- Email: [email protected]
Experimental investigation of crack dynamic evolution induced by pulsating hydraulic fracturing in coalbed methane reservoir1
Journal of Natural Gas Science and Engineering 75 (2020) 103159
Contents lists available at ScienceDirect
Journal of Natural Gas Science and Engineering journal homepage: http://www.elsevier.com/locate/jngse
Experimental investigation of crack dynamic evolution induced by pulsating hydraulic fracturing in coalbed methane reservoir1 Jingjing Wu a, Shaohe Zhang b, Han Cao b, *, Mingming Zheng c, **, Feilong Qu d, Canwei Peng d a
College of Civil Engineering, Hunan University of Technology, Zhuzhou, 412007, China Key Laboratory of Metallogenic Prediction of Nonferrous Metals and Geological Environment Monitoring Ministry of Education, School of Geoscience and Infophysics, Central South University, Changsha, 410083, China c State Key Laboratory of Geohazard Prevention and Geoenvironment Protection, Chengdu University of Technology, Chengdu, Sichuan, 610059, China d Zhongye Changtian International Engineering Co., Ltd, Changsha, 410205, China b
A R T I C L E I N F O
A B S T R A C T
Keywords: Pulsating hydraulic fracturing Coalbed methane reservoir Pulsating frequency Crack initiation Crack propagation Acoustic emission
To evaluate the crack initiation and propagation behavior during pulsating hydraulic fracturing (PHF) process, lab tests on PHF of coal samples were performed by using a self-developed true triaxial testing system. The results demonstrate that: the crack could be better developed under pulsating hydraulic load. And as PF increase, the initiation pressure decreases firstly, then increases and reached minimum when the PF is 4 Hz. Besides, the changes of acoustic emission (AE) behavior can be divided into four stages: quiet phase, acceleration phase, lifting phase and resting phase. And the accumulated energy of acceleration and lifting phase increased respectively about 4.81 and 17.21 times (average) in comparison with quiet phase. Our results also show that the proportion of minor AE events and great AE events were higher in acceleration phase, and lifting and resting phase respectively. And when the PF is 4 Hz, the proportion of smaller AE events is larger.
1. Introduction Most of coalbed methane (CBM) reservoirs in China have the char acteristics with low permeability, resulting in difficult exploitation of CBM (Huang et al., 2009; Song et al., 2012; Karacan et al., 2011). Recently, a new method was introduced to improve the effective permeability of coalbed methane reservoir using pulse hydraulic frac turing (PHF) technology (Wu et al., 2016). The fundamental mechanism of PHF is to pump the fracturing fluid into the coal seams in the form of dynamic load to generate the alternating load, resulting in fatigue damage in the coal bodies and forming a mutually intersecting fracture network (Li et al., 2014). Previous studies (Lu et al., 2015a, 2015b; Li, 2015) had confirmed that the PHF, which also includes the cumulative fatigue damage theory, is more complicated and has better fracturing effects comparing to the traditional hydraulic fracturing (HF). More parameters especially for the pulsating frequency and AE parameters need to be explored. Initially, the PHF technology was applied to the coal seam pressure relief and permeability-enhancing construction in domestic coal mines, and the gas drainage effect was improved obviously, which accumulated
a certain industrial application basis. Zhao (2008) and Zhang (2009) conducted pulsating water injection tests in spot and found that the coal seam permeability was greatly enhanced and the gas production was increased. By injecting the pulsating water with high pressure into for mations, Nie and He (2009) found that the water could seep into different types of cracks and pores. This can change mechanical prop erties of the coal to utmost extent and improve gas emissions, thus reducing or eliminating the coal seam outburst and impact risk. Lin et al. (2011) first put forward the coal seam pressure relief and permeability-enhancing technology on the basis of PHF, and applied it to the SV719 working face of Daxing coal mine, and found the gas permeability was effectively improved. Li et al. (2013) found that the reflection and superposition of stress waves, as well as the energy accumulation effect in coal are the key to breaking rocks, this is favor able to form fracture network at a lower pressure comparing to the HF. Considering the complicated multi-field coupling process of PHF, numerical simulation can be a effective method for studying the inducecrack characteristics of coal (Zhu et al., 2013), the dynamic liquid-solid coupling of stress and flow field (Meng, 2011), coal and stress distur bance features (Lu et al., 2015a, 2015c). In these studies, the methods
* Corresponding author. ** Corresponding author. E-mail addresses: [email protected] (H. Cao), [email protected] (M. Zheng). https://doi.org/10.1016/j.jngse.2020.103159 Received 20 August 2019; Received in revised form 27 November 2019; Accepted 9 January 2020 Available online 10 January 2020 1875-5100/© 2020 Elsevier B.V. All rights reserved.
J. Wu et al.
Journal of Natural Gas Science and Engineering 75 (2020) 103159
were all adopted in 2D. With the increasing concern on the PHF, some researches have been conducted for the PHF applied in gas extraction from the laboratory. Li et al. (2013) and Zhai et al. (2011) experimentally investigated the principle of PHF viewed from fatigue effect and pulsating pressure wave, without considering the effect of confining pressure. Li (2015) con ducted a number of laboratory tests for the impact of loading order and frequency on rock breakage by using a simplified technical system. Ni (2015) concluded that the pulsating water could invaded into the micro-pore structure at much smaller size, and the influence of pulsating load on coal porosity was greater than that of constant load. However, there are very few attempts to systematically evaluate the characteristic of crack initiation and propagation behavior during the PHF process. Based on the previous reviews, PHF can effectively improve the reser voir permeability is attributed to the particular load, the pulsating water pressure. It makes the rock breaking much easier. The push for PHF to better understand the key drivers in effective crack is very important. Therefore, any investigations concerning how to promote the internal micro crack initiation, propagation and penetration is the key for PHF, especially on the basis of laboratory facilities. Thus, an experimental methodology concentrating on crack dynamic evolution in coalbed methane reservoir during the PHF process is introduced in this study. A newly improved device allows conducting PHF tests for the coal specimen under simulated crustal stresses, and the pulsating frequency considered as a key factor. The initiation and growth of cracks in rock can be monitored on real-time, and the crack morphology can be observed by acoustic emission (AE) system. And according to the above-mentioned test data, macro-features of hydraulic fractures, the pressure and AE parameters evolution characteristics are studied. The crack initiation and propagation law is subsequently analyzed and described. Finally, the dynamic feature of b-value (relation between magnitude and frequency) are discussed to reveal the distri bution of micro-cracks during the PHF process. All the results are ex pected to provide some valuable information for field application of PHF technology in CBM wells.
2. Experimental methodology 2.1. Experimental system A self-developed large size true triaxial PHF testing system (Fig. 1), including a triaxial loading system, a pulsating generator, a controlling system and a monitoring system, was built. The detailed characteristics of the four parts are as follows: (1) The triaxial loading system is used to simulate a ground stress environment, including a true triaxial loading platform and a hydraulic console. The pressure from the loading plate in vertical direction and horizontal direction could reach 1000 kN and 600 kN respectively. This meant the stress in both horizontal di rections could be varied independently from 0 to 6.5Mpa while the vertical stress could be raised up to 11 MPa for a cubic sample of size 300 � 300 � 300 mm3. In this study, the influence of ground stress on crack dynamic evolution was not to be consid ered, thus building a unified geo-stress environment according to the actual geo-stress conditions. The horizontal stress, σ2 and σ3 was respectively 3 MPa and 1 MPa, and the vertical stress σ1 was 5.5 MPa, due to the in-situ horizontal principal stress difference is about 2 MPa. (2) Pulsating fluid signal generating device, including a small control box and two pilot-operated solenoid valves, can offer a new way to provide different frequencies to the fracturing fluid. The small control box contained a voltage stabilizing transformer, a pulse signal generator, two pieces of solid-state relays and a breaker. All the devices assembled together to realize the continuous flow output and the frequency conversion function. The steady-state test and dynamic test for this pulsating generator indicated that the pulsating control signal range of this device was 0–10 Hz. In this paper, 0 Hz, 2 Hz, 4 Hz, 6 Hz, 8 Hz were chosen as the test frequency on the basis of adjustment capacity of the device. (3) The control system consisted of the pressure control and flow control methods. The pressure control system included the metering pump, the water tank, the overflow value and the bumper. The pressure of the injecting fluid depended on the
Fig. 1. Schematic diagram of testing system. 2
J. Wu et al.
Journal of Natural Gas Science and Engineering 75 (2020) 103159
pump whose rated working pressure and flow was 12 MPa and 80 L/h respectively. On the other hand, two-stage flow control was used in this system: the low precise control, carried out by the metering pump and the control precision was 5%, and the high precise control, implemented by the shutoff valve combined with the flow sensor. This two-stage flow control method was used to determine the optimal flow for each test. (4) The PCI-2 AE system is the main monitoring device in this study. It can continuously monitor micro cracks generated inside coal samples in real time during the PHF process (Polito et al., 2010), which helps to identify morphological changes that occur inside the rock. In this study, 6 sensors will participate in the entire monitoring process and will be well deployed pre-experiment (Fig. 2(e)). Also, some parameters of the AE system for the tests were set as: the wave velocity, tested by the AST, was 500000 mm/s and the sampling frequency was 1 MHz; the analog filter worked over the range 100 kHz~3 MHz and the preamplifier sensor gain was 40 dB.
Five sets of experiments were performed as summarized in Table 2. All the specimens (i.e. A1, A2, A3, A4 and A5) were conducted to evaluate the fracture dynamic evolution during the PHF process, and analyze variation difference of the pressure and AE behavior between different frequencies. 3. Test results and analysis 3.1. Hydraulic macroscopic crack characteristics under the pulsating load Fig. 3 shows the shape of the five tested coal specimens at the end of the experiments. It can be seen that macroscopic main cracks distributed on the specimen surface and extended in the direction of the injection pipe (σ2) pointing to the edge of specimen. This satisfies the conditions of rock failure criterion, i.e. main cracks of the specimen always initiate and propagate along the direction perpendicular to the minimum prin cipal stress (σ3) (Su et al., 2005). But it can be also found that the main crack of A1 (f ¼ 0 Hz) only propagated on one side of the fracturing pipe, which indicates that the pulsating shock has some influence on crack formation, but the main crack is mainly caused by the stress field. On the other hand, we can also see from Fig. 3, the fracture shape of five specimens were different, thus, the specimens were sectioned along the fracture plane to investigate their internal information, as shown in Fig. 4. As presented in Fig. 4, it could be clearly found the infiltration distribution for each specimen. Fig. 4 (A1), Fig. 4 (A2), Fig. 4 (A3), Fig. 4 (A4) and Fig. 4 (A5) represents the rupture plane morphology of spec imens at pulsating frequency of 0 Hz, 2 Hz, 4 Hz, 6 Hz and 8 Hz, and we can differentiate different areas affected by the fracturing fluid by using with the ImageJ software to get the binary image. Then the affected areas can be divided into three parts: Grey (A zone), Dark grey (B zone) and bright black area (C zone). Grey (A zone) indicates that fracture surface was dry, that is, there is no fracturing fluid available to infiltrate into this section. Dark grey (B zone) indicates the area where the frac turing fluid invaded. And bright black area (C zone) in B area indicates that the infiltration amount of the fracturing fluid were the maximum. Therefore, as can be seen from Fig. 4, the specimen A1 (f ¼ 0 Hz) shows the smallest area of B zone. This means the distribution area of frac turing fluid using conventional fracturing (A1) is smaller than that of using PHF (A2, A3, A4 and A5), namely, the crack can be better devel oped under pulsating hydraulic load. Besides, the area of B zone for the specimen A2 (f ¼ 2 Hz), A3 (f ¼ 4 Hz) and A4 (f ¼ 6 Hz) accounts about 4/5 of the whole fracture plane respectively, but the specimen A5 (f ¼ 8 Hz) shows a smaller area of B zone. This experimental phenomenon indicates that the distribution of fracturing fluid and the crack devel opment are both different with the different pulse frequencies. Furthermore, it can also be seen from the area of C zone for each spec imen that, the fracturing fluid is not infiltrated along the same direction on one rupture plane, that is, its propagation direction is not single. We can find that, the C zone of specimen A5 (f ¼ 8 Hz) is mainly distributed on the right side of the fracturing pipe and its C zone color is lighter. Also, compared to specimen A5 (f ¼ 8 Hz), the C zone of specimen A2 (f ¼ 2 Hz), A3 (f ¼ 4 Hz) or A4 (f ¼ 6 Hz) is distributed more unevenly, and specimen A3 (f ¼ 4 Hz) shows the most unevenly distributed C zone. It is worth to note that the more uneven distribution of C zone, the more complicated of crack development will be. Hence, it can be concluded that the internal fracture network complexity of specimen is highest when the pulsating frequency is 4 Hz.
2.2. Specimen design and production Concrete specimens were made to replace the actual coal by using similar principles. The specimen was mainly composed of quartz sand, pulverized coal, cement and gypsum, and a small amount of calcium sulphate retarder to regular the curing time. The best addition ratio of the concrete materials was determined by lab experiments and me chanical experiments, therefore, it was finally determined to be quartz sand: pulverized coal: cement: gypsum ¼ 36:18:23:23. The mechanical properties of the concrete samples are shown in Table 1. For the tests in this paper, several 300 mm � 300 mm � 300 mm cubic specimens (Fig. 2 (b)) were made. Fig. 3 shows the test samples used in the tests. After the specimen was cured, a 160 mm deep borehole was drilled in the center of the top face using a ϕ32mm drill bit (Fig. 2 (c)), in which a 6 mm inner diameter fracturing pipe was placed. The fracturing section is 30 mm long and 32 mm in diameter, and it was formed by sealing the 130 mm long hole with AB adhesive in the middle (Fig. 2 (d)) (Chen et al., 2000; Lin and Du, 2011; Li et al., 2014). Besides, to prevent the fracturing section from being blocked, we inserted a small steel spacer and a rubber blanket 30 mm away from the end of the pipe. 2.3. Experiment set-up and procedure The PHF process is implemented as follows: firstly, put the coal block in the triaxial loading frame, with three oil cylinders applying load on six steel plates on the platform (Fig. 2(f)). Next, after the stabilization of the stresses in three directions, set a frequency for the test and start all the monitoring systems and pumping system. At this moment, the fracturing fluid is pumped out from the water tank and flow through the metering pump. Then one part of fluid flows back to the water tank (Fig. 1-①), and the other part flows into the pilot-operated solenoid valves (Fig. 1-②), thus running out with the predefined frequency. Finally, after the fluid in Line ② flow through the one-way value and shut-off value, it enters into the sample. The test will be terminated when fluid flowed out of the specimen, which indicates the rupture of the specimen. And during testing, the data collection system will auto matically monitor and record pressure, AE signal, time, and also export curves of time-pressure and time-acoustic emission signal in real time. Five frequencies, 0 Hz, 2 Hz, 4 Hz, 6 Hz and 8 Hz, are chosen to test on specimen 1–5, respectively.
3.2. Response characteristics of pressures and AE parameters during the PHF process
Table 1 Mechanical properties of the test samples (average). Uniaxial compression strength/MPa
Elasticity modulus/GPa
Poisson’s ratio
Tensile Strength/MPa
7.31
3.35
0.191
0.807
During the whole PHF process, the initiation, extension and final rupture of the specimen will always be accompanied by the accumula tion and release of energy. The cumulative AE energy is the accumula tion of energy released from the beginning of the test to a certain time. It can help us to deeply understand the fracture evolution law. Therefore, 3
J. Wu et al.
Journal of Natural Gas Science and Engineering 75 (2020) 103159
Fig. 2. Test specimen states. (a) The sketch diagram of the specimen, (b) the cubic specimen ready for drilling, (c) specimen drilling in the center of the specimen, (d) sealed specimen, (e) positions of AE sensors and (f) the specimen was placed in the triaxial loading system, in which AE sensors were well arranged.
we can see from Fig. 5, based on the cumulative AE energy, the PHF process can be divided into four stages:
plastic deformation. Meanwhile, under the effect of the confining pressure and the pulsed pressure, the AE phenomenon tended to be more active because of the continuous evolution of the internal micro-cracks. Finally, the fatigue damage of the coal specimens continued to accumulate until the pressure reached the speci mens’ fracture pressure. At that time, the AE energy rate reached the maximum value. Also, the slope of the energy curve began to increase. It was worth to note that the accumulated energy of this period increased about 4.81 times (average) in comparison with the quiet phase. (c) Lifting phase③: After the initiation of the fracture, the fracturing fluid entered into the fracture and the pressure would drop gradually. However, under the continuous pulsating effect, the original cracks and the new cracks in the coal specimens grew and developed further. And then, the cracks were gradually con nected and interpenetrated. A lot of cracks were developed into macro cracks and finally the coal specimen was fractured. At this
(a) Quiet phase①: During this period, the fracturing fluid entered into the pre-existing fractures and pores in the specimens through a porous flow pattern. The pressure did not rise and there was no significant damage to the specimen. So only sporadic acoustic emission events appeared at this stage, this may be due to friction between the pulsating flow and the pipe. (b) Acceleration phase②: At this stage, after the crack and pores were full of fluid, a cyclical impact with a fixed frequency started to act on the specimens with the increase of injection pressure. Some fatigue micro-cracks were generated in coal samples around the hole. The cumulative AE energy started to increase due to the initial damage in the coal specimens. Then, as the water pressure in the hole increases, fatigue damage was induced in the coal specimens, and the specimens entered the stage of 4
J. Wu et al.
Journal of Natural Gas Science and Engineering 75 (2020) 103159
Fig. 3. Shape of cracks on specimen surfaces.
under different frequencies. As presented in Fig. 7, the initiation pres sures of the five specimens were about 7~8 MPa and showed little change under different frequencies. This was because the sphere of ac tion of fracturing fluid was limited to the perforation section with small area. And compared to the whole specimen, the perforation section looked minor. Thus, the fracturing fluid would fill up the perforation section rapidly and the crack occurred within a very short time. In this case, only little pulsating effect acted on the specimen, resulting in the pressure with little change under different frequencies. Besides, it can be seen from Fig. 7, crack initiation pressure of specimen with 0 Hz is larger than that of specimens with other frequencies. This means that the crack initiation pressure of PHF is smaller than that of conventional HF. This is in accord with the existing results (Zhai et al., 2011). On the other hand, though the initiation pressures showed little change under different frequencies, it could still be found that the initiation pressures firstly declined and then rised with the increase of frequency, and reached the minimum value when the pulsating fre quency was 4 Hz. Actually, compared to the HF, the PHF also includes the pulsating effect part. This part will cause fatigue damage on the specimen, and the damage is related to the pulsating frequency and the amplitude. Xiao et al. (2009) and Meng (2011) indicated that the higher the pulsating frequency, the more damage the specimen accumulated, and the larger the amplitude, the more damage the specimen accumu lated when crack extending. It is observed that the increase of pulsating frequency or the amplitude is beneficial to the crack expansion and the rock breaking efficiency. However, in the actual PHF process, it is difficult to implement overlarge frequency or overlarge amplitude, in which case the equipment is also highly demanding. What’s more, high frequency or high amplitude means a raise in loading rate (Xiao, 2009), resulting in fracturing process similar to conventional HF. And in this case, the crack has no enough time to propagate and only extends, connects and even broken along the main cracks. But the goal of the PHF is attempt to form more complicated crack network. As a result, optimal pulsating frequency and its suitable amplitude can make the specimen forming a complex crack network. In this paper, the pulsating generator of the experimental system has a typical character: the higher the fre quency, the smaller the amplitude. In other words, when the pulsating
Table 2 Summary of experimental parameters. Sample profile
Vertical stress (σ1)/ MPa
Maximum horizontal stresses (σ2)/MPa
Minimum horizontal stresses (σ3)/MPa
Injection flow rate/ (mL⋅min 1)
Frequencies (f)/Hz
A1 A2 A3 A4 A5
5.5 5.5 5.5 5.5 5.5
3 3 3 3 3
1 1 1 1 1
10 10 10 10 10
0 2 4 6 8
stage, the intensifying interactions between cracks, and the highly active AE activity necessitated a radical increase of the cumulative energy. Therefore, the slope of the energy curve increased obviously. It was worth to note that the accumulated energy of this period increased about 17.21 times (average) in comparison with the quiet phase. (d) Resting phase④: At this stage, the specimen was completely broken and the extreme structural motion stopped, so the AE phenomenon began to flatten. 3.3. Crack initiation and propagation characteristics during PHF process The crack initiation and propagation law during the PHF are the two basic and key questions to study the crack dynamic evolution (Li, 2010). And it is noted that the crack inner pressure vs. time for coal specimen during PHF can better reflect the crack initiation and propagation characteristics. So the objective of this section is to study the crack initiation and propagation characteristics by the pressure vs. time curve showed in Fig. 6. As shown in Fig. 6, the crack initiation stage and propagation stage correspond to stage A and stage B respectively in Fig. 6. 3.3.1. The effect of pulsating frequency on the crack initiation pressure According to Fig. 6, it is easy to know that each pulsating frequency corresponds to an initiation pressure. Fig. 7 shows the initiation pressure 5
J. Wu et al.
Journal of Natural Gas Science and Engineering 75 (2020) 103159
Fig. 4. Specimen shape after pulsating hydraulic fracturing.
frequency is small, the pulsation number is small but the strength of a single pulsation is great. On the contrary, when the pulsating frequency is larger, the pulsation number is large but the strength of a single pulsation is small. This can be considered that there is a threshold for the pulsating frequency with the increase of the frequency. When the fre quency is less than the threshold, the strain energy of the specimens will increase as the increase of the frequency. On the contrary, when the frequency is larger than the threshold, the strain energy will decrease as the increase of the frequency. Hence, it can be concluded that in this test case, the threshold of pulsating frequency was 4 Hz. When the pulsating frequency was less than 4 Hz, the damage accumulation of specimens would decrease with the increase of frequency, resulting in smaller initiation pressure. On the contrary, the initiation pressure became correspondingly larger and larger. This is why the initiation pressure exhibited minimum when the pulsating frequency was 4 Hz, using this set-up.
pressure cycles was shown in Fig. 8 (Fjær and Holt, 1994). When the elastic deformation occurred in the formation around borehole under the action of fracturing fluid, the crack started to initiate (the sharp point in Fig. 8), then the pressure was released and dropped rapidly. In the second period, the tensile strength of specimen was 0 due to the existing of cracks, thus, the crack could propagate further only required to resist stress concentration around borehole. Therefore, during the PHF pro cess, the rock mass around borehole was subjected to dynamic fluctu ating loads, which can easily made multiple cracks initiate and grow. And after the crack initiation, the tensile strength of specimen would decrease, thus the initiation pressure of the subsequent hydraulic crack would decrease as well. 3.4. Dynamic characteristic of b-value during the PHF process Amplitude, which is the largest measured voltage in a waveform (expressed in dB), can represent the strength of the AE events (Zhang et al., 2011). AE amplitude has a close relationship with the frequency-magnitude coefficient b (Li et al., 2009; Yan, 1994). It is known from the literature (Li et al., 2010) that the b-value is the function of the extension size of cracks and can indicate the distribution of micro-cracks. It is also found that the AE events during the rock failure process can be considered as seismic activity (micro-quake) and the measurement of AE is similar to the measurement of some seismic physical quantities. The b-value of AE events (Zhang et al., 2015) can be calculated through Eq. (1):
3.3.2. The effect of pulsating frequency on the crack propagation stage As presented in Fig. 6, specimen A1 (f ¼ 0 Hz) showed few recurrent fluctuation at the B stage. And the pressure of specimen A2 (f ¼ 2 Hz), A3 (f ¼ 4 Hz), A4 (f ¼ 6 Hz) and A5 (f ¼ 8 Hz) had some fluctuation, and their fluctuation numbers were about 3, 5, 3 and 1 respectively. One fluctuation meant the crack changed from closed state to open state. Thus, it indicated that the probability and frequency of the crack propagation in the process of PHF occurred more often than that in the process of HF. That is to say, the efficiency of sample rupture was higher when using PHF, and the most fluctuation number of A3 (f ¼ 4 Hz) meant the high working efficiency when the pulsating frequency was 4 Hz. On the other hand, when the pulsating frequency was 8 Hz, the injection rate was relatively large. The PHF process was similar to the traditional HF, this meant the fluctuation type of specimen A5 (f ¼ 8 Hz) was similar to the specimen A1 (f ¼ 0 Hz). This was why the specimen A5 (f ¼ 8 Hz) showed only one obvious fluctuation. On the other hand, from Fig. 6, it is worth to note that the next fluctuation was always “lower” than that of the previous fluctuation when under pulsating effect. In other words, after one crack initiation, the initiation pressure of the new crack would become smaller than that of the existing crack. It can be explained, that as the fracturing fluid was pumped into the specimen, the borehole pressure changes in two
logðNðCÞÞ ¼ a
blogðCÞ
(1)
where C is the amplitude of AE events during the PHF process, N(C) is sum of AE events in the group whose amplitude is greater (or equal to) than C, and a and b are constants. As found in Zeng et al. (1995), an increase in the b-value means a rise of the proportion of minor events and micro-cracks of smaller size. A constant b-value means the relative balanced distribution of micro-cracks with different sizes. And a drop of b-value means the in crease of a proportion of large events and micro-cracks in large-size. In this paper, we explore the dynamic feature of b-value to illustrate the crack evolution characteristics during the PHF process and compare its evolution difference under different pulsating frequency. 6
J. Wu et al.
Journal of Natural Gas Science and Engineering 75 (2020) 103159
Fig. 5. Changes in pressure and AE parameters during the PHF process under (a) f ¼ 0 Hz, (b) f ¼ 2 Hz, (c) f ¼ 4 Hz, (d) f ¼ 6 Hz, (e) f ¼ 8 Hz
According to Eq. (1) and data from the lab tests, b-values were ob tained by using the least square method where 200 data was selected as a sampling group and 50 data was used as the step-size. Fig. 9 gives the relationship of b-value with time (turning point of four stages) for the PHF process with different frequencies. We observe from Fig. 9 that the overall trend of the b-value changes under different frequencies was similar in the whole process. It meant that the b-value increased in the acceleration phase, then decreased in lifting phase and finally basically unchanged in steady period. This in dicates that the proportion of minor AE events was higher in accelera tion period, and the proportion of great AE events was higher in boosting and steady period. It can be explained, that the initiation and extension of micro-cracks is dominant in acceleration phase, leading to a sharp increase of micro-cracks in small size but a slow increase of micro-cracks in large size. The b-value would increase in this case. Then cracks began to become connected and interpenetrated and many cracks developed
into macro cracks in the lifting phase. In this case, the increment speed of micro-cracks in large size exceeded the increment speed of microcracks in small size, which meant a decrease of b-value. And finally the coal specimen was completely fractured. The size distribution of micro-cracks was relative balanced, so the b-value remained stable at the final stage. As presented in Fig. 9, the b values have different characteristics under different frequencies. Specimen A3 (f ¼ 4 Hz) showed the most obvious features of b-value at three stages, and the b value was maximum during the whole fracturing process. This indicated that the proportion of small AE events was higher at this pulsating frequency than that of other frequencies, and also the change of its AE amplitude distribution was more “violent”. The three-stage characteristic of spec imen A5 (f ¼ 8 Hz) became less apparent, and the b value was minimum and showed a relatively smooth tendency in most of the fracturing process. Namely, during the fracturing process of specimen A5, the 7
J. Wu et al.
Journal of Natural Gas Science and Engineering 75 (2020) 103159
Fig. 6. Crack inner water pressure vs. time for coal samples during PHF, all of which are completed in three steps: crack initiation stage (A), crack propagation stage (B), break stage (C).
8
J. Wu et al.
Journal of Natural Gas Science and Engineering 75 (2020) 103159
proportion of great AE events was higher at this pulsating frequency than that of at other frequencies. Also, the whole process embodies an incremental stable extension process. Besides, the b-value characteris tics of specimen A2 (f ¼ 2 Hz) and specimen A4 (f ¼ 6 Hz) were similar, and they were intermediate between specimen A3 and specimen A5. However, the small AE events proportion of specimen A4 was slightly higher than that of specimen A2, also the AE amplitude of specimen A4 changed a little more drastic. All of these experimental phenomena may be caused by the following reasons, and here taking typical specimen A5 (f ¼ 8 Hz) and specimen A3 (f ¼ 4 Hz) for example. When the pulsating frequency was 8 Hz, high frequency means a raise in loading rate (Xiao, 2009), resulting in a similar fracturing process as the traditional HF. In this case, the pulsating fluid was continuously injected into the specimen at high pressure, leading to single main fracture developing inside the specimen and relatively stable process of crack propagation. Thus, bvalue was small and it changed within a small range. On the other hand, when the pulsating frequency was 4 Hz, the initiation pressure of specimen A3 (f ¼ 4 Hz) was the smallest, in which case the micro-crack had a longer time to propagate further to form a complicated fracture network. Hence the proportion of small AE events gradually increased. After reaching the fatigue strength of specimen A3, lots of micro-cracks become connected and interpenetrated and many cracks developed into macro cracks, and thus large AE events occurred. This whole process seemed to be a relatively “intense” activity and the AE amplitude changed obviously. Similarly, the analyzed method can be used to explain the above phenomena for specimen A2 (f ¼ 2 Hz) and specimen A4 (f ¼ 6 Hz).
Fig. 7. Initiation pressure under different frequencies.
4. Conclusions This paper presented an experimental methodology to evaluate the crack generation ability of PHF technology in coalbed methane reser voir. Several cubic specimens (300 � 300 � 300 mm3) were made. Five sets of tests, where the pulsating frequency was considered as a signif icant factor, were conducted to study the crack dynamic evolution characteristics. The main conclusions are as follows: (1) Hydraulic macroscopic crack characteristics under the pulsating load show that, the crack is perpendicular to the direction of the minimum principal stress (σ3). And the affected area of fracturing fluid using conventional fracturing (A1) is smaller than that of using PHF, which means the crack could be better developed under pulsating hydraulic load. Also, the internal fracture network complexity of specimen is highest when the pulsating frequency is 4 Hz, using this setup. (2) The evolution of cumulative AE energy during the PHF process can be divided into four stages: quiet phase, acceleration phase, lifting phase and resting phase. It was worth to note that the accumulated energy of acceleration period and boosting period increased respectively about 4.81 and 17.21 times (average) in comparison with the tranquil period. (3) Comparing to the traditional HF, the initiation pressure of the fracture was smaller and the efficiency of sample rupture was higher when using the PHF technology. Meanwhile, the initiation pressure decreased firstly and then increased with the increase of pulsating frequency, and it reached its minimum when the pul sating frequency was 4 Hz. Also at this pulsating frequency (4 Hz), the probability and frequency of the crack propagation is large, thus the working efficiency is high in this case. (4) The variation of the b-value was similar under different fre quencies during the whole process. The proportion of small-size cracks was greater in the acceleration period, while the propor tion of large-size cracks was higher in the boosting and steady periods. Our results also show a great change in AE amplitude distribution when the pulsating frequency was 4 Hz, namely, the growth and development of the micro-crack was a little more
Fig. 8. Borehole pressure changes in two pressure cycles under ideal conditions (Fjær and Holt, 1994).
Fig. 9. b value vs. time for pulse fracturing process with different frequencies.
9
J. Wu et al.
Journal of Natural Gas Science and Engineering 75 (2020) 103159
“violent”. Meanwhile, the b value was large at this pulsating frequency, which means a great proportion of small AE events occurred throughout the fracturing process.
Li, Q.G., 2015. Research on the Evolution Law of Coal Fracture under Pulse Loading and its Application on Gas Extraction. PhD theses. China University of Mining and Technology, pp. 22–34. Lin, B.Q., Li, Z.W., Zhai, C., Bi, Q., Wen, Y.Q., 2011. Pressure relief and permeabilityincreasing technology based on high pressure pulsating hydraulic fracturing and its application. Journal of Mining & Safety Engineering 28 (3), 452–455. Lin, H.X., Du, C.Z., 2011. Experimental research on the quasi three-axis hydraulic fracturing of coal. J. China Coal Soc. 36 (36), 1801–1805. Lu, P.Q., Li, G.S., Huang, Z.W., Li, X.J., Tian, S.C., Song, X.Z., Sheng, M., 2015a. A numerical analysis of the features of the stress disturbance during pulsating hydraulic fracturing in coal seams. In: SPE Technical Conference and Exhibition, Houston, Texas, USA, pp. 1–15. Lu, P.Q., Li, G.S., Huang, Z.W., Tian, S.C., Shen, Z.H., Li, X.J., 2015b. Numerical simulation for stress disturbance features of coal beds during pulsating hydrofracturing. J. Vib. Shock 34 (21), 210–216. Lu, P.Q., Li, G.S., Huang, Z.W., He, Z.G., Li, X.J., Zhang, H.Y., 2015c. Modeling and parameters analysis on a pulsating hydro-fracturing stress disturbance in a coal seam. J. Nat. Gas Sci. Eng. 26, 253–263. Meng, J.Q., 2011. Study on the Theory and Technology of Coal Seam High Pressure Pulsation Water Infusion Preventing Coal and Gas Outburst. PhD theses. China University of Mining and Technology, Beijing), pp. 82–86. Ni, G.H., 2015. Microscopic Kinetics Characteristics of Methane under Pulsating Hydraulic Fracturing and Mechanism of Liquid Retention Effect. PhD theses. China University of Mining and Technology, pp. 18–30. Nie, B., He, X.Q., 2009. Methods and Equipment for Prevention of Coal Dynamic Disasters by High Pressure Pulsating Water Injection. Chinese Patent. CN 101539028 A. Polito, F., Petri, A., Pontuale, G., Dalton, F., 2010. Analysis of metal cutting acoustic emissions by time series models. Int. J. Adv. Manuf. Technol. 48 (9–12), 897–903. Song, Y., Liu, S.B., Hong, F., Qin, S.F., 2012. Geochemical characteristics and genesis of coalbed methane in China. Acta Pet. Sin. 33 (s1), 99–106, 2012. Su, X.B., Ning, C., Hua, S.L., 2005. Fluid pressure-dependent fractures in coal-bed gas reservoirs. Nat. Gas. Ind. 25 (1), 127–129. Wu, J.J., Zhang, S.H., Cao, H., Kemeny, J., 2016. The effect of pulse frequency on the acoustic emission characteristics in coal bed hydraulic fracturing. In: 50th U.S. Rock Mechanics/Geomechanics Symposium, Houston, Texas, USA. Xiao, J.Q., 2009. Theoretical and Experimental Investigation on Fatigue Properties of Rock under Cyclic Loading. PhD theses. Central South University, pp. 63–64. Yan, Z.G., 1994. Robust estimation of the b-value of magnitude-frequency relationship. Earthq. Res. China 10 (4), 320–326. Zeng, Z.W., Ma, J., Liu, L.Q., Liu, T.C., 1995. AE b-Value dynamic features during rock mass fracturing and their significances. Seismol. Geol. 17 (1), 7–12. Zhai, C., Li, Z.X., Li, Q.G., 2011. Research and application of coal seam pulse hydraulic fracturing technology. J. Chin. Soc. 36 (12), 1996–2001. Zhang, J.S., 2009. Project Report of High Pressure Pulsating Hydraulic Fracturing Technology for Difficult Drawing Coal Seam. China University of Mining and Technology, pp. 57–58. Zhang, H.H., Liu, F., Chang, F.Q., 2011. Experimental study of acoustic emission characteristics during rock failure process and energy properties. J. Highw. Transp. Res. Dev. 28 (3), 48–54. Zhang, Z.P., Zhang, R., Li, H.G., 2015. Experimental research on effects of bedding plane direction on coal acoustic emission characteristics under uniaxial compression. Chin. J. Rock Mech. Eng. 34 (4), 1–9. Zhao, Z.B., 2008. Study of technology of variable-frequency pulse water infusion into coal seam. J. Min. Saf. Eng. 25 (4), 486–489. Zhu, H.Q., Shen, J., Zhang, M.B., Feng, S.L., Xu, J.Y., 2013. Simulation of coal inducecrack characteristics on the conditions of pulsatile and constant pressure water injections. Sci. Technol. Rev. 31 (18), 57–62.
Authors’ contributions Jingjing Wu: Conceived and designed the study, design of meth odology, data analysis, draft manuscript writing. Shaohe Zhang: Oversight and leadership responsibility for the research activity plan ning and execution, acquisition of the financial support, design of methodology, data analysis, manuscript review and editing. Han Cao: Conceived and designed the study, data analysis, manuscript review and editing. Mingming Zheng: Design of methodology, data analysis, manuscript review and editing. Feilong Qu: Field sampling, performed the experiments, manuscript review and editing. Canwei Peng: Artifi cial samples preparation, performed the experiments, manuscript re view and editing. All authors read and approved the manuscript. Acknowledgments This work is sponsored by the National Natural Science Foundation of China (No. 41872186, 41602372, 41702389); and Open Research Fund Program of Key Laboratory of Metallogenic Prediction of Nonfer rous Metals and Geological Environment Monitoring (Central South University), Ministry of Education (No. 2019YSJS05, 2018YSJS09); and Scientific Research Fund of Hunan Provincial Education Department (No. 18C0541). References Chen, M., Pang, F., Jin, Y., 2000. Experimental and analysis on hydraulic fracturing by a large-size triaxial simulator. Chin. J. Rock Mech. Eng. 19 (s1), 868–872. Fjær, E., Holt, R.M., 1994. Rock acoustics and rock mechanics: their link in petroleum engineering. Lead. Edge 31 (31), 255–258. Huang, S.C., et al., 2009. Coalbed methane development and utilization in China: status and future development. China Coal 35 (1), 5–10. € Ruiz, F.A., Cot�e, M., Phipps, S., 2011. Coal mine methane: a review of Karacan, C.O., capture and utilization practices with benefits to mining safety and to greenhouse gas reduction. Int. J. Coal Geol. 86 (2), 121–156. Li, Y.H., Liu, J.P., Zhao, X.D., Yang, Y.J., 2009. Study on b-value and fractal dimension of acoustic emission during rock failure process. Rock Soil Mech. 30 (9), 2559–2574. Li, W., 2010. Research on Reservoir Characteristics and Fracture Initiation Mechanism Based on Fractal Theory. Master theses. Daqing Petroleum Institute, p. 5p. Li, S.Y., He, T.M., Yin, X.C., 2010. Study on the nucleation evolution of micro fracture using acoustic emission method. In: Introduction of Rock Fracture Mechanics, China, pp. 372–374. Li, Z.X., Lin, B.Q., Zhai, C., Li, Q.G., Ni, G.H., 2013. The mechanism of breaking coal and rock by pulsating pressure wave in single low permeability seam. J. China Coal Soc. 38 (6), 918–923. Li, Q.G., Lin, B.Q., Zhai, C., 2014. The effect of pulse frequency on the fracture extension during hydraulic fracturing. J. Nat. Gas Sci. Eng. 21, 296–303.
10
Contents lists available at ScienceDirect
Journal of Natural Gas Science and Engineering journal homepage: http://www.elsevier.com/locate/jngse
Experimental investigation of crack dynamic evolution induced by pulsating hydraulic fracturing in coalbed methane reservoir1 Jingjing Wu a, Shaohe Zhang b, Han Cao b, *, Mingming Zheng c, **, Feilong Qu d, Canwei Peng d a
College of Civil Engineering, Hunan University of Technology, Zhuzhou, 412007, China Key Laboratory of Metallogenic Prediction of Nonferrous Metals and Geological Environment Monitoring Ministry of Education, School of Geoscience and Infophysics, Central South University, Changsha, 410083, China c State Key Laboratory of Geohazard Prevention and Geoenvironment Protection, Chengdu University of Technology, Chengdu, Sichuan, 610059, China d Zhongye Changtian International Engineering Co., Ltd, Changsha, 410205, China b
A R T I C L E I N F O
A B S T R A C T
Keywords: Pulsating hydraulic fracturing Coalbed methane reservoir Pulsating frequency Crack initiation Crack propagation Acoustic emission
To evaluate the crack initiation and propagation behavior during pulsating hydraulic fracturing (PHF) process, lab tests on PHF of coal samples were performed by using a self-developed true triaxial testing system. The results demonstrate that: the crack could be better developed under pulsating hydraulic load. And as PF increase, the initiation pressure decreases firstly, then increases and reached minimum when the PF is 4 Hz. Besides, the changes of acoustic emission (AE) behavior can be divided into four stages: quiet phase, acceleration phase, lifting phase and resting phase. And the accumulated energy of acceleration and lifting phase increased respectively about 4.81 and 17.21 times (average) in comparison with quiet phase. Our results also show that the proportion of minor AE events and great AE events were higher in acceleration phase, and lifting and resting phase respectively. And when the PF is 4 Hz, the proportion of smaller AE events is larger.
1. Introduction Most of coalbed methane (CBM) reservoirs in China have the char acteristics with low permeability, resulting in difficult exploitation of CBM (Huang et al., 2009; Song et al., 2012; Karacan et al., 2011). Recently, a new method was introduced to improve the effective permeability of coalbed methane reservoir using pulse hydraulic frac turing (PHF) technology (Wu et al., 2016). The fundamental mechanism of PHF is to pump the fracturing fluid into the coal seams in the form of dynamic load to generate the alternating load, resulting in fatigue damage in the coal bodies and forming a mutually intersecting fracture network (Li et al., 2014). Previous studies (Lu et al., 2015a, 2015b; Li, 2015) had confirmed that the PHF, which also includes the cumulative fatigue damage theory, is more complicated and has better fracturing effects comparing to the traditional hydraulic fracturing (HF). More parameters especially for the pulsating frequency and AE parameters need to be explored. Initially, the PHF technology was applied to the coal seam pressure relief and permeability-enhancing construction in domestic coal mines, and the gas drainage effect was improved obviously, which accumulated
a certain industrial application basis. Zhao (2008) and Zhang (2009) conducted pulsating water injection tests in spot and found that the coal seam permeability was greatly enhanced and the gas production was increased. By injecting the pulsating water with high pressure into for mations, Nie and He (2009) found that the water could seep into different types of cracks and pores. This can change mechanical prop erties of the coal to utmost extent and improve gas emissions, thus reducing or eliminating the coal seam outburst and impact risk. Lin et al. (2011) first put forward the coal seam pressure relief and permeability-enhancing technology on the basis of PHF, and applied it to the SV719 working face of Daxing coal mine, and found the gas permeability was effectively improved. Li et al. (2013) found that the reflection and superposition of stress waves, as well as the energy accumulation effect in coal are the key to breaking rocks, this is favor able to form fracture network at a lower pressure comparing to the HF. Considering the complicated multi-field coupling process of PHF, numerical simulation can be a effective method for studying the inducecrack characteristics of coal (Zhu et al., 2013), the dynamic liquid-solid coupling of stress and flow field (Meng, 2011), coal and stress distur bance features (Lu et al., 2015a, 2015c). In these studies, the methods
* Corresponding author. ** Corresponding author. E-mail addresses: [email protected] (H. Cao), [email protected] (M. Zheng). https://doi.org/10.1016/j.jngse.2020.103159 Received 20 August 2019; Received in revised form 27 November 2019; Accepted 9 January 2020 Available online 10 January 2020 1875-5100/© 2020 Elsevier B.V. All rights reserved.
J. Wu et al.
Journal of Natural Gas Science and Engineering 75 (2020) 103159
were all adopted in 2D. With the increasing concern on the PHF, some researches have been conducted for the PHF applied in gas extraction from the laboratory. Li et al. (2013) and Zhai et al. (2011) experimentally investigated the principle of PHF viewed from fatigue effect and pulsating pressure wave, without considering the effect of confining pressure. Li (2015) con ducted a number of laboratory tests for the impact of loading order and frequency on rock breakage by using a simplified technical system. Ni (2015) concluded that the pulsating water could invaded into the micro-pore structure at much smaller size, and the influence of pulsating load on coal porosity was greater than that of constant load. However, there are very few attempts to systematically evaluate the characteristic of crack initiation and propagation behavior during the PHF process. Based on the previous reviews, PHF can effectively improve the reser voir permeability is attributed to the particular load, the pulsating water pressure. It makes the rock breaking much easier. The push for PHF to better understand the key drivers in effective crack is very important. Therefore, any investigations concerning how to promote the internal micro crack initiation, propagation and penetration is the key for PHF, especially on the basis of laboratory facilities. Thus, an experimental methodology concentrating on crack dynamic evolution in coalbed methane reservoir during the PHF process is introduced in this study. A newly improved device allows conducting PHF tests for the coal specimen under simulated crustal stresses, and the pulsating frequency considered as a key factor. The initiation and growth of cracks in rock can be monitored on real-time, and the crack morphology can be observed by acoustic emission (AE) system. And according to the above-mentioned test data, macro-features of hydraulic fractures, the pressure and AE parameters evolution characteristics are studied. The crack initiation and propagation law is subsequently analyzed and described. Finally, the dynamic feature of b-value (relation between magnitude and frequency) are discussed to reveal the distri bution of micro-cracks during the PHF process. All the results are ex pected to provide some valuable information for field application of PHF technology in CBM wells.
2. Experimental methodology 2.1. Experimental system A self-developed large size true triaxial PHF testing system (Fig. 1), including a triaxial loading system, a pulsating generator, a controlling system and a monitoring system, was built. The detailed characteristics of the four parts are as follows: (1) The triaxial loading system is used to simulate a ground stress environment, including a true triaxial loading platform and a hydraulic console. The pressure from the loading plate in vertical direction and horizontal direction could reach 1000 kN and 600 kN respectively. This meant the stress in both horizontal di rections could be varied independently from 0 to 6.5Mpa while the vertical stress could be raised up to 11 MPa for a cubic sample of size 300 � 300 � 300 mm3. In this study, the influence of ground stress on crack dynamic evolution was not to be consid ered, thus building a unified geo-stress environment according to the actual geo-stress conditions. The horizontal stress, σ2 and σ3 was respectively 3 MPa and 1 MPa, and the vertical stress σ1 was 5.5 MPa, due to the in-situ horizontal principal stress difference is about 2 MPa. (2) Pulsating fluid signal generating device, including a small control box and two pilot-operated solenoid valves, can offer a new way to provide different frequencies to the fracturing fluid. The small control box contained a voltage stabilizing transformer, a pulse signal generator, two pieces of solid-state relays and a breaker. All the devices assembled together to realize the continuous flow output and the frequency conversion function. The steady-state test and dynamic test for this pulsating generator indicated that the pulsating control signal range of this device was 0–10 Hz. In this paper, 0 Hz, 2 Hz, 4 Hz, 6 Hz, 8 Hz were chosen as the test frequency on the basis of adjustment capacity of the device. (3) The control system consisted of the pressure control and flow control methods. The pressure control system included the metering pump, the water tank, the overflow value and the bumper. The pressure of the injecting fluid depended on the
Fig. 1. Schematic diagram of testing system. 2
J. Wu et al.
Journal of Natural Gas Science and Engineering 75 (2020) 103159
pump whose rated working pressure and flow was 12 MPa and 80 L/h respectively. On the other hand, two-stage flow control was used in this system: the low precise control, carried out by the metering pump and the control precision was 5%, and the high precise control, implemented by the shutoff valve combined with the flow sensor. This two-stage flow control method was used to determine the optimal flow for each test. (4) The PCI-2 AE system is the main monitoring device in this study. It can continuously monitor micro cracks generated inside coal samples in real time during the PHF process (Polito et al., 2010), which helps to identify morphological changes that occur inside the rock. In this study, 6 sensors will participate in the entire monitoring process and will be well deployed pre-experiment (Fig. 2(e)). Also, some parameters of the AE system for the tests were set as: the wave velocity, tested by the AST, was 500000 mm/s and the sampling frequency was 1 MHz; the analog filter worked over the range 100 kHz~3 MHz and the preamplifier sensor gain was 40 dB.
Five sets of experiments were performed as summarized in Table 2. All the specimens (i.e. A1, A2, A3, A4 and A5) were conducted to evaluate the fracture dynamic evolution during the PHF process, and analyze variation difference of the pressure and AE behavior between different frequencies. 3. Test results and analysis 3.1. Hydraulic macroscopic crack characteristics under the pulsating load Fig. 3 shows the shape of the five tested coal specimens at the end of the experiments. It can be seen that macroscopic main cracks distributed on the specimen surface and extended in the direction of the injection pipe (σ2) pointing to the edge of specimen. This satisfies the conditions of rock failure criterion, i.e. main cracks of the specimen always initiate and propagate along the direction perpendicular to the minimum prin cipal stress (σ3) (Su et al., 2005). But it can be also found that the main crack of A1 (f ¼ 0 Hz) only propagated on one side of the fracturing pipe, which indicates that the pulsating shock has some influence on crack formation, but the main crack is mainly caused by the stress field. On the other hand, we can also see from Fig. 3, the fracture shape of five specimens were different, thus, the specimens were sectioned along the fracture plane to investigate their internal information, as shown in Fig. 4. As presented in Fig. 4, it could be clearly found the infiltration distribution for each specimen. Fig. 4 (A1), Fig. 4 (A2), Fig. 4 (A3), Fig. 4 (A4) and Fig. 4 (A5) represents the rupture plane morphology of spec imens at pulsating frequency of 0 Hz, 2 Hz, 4 Hz, 6 Hz and 8 Hz, and we can differentiate different areas affected by the fracturing fluid by using with the ImageJ software to get the binary image. Then the affected areas can be divided into three parts: Grey (A zone), Dark grey (B zone) and bright black area (C zone). Grey (A zone) indicates that fracture surface was dry, that is, there is no fracturing fluid available to infiltrate into this section. Dark grey (B zone) indicates the area where the frac turing fluid invaded. And bright black area (C zone) in B area indicates that the infiltration amount of the fracturing fluid were the maximum. Therefore, as can be seen from Fig. 4, the specimen A1 (f ¼ 0 Hz) shows the smallest area of B zone. This means the distribution area of frac turing fluid using conventional fracturing (A1) is smaller than that of using PHF (A2, A3, A4 and A5), namely, the crack can be better devel oped under pulsating hydraulic load. Besides, the area of B zone for the specimen A2 (f ¼ 2 Hz), A3 (f ¼ 4 Hz) and A4 (f ¼ 6 Hz) accounts about 4/5 of the whole fracture plane respectively, but the specimen A5 (f ¼ 8 Hz) shows a smaller area of B zone. This experimental phenomenon indicates that the distribution of fracturing fluid and the crack devel opment are both different with the different pulse frequencies. Furthermore, it can also be seen from the area of C zone for each spec imen that, the fracturing fluid is not infiltrated along the same direction on one rupture plane, that is, its propagation direction is not single. We can find that, the C zone of specimen A5 (f ¼ 8 Hz) is mainly distributed on the right side of the fracturing pipe and its C zone color is lighter. Also, compared to specimen A5 (f ¼ 8 Hz), the C zone of specimen A2 (f ¼ 2 Hz), A3 (f ¼ 4 Hz) or A4 (f ¼ 6 Hz) is distributed more unevenly, and specimen A3 (f ¼ 4 Hz) shows the most unevenly distributed C zone. It is worth to note that the more uneven distribution of C zone, the more complicated of crack development will be. Hence, it can be concluded that the internal fracture network complexity of specimen is highest when the pulsating frequency is 4 Hz.
2.2. Specimen design and production Concrete specimens were made to replace the actual coal by using similar principles. The specimen was mainly composed of quartz sand, pulverized coal, cement and gypsum, and a small amount of calcium sulphate retarder to regular the curing time. The best addition ratio of the concrete materials was determined by lab experiments and me chanical experiments, therefore, it was finally determined to be quartz sand: pulverized coal: cement: gypsum ¼ 36:18:23:23. The mechanical properties of the concrete samples are shown in Table 1. For the tests in this paper, several 300 mm � 300 mm � 300 mm cubic specimens (Fig. 2 (b)) were made. Fig. 3 shows the test samples used in the tests. After the specimen was cured, a 160 mm deep borehole was drilled in the center of the top face using a ϕ32mm drill bit (Fig. 2 (c)), in which a 6 mm inner diameter fracturing pipe was placed. The fracturing section is 30 mm long and 32 mm in diameter, and it was formed by sealing the 130 mm long hole with AB adhesive in the middle (Fig. 2 (d)) (Chen et al., 2000; Lin and Du, 2011; Li et al., 2014). Besides, to prevent the fracturing section from being blocked, we inserted a small steel spacer and a rubber blanket 30 mm away from the end of the pipe. 2.3. Experiment set-up and procedure The PHF process is implemented as follows: firstly, put the coal block in the triaxial loading frame, with three oil cylinders applying load on six steel plates on the platform (Fig. 2(f)). Next, after the stabilization of the stresses in three directions, set a frequency for the test and start all the monitoring systems and pumping system. At this moment, the fracturing fluid is pumped out from the water tank and flow through the metering pump. Then one part of fluid flows back to the water tank (Fig. 1-①), and the other part flows into the pilot-operated solenoid valves (Fig. 1-②), thus running out with the predefined frequency. Finally, after the fluid in Line ② flow through the one-way value and shut-off value, it enters into the sample. The test will be terminated when fluid flowed out of the specimen, which indicates the rupture of the specimen. And during testing, the data collection system will auto matically monitor and record pressure, AE signal, time, and also export curves of time-pressure and time-acoustic emission signal in real time. Five frequencies, 0 Hz, 2 Hz, 4 Hz, 6 Hz and 8 Hz, are chosen to test on specimen 1–5, respectively.
3.2. Response characteristics of pressures and AE parameters during the PHF process
Table 1 Mechanical properties of the test samples (average). Uniaxial compression strength/MPa
Elasticity modulus/GPa
Poisson’s ratio
Tensile Strength/MPa
7.31
3.35
0.191
0.807
During the whole PHF process, the initiation, extension and final rupture of the specimen will always be accompanied by the accumula tion and release of energy. The cumulative AE energy is the accumula tion of energy released from the beginning of the test to a certain time. It can help us to deeply understand the fracture evolution law. Therefore, 3
J. Wu et al.
Journal of Natural Gas Science and Engineering 75 (2020) 103159
Fig. 2. Test specimen states. (a) The sketch diagram of the specimen, (b) the cubic specimen ready for drilling, (c) specimen drilling in the center of the specimen, (d) sealed specimen, (e) positions of AE sensors and (f) the specimen was placed in the triaxial loading system, in which AE sensors were well arranged.
we can see from Fig. 5, based on the cumulative AE energy, the PHF process can be divided into four stages:
plastic deformation. Meanwhile, under the effect of the confining pressure and the pulsed pressure, the AE phenomenon tended to be more active because of the continuous evolution of the internal micro-cracks. Finally, the fatigue damage of the coal specimens continued to accumulate until the pressure reached the speci mens’ fracture pressure. At that time, the AE energy rate reached the maximum value. Also, the slope of the energy curve began to increase. It was worth to note that the accumulated energy of this period increased about 4.81 times (average) in comparison with the quiet phase. (c) Lifting phase③: After the initiation of the fracture, the fracturing fluid entered into the fracture and the pressure would drop gradually. However, under the continuous pulsating effect, the original cracks and the new cracks in the coal specimens grew and developed further. And then, the cracks were gradually con nected and interpenetrated. A lot of cracks were developed into macro cracks and finally the coal specimen was fractured. At this
(a) Quiet phase①: During this period, the fracturing fluid entered into the pre-existing fractures and pores in the specimens through a porous flow pattern. The pressure did not rise and there was no significant damage to the specimen. So only sporadic acoustic emission events appeared at this stage, this may be due to friction between the pulsating flow and the pipe. (b) Acceleration phase②: At this stage, after the crack and pores were full of fluid, a cyclical impact with a fixed frequency started to act on the specimens with the increase of injection pressure. Some fatigue micro-cracks were generated in coal samples around the hole. The cumulative AE energy started to increase due to the initial damage in the coal specimens. Then, as the water pressure in the hole increases, fatigue damage was induced in the coal specimens, and the specimens entered the stage of 4
J. Wu et al.
Journal of Natural Gas Science and Engineering 75 (2020) 103159
Fig. 3. Shape of cracks on specimen surfaces.
under different frequencies. As presented in Fig. 7, the initiation pres sures of the five specimens were about 7~8 MPa and showed little change under different frequencies. This was because the sphere of ac tion of fracturing fluid was limited to the perforation section with small area. And compared to the whole specimen, the perforation section looked minor. Thus, the fracturing fluid would fill up the perforation section rapidly and the crack occurred within a very short time. In this case, only little pulsating effect acted on the specimen, resulting in the pressure with little change under different frequencies. Besides, it can be seen from Fig. 7, crack initiation pressure of specimen with 0 Hz is larger than that of specimens with other frequencies. This means that the crack initiation pressure of PHF is smaller than that of conventional HF. This is in accord with the existing results (Zhai et al., 2011). On the other hand, though the initiation pressures showed little change under different frequencies, it could still be found that the initiation pressures firstly declined and then rised with the increase of frequency, and reached the minimum value when the pulsating fre quency was 4 Hz. Actually, compared to the HF, the PHF also includes the pulsating effect part. This part will cause fatigue damage on the specimen, and the damage is related to the pulsating frequency and the amplitude. Xiao et al. (2009) and Meng (2011) indicated that the higher the pulsating frequency, the more damage the specimen accumulated, and the larger the amplitude, the more damage the specimen accumu lated when crack extending. It is observed that the increase of pulsating frequency or the amplitude is beneficial to the crack expansion and the rock breaking efficiency. However, in the actual PHF process, it is difficult to implement overlarge frequency or overlarge amplitude, in which case the equipment is also highly demanding. What’s more, high frequency or high amplitude means a raise in loading rate (Xiao, 2009), resulting in fracturing process similar to conventional HF. And in this case, the crack has no enough time to propagate and only extends, connects and even broken along the main cracks. But the goal of the PHF is attempt to form more complicated crack network. As a result, optimal pulsating frequency and its suitable amplitude can make the specimen forming a complex crack network. In this paper, the pulsating generator of the experimental system has a typical character: the higher the fre quency, the smaller the amplitude. In other words, when the pulsating
Table 2 Summary of experimental parameters. Sample profile
Vertical stress (σ1)/ MPa
Maximum horizontal stresses (σ2)/MPa
Minimum horizontal stresses (σ3)/MPa
Injection flow rate/ (mL⋅min 1)
Frequencies (f)/Hz
A1 A2 A3 A4 A5
5.5 5.5 5.5 5.5 5.5
3 3 3 3 3
1 1 1 1 1
10 10 10 10 10
0 2 4 6 8
stage, the intensifying interactions between cracks, and the highly active AE activity necessitated a radical increase of the cumulative energy. Therefore, the slope of the energy curve increased obviously. It was worth to note that the accumulated energy of this period increased about 17.21 times (average) in comparison with the quiet phase. (d) Resting phase④: At this stage, the specimen was completely broken and the extreme structural motion stopped, so the AE phenomenon began to flatten. 3.3. Crack initiation and propagation characteristics during PHF process The crack initiation and propagation law during the PHF are the two basic and key questions to study the crack dynamic evolution (Li, 2010). And it is noted that the crack inner pressure vs. time for coal specimen during PHF can better reflect the crack initiation and propagation characteristics. So the objective of this section is to study the crack initiation and propagation characteristics by the pressure vs. time curve showed in Fig. 6. As shown in Fig. 6, the crack initiation stage and propagation stage correspond to stage A and stage B respectively in Fig. 6. 3.3.1. The effect of pulsating frequency on the crack initiation pressure According to Fig. 6, it is easy to know that each pulsating frequency corresponds to an initiation pressure. Fig. 7 shows the initiation pressure 5
J. Wu et al.
Journal of Natural Gas Science and Engineering 75 (2020) 103159
Fig. 4. Specimen shape after pulsating hydraulic fracturing.
frequency is small, the pulsation number is small but the strength of a single pulsation is great. On the contrary, when the pulsating frequency is larger, the pulsation number is large but the strength of a single pulsation is small. This can be considered that there is a threshold for the pulsating frequency with the increase of the frequency. When the fre quency is less than the threshold, the strain energy of the specimens will increase as the increase of the frequency. On the contrary, when the frequency is larger than the threshold, the strain energy will decrease as the increase of the frequency. Hence, it can be concluded that in this test case, the threshold of pulsating frequency was 4 Hz. When the pulsating frequency was less than 4 Hz, the damage accumulation of specimens would decrease with the increase of frequency, resulting in smaller initiation pressure. On the contrary, the initiation pressure became correspondingly larger and larger. This is why the initiation pressure exhibited minimum when the pulsating frequency was 4 Hz, using this set-up.
pressure cycles was shown in Fig. 8 (Fjær and Holt, 1994). When the elastic deformation occurred in the formation around borehole under the action of fracturing fluid, the crack started to initiate (the sharp point in Fig. 8), then the pressure was released and dropped rapidly. In the second period, the tensile strength of specimen was 0 due to the existing of cracks, thus, the crack could propagate further only required to resist stress concentration around borehole. Therefore, during the PHF pro cess, the rock mass around borehole was subjected to dynamic fluctu ating loads, which can easily made multiple cracks initiate and grow. And after the crack initiation, the tensile strength of specimen would decrease, thus the initiation pressure of the subsequent hydraulic crack would decrease as well. 3.4. Dynamic characteristic of b-value during the PHF process Amplitude, which is the largest measured voltage in a waveform (expressed in dB), can represent the strength of the AE events (Zhang et al., 2011). AE amplitude has a close relationship with the frequency-magnitude coefficient b (Li et al., 2009; Yan, 1994). It is known from the literature (Li et al., 2010) that the b-value is the function of the extension size of cracks and can indicate the distribution of micro-cracks. It is also found that the AE events during the rock failure process can be considered as seismic activity (micro-quake) and the measurement of AE is similar to the measurement of some seismic physical quantities. The b-value of AE events (Zhang et al., 2015) can be calculated through Eq. (1):
3.3.2. The effect of pulsating frequency on the crack propagation stage As presented in Fig. 6, specimen A1 (f ¼ 0 Hz) showed few recurrent fluctuation at the B stage. And the pressure of specimen A2 (f ¼ 2 Hz), A3 (f ¼ 4 Hz), A4 (f ¼ 6 Hz) and A5 (f ¼ 8 Hz) had some fluctuation, and their fluctuation numbers were about 3, 5, 3 and 1 respectively. One fluctuation meant the crack changed from closed state to open state. Thus, it indicated that the probability and frequency of the crack propagation in the process of PHF occurred more often than that in the process of HF. That is to say, the efficiency of sample rupture was higher when using PHF, and the most fluctuation number of A3 (f ¼ 4 Hz) meant the high working efficiency when the pulsating frequency was 4 Hz. On the other hand, when the pulsating frequency was 8 Hz, the injection rate was relatively large. The PHF process was similar to the traditional HF, this meant the fluctuation type of specimen A5 (f ¼ 8 Hz) was similar to the specimen A1 (f ¼ 0 Hz). This was why the specimen A5 (f ¼ 8 Hz) showed only one obvious fluctuation. On the other hand, from Fig. 6, it is worth to note that the next fluctuation was always “lower” than that of the previous fluctuation when under pulsating effect. In other words, after one crack initiation, the initiation pressure of the new crack would become smaller than that of the existing crack. It can be explained, that as the fracturing fluid was pumped into the specimen, the borehole pressure changes in two
logðNðCÞÞ ¼ a
blogðCÞ
(1)
where C is the amplitude of AE events during the PHF process, N(C) is sum of AE events in the group whose amplitude is greater (or equal to) than C, and a and b are constants. As found in Zeng et al. (1995), an increase in the b-value means a rise of the proportion of minor events and micro-cracks of smaller size. A constant b-value means the relative balanced distribution of micro-cracks with different sizes. And a drop of b-value means the in crease of a proportion of large events and micro-cracks in large-size. In this paper, we explore the dynamic feature of b-value to illustrate the crack evolution characteristics during the PHF process and compare its evolution difference under different pulsating frequency. 6
J. Wu et al.
Journal of Natural Gas Science and Engineering 75 (2020) 103159
Fig. 5. Changes in pressure and AE parameters during the PHF process under (a) f ¼ 0 Hz, (b) f ¼ 2 Hz, (c) f ¼ 4 Hz, (d) f ¼ 6 Hz, (e) f ¼ 8 Hz
According to Eq. (1) and data from the lab tests, b-values were ob tained by using the least square method where 200 data was selected as a sampling group and 50 data was used as the step-size. Fig. 9 gives the relationship of b-value with time (turning point of four stages) for the PHF process with different frequencies. We observe from Fig. 9 that the overall trend of the b-value changes under different frequencies was similar in the whole process. It meant that the b-value increased in the acceleration phase, then decreased in lifting phase and finally basically unchanged in steady period. This in dicates that the proportion of minor AE events was higher in accelera tion period, and the proportion of great AE events was higher in boosting and steady period. It can be explained, that the initiation and extension of micro-cracks is dominant in acceleration phase, leading to a sharp increase of micro-cracks in small size but a slow increase of micro-cracks in large size. The b-value would increase in this case. Then cracks began to become connected and interpenetrated and many cracks developed
into macro cracks in the lifting phase. In this case, the increment speed of micro-cracks in large size exceeded the increment speed of microcracks in small size, which meant a decrease of b-value. And finally the coal specimen was completely fractured. The size distribution of micro-cracks was relative balanced, so the b-value remained stable at the final stage. As presented in Fig. 9, the b values have different characteristics under different frequencies. Specimen A3 (f ¼ 4 Hz) showed the most obvious features of b-value at three stages, and the b value was maximum during the whole fracturing process. This indicated that the proportion of small AE events was higher at this pulsating frequency than that of other frequencies, and also the change of its AE amplitude distribution was more “violent”. The three-stage characteristic of spec imen A5 (f ¼ 8 Hz) became less apparent, and the b value was minimum and showed a relatively smooth tendency in most of the fracturing process. Namely, during the fracturing process of specimen A5, the 7
J. Wu et al.
Journal of Natural Gas Science and Engineering 75 (2020) 103159
Fig. 6. Crack inner water pressure vs. time for coal samples during PHF, all of which are completed in three steps: crack initiation stage (A), crack propagation stage (B), break stage (C).
8
J. Wu et al.
Journal of Natural Gas Science and Engineering 75 (2020) 103159
proportion of great AE events was higher at this pulsating frequency than that of at other frequencies. Also, the whole process embodies an incremental stable extension process. Besides, the b-value characteris tics of specimen A2 (f ¼ 2 Hz) and specimen A4 (f ¼ 6 Hz) were similar, and they were intermediate between specimen A3 and specimen A5. However, the small AE events proportion of specimen A4 was slightly higher than that of specimen A2, also the AE amplitude of specimen A4 changed a little more drastic. All of these experimental phenomena may be caused by the following reasons, and here taking typical specimen A5 (f ¼ 8 Hz) and specimen A3 (f ¼ 4 Hz) for example. When the pulsating frequency was 8 Hz, high frequency means a raise in loading rate (Xiao, 2009), resulting in a similar fracturing process as the traditional HF. In this case, the pulsating fluid was continuously injected into the specimen at high pressure, leading to single main fracture developing inside the specimen and relatively stable process of crack propagation. Thus, bvalue was small and it changed within a small range. On the other hand, when the pulsating frequency was 4 Hz, the initiation pressure of specimen A3 (f ¼ 4 Hz) was the smallest, in which case the micro-crack had a longer time to propagate further to form a complicated fracture network. Hence the proportion of small AE events gradually increased. After reaching the fatigue strength of specimen A3, lots of micro-cracks become connected and interpenetrated and many cracks developed into macro cracks, and thus large AE events occurred. This whole process seemed to be a relatively “intense” activity and the AE amplitude changed obviously. Similarly, the analyzed method can be used to explain the above phenomena for specimen A2 (f ¼ 2 Hz) and specimen A4 (f ¼ 6 Hz).
Fig. 7. Initiation pressure under different frequencies.
4. Conclusions This paper presented an experimental methodology to evaluate the crack generation ability of PHF technology in coalbed methane reser voir. Several cubic specimens (300 � 300 � 300 mm3) were made. Five sets of tests, where the pulsating frequency was considered as a signif icant factor, were conducted to study the crack dynamic evolution characteristics. The main conclusions are as follows: (1) Hydraulic macroscopic crack characteristics under the pulsating load show that, the crack is perpendicular to the direction of the minimum principal stress (σ3). And the affected area of fracturing fluid using conventional fracturing (A1) is smaller than that of using PHF, which means the crack could be better developed under pulsating hydraulic load. Also, the internal fracture network complexity of specimen is highest when the pulsating frequency is 4 Hz, using this setup. (2) The evolution of cumulative AE energy during the PHF process can be divided into four stages: quiet phase, acceleration phase, lifting phase and resting phase. It was worth to note that the accumulated energy of acceleration period and boosting period increased respectively about 4.81 and 17.21 times (average) in comparison with the tranquil period. (3) Comparing to the traditional HF, the initiation pressure of the fracture was smaller and the efficiency of sample rupture was higher when using the PHF technology. Meanwhile, the initiation pressure decreased firstly and then increased with the increase of pulsating frequency, and it reached its minimum when the pul sating frequency was 4 Hz. Also at this pulsating frequency (4 Hz), the probability and frequency of the crack propagation is large, thus the working efficiency is high in this case. (4) The variation of the b-value was similar under different fre quencies during the whole process. The proportion of small-size cracks was greater in the acceleration period, while the propor tion of large-size cracks was higher in the boosting and steady periods. Our results also show a great change in AE amplitude distribution when the pulsating frequency was 4 Hz, namely, the growth and development of the micro-crack was a little more
Fig. 8. Borehole pressure changes in two pressure cycles under ideal conditions (Fjær and Holt, 1994).
Fig. 9. b value vs. time for pulse fracturing process with different frequencies.
9
J. Wu et al.
Journal of Natural Gas Science and Engineering 75 (2020) 103159
“violent”. Meanwhile, the b value was large at this pulsating frequency, which means a great proportion of small AE events occurred throughout the fracturing process.
Li, Q.G., 2015. Research on the Evolution Law of Coal Fracture under Pulse Loading and its Application on Gas Extraction. PhD theses. China University of Mining and Technology, pp. 22–34. Lin, B.Q., Li, Z.W., Zhai, C., Bi, Q., Wen, Y.Q., 2011. Pressure relief and permeabilityincreasing technology based on high pressure pulsating hydraulic fracturing and its application. Journal of Mining & Safety Engineering 28 (3), 452–455. Lin, H.X., Du, C.Z., 2011. Experimental research on the quasi three-axis hydraulic fracturing of coal. J. China Coal Soc. 36 (36), 1801–1805. Lu, P.Q., Li, G.S., Huang, Z.W., Li, X.J., Tian, S.C., Song, X.Z., Sheng, M., 2015a. A numerical analysis of the features of the stress disturbance during pulsating hydraulic fracturing in coal seams. In: SPE Technical Conference and Exhibition, Houston, Texas, USA, pp. 1–15. Lu, P.Q., Li, G.S., Huang, Z.W., Tian, S.C., Shen, Z.H., Li, X.J., 2015b. Numerical simulation for stress disturbance features of coal beds during pulsating hydrofracturing. J. Vib. Shock 34 (21), 210–216. Lu, P.Q., Li, G.S., Huang, Z.W., He, Z.G., Li, X.J., Zhang, H.Y., 2015c. Modeling and parameters analysis on a pulsating hydro-fracturing stress disturbance in a coal seam. J. Nat. Gas Sci. Eng. 26, 253–263. Meng, J.Q., 2011. Study on the Theory and Technology of Coal Seam High Pressure Pulsation Water Infusion Preventing Coal and Gas Outburst. PhD theses. China University of Mining and Technology, Beijing), pp. 82–86. Ni, G.H., 2015. Microscopic Kinetics Characteristics of Methane under Pulsating Hydraulic Fracturing and Mechanism of Liquid Retention Effect. PhD theses. China University of Mining and Technology, pp. 18–30. Nie, B., He, X.Q., 2009. Methods and Equipment for Prevention of Coal Dynamic Disasters by High Pressure Pulsating Water Injection. Chinese Patent. CN 101539028 A. Polito, F., Petri, A., Pontuale, G., Dalton, F., 2010. Analysis of metal cutting acoustic emissions by time series models. Int. J. Adv. Manuf. Technol. 48 (9–12), 897–903. Song, Y., Liu, S.B., Hong, F., Qin, S.F., 2012. Geochemical characteristics and genesis of coalbed methane in China. Acta Pet. Sin. 33 (s1), 99–106, 2012. Su, X.B., Ning, C., Hua, S.L., 2005. Fluid pressure-dependent fractures in coal-bed gas reservoirs. Nat. Gas. Ind. 25 (1), 127–129. Wu, J.J., Zhang, S.H., Cao, H., Kemeny, J., 2016. The effect of pulse frequency on the acoustic emission characteristics in coal bed hydraulic fracturing. In: 50th U.S. Rock Mechanics/Geomechanics Symposium, Houston, Texas, USA. Xiao, J.Q., 2009. Theoretical and Experimental Investigation on Fatigue Properties of Rock under Cyclic Loading. PhD theses. Central South University, pp. 63–64. Yan, Z.G., 1994. Robust estimation of the b-value of magnitude-frequency relationship. Earthq. Res. China 10 (4), 320–326. Zeng, Z.W., Ma, J., Liu, L.Q., Liu, T.C., 1995. AE b-Value dynamic features during rock mass fracturing and their significances. Seismol. Geol. 17 (1), 7–12. Zhai, C., Li, Z.X., Li, Q.G., 2011. Research and application of coal seam pulse hydraulic fracturing technology. J. Chin. Soc. 36 (12), 1996–2001. Zhang, J.S., 2009. Project Report of High Pressure Pulsating Hydraulic Fracturing Technology for Difficult Drawing Coal Seam. China University of Mining and Technology, pp. 57–58. Zhang, H.H., Liu, F., Chang, F.Q., 2011. Experimental study of acoustic emission characteristics during rock failure process and energy properties. J. Highw. Transp. Res. Dev. 28 (3), 48–54. Zhang, Z.P., Zhang, R., Li, H.G., 2015. Experimental research on effects of bedding plane direction on coal acoustic emission characteristics under uniaxial compression. Chin. J. Rock Mech. Eng. 34 (4), 1–9. Zhao, Z.B., 2008. Study of technology of variable-frequency pulse water infusion into coal seam. J. Min. Saf. Eng. 25 (4), 486–489. Zhu, H.Q., Shen, J., Zhang, M.B., Feng, S.L., Xu, J.Y., 2013. Simulation of coal inducecrack characteristics on the conditions of pulsatile and constant pressure water injections. Sci. Technol. Rev. 31 (18), 57–62.
Authors’ contributions Jingjing Wu: Conceived and designed the study, design of meth odology, data analysis, draft manuscript writing. Shaohe Zhang: Oversight and leadership responsibility for the research activity plan ning and execution, acquisition of the financial support, design of methodology, data analysis, manuscript review and editing. Han Cao: Conceived and designed the study, data analysis, manuscript review and editing. Mingming Zheng: Design of methodology, data analysis, manuscript review and editing. Feilong Qu: Field sampling, performed the experiments, manuscript review and editing. Canwei Peng: Artifi cial samples preparation, performed the experiments, manuscript re view and editing. All authors read and approved the manuscript. Acknowledgments This work is sponsored by the National Natural Science Foundation of China (No. 41872186, 41602372, 41702389); and Open Research Fund Program of Key Laboratory of Metallogenic Prediction of Nonfer rous Metals and Geological Environment Monitoring (Central South University), Ministry of Education (No. 2019YSJS05, 2018YSJS09); and Scientific Research Fund of Hunan Provincial Education Department (No. 18C0541). References Chen, M., Pang, F., Jin, Y., 2000. Experimental and analysis on hydraulic fracturing by a large-size triaxial simulator. Chin. J. Rock Mech. Eng. 19 (s1), 868–872. Fjær, E., Holt, R.M., 1994. Rock acoustics and rock mechanics: their link in petroleum engineering. Lead. Edge 31 (31), 255–258. Huang, S.C., et al., 2009. Coalbed methane development and utilization in China: status and future development. China Coal 35 (1), 5–10. € Ruiz, F.A., Cot�e, M., Phipps, S., 2011. Coal mine methane: a review of Karacan, C.O., capture and utilization practices with benefits to mining safety and to greenhouse gas reduction. Int. J. Coal Geol. 86 (2), 121–156. Li, Y.H., Liu, J.P., Zhao, X.D., Yang, Y.J., 2009. Study on b-value and fractal dimension of acoustic emission during rock failure process. Rock Soil Mech. 30 (9), 2559–2574. Li, W., 2010. Research on Reservoir Characteristics and Fracture Initiation Mechanism Based on Fractal Theory. Master theses. Daqing Petroleum Institute, p. 5p. Li, S.Y., He, T.M., Yin, X.C., 2010. Study on the nucleation evolution of micro fracture using acoustic emission method. In: Introduction of Rock Fracture Mechanics, China, pp. 372–374. Li, Z.X., Lin, B.Q., Zhai, C., Li, Q.G., Ni, G.H., 2013. The mechanism of breaking coal and rock by pulsating pressure wave in single low permeability seam. J. China Coal Soc. 38 (6), 918–923. Li, Q.G., Lin, B.Q., Zhai, C., 2014. The effect of pulse frequency on the fracture extension during hydraulic fracturing. J. Nat. Gas Sci. Eng. 21, 296–303.
10