double-bubble ultrasonic cavitation. Acta Phys Sin 2011;60(4):1–5. [3] Li HX. Experimental research on the anti-scale and scale-removal by ultrasonic cavitation and anti-scale and scale-removal and enhancing heat transfer by hydrodynamic cavitation. Beijing: Chinese Academy of Sciences; 2009. p. 16–30. [4] Ma KJ, Jia DZ, Sun WL, Bao WZ, Zhao WX, Jin DM. Mass transfer induced by coalescence and slip of ultrasonic cavitation bubbles at phase interface. Tech Acoust 2009;28(6):742–6. [5] Mo RY, Lin SY, Wang CH. Methods of study on sound cavitation. Appl Acoust 2009;28(5):389–400. [6] Wang CH, Lin SY. The nonlinear oscillation of bubbles in the ultrasonic field. Chin J Theor Appl Mech 2010;42(6):1050–9. [7] Jimenez-Fernandez J, Crespo A. Bubble oscillation and inertial cavitation in viscoelastic fluids. Ultrasonics 2005;43(8):643–51. [8] Ilinskii YA, Wilson PS, Hamilton MF. Bubble growth by rectified diffusion at high gas supersaturation levels. Acoust Soc Am 2008;124(4):1950–5. [9] Yang Y, Zong FD. Influence of exciting parameters on the vibration of single cavitation bubble driven by intensive sound. Chin J Theor Appl Mech 2009;41 (1):8–14. [10] Huang W, Chen WZ, Liu YN. The evolution of the cavitation bubble driven by different sound pressure. Ultrasonics 2006;44(Suppl. 1):407–10. [11] Jiang YD, Xian XF, Yi J. Experimental and mechanism of acoustic shock method promote methane desorption law. J Chin Coal Soc 2008;33(6):675–80. [12] Li JL, Yan JP, Wang LB. Study of coal gas emission characteristics under the action of sound wave field. J Chin Coal Soc 2012;37(2):264–7.
[13] Li SG, Zhao Y, Zhang TJ. The effect of low frequency mechanical vibration on coal sample adsorption characteristics. J Chin Univ Min Technol 2012;41 (6):873–7. [14] Klobes P, Riesemier H, Meyer K, Goebbels J, Hellmuth KH. Rock porosity determination by combination of X-ray computerized tomography with mercury porosimetry. J Anal Chem 1997;357(5):543–7. [15] Goodwin AK, O’Nell MA, Anderson WF. The use of X-ray computer topography to investigate particulate interactions within opencast coal mine backfills. Eng Geol 2003;70(3–4):331–41. [16] Ren WJ, Yuan XD, Pan YS. Experimental study on the influence of power ultrasound on the mechanical properties of coal and rock. J Liaoning Tech Univ 2001;20(6):773–6. [17] Yu YJ, Zhang CH, Wang LG. Mechanism of improving coal bed gas drainage rate by ultrasonic interference. J Liaoning Tech Univ 2008;6(27):805–8. [18] Shao CJ, Luo RZ, Sun RY. Basic research on improving oil production by using physical field. Petrol J 1998;18(3):63–70. [19] Liu Q, Guo YS, An FH, Lin LY, Lai YM. Water blocking effect caused by the use of hydraulic methods for permeability enhancement in coal seams and methods for its removal. Int J Min Sci Technol 2016;26(4):615–21. [20] Xiang WY, Lu YY, Li XG. Experimental study on the effect of cavitation jet on rock fragmentation. Rock Soil Mech 2006;9(27):1505–8. [21] Ge ZL, Lu YY, Zhou DP. Study on the law and mechanism of the effect of cavitation water jet acoustic shock effect on methane desorption. J Chin Coal Soc 2011;7(36):1150–5. [22] Lu YY, Wang JH, Huang F. Improvement of the methane desorption and percolation flow test device by the acoustic shock effect of the cavitation water jet. J Chin Coal Soc 2013;9(38):1604–10. [23] Ni XM, Li QZ, Wang YB, Sha S. Permeability variation characteristics of coal after injecting carbon dioxide into a coal seam. Int J Min Sci Technol 2015;25 (4):665–70. [24] Lu YY, Ding H, Ge ZL. The effect of thermal effect of cavitation water jet on coal permeability. Rock Soil Mech 2014;5(35):1247–54. [25] Vladimir OA, Marat SM, Anna VA, Lgor BE, Timothy JM. Ultrasonic technology for enhanced oil recovery from failing oil wells and the equipment for its implemention. Ultrason Sonochem 2013;20(5):1289–95. [26] Emad A, Mohammad A, Mohammad A. Experimental application of ultrasound waves to improved oil recovery during water flooding. J King Saud Univ 2014;26(1):103–10. [27] Liu HJ, An Y. Pressure distribution outside a single cavitation bubble. Acta Phys Sin 2004;53(5):1406–12. [28] Tseng CC. Turbulence and cavitation models for time-dependent turbulent cavitating flows. Acta Mech Sin 2011;27(4):473–87. [29] Huang JT, Zhang HS. Level set method for numerical simulation of a cavitation bubble, its growth, collapse and rebound near a rigid wall. Acta Mech Sin 2007;23(6):645–53.
Contents lists available at ScienceDirect
International Journal of Mining Science and Technology journal homepage: www.elsevier.com/locate/ijmst
Ultrasonic vibrations and coal permeability: Laboratory experimental investigations and numerical simulations Zhang Junwen a,⇑, Li Yulin a,b a b
Heilongjiang University of Science and Technology, School of Mining Engineering, Harbin 150022, China China University of Mining and Technology, School of Mechanics and Architectural Engineering, Beijing 100083, China
a r t i c l e
i n f o
Article history: Received 2 April 2015 Received in revised form 12 July 2015 Accepted 2 September 2015 Available online 26 January 2017 Keywords: Ultrasonic cavitation Rock-coal Coal fracture Permeability Experimental analysis
a b s t r a c t Ultrasonic vibrations in coal lead to cavitation bubble oscillation, growth, shrinkage, and collapse, and the strong vibration of cavitation bubbles not only makes coal pores break and cracks propagate, but plays an important role in enhancing the permeability of coal. In this paper, the influence of ultrasonic cavitation on coal and the effects of the sonic waves on crack generation, propagation, connection, as well as the effect of cracks on the coal permeability, are studied. The experimental results show that cracks in coal are generated even connected rapidly after ultrasonic cavitation. Under the effect of ultrasonic cavitation, the permeability increases between 30% and 60%, and the number of cracks in coal also significantly increased. Numerical experiments show that the effective sound pressure is beneficial to fracture propagation and connection, and it is closely related to the permeability. Moreover, the numerical simulations and physical experiments provide a guide for the coal permeability improvement. Ó 2017 Published by Elsevier B.V. on behalf of China University of Mining & Technology. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).
1. Introduction Ultrasonic waves can promote mass transfer and accelerate chemical reactions [1–5]. Ultrasonic waves have been applied in hydraulic drilling and industrial cleaning. In coal, these waves cause cavitation bubbles to oscillate, grow, shrink, and collapse, and the strong bubble vibrations cause the pores in the coal to break and cracks to propagate, which play an important role in enhancing the coal’s permeability. Ultrasonic cavitation refers to the activation of the micro-bubble’s nucleus by ultrasonic waves in a liquid [6–10]. This activation is the dynamic oscillation, growth, shrinkage, and collapse of the bubble nucleus. Many investigators have conducted research on ultrasonic cavitation and the research has embraced both different physical research methods and mathematical modeling. Jiang et al. [11] carried out an experiment to study acoustic vibrations to promote methane desorption from coal. Li et al. [12] carried out research on coal-bed methane desorption and emissions under the action of sound waves and obtained coal methane gas desorption and diffusion parameters. Li et al. [13] developed a mechanical coal sorption/desorption test system and used it to study coal adsorption under low frequency vibrations. Klobes et al. [14] determined rock porosity with a combination of X-ray computerized ⇑ Corresponding author. E-mail address: [email protected] (J. Zhang).
tomography and mercury porosimetry. Goodwin et al. [15] investigated particulate interactions within opencast coal mine backfill by the use of X-ray computer tomography. Ren et al. [16] performed an experimental study on the effect of power ultrasound on coal and rock. That study looked at crack generation and development and the change in the stress state and the mechanical properties of the coal and rock. The study also presented a qualitative analysis of the changes in compressive strength and elastic modulus of coal and rock under ultrasonic vibration. This study provided an experimental basis for the changes of coal mechanical properties caused by cracks which are generated by ultrasonic vibration. Jiang et al. [11] studied the desorption of methane gas in coal under ultrasonic waves and the thermal effect of the waves. Li [12] using a selfdeveloped acoustic field coal gas desorption device, established a physical model for coal gas emissions and monitored the emission of total pressure, gas pressure, and gas emission rates. Nie et al. analyzed the influence of changes in the porosity and permeability of coal caused by power ultrasonic waves on coal-bed methane. Yu et al. [17] explored ultrasonic interference for improving the permeability of coal reservoirs and increasing the coal-bed methane desorption rate. Using ultrasonic waves at 20 kHz, Shao et al. [18] studied the effect of ultrasound on rock permeability. A number of other investigations conducted by Li et al. have looked at how to solve the problems of coal bed micro cracks, low permeability, high gas adsorption, and the difficulty of draining methane by using cavitation water jet acoustic shocks to improve methane
http://dx.doi.org/10.1016/j.ijmst.2017.01.001 2095-2686/Ó 2017 Published by Elsevier B.V. on behalf of China University of Mining & Technology. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).
222
J. Zhang, Y. Li / International Journal of Mining Science and Technology 27 (2017) 221–228
desorption and seepage flow [19–23]. Lu et al. [24] determined how temperature affects coal permeability by studying the thermal effects produced at the moment of cavitation bubble breaking. They also studied the cavitation noise sound field produced during tests of coal temperature and tested methane permeability under different cavitation conditions (pump pressure and cavitation pressure). Abramov et al. [25] found that when used on oil wells with a permeability greater than 20 md and a porosity of more than 15%, ultrasonic technology could increase oil production by 30–50% or more. Another experimental study, by Alhomadhi et al. [26] was done to investigate improving oil recovery by using ultrasonic waves during water flooding. Many previous studies have focused on the effects of ultrasonic waves on coal methane desorption and diffusion, while less attention has been paid to research the effects of sound waves on crack propagation and permeability in coal. The way in which strong ultrasonic cavitation enhances permeability in coal is still not very clear. In this paper, the method used combines theoretical analysis with physical experiments to analyze cavitation bubble oscillation, growth, shrinkage, and collapse, and discuss ultrasonic cavitation in coal. The experiments determined the permeability of the coal and used computed tomography (CT) to study coal crack generation, propagation, and connectivity along with the changes in permeability under ultrasonic cavitation. In addition, stress wave loading reproduced ruptures in the coal caused by ultrasonic mechanical effects. The study showed that sound pressure promotes crack propagation and connection. The experimental results can provide a foundation for the study of ultrasonic cavitation rupture to enhance coal permeability.
2. Ultrasonic cavitation experiments on coal rupture and permeability enhancement
0.02–0.90 m thick. The longwall face on the No. 3 bed is about 350 m long. The coal samples used for this study were rectangular blocks 50 mm on each side and 100 mm long. The purpose of the experiment was to study the effect of ultrasonic waves on the evolution of the cracks in the coal and the ultrasound’s effect on the coal permeability. Some of the coal samples used for these experiments are shown in Fig. 1. 2.2. Experimental apparatus The device used for ultrasonic excitation of the coal was a ZJS500N type ultrasonic generator, which will work in an indoor environment if the humidity is less than or equal to 85% relative humidity and the temperature is between 0 °C and 40 °C. The specific components of the ultrasonic device include: (1) an ultrasonic vibration source (drive power): this converts the 50–60 Hz current to a higher power and frequency (15–100 kHz) to supply the transducer; (2) a transducer (controller transducer): this converts the high frequency electric current into mechanical vibration energy; (3) an amplitude transformer: connected to the fixed transducer and the tool head, this device amplifies the vibrations from the transducer and sends them to the tool head; (4) the tool head (guide rod): this transfers the mechanical energy and the pressure to the sample. It also independently increases the amplitude; (5) connecting bolts: these connect the components mentioned above. The three axis penetration apparatus and ultrasonic probe component connect the ultrasonic generator to the experimental equipment. The three axis penetration instrument, the precision digital flow meter, the ZJS-500N ultrasonic generator, and a schematic diagram of experimental assembly used for the experiments are shown in Fig. 2. 2.3. Experimental design
2.1. Sample description Coal samples used in these experiments were collected from the Jincheng Tiandiwangpo No. 3 coal bed, Shanxi Province, China. The coal mines there are high methane mines but the excavated coal is not prone to spontaneous combustion. The Wangpo coal mine is located in the southern Taihang Mountain area, the watershed of the Qin and Long Rivers. The area is moderately mountainous and dissected by gullies. Terrain slopes are more than 20–30°. The highest areas in the Wangpo mine are in the northeast central area, the lowest are in the north, south, and east areas. The highest topographic point is on the northeast ridge at an elevation of 1327.5 m, the low point is in the Long River valley, elevation 877.2 m. The No. 3 coal bed is hosted by the lower Permian Shanxi Formation in the lower part of the mine. The No. 3 coal bed ranges from 4.10 to 6.70 m in thickness (averaging 5.76 m) and it has simple structure only containing 0–2 layers of carbonaceous shale
To analyze the effects of ultrasonic cavitation, confining pressure, and pore pressure on the coal’s permeability, the experiments were run under two sets of experimental conditions. First, the experiments were run both with and without ultrasound with the confining pressure set to 4.0 MPa and pore pressures of 0.2, 0.3, 0.4, 0.5, 0.6, 0.8, and 0.7 MPa [27,28]. Then, the confining pressure was increased to 12 MPa and the experiments were run again at pore pressures of 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8, 1.1, and 1.6 MPa. The experimental scheme is shown in Table 1. 2.4. Ultrasonic coal permeability analysis A triaxial permeability measuring system, based on the principle of a steady state method, was used to determine the permeability of the coal samples to compare the permeability of the samples before and after ultrasonic treatment. According to
Fig. 1. Photographs of some of the coal samples used in the experiments.
223
J. Zhang, Y. Li / International Journal of Mining Science and Technology 27 (2017) 221–228
(a) Three axis penetration instrument and ultrasonic probe
(b) Precision digital flow meter Six way valve
Six way valve
High pressure regulating valve
Pressure gauge Confining pressure inlet
Ultrasonic probe
Three axis penetration instrument
Gas cylinders
(c) Ultrasonic generator
Axial compression inlet Pressure pump
Collection device
Power
Ultrasonic generator
(d) Schematic diagramof experimental assembly
Fig. 2. Ultrasonic equipment and schematic diagram of the experimental apparatus.
Table 1 Experimental conditions. Project
Confining pressure (MPa)
Pore pressure (MPa)
Without ultrasound Ultrasonic Without ultrasound Ultrasonic
4.0 4.0 12.0 12.0
0.2 0.2 0.2 0.2
0.3 0.3 0.3 0.3
0.4 0.4 0.4 0.4
Permeability measured experiment 0.5 0.5 0.5 0.5
0.6 0.6 0.6 0.6
0.7 0.7 0.7 0.7
0.8 0.8 0.8 0.8
1.1 1.1
Comparison of permeability of coal samples before and after ultrasonic test Comparison of permeability of coal samples before and after ultrasonic test
1.6 1.6
0.9
No ultrasonic cavitate
Permeability (µm2 )
Permeability (µm2)
0.26
Ultrasonic cavitate
0.8 0.7 0.6 0.5 0.4 0.3
0.22 0.18 0.14 0.10 0.06
0.2
0.3
0.4
0.5
0.6
0.7
0.8
Pore pressure (MPa) (a) At a confining pressure of 4 MPa
0
0.2
0.4
0.6
0.8
1.0
1.2
1.4
1.6
1.8
Pore pressure (MPa) (b) At a confining pressure of 12 MPa
Fig. 3. Graph showing the relationship between permeability and pore pressure at confining pressure 4 MPa and 12 MPa.
the experiments, the permeability of the coal after ultrasonic cavitation was greatly improved. When the confining pressure was 4 MPa and the pore pressure was increased from 0.2 to 0.8 MPa, the permeability of coal was between 0.68 and 0.29 lm2. However, after ultrasonic cavitation, the permeability of coal for those same pore pressures was between 0.83 and 0.42 lm2 (Fig. 3a). When the confining pressure was 12 MPa and the pore pressure was increased from 0.2 to 1.6 MPa, the permeability of coal before
ultrasonic cavitation was between 0.21 and 0.10 lm2, but after ultrasonic cavitation the permeability ranged from 0.27 to 0.17 lm2 (Fig. 3b). These coal permeability experiments show that, under the same conditions, when confining pressure increases, the permeability decreases significantly. The confining pressure effect on crack pore squeezing is obvious. After ultrasonic cavitation, coal permeability improves significantly and the reason is that the coal inter-crack has been fully developed and broken through [29],
224
J. Zhang, Y. Li / International Journal of Mining Science and Technology 27 (2017) 221–228
resulting in a significant permeability increase. This phenomenon shows that the ultrasonic cavitation effect on the coal favored crack extension and this improved the permeability.
matched those of the coal samples used for the physical experiments. Those mechanical parameters and the associated control conditions are shown in Table 3.
3. Numerical simulations of ultrasonic rupture in coal to enhance permeability
3.3. Analysis of effective sound pressure
3.1. Numerical model and experimental design To analyze the effects of ultrasonic waves on cracks produced during the physical experiments in more detail, numerical simulations were employed to study crack growth related to acoustic energy emissions. The numerical model was the same size (50 mm 50 mm 100 mm) as the physical coal sample, (Fig. 4) and the geometric model was divided into 20,000 cells. The Weibull distribution was used to represent sample in homogeneity and the Moire-Kulun strength theory was chosen to act as a sample failure criterion. The effect of ultrasonic waves on a crack in the coal was represented by the effective sound pressure, the stress wave method was applied to the load, and the waveform chosen was a sine wave. Table 2 presents the conditions used for the numerical experiments. 3.2. Coal mechanical parameters and control conditions To make the results of the numerical experiments applicable, the mechanical parameters chosen for the numerical simulations Ultrasonic probe
The numerical experiments were run to determine the shape of the Y-axis stress curve with an increasing acoustic emission load. The results are shown in Fig. 5. Fig. 5 shows that, at the initial stage of loading, the acoustic emission frequency was very low and, because of the viscous nature of the coal, the Y-axial stress is attenuated. That is because the closely spaced and new generation of crack-fractures and the generation of acoustic emissions all need to absorb energy at this stage, while the energy accumulation remains in local dynamic equilibrium, so that the acoustic emission frequency is not very high. However, when the energy accumulates and the stress concentrations reach a specific threshold, acoustic emissions will be produced as the crack-fractures begin to extend. With the increasing number of acoustic emissions, energy is gradually released and the value of the Y-axis stress also increases gradually from its low initial state. The curves for the acoustic emission frequency and the Y-axis stress are nearly parallel and it can be seen that when the rate of change of stress accelerates, the number of acoustic emissions also increases. The frequency of sound emissions moderates when the stress rate decreases. As the stress increases and decreases and energy emitted by the acoustic emissions changes, the stresses in the crack-fracture area gradually converge, thus promoting the initiation and propagation of crack-fractures.
100 mm
3.4. Analysis of crack extension and coal deformation
50 mm
Fig. 4. Diagram showing the numerical simulation model.
Table 2 Conditions for the numerical simulations. Scheme
Voltage (V)
Electric current (A)
Effective sound pressure (MPa)
I II III IV V
220 220 220 220 220
4.09 5.91 6.82 8.64 10.00
2.64 3.81 4.39 5.56 6.44
Table 3 Mechanical parameters and control conditions for the numerical model. Parameter
Numerical value
Coefficient of homogeneity Mean value of elastic modulus (GPa) Mean value of compressive strength (MPa) Density (kg/m3) Poisson ratio Internal friction angle (°) Time step (s)
5 15 13 1350 0.25 37 2 10
7
The numerical simulations can produce maximum principal stress gradient maps showing the changes in maximum principal stress as the experiment progresses. Data from the simulations can also be used to generate the corresponding maximum principal stress versus time curves and the X-axis displacement versus time curves. These maps and graphs were generated from the simulation results for effective sound pressures of 2.64, 4.39, and 6.44 MPa and the resulting maps and graphs are shown in Figs. 6 and 7. In Fig. 6, blue represent coal; red, orange, and green indicate stress from high to low, and empty is a crack. Fig. 6 shows that with an increase in the effective sound pressure, the crack in the coal is formed in advance and the crack breakthrough increases. Fig. 7a shows that the maximum principal stress increases with an increase in the effective sound pressure and the maximum principal stress fluctuation is ahead of time, that is, the mutation rate of stress changes in advance. In the phase where the maximum principal stress suddenly changes, which is the frequency phase of acoustic emissions, the acoustic emission is produced by a frequency-generated surface crack-fracture which is initiated and gradually expands to breakthrough. In Fig. 7b, the X-axis displacement directly reflects the total width of cracks plus fractures and this sum increases at greater effective sound pressures. The Xaxis displacement curve changes from linear to nonlinear at higher sound pressures and the higher the effective sound pressure, the faster the displacement rate increases. That is, the faster the crack-fractures propagate, the more consistent the data shown in Fig. 7b are with the experimental data from the CT observations. Fig. 7a and b shows that, when the ultrasonic generator frequency has been determined, ultrasonic cavitation is a feasible tool that can be used effectively to improve the permeability of a coal-bed methane reservoir by increasing the ultrasonic power to an effective sound pressure level.
225
1.2
200
0.6
100
20
30
40
50
200 1.2 100
0
60
(a) Effective sound pressure 2.64 MPa
2.4 200
10
20
30
1.2 0.6
40
0
50
0
5 300 4 200
3 2
100 1 0 20
10
30
20
30
40
Calculation time step (c) Effective sound pressure 4.39 MPa
300
6
10
1.8
100
(b) Effective sound pressure 3.81 MPa
400
0
3.0 300
Calculation time step
Calculation time step
Acoustic emission frequency
0.6
Acoustic emission frequency
10
1.8
3.6
0
0 0
2.4
300
400
Stress (MPa)
300
400
3.0
4.8 4.2 3.6
200
3.0 2.4 1.8
100
Stress (MPa)
1.8
3.6
Acoustic emission frequency
Y-axial stress curve
500
Stress (MPa)
2.4
Stress (MPa)
400
Acoustic emission frequency
Stress (MPa)
500
Acoustic emission frequency
Acoustic emission frequency
J. Zhang, Y. Li / International Journal of Mining Science and Technology 27 (2017) 221–228
1.2 0.6 0
10
20
30
0
Calculation time step
Calculation time step
(e) Effective sound pressure 6.44 MPa
(d) Effective sound pressure 5.56 MPa
Fig. 5. Acoustic emission and Y-axial stress curves for sound pressures of 2.64, 3.81, 4.39, 5.56, and 6.44 MPa.
(a) Effective sound pressure 2.64 MPa
(b) Effective sound pressure 4.39 MPa
(c) Effective sound pressure 6.44 MPa
Fig. 6. Stress gradient maps for effective sound pressures at (a) 2.64 MPa, (b) 4.39 MPa, and (c) 6.44 MPa.
3.5. Analysis of crack propagation in coal under confining pressure A numerical analysis of crack generation in coal under the combined influence of sound pressure and the surrounding rock stress was carried out. The influence of wall rock stress on crack initiation and propagation was done as part of the investigation of power ultrasonic cracking of coal to improve coal-bed methane extraction from the actual coal seam. Table 4 presents experimental conditions for the numerical simulations under the effects of different sound pressures and confining pressures.
Fig. 8 shows the acoustic emissions from coal under different confining pressures with an effective sound pressure of 4.39 MPa. The red lines outline the destroyed areas and the white lines outline the areas of pressure damage. In Fig. 9, only the displacements of the surrounding rock under confining pressures of 0.2, 0.4, and 0.8 MPa are shown. It can be shown from the way the coal breaks that, because of the stress in the surrounding rock, the mode of damage in the coal gradually changes from tensile failure to compressive shear failure. The greater the stress surrounding the coal, the smaller the area of influence of the ultrasound cracking will be.
J. Zhang, Y. Li / International Journal of Mining Science and Technology 27 (2017) 221–228
Maximum principal stress (MPa)
20
Displacement curve (6.44MPa)
5
Maximum principal stress curve (6.44 MPa)
Maximum principal stress curve (4.39 MPa)
15
X-axial displacement (μ )
226
10
5
Maximum principal stress curve (2.64 MPa)
0
4 Displacement curve (4.39MPa)
3
Displacement curve (2.64 MPa)
2 1
0 10
20
30
40
10
50
20
30
40
Calculation time step
Calculation time step
(a) Maximum principal stresses versustime
(b) X-axis displacement versustime
50
Fig. 7. Graph showing maximum principal stresses versus time and X-axis displacement versus time for three different effective sound pressures (the sound pressures are shown in the box in the upper right corner of the graph).
Table 4 Numerical simulation experimental conditions. Scheme
Voltage (V)
Electric current (A)
Effective sound pressure (MPa)
Elastic modulus (GPa)
Compressive strength (MPa)
Confining pressure (MPa)
I
220
4.09
4.39
15
15
II III
220 220
5.91 6.82
5.56 6.44
15 15
15 15
0.2 0.4 0.8 3.2 3.2 3.2
(a) Confining pressure 0.2 MPa
(b) Confining pressure 0.4 MPa
(c) Confining pressure 0.8 MPa
(d) Confining pressure 3.2 MPa
Fig. 8. Maps showing the effect of different confining pressures on the acoustic emissions from coal under an effective sound pressure of 4.39 MPa (the red lines outline the destroyed areas; the white lines outline the areas of pressure damage).
8
Confining pressure 0.2 MPa
Y-axial displacement (μ )
X-axial displacement (μ )
0.6
Confining pressure 0.8 MPa
0.4
Confining pressure 0.4 MPa
0.2
6
4
2
0
0 0
50
100
150
200
250
Calculation time step
0
50
100
150
200
250
Calculation time step
Fig. 9. Graphs showing the effect of three different confining pressures on X- and Y-axis displacement during coal rock fracturing under an effective sound pressure of 4.39 MPa.
Therefore, it is necessary to increase the ultrasonic power to enlarge the area of cracked coal enough so that the amount of coal-bed gas collected can be increased. To analyze the inhibiting effect of the confining pressure on ultrasonic cracking, it is necessary to set the confining pressure to 3.2 MPa. The threshold for
effective sound pressure increases gradually but the observed acoustic emission rule of the coal rupture process can provide a basis for the selection of a reasonably effective sound pressure for the gas in a low permeability coal seam. The experimental results are shown in Figs. 10 and 11.
227
J. Zhang, Y. Li / International Journal of Mining Science and Technology 27 (2017) 221–228
(a) Sound pressure 4.39 MPa
(b) Sound pressure 5.56 MPa
(c) Sound pressure 6.44 MPa
(d) Sound pressure 4.39-6.44 MPa
Fig. 10. Maps showing numerically simulated acoustic emissions from coal samples being fractured by different sonic effective pressure at a constant confining pressure.
Sonic effective pressure 5.56 MPa
10
Sonic effective pressure 6.44 MPa
0.6
0.4 Sonic effective pressure 4.39 MPa
0.2
Y-axial displacement (μ )
X-axial displacement (μ )
0.8
8 6 4 2 0
0 0
50
100
150
200
0
50
100
150
200
Calculation time step
Calculation time step
Fig. 11. Graphs showing the effect of three different sonic pressures (4.30 MPa, 5.56 MPa, 6.44 MPa) on X- and Y-axis displacement during coal rock fracturing at a constant confining pressure of 3.2 MPa.
Fig. 10 shows diagrams for numerically simulated acoustic emissions from coal samples subjected to different sound pressures at constant confining pressure. Fig. 11 shows the X- and Y-axis displacement diagrams for the corresponding coal deformation. The major load in coal breakage is the dynamic load applied by the high frequency tapping on the coal body of the ultrasonic probe. Tensile failure can be regarded as the macroscopic manifestation of a crack opening and this can be represented by X-axis displacement. The slope of the Y-axis displacement curve reflects the slip friction of the coal during crack propagation. The more intense the tensile failure, the greater the X displacement; increased slope on the Y displacement curve is mainly caused by shear failure. The experimental results show that the deformation in the coal gradually changes from compressive shear failure to tensile failure after the ultrasonic pressure is increased. When the ultrasonic pressure is increased, a pressure shear failure zone appears in the coal at the end of the probe and the internal damage to the coal is mostly due to tensile failure. The position of the tensile rupture zone in the coal is far away from the probe and the crack area gradually forms continuous cracks. The acoustic emissions from the cracks in the coal can be well reflected by the ultrasonic wave, which can reflect the effect of the ultrasonic wave on the coal and how the acoustic wave propagates. Through numerical simulation of the effect of ultrasonic sound pressure on coal, the effective sound pressure was used to determine the working power of the ultrasonic transmitter. After using several different ultrasonic sound pressure schemes to crack coal, the way in which the cracks were produced and the way they expanded were used to determine the infiltration effect. Fig. 5 showed the mesh produced by crack initiation and continued crack development. Some aspects of acoustic emission variations during
the ultrasonic fracturing of coal led to the definition of the effective sound pressure and other important engineering parameters. These definitions further clarified the mechanisms by which ultrasonic waves crack coal and thus the mechanisms that may allow the permeability of coal to promote the extraction of coal-bed methane. 4. Conclusions The following conclusions can be obtained, based on the analysis of the crack propagation experimental results: (1) The experimental results on coal permeability show rapid crack development and connection after ultrasonic cavitation, and the average permeability increases by between 30% and 60%. Ultrasonic cavitation greatly increases the permeability of coal. (2) There is a close relationship between the acoustic emissions from coal and the stress generated by the passage of ultrasonic waves. The alternating change of the rate of change of the stresses and the acoustic emissions can increase the wave stress and cause the stress to converge at the tip of a crack in the coal, which can promote both fracture formation and crack propagation. (3) Coal rupture is reproduced well by the numerical simulations. The numerical results show that effective sound pressure is closely related to permeability and increasing effective sound pressure is beneficial to initial fracturing, crack production, and crack expansion and connection. (4) Changes in the acoustic emissions during ultrasonic fracturing of coal result in effective sound pressure change, which is an important engineering parameter for the propagation
228
J. Zhang, Y. Li / International Journal of Mining Science and Technology 27 (2017) 221–228
of ultrasonic waves in coal. This plays an important role in guiding the selection of the appropriate engineering parameters for successful coal infiltration.
Acknowledgments This research was supported by the National Natural Science Foundation of China (No. 51574114), and the National Key Research and Development Program (No. 2016YFC0600901). References [1] Liu XY, Zhang SM, Shi MH. Numerical analysis on single bubble dynamics equation of the real gas in ultrasonic cavitation. J Anhui Normal Univ 2010;33 (5):446–50. [2] Lu YG, Wu XH. Computational analysis of double-bubble ultrasonic cavitation. Acta Phys Sin 2011;60(4):1–5. [3] Li HX. Experimental research on the anti-scale and scale-removal by ultrasonic cavitation and anti-scale and scale-removal and enhancing heat transfer by hydrodynamic cavitation. Beijing: Chinese Academy of Sciences; 2009. p. 16–30. [4] Ma KJ, Jia DZ, Sun WL, Bao WZ, Zhao WX, Jin DM. Mass transfer induced by coalescence and slip of ultrasonic cavitation bubbles at phase interface. Tech Acoust 2009;28(6):742–6. [5] Mo RY, Lin SY, Wang CH. Methods of study on sound cavitation. Appl Acoust 2009;28(5):389–400. [6] Wang CH, Lin SY. The nonlinear oscillation of bubbles in the ultrasonic field. Chin J Theor Appl Mech 2010;42(6):1050–9. [7] Jimenez-Fernandez J, Crespo A. Bubble oscillation and inertial cavitation in viscoelastic fluids. Ultrasonics 2005;43(8):643–51. [8] Ilinskii YA, Wilson PS, Hamilton MF. Bubble growth by rectified diffusion at high gas supersaturation levels. Acoust Soc Am 2008;124(4):1950–5. [9] Yang Y, Zong FD. Influence of exciting parameters on the vibration of single cavitation bubble driven by intensive sound. Chin J Theor Appl Mech 2009;41 (1):8–14. [10] Huang W, Chen WZ, Liu YN. The evolution of the cavitation bubble driven by different sound pressure. Ultrasonics 2006;44(Suppl. 1):407–10. [11] Jiang YD, Xian XF, Yi J. Experimental and mechanism of acoustic shock method promote methane desorption law. J Chin Coal Soc 2008;33(6):675–80. [12] Li JL, Yan JP, Wang LB. Study of coal gas emission characteristics under the action of sound wave field. J Chin Coal Soc 2012;37(2):264–7.
[13] Li SG, Zhao Y, Zhang TJ. The effect of low frequency mechanical vibration on coal sample adsorption characteristics. J Chin Univ Min Technol 2012;41 (6):873–7. [14] Klobes P, Riesemier H, Meyer K, Goebbels J, Hellmuth KH. Rock porosity determination by combination of X-ray computerized tomography with mercury porosimetry. J Anal Chem 1997;357(5):543–7. [15] Goodwin AK, O’Nell MA, Anderson WF. The use of X-ray computer topography to investigate particulate interactions within opencast coal mine backfills. Eng Geol 2003;70(3–4):331–41. [16] Ren WJ, Yuan XD, Pan YS. Experimental study on the influence of power ultrasound on the mechanical properties of coal and rock. J Liaoning Tech Univ 2001;20(6):773–6. [17] Yu YJ, Zhang CH, Wang LG. Mechanism of improving coal bed gas drainage rate by ultrasonic interference. J Liaoning Tech Univ 2008;6(27):805–8. [18] Shao CJ, Luo RZ, Sun RY. Basic research on improving oil production by using physical field. Petrol J 1998;18(3):63–70. [19] Liu Q, Guo YS, An FH, Lin LY, Lai YM. Water blocking effect caused by the use of hydraulic methods for permeability enhancement in coal seams and methods for its removal. Int J Min Sci Technol 2016;26(4):615–21. [20] Xiang WY, Lu YY, Li XG. Experimental study on the effect of cavitation jet on rock fragmentation. Rock Soil Mech 2006;9(27):1505–8. [21] Ge ZL, Lu YY, Zhou DP. Study on the law and mechanism of the effect of cavitation water jet acoustic shock effect on methane desorption. J Chin Coal Soc 2011;7(36):1150–5. [22] Lu YY, Wang JH, Huang F. Improvement of the methane desorption and percolation flow test device by the acoustic shock effect of the cavitation water jet. J Chin Coal Soc 2013;9(38):1604–10. [23] Ni XM, Li QZ, Wang YB, Sha S. Permeability variation characteristics of coal after injecting carbon dioxide into a coal seam. Int J Min Sci Technol 2015;25 (4):665–70. [24] Lu YY, Ding H, Ge ZL. The effect of thermal effect of cavitation water jet on coal permeability. Rock Soil Mech 2014;5(35):1247–54. [25] Vladimir OA, Marat SM, Anna VA, Lgor BE, Timothy JM. Ultrasonic technology for enhanced oil recovery from failing oil wells and the equipment for its implemention. Ultrason Sonochem 2013;20(5):1289–95. [26] Emad A, Mohammad A, Mohammad A. Experimental application of ultrasound waves to improved oil recovery during water flooding. J King Saud Univ 2014;26(1):103–10. [27] Liu HJ, An Y. Pressure distribution outside a single cavitation bubble. Acta Phys Sin 2004;53(5):1406–12. [28] Tseng CC. Turbulence and cavitation models for time-dependent turbulent cavitating flows. Acta Mech Sin 2011;27(4):473–87. [29] Huang JT, Zhang HS. Level set method for numerical simulation of a cavitation bubble, its growth, collapse and rebound near a rigid wall. Acta Mech Sin 2007;23(6):645–53.