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Experimental study of pulsating water pressure propagation in CBM reservoirs during pulse hydraulic fracturing
Journal of Natural Gas Science and Engineering 25 (2015) 15e22
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Experimental study of pulsating water pressure propagation in CBM reservoirs during pulse hydraulic fracturing Cheng Zhai a, b, Xu Yu a, b, *, Xianwei Xiang a, b, Quangui Li a, b, Shiliang Wu a, b, Jizhao Xu a, b a b
State Key Laboratory of Coal Resources and Safe Mining. Xuzhou, Jiangsu, 221116, China School of Safety Engineering, China University of Mining & Technology, Xuzhou, Jiangsu, 221116, China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 16 December 2014 Received in revised form 20 April 2015 Accepted 21 April 2015 Available online
Pulse hydraulic fracturing (PHF) technology can effectively improve reservoir permeability for coal bed methane (CBM) extraction. In this work, laboratory experiments were conducted to study the laws regarding pulsating water pressure (PWP) propagation during PHF in a CBM reservoir at different pulse frequencies and plugging rates. The results demonstrate that the average PWP is stable over the length of the reservoir fissure, and the PWP peak primarily depends on the PWP amplitude. The experimental results show that for a non-plugging fissure, the PWP peak pressure first decreases and subsequently increases with the maximum pressure occurring at the fissure tip. When the fissure is blocked, a portion of the pressure waves are reflected in front of the blockage, forming reflection waves, which interact with the incident waves and lead to an increase in the PWP peak pressure. When the plugging rate is 95%, there is a significant difference in the PWP peak pressure in front of and behind the blockage because of the pressure wave reflections and interactions. As the plugging rate decreases, the blockage effects are reduced, and the wave proportion that is reflected decreases such that the transmission and creeping wave components increase. When the plugging rate is 65%, the PWP peak pressure behind the blockage is larger than that prior to the blockage. The findings build a basis to predict and control fissure extensions to improve the effect of PHF by optimizing the technical parameters. © 2015 Elsevier B.V. All rights reserved.
Keywords: Pulse hydraulic fracturing Pulsating water pressure Parallel-straight fissure Coal bed methane Plugging rate
1. Introduction The proven reserves of the CBMs in China are 37 trillion cubic meters, ranking third in the world. Unfortunately, the Chinese CBM reservoirs have low permeability, low porosity and high in-situ stress owing to the effects of a complex geologic structure and burial conditions. This leads to low concentrations, low flow and the fast decay of the CBM extraction. These problems make it difficult to exploit CBM in China. In 2013, the CBM production was 15.6 billion cubic meters, whereas the effective CBM utilization ratio was less than 50% for low concentrations. The percentage of underground CBM output is 80.8%, and the percentage of surface drilling output is less than 20%; therefore, the CBM extraction methods used in China are presently performed largely via underground gas extraction in coal mines (Li et al., 2015; Luo and Dai, 2009; Xia et al., 2014; Xu et al., 2013). Underground gas extraction
* Corresponding author. Daxue Road, Xuzhou City, Jiangsu Province, 221116, China. E-mail address: [email protected] (X. Yu). http://dx.doi.org/10.1016/j.jngse.2015.04.027 1875-5100/© 2015 Elsevier B.V. All rights reserved.
is also an important method to prevent gas hazards in coal mines. However, ordinary gas drilling is characterized by a small extraction radius, a small flow and fast attenuation due to the low permeability of the coal seam. Many scholars, both in China and abroad, have put forth hydraulic measures to improve coal seam permeability, such as hydraulic fracturing (Huang et al., 2012, 2014, 2011; Majdi et al., 2012; Wang et al., 2014; Zhang and Chen, 2010), hydraulic flushing (Lu et al., 2010; Wang and Li, 2012), hydraulic slotting (Shen et al., 2012; Zou et al., 2014), etc., which have yielded some positive results. However, with improvements in the level of safety requirements in underground coal mining, these technologies have yielded some negative effects, the most outstanding of which is the high pressure required that increases the number of underground hazards. To avoid using high pressure, Lin et al. introduced pulse hydraulic fracturing (PHF) technology that integrates the advantages of pulsating loading and hydraulic fracturing. The main characteristic of PHF is that coal fissures can be extended by the pulsating loading of pulsating water pressure (PWP) using a lower pressure to accelerate the extension of a fissure and effectively improve the permeability of a coal seam (Li
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et al., 2013a, b; Lin et al., 2012; Zhai et al., 2011). Little research has been conducted on PHF technologies used in CBM extraction, and the existing results come mostly from field experiments that examine the macroscopic characteristics of the PHF including the initial fracture pressure, the fracturing radius and the fissure extending process. These results have led to many meaningful conclusions (LI et al., 2013; Li et al., 2014); however, the laws of PWP propagation in coal fissures of CBM reservoirs have not yet been studied. Similar studies appeared in the hydraulic engineering and water resource fields (Jiang and Liang, 1983; Li and Liu, 2006, 2008; Qian et al., 2005; Zhao and Liang, 1988), including a single fissure model of a dam batholith to examine the break mechanism, (Li et al., 2011; M. and S., 1995; Qian et al., 2005; Zhang et al., 2013a, 2013b) adopted a approach to address the groundwater flow laws in a narrow channel. These references are very valuable and allow a model to be established for the coal fissures in this paper. The fissures of CBM reservoirs are random in shape and size; they are classified into coal seam beddings, joints and other types. Different types of fissures may cross each other to form a complex fissure network. Existing studies show that the CBM reservoirs are first fractured along the coal seam fissures (Ide et al., 2010; Zhang et al., 2013a, 2013b). To study the PWP propagation in CBM reservoirs, the process of PWP propagation must be simulated in coal seam fissures. A fissure with a blind end is chosen, which acts as a representative type of coal fissures, and it was simplified into “double parallel planes” with a closed tip (namely, a parallelstraight fissure), as shown in Fig. 2. It was found that the fissures are filled with water prior to extension. When PHF is implemented in a coal mine, the PWP propagates in a wave-form through the water medium and is affected by the fissure size changes and blockage state (Ashour, 2000; Ionov, 2007). When mining or drilling occurs in an underground coal mine, crustal stress is redistributed, and coal particles flow into the fissures, causing coal fissure size and shape changes. This occurs because of the fissure cross section being shrunken or blocked. According to Kabir et al. (2004, 2006) and Teyssedou et al. (2005), blockages have a significant influence on the PWP propagation in fissures. Therefore, it is necessary to perform research on the laws of PWP propagation for different blockage states in CBM reservoirs. In this paper, PWP propagation experiments were conducted using a parallel-straight fissure model. The experiments were performed for no blockage conditions and different plugging rates. The results provide a theoretical basis for optimizing the PHF technology parameters and improving overall CBM extraction. 2. Materials and methods 2.1. Experimental setups The experimental setup for PWP propagation in a parallelstraight fissure includes the PWP generator, the parallel-straight fissure model, a data collector and a high-frequency pressure sensor, as shown in Fig. 1. The PWP generator consists of a pulsating pump, a frequency controller, a water tank, an overflow valve and a pressure meter. The pulsating pump outputs pressure water, namely PWP, from 0 to 6 MPa. The frequency controller is used to change the input frequency of the pulsating pump over a range of 0e20 Hz with an adjustable accuracy of 0.4 Hz. The overflow control valve adjusts the export flow of the system, controlling the PWP value. The mean pressure represents the pressure meter reading and is an important controlling parameter for the PHF process in a real scenario. The parallel-straight fissure model is welded from 304 stainless steel. In this work, the CBM reservoir was assumed to be a closed system, leakage of the fracture liquid
was ignored. Therefore, stainless steel is used as the model material. The model is 4.5 m in length and has a rectangular crosssection of 100 mm 5 mm. The largest pressure the pipe can handle is 10 MPa. The experiments were divided into two parts: (1) a test of the PWP propagation in a non-plugged fissure, and (2) a test of the PWP propagation for various plugging rates. The plugging rate is defined as the difference between the cross-sectional area A of the fissure and the opening area A1 of the blockage divided by A, namely d ¼ (A A1)/A, of the parallel-straight fissure. There are seven pressure ports on the fissure, P1 ~ P7 shown in Fig. 1, that measure the changes in the PWP propagating in the direction of the fissure. The pressures at the seven points are measured using high-frequency digital sensors with a measurement error of less than 1%. The data are simultaneously collected for the seven different sensors and transferred in real-time to a computer. 2.2. Experimental methods Fig. 2 shows a profile of the parallel-straight fissure model (shown in Fig. 1). The model is divided into two parts: the parallel fissure and the fissure end. 5 mm is the fissure opening value. The experiments were conducted under the same operating conditions to investigate the laws of PWP propagation in the fissure at the same frequencies. The opening of overflow valve is fixed during the experiments. So the PWP changing, as a result of changing the operating conditions, is ignored. The experiments were divided into two groups: (1) the PWP propagated in a non-plugging fissure at a pulsation frequency of 16 Hz with two verification tests at 12 and 20 Hz; (2) PWP propagated in a plugging fissure at various plugging rates, 65%, 75%, 85% and 95%. The process used was as follows: (1) Experiments on PWP propagation in a non-plugging fissure The non-plugging fissure model is defined as having no blockages (Fig. 2). Seven high-frequency pressure sensors were installed on the fissure model with the parameters shown in Table 1. Prior to performing the experiments, preparation is required. At the beginning of a test, power is turned on, and the vacuum pump is allowed to run for half an hour to remove any air from the fissure model under a negative pressure of 0.1 MPa. Then, the input frequency was set to 20 Hz by the frequency controller. The pulsating pump is started for a certain set of conditions. After the pump stabilized, data collection was activated. The pulsating pump and data collector are stopped in an orderly manner every 2.5 min when the data collector functioned properly. Every test group was repeated three times to reduce errors. To verify the experimental results, two contrasting tests were performed at 12 and 20 Hz. (2) Experiments on PWP propagation in a plugging fissure From Fig. 2, the plugging fissures were defined in the fracture model as fissures with different blockages. The plugging rate is the only variable in experiments; all other parameters, such as the frequency (16 Hz) and the overflow valve opening, remained the same. The blockage in the fissure was simulated using flappers with different opening sizes. To eliminate any effects at the opening position, the openings were symmetrically distributed on the flappers. Four plugging rates of 65%, 75%, 85% and 95% were chosen to function as blockages (Table 2). The tests were conducted identically as the experiments in the non-plugging fissure. To verify the experimental results at different frequencies, a test was designed at a frequency of 12 Hz over the four plugging rates. Finally, to eliminate the influence of the fissure opening value, a contrasting test
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Fig. 1. Experimental setup of the PWP propagation system.
Fig. 2. Profile of the parallel-straight fissure model.
was conducted with the opening value changed to 3 mm. The blockage sizes and location are shown in Table 2. 3. Results 3.1. PWP propagation in a non-plugging fissure Fig. 3 shows the experimental results for the PWP propagation in a non-plugging fissure at a pulsating frequency of 16 Hz. The curves P1, P3, P5, P6 and P7 represent the results at the corresponding measurement points on the fissure model (Fig. 2). The experiments mainly examine the laws of PWP propagation for identical operating conditions or the same output PWP in a group of tests. As shown in Fig. 3, the mean PWP pressure remains identical, but the peak pressure and phase of the PWP curve changes as the PWP spreads out in the fissure. Because of the PWP peak pressure equaling the mean pressure plus a half-amplitude, the peak
Table 1 High-frequency sensor parameters. Sensor no.
Testing pressure range (MPa)
Distance from the entrance (m)
Opening value of fissure (mm)
P1 P2 P3 P4 P5 P6 P7
0e10 0e10 0e10 0e10 0e10 0e10 0e10
0.05 1.05 2.05 2.55 3.05 3.55 4.15
5 5 5 5 5 4.75 1.75
changes depend on the pressure amplitude changes. The peak pressure first decreases and then increases as the distance increases from the fissure entrance. Sensor P1 sits at the entrance of the fissure, so the PWP curve at P1 is treated as the initial dataset. The shape of the initial curve is similar to a sine wave. When the PWP reaches sensor P2, the PWP amplitude decreases by 30%. When the PWP reaches sensor P5, the PWP amplitude increases by 42.9% from P2. Additionally, PWP amplitude increases rapidly as it spreads to points P6 and P7 at the end of the fissure. The growth rate of the PWP amplitude is 70% between sensors P5 and P7. Therefore, it can be inferred that the PWP peak pressure is maximized at the fissure tip. In general, as the measuring point moves closer to the fissure tip, the observed peak pressure increases. The reason for this phenomenon is likely because the PWP is reflected by the walls of fissure end, forming a reflection wave that interacts with the incident wave and leads to an increase in the PWP peak pressure. To verify the experimental results, two additional tests were conducted on the PWP propagation at frequencies of 12 and 20 Hz. The experimental results are shown in Figs. 4 and 5, which are similar to those in Fig. 3. The PWP peak pressure also first decreases before increasing. Meanwhile, the peak pressure is larger at the fissure end. By comparing the three figures (Figs. 3e5), it is clear that the PWP increases as the pulsing frequency increases. However, this is not discussed in this paper and is intended to be a direction of future research. 3.2. Effects of plugging rate on PWP propagation To study the effect of various plugging rates on PWP
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Table 2 Experiment parameters in the plugging fissure. Experiment no.
Pulse frequency (Hz)
Cross-sectional area (mm2)
1 2 3 4 5 6 7 8 9 10 11 12
16 16 16 16 12 12 12 12 16 16 16 16
5 5 5 5 5 5 5 5 3 3 3 3
100 100 100 100 100 100 100 100 100 100 100 100
propagation in blocked fissures, tests were completed at a frequency of 16 Hz; the results are shown in Fig. 6. These results indicate that the PWP causes severe changes in the plugging area of the fissure. The shape of the initial PWP curve is a sine wave with a frequency of 16 Hz. When the plugging rate of the fissure is 95%, the effects stemming from the blockage are obvious. The peak PWP pressure of the P1 curve is four times greater than that of the P2 curve, with curves P1 and P2 representing the PWP in front and behind the blockage, respectively. The peak PWP pressure of all of the measured points behind the blockage is small. When the plugging rate of the fissure is 85%, the shape of the P1 PWP curve is still similar to a sine wave, and there is a great reduction in the peak PWP pressure between points P1 and P2. The PWP amplitude of P1 decreases by 40%, compared to that at a plugging rate of 95%. The PWP amplitude at P1 is three times as large as the PWP amplitude at P2, and the peak pressure of the measuring points behind P2
Openings area (mm2)
Plugging rate (%)
Distance from the entrance(m)
25 75 125 175 25 75 125 175 15 45 75 105
95% 85% 75% 65% 95% 85% 75% 65% 95% 85% 75% 65%
1 1 1 1 1 1 1 1 1 1 1 1
increases and gradually reaches the P1 value. As the plugging rate decreases further, the peak pressure of P1 reduces to 1450 kPa. The differences in the peak pressure between P1 and P2 decreases gradually until the peak pressure at P1 equals that at P2 for a plugging rate of 65%. The PWP peak at P5 behind the blockage is greater than at P1 when the plugging rate is 65%, which is similar to PWP changing trend in the non-plugging fissure. It can be inferred that the blockage effects can be neglected when the plugging rate is less than 65%. By comparing the four figures (Fig. 6) showing the different plugging rates, it is evident that the degree of blockage in the fissure has a great deal of influence on the spreading of the PWP. The PWP peak is larger prior to the blockage than that behind the blockage. The peak pressure of P1 increases with the plugging rate increasing, causing a peak pressure decrease behind the fissure blockage. The maximum PWP value is in front of the blockage under these conditions. These results show that coal will be fractured first in front of the blockage for plugging rates greater than 75%. As the plugging rate decreases, the PWP peak decreases in front of the fissure blockage and increases behind the blockage. If the plugging rate is 65%, the maximum PWP value is at the fissure tip. Therefore, the coal will fissure first around the fissure tip. To verify these results on the effects of different plugging rates on PWP spreading, a contrasting test was conducted. The operating conditions were changed by adjusting the pulse frequency. Fig. 7 shows the results of the experiments completed at a frequency of 12 Hz. The same conclusions were observed from the results shown in Fig. 7. When the plugging rate is 95%, the PWP at P1 is the highest. The PWP peak in front of the fissure blockage decreases gradually before increasing behind the blockage as the plugging rate is reduced. However, in this experiment, it was also found that the pulse frequency was locally abnormal, and frequency
Fig. 3. Changes in the PWP at a frequency of 16 Hz.
Fig. 4. Changes in the PWP at a frequency of 12 Hz.
Fig. 5. Changes in the PWP at a frequency of 20 Hz.
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Fig. 6. Changes in the PWP at 16 Hz under various plugging rates at the opening value of 5 mm.
amplification occurs (frequency doubling). This phenomenon is not discussed in this paper, but it will be addressed in future work. To eliminate the effect of opening value on PWP propagation, another test was completed using a different fissure opening value. The opening value was changed to 3 mm at a frequency of 16 Hz, with the results shown in Fig. 8. The results also indicate that the plugging rate has a great influence on PWP. The same conclusions, as shown in Fig. 6, were observed from the results in Fig. 8. When the plugging rate is 95%, the peak pressure of P1 is the highest, and it decreases gradually with a reduced plugging rate. However, the PWP at the same position in Fig. 8 is larger than that in Fig. 6.
4. Discussion (1) Effects of fissure tip on PWP Water flows into coal rock and seeps into coal fissures during PHF. The coal fissures are filled with water prior to fissure extension, and the PWP travels through the water in the fissures (Ashour, 2000; Ionov, 2007). The fissure tip has a great influence on PWP propagation, especially on the PWP peak in parallel-straight fissures. The closer the distance between the fissure tip and the pressure monitoring point is, the larger the peak pressure will be, indicating that a stress concentration will appear around the fissure tip. This verifies the conclusion that PHF can cause the fissures to extend effectively. This conclusion is explained at the micro-scale where PWP propagation meets the law of wave propagation. A pressure wave will be reflected when it propagates into a fissure end, as described in wave theory. This forms a reflection wave that interacts constructively with an incident wave, leading to an increase in the PWP peak at the end of the fissure. This result may be interpreted using the wave equation. Previous work (LI et al., 2013; Zhang and Tian, 2005) examined pressure wave propagation in rock and derived a pressure wave equation as a function of a sine wave (Eq. (1)):
i h x þ4 ; P ¼ A sin 2pf t c
(1)
where P is the transient pressure loaded on the coal in kPa, A is the
PWP peak in kPa, f is the PWP frequency in Hz, c is the velocity of the wave in m/s, 4 is the initial phase, x is the position in m and t is the transient time in s. The shape of the curve generated by Eq. (1) is identical to the initial experimental pressure; however, the values are not identical on the y axis (Fig. 9). As Fig. 9 shows, there is a fixed difference between the curve derived from Eq. (1) and the initial experimental pressure. A constant P0 is added to Eq. (1) to correct this imbalance and to obtain Eq. (2):
i h x þ4 ; P ¼ P0 þ A sin u t c
(2)
where P0 represents the average of the PWP that did not change during the PWP propagation period. Fig. 9 shows pressure result for P1 at 20 Hz and the two curves fitted using Eqs. (1) and (2). The relative coefficient of the fitted curve using Eq. (2) is 0.99, which demonstrates the validity of Eq. (2). The PWP is function of t, x and f for the same PWP generator based on Eq. (2), which agrees with the experimental results (Fig. 3). When the pressure wave is reflected in the fissure end, the reflection and incident waves meet the conditions required for constructive wave interference according to the wave reflection theory. It was assumed that the incident and reflection waves propagate from S1 and S2 in Fig. 10, encountering each other and interacting at point P. The new wave synthesized from these two individual waves may be expressed as:
i h x þ 40 ; P 0 ¼ P0 þ A0 sin 2pf t c
(3)
0 is the PW peak of the synthesized wave and is defined as where qA ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi A0 ¼ A21 þ A22 þ 2A1 A2 cosD4. For instance, A0 ¼ 610 kPa at the position for P6 (20 Hz). A1 and A2 are the amplitude of the incident and reflected waves, respectively, and D4 is the phase difference between the two waves. A quantitative relationship among the synthesized, incident and reflected waves was not determined because the incident and reflected waves could not be measured separately. However, from Fig. 3, the PWP peak was determined to increase at the fissure end, indicating that the incident and reflection waves experienced constructive interference, namely
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Fig. 7. Changes in PWP at 12 Hz for various plugging rates at the opening value of 5 mm.
Fig. 8. Changes in PWP at 16 Hz for various plugging rates at the opening value of 3 mm.
A0 > A1 and A0 > A2. (2) Effects of blockage on the PWP propagation Based on wave theory, the PWP spreads in the form of a pressure wave in the water-filled fissure. The pressure wave encounters the blockage and splits into reflection, transmission and creeping wave components, as shown in Fig. 11. When the plugging rate of the fissure is 95%, most of pressure wave is reflected, and the two other wave components are very small, resulting in a large difference before and after the blockage. Fig. 6 also shows that a pressure difference emerges prior to the blockage for the reflection and incident waves that experience constructive interference. When the plugging rate is very large (such as 95%, as shown in Fig. 6), the maximum peak pressure is prior to the blockage. This indicates that
Fig. 9. Comparison between the fitted curves from Eqs. (1) and (2).
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data feedback of a worksite to enhance coal bed permeability; therefore, CBM extraction can be improved. Acknowledgments
Fig. 10. Sketch of the interference process.
This work was financially supported by the Fundamental Research Funds for the Central Universities (No. 2014ZDPY04), the National Natural Science Foundation of China (Grant No. 51274195, U1361106), the Natural Science Foundation of Jiangsu Province (Grant No. BK2012571), the National Major Scientific Instrument and Equipment Development Project (Grant No. 2013YQ17046309), the Program for New Century Excellent Talents in University (Grant No. NCET-12-0959), the National Basic Research Program of China (973 Project) (Grant No. 2011CB201205) and the Qing Lan Project. References
Fig. 11. Pressure wave propagation at a blockage.
the coal rock prior to the blockage will be fractured first. However, the pressure difference decreases with plugging rate reduced. The point of maximum peak pressure moves behind the blockage when the plugging rate is less than 65%. Fig. 6 shows that this new maximum should be at the fissure tip. When the PWP propagates into the fissure, fracturing will occur first at the fissure tip.
5. Conclusions In this study, an experimental PWP propagation system was constructed with a PWP generator, a parallel-straight fissure model and a data collector. The fissure plugging rate plays a very important role in PWP propagation in coal fissures because the blockage may reflect the pressure wave. Upon reflection, the wave may interact with another incident wave and experience constructive interference. Some of the major conclusions from this work are as follows: (1) In a non-plugging fissure, the PWP peak pressure first decreases and subsequently increases. When the PWP stabilizes, the maximum peak pressure emerges at the fissure tip. The coal around the fissure tip will be fractured first during PHF period. The PWP peak increases rapidly when the PWP enters the end of the fissure, due to the reflection wave generated in the fissure end interacting constructively with the PWP incident wave. (2) In a plugging fissure, the plugging rate has a great effect on PWP propagation of PHF. When the plugging rate is larger than 75%, the maximum peak pressure occurs before the blockage; when the plugging rate is less than 65%, the maximum peak pressure occurs behind the blockage. The results demonstrate that the position of initial fracturing depends on the plugging rate. (3) The results of this work have given a basis for predicting and assessing the positions and directions of initial fracturing. According to this work, PHF parameters can be adjusted by
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Journal of Natural Gas Science and Engineering journal homepage: www.elsevier.com/locate/jngse
Experimental study of pulsating water pressure propagation in CBM reservoirs during pulse hydraulic fracturing Cheng Zhai a, b, Xu Yu a, b, *, Xianwei Xiang a, b, Quangui Li a, b, Shiliang Wu a, b, Jizhao Xu a, b a b
State Key Laboratory of Coal Resources and Safe Mining. Xuzhou, Jiangsu, 221116, China School of Safety Engineering, China University of Mining & Technology, Xuzhou, Jiangsu, 221116, China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 16 December 2014 Received in revised form 20 April 2015 Accepted 21 April 2015 Available online
Pulse hydraulic fracturing (PHF) technology can effectively improve reservoir permeability for coal bed methane (CBM) extraction. In this work, laboratory experiments were conducted to study the laws regarding pulsating water pressure (PWP) propagation during PHF in a CBM reservoir at different pulse frequencies and plugging rates. The results demonstrate that the average PWP is stable over the length of the reservoir fissure, and the PWP peak primarily depends on the PWP amplitude. The experimental results show that for a non-plugging fissure, the PWP peak pressure first decreases and subsequently increases with the maximum pressure occurring at the fissure tip. When the fissure is blocked, a portion of the pressure waves are reflected in front of the blockage, forming reflection waves, which interact with the incident waves and lead to an increase in the PWP peak pressure. When the plugging rate is 95%, there is a significant difference in the PWP peak pressure in front of and behind the blockage because of the pressure wave reflections and interactions. As the plugging rate decreases, the blockage effects are reduced, and the wave proportion that is reflected decreases such that the transmission and creeping wave components increase. When the plugging rate is 65%, the PWP peak pressure behind the blockage is larger than that prior to the blockage. The findings build a basis to predict and control fissure extensions to improve the effect of PHF by optimizing the technical parameters. © 2015 Elsevier B.V. All rights reserved.
Keywords: Pulse hydraulic fracturing Pulsating water pressure Parallel-straight fissure Coal bed methane Plugging rate
1. Introduction The proven reserves of the CBMs in China are 37 trillion cubic meters, ranking third in the world. Unfortunately, the Chinese CBM reservoirs have low permeability, low porosity and high in-situ stress owing to the effects of a complex geologic structure and burial conditions. This leads to low concentrations, low flow and the fast decay of the CBM extraction. These problems make it difficult to exploit CBM in China. In 2013, the CBM production was 15.6 billion cubic meters, whereas the effective CBM utilization ratio was less than 50% for low concentrations. The percentage of underground CBM output is 80.8%, and the percentage of surface drilling output is less than 20%; therefore, the CBM extraction methods used in China are presently performed largely via underground gas extraction in coal mines (Li et al., 2015; Luo and Dai, 2009; Xia et al., 2014; Xu et al., 2013). Underground gas extraction
* Corresponding author. Daxue Road, Xuzhou City, Jiangsu Province, 221116, China. E-mail address: [email protected] (X. Yu). http://dx.doi.org/10.1016/j.jngse.2015.04.027 1875-5100/© 2015 Elsevier B.V. All rights reserved.
is also an important method to prevent gas hazards in coal mines. However, ordinary gas drilling is characterized by a small extraction radius, a small flow and fast attenuation due to the low permeability of the coal seam. Many scholars, both in China and abroad, have put forth hydraulic measures to improve coal seam permeability, such as hydraulic fracturing (Huang et al., 2012, 2014, 2011; Majdi et al., 2012; Wang et al., 2014; Zhang and Chen, 2010), hydraulic flushing (Lu et al., 2010; Wang and Li, 2012), hydraulic slotting (Shen et al., 2012; Zou et al., 2014), etc., which have yielded some positive results. However, with improvements in the level of safety requirements in underground coal mining, these technologies have yielded some negative effects, the most outstanding of which is the high pressure required that increases the number of underground hazards. To avoid using high pressure, Lin et al. introduced pulse hydraulic fracturing (PHF) technology that integrates the advantages of pulsating loading and hydraulic fracturing. The main characteristic of PHF is that coal fissures can be extended by the pulsating loading of pulsating water pressure (PWP) using a lower pressure to accelerate the extension of a fissure and effectively improve the permeability of a coal seam (Li
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et al., 2013a, b; Lin et al., 2012; Zhai et al., 2011). Little research has been conducted on PHF technologies used in CBM extraction, and the existing results come mostly from field experiments that examine the macroscopic characteristics of the PHF including the initial fracture pressure, the fracturing radius and the fissure extending process. These results have led to many meaningful conclusions (LI et al., 2013; Li et al., 2014); however, the laws of PWP propagation in coal fissures of CBM reservoirs have not yet been studied. Similar studies appeared in the hydraulic engineering and water resource fields (Jiang and Liang, 1983; Li and Liu, 2006, 2008; Qian et al., 2005; Zhao and Liang, 1988), including a single fissure model of a dam batholith to examine the break mechanism, (Li et al., 2011; M. and S., 1995; Qian et al., 2005; Zhang et al., 2013a, 2013b) adopted a approach to address the groundwater flow laws in a narrow channel. These references are very valuable and allow a model to be established for the coal fissures in this paper. The fissures of CBM reservoirs are random in shape and size; they are classified into coal seam beddings, joints and other types. Different types of fissures may cross each other to form a complex fissure network. Existing studies show that the CBM reservoirs are first fractured along the coal seam fissures (Ide et al., 2010; Zhang et al., 2013a, 2013b). To study the PWP propagation in CBM reservoirs, the process of PWP propagation must be simulated in coal seam fissures. A fissure with a blind end is chosen, which acts as a representative type of coal fissures, and it was simplified into “double parallel planes” with a closed tip (namely, a parallelstraight fissure), as shown in Fig. 2. It was found that the fissures are filled with water prior to extension. When PHF is implemented in a coal mine, the PWP propagates in a wave-form through the water medium and is affected by the fissure size changes and blockage state (Ashour, 2000; Ionov, 2007). When mining or drilling occurs in an underground coal mine, crustal stress is redistributed, and coal particles flow into the fissures, causing coal fissure size and shape changes. This occurs because of the fissure cross section being shrunken or blocked. According to Kabir et al. (2004, 2006) and Teyssedou et al. (2005), blockages have a significant influence on the PWP propagation in fissures. Therefore, it is necessary to perform research on the laws of PWP propagation for different blockage states in CBM reservoirs. In this paper, PWP propagation experiments were conducted using a parallel-straight fissure model. The experiments were performed for no blockage conditions and different plugging rates. The results provide a theoretical basis for optimizing the PHF technology parameters and improving overall CBM extraction. 2. Materials and methods 2.1. Experimental setups The experimental setup for PWP propagation in a parallelstraight fissure includes the PWP generator, the parallel-straight fissure model, a data collector and a high-frequency pressure sensor, as shown in Fig. 1. The PWP generator consists of a pulsating pump, a frequency controller, a water tank, an overflow valve and a pressure meter. The pulsating pump outputs pressure water, namely PWP, from 0 to 6 MPa. The frequency controller is used to change the input frequency of the pulsating pump over a range of 0e20 Hz with an adjustable accuracy of 0.4 Hz. The overflow control valve adjusts the export flow of the system, controlling the PWP value. The mean pressure represents the pressure meter reading and is an important controlling parameter for the PHF process in a real scenario. The parallel-straight fissure model is welded from 304 stainless steel. In this work, the CBM reservoir was assumed to be a closed system, leakage of the fracture liquid
was ignored. Therefore, stainless steel is used as the model material. The model is 4.5 m in length and has a rectangular crosssection of 100 mm 5 mm. The largest pressure the pipe can handle is 10 MPa. The experiments were divided into two parts: (1) a test of the PWP propagation in a non-plugged fissure, and (2) a test of the PWP propagation for various plugging rates. The plugging rate is defined as the difference between the cross-sectional area A of the fissure and the opening area A1 of the blockage divided by A, namely d ¼ (A A1)/A, of the parallel-straight fissure. There are seven pressure ports on the fissure, P1 ~ P7 shown in Fig. 1, that measure the changes in the PWP propagating in the direction of the fissure. The pressures at the seven points are measured using high-frequency digital sensors with a measurement error of less than 1%. The data are simultaneously collected for the seven different sensors and transferred in real-time to a computer. 2.2. Experimental methods Fig. 2 shows a profile of the parallel-straight fissure model (shown in Fig. 1). The model is divided into two parts: the parallel fissure and the fissure end. 5 mm is the fissure opening value. The experiments were conducted under the same operating conditions to investigate the laws of PWP propagation in the fissure at the same frequencies. The opening of overflow valve is fixed during the experiments. So the PWP changing, as a result of changing the operating conditions, is ignored. The experiments were divided into two groups: (1) the PWP propagated in a non-plugging fissure at a pulsation frequency of 16 Hz with two verification tests at 12 and 20 Hz; (2) PWP propagated in a plugging fissure at various plugging rates, 65%, 75%, 85% and 95%. The process used was as follows: (1) Experiments on PWP propagation in a non-plugging fissure The non-plugging fissure model is defined as having no blockages (Fig. 2). Seven high-frequency pressure sensors were installed on the fissure model with the parameters shown in Table 1. Prior to performing the experiments, preparation is required. At the beginning of a test, power is turned on, and the vacuum pump is allowed to run for half an hour to remove any air from the fissure model under a negative pressure of 0.1 MPa. Then, the input frequency was set to 20 Hz by the frequency controller. The pulsating pump is started for a certain set of conditions. After the pump stabilized, data collection was activated. The pulsating pump and data collector are stopped in an orderly manner every 2.5 min when the data collector functioned properly. Every test group was repeated three times to reduce errors. To verify the experimental results, two contrasting tests were performed at 12 and 20 Hz. (2) Experiments on PWP propagation in a plugging fissure From Fig. 2, the plugging fissures were defined in the fracture model as fissures with different blockages. The plugging rate is the only variable in experiments; all other parameters, such as the frequency (16 Hz) and the overflow valve opening, remained the same. The blockage in the fissure was simulated using flappers with different opening sizes. To eliminate any effects at the opening position, the openings were symmetrically distributed on the flappers. Four plugging rates of 65%, 75%, 85% and 95% were chosen to function as blockages (Table 2). The tests were conducted identically as the experiments in the non-plugging fissure. To verify the experimental results at different frequencies, a test was designed at a frequency of 12 Hz over the four plugging rates. Finally, to eliminate the influence of the fissure opening value, a contrasting test
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Fig. 1. Experimental setup of the PWP propagation system.
Fig. 2. Profile of the parallel-straight fissure model.
was conducted with the opening value changed to 3 mm. The blockage sizes and location are shown in Table 2. 3. Results 3.1. PWP propagation in a non-plugging fissure Fig. 3 shows the experimental results for the PWP propagation in a non-plugging fissure at a pulsating frequency of 16 Hz. The curves P1, P3, P5, P6 and P7 represent the results at the corresponding measurement points on the fissure model (Fig. 2). The experiments mainly examine the laws of PWP propagation for identical operating conditions or the same output PWP in a group of tests. As shown in Fig. 3, the mean PWP pressure remains identical, but the peak pressure and phase of the PWP curve changes as the PWP spreads out in the fissure. Because of the PWP peak pressure equaling the mean pressure plus a half-amplitude, the peak
Table 1 High-frequency sensor parameters. Sensor no.
Testing pressure range (MPa)
Distance from the entrance (m)
Opening value of fissure (mm)
P1 P2 P3 P4 P5 P6 P7
0e10 0e10 0e10 0e10 0e10 0e10 0e10
0.05 1.05 2.05 2.55 3.05 3.55 4.15
5 5 5 5 5 4.75 1.75
changes depend on the pressure amplitude changes. The peak pressure first decreases and then increases as the distance increases from the fissure entrance. Sensor P1 sits at the entrance of the fissure, so the PWP curve at P1 is treated as the initial dataset. The shape of the initial curve is similar to a sine wave. When the PWP reaches sensor P2, the PWP amplitude decreases by 30%. When the PWP reaches sensor P5, the PWP amplitude increases by 42.9% from P2. Additionally, PWP amplitude increases rapidly as it spreads to points P6 and P7 at the end of the fissure. The growth rate of the PWP amplitude is 70% between sensors P5 and P7. Therefore, it can be inferred that the PWP peak pressure is maximized at the fissure tip. In general, as the measuring point moves closer to the fissure tip, the observed peak pressure increases. The reason for this phenomenon is likely because the PWP is reflected by the walls of fissure end, forming a reflection wave that interacts with the incident wave and leads to an increase in the PWP peak pressure. To verify the experimental results, two additional tests were conducted on the PWP propagation at frequencies of 12 and 20 Hz. The experimental results are shown in Figs. 4 and 5, which are similar to those in Fig. 3. The PWP peak pressure also first decreases before increasing. Meanwhile, the peak pressure is larger at the fissure end. By comparing the three figures (Figs. 3e5), it is clear that the PWP increases as the pulsing frequency increases. However, this is not discussed in this paper and is intended to be a direction of future research. 3.2. Effects of plugging rate on PWP propagation To study the effect of various plugging rates on PWP
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Table 2 Experiment parameters in the plugging fissure. Experiment no.
Pulse frequency (Hz)
Cross-sectional area (mm2)
1 2 3 4 5 6 7 8 9 10 11 12
16 16 16 16 12 12 12 12 16 16 16 16
5 5 5 5 5 5 5 5 3 3 3 3
100 100 100 100 100 100 100 100 100 100 100 100
propagation in blocked fissures, tests were completed at a frequency of 16 Hz; the results are shown in Fig. 6. These results indicate that the PWP causes severe changes in the plugging area of the fissure. The shape of the initial PWP curve is a sine wave with a frequency of 16 Hz. When the plugging rate of the fissure is 95%, the effects stemming from the blockage are obvious. The peak PWP pressure of the P1 curve is four times greater than that of the P2 curve, with curves P1 and P2 representing the PWP in front and behind the blockage, respectively. The peak PWP pressure of all of the measured points behind the blockage is small. When the plugging rate of the fissure is 85%, the shape of the P1 PWP curve is still similar to a sine wave, and there is a great reduction in the peak PWP pressure between points P1 and P2. The PWP amplitude of P1 decreases by 40%, compared to that at a plugging rate of 95%. The PWP amplitude at P1 is three times as large as the PWP amplitude at P2, and the peak pressure of the measuring points behind P2
Openings area (mm2)
Plugging rate (%)
Distance from the entrance(m)
25 75 125 175 25 75 125 175 15 45 75 105
95% 85% 75% 65% 95% 85% 75% 65% 95% 85% 75% 65%
1 1 1 1 1 1 1 1 1 1 1 1
increases and gradually reaches the P1 value. As the plugging rate decreases further, the peak pressure of P1 reduces to 1450 kPa. The differences in the peak pressure between P1 and P2 decreases gradually until the peak pressure at P1 equals that at P2 for a plugging rate of 65%. The PWP peak at P5 behind the blockage is greater than at P1 when the plugging rate is 65%, which is similar to PWP changing trend in the non-plugging fissure. It can be inferred that the blockage effects can be neglected when the plugging rate is less than 65%. By comparing the four figures (Fig. 6) showing the different plugging rates, it is evident that the degree of blockage in the fissure has a great deal of influence on the spreading of the PWP. The PWP peak is larger prior to the blockage than that behind the blockage. The peak pressure of P1 increases with the plugging rate increasing, causing a peak pressure decrease behind the fissure blockage. The maximum PWP value is in front of the blockage under these conditions. These results show that coal will be fractured first in front of the blockage for plugging rates greater than 75%. As the plugging rate decreases, the PWP peak decreases in front of the fissure blockage and increases behind the blockage. If the plugging rate is 65%, the maximum PWP value is at the fissure tip. Therefore, the coal will fissure first around the fissure tip. To verify these results on the effects of different plugging rates on PWP spreading, a contrasting test was conducted. The operating conditions were changed by adjusting the pulse frequency. Fig. 7 shows the results of the experiments completed at a frequency of 12 Hz. The same conclusions were observed from the results shown in Fig. 7. When the plugging rate is 95%, the PWP at P1 is the highest. The PWP peak in front of the fissure blockage decreases gradually before increasing behind the blockage as the plugging rate is reduced. However, in this experiment, it was also found that the pulse frequency was locally abnormal, and frequency
Fig. 3. Changes in the PWP at a frequency of 16 Hz.
Fig. 4. Changes in the PWP at a frequency of 12 Hz.
Fig. 5. Changes in the PWP at a frequency of 20 Hz.
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Fig. 6. Changes in the PWP at 16 Hz under various plugging rates at the opening value of 5 mm.
amplification occurs (frequency doubling). This phenomenon is not discussed in this paper, but it will be addressed in future work. To eliminate the effect of opening value on PWP propagation, another test was completed using a different fissure opening value. The opening value was changed to 3 mm at a frequency of 16 Hz, with the results shown in Fig. 8. The results also indicate that the plugging rate has a great influence on PWP. The same conclusions, as shown in Fig. 6, were observed from the results in Fig. 8. When the plugging rate is 95%, the peak pressure of P1 is the highest, and it decreases gradually with a reduced plugging rate. However, the PWP at the same position in Fig. 8 is larger than that in Fig. 6.
4. Discussion (1) Effects of fissure tip on PWP Water flows into coal rock and seeps into coal fissures during PHF. The coal fissures are filled with water prior to fissure extension, and the PWP travels through the water in the fissures (Ashour, 2000; Ionov, 2007). The fissure tip has a great influence on PWP propagation, especially on the PWP peak in parallel-straight fissures. The closer the distance between the fissure tip and the pressure monitoring point is, the larger the peak pressure will be, indicating that a stress concentration will appear around the fissure tip. This verifies the conclusion that PHF can cause the fissures to extend effectively. This conclusion is explained at the micro-scale where PWP propagation meets the law of wave propagation. A pressure wave will be reflected when it propagates into a fissure end, as described in wave theory. This forms a reflection wave that interacts constructively with an incident wave, leading to an increase in the PWP peak at the end of the fissure. This result may be interpreted using the wave equation. Previous work (LI et al., 2013; Zhang and Tian, 2005) examined pressure wave propagation in rock and derived a pressure wave equation as a function of a sine wave (Eq. (1)):
i h x þ4 ; P ¼ A sin 2pf t c
(1)
where P is the transient pressure loaded on the coal in kPa, A is the
PWP peak in kPa, f is the PWP frequency in Hz, c is the velocity of the wave in m/s, 4 is the initial phase, x is the position in m and t is the transient time in s. The shape of the curve generated by Eq. (1) is identical to the initial experimental pressure; however, the values are not identical on the y axis (Fig. 9). As Fig. 9 shows, there is a fixed difference between the curve derived from Eq. (1) and the initial experimental pressure. A constant P0 is added to Eq. (1) to correct this imbalance and to obtain Eq. (2):
i h x þ4 ; P ¼ P0 þ A sin u t c
(2)
where P0 represents the average of the PWP that did not change during the PWP propagation period. Fig. 9 shows pressure result for P1 at 20 Hz and the two curves fitted using Eqs. (1) and (2). The relative coefficient of the fitted curve using Eq. (2) is 0.99, which demonstrates the validity of Eq. (2). The PWP is function of t, x and f for the same PWP generator based on Eq. (2), which agrees with the experimental results (Fig. 3). When the pressure wave is reflected in the fissure end, the reflection and incident waves meet the conditions required for constructive wave interference according to the wave reflection theory. It was assumed that the incident and reflection waves propagate from S1 and S2 in Fig. 10, encountering each other and interacting at point P. The new wave synthesized from these two individual waves may be expressed as:
i h x þ 40 ; P 0 ¼ P0 þ A0 sin 2pf t c
(3)
0 is the PW peak of the synthesized wave and is defined as where qA ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi A0 ¼ A21 þ A22 þ 2A1 A2 cosD4. For instance, A0 ¼ 610 kPa at the position for P6 (20 Hz). A1 and A2 are the amplitude of the incident and reflected waves, respectively, and D4 is the phase difference between the two waves. A quantitative relationship among the synthesized, incident and reflected waves was not determined because the incident and reflected waves could not be measured separately. However, from Fig. 3, the PWP peak was determined to increase at the fissure end, indicating that the incident and reflection waves experienced constructive interference, namely
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Fig. 7. Changes in PWP at 12 Hz for various plugging rates at the opening value of 5 mm.
Fig. 8. Changes in PWP at 16 Hz for various plugging rates at the opening value of 3 mm.
A0 > A1 and A0 > A2. (2) Effects of blockage on the PWP propagation Based on wave theory, the PWP spreads in the form of a pressure wave in the water-filled fissure. The pressure wave encounters the blockage and splits into reflection, transmission and creeping wave components, as shown in Fig. 11. When the plugging rate of the fissure is 95%, most of pressure wave is reflected, and the two other wave components are very small, resulting in a large difference before and after the blockage. Fig. 6 also shows that a pressure difference emerges prior to the blockage for the reflection and incident waves that experience constructive interference. When the plugging rate is very large (such as 95%, as shown in Fig. 6), the maximum peak pressure is prior to the blockage. This indicates that
Fig. 9. Comparison between the fitted curves from Eqs. (1) and (2).
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data feedback of a worksite to enhance coal bed permeability; therefore, CBM extraction can be improved. Acknowledgments
Fig. 10. Sketch of the interference process.
This work was financially supported by the Fundamental Research Funds for the Central Universities (No. 2014ZDPY04), the National Natural Science Foundation of China (Grant No. 51274195, U1361106), the Natural Science Foundation of Jiangsu Province (Grant No. BK2012571), the National Major Scientific Instrument and Equipment Development Project (Grant No. 2013YQ17046309), the Program for New Century Excellent Talents in University (Grant No. NCET-12-0959), the National Basic Research Program of China (973 Project) (Grant No. 2011CB201205) and the Qing Lan Project. References
Fig. 11. Pressure wave propagation at a blockage.
the coal rock prior to the blockage will be fractured first. However, the pressure difference decreases with plugging rate reduced. The point of maximum peak pressure moves behind the blockage when the plugging rate is less than 65%. Fig. 6 shows that this new maximum should be at the fissure tip. When the PWP propagates into the fissure, fracturing will occur first at the fissure tip.
5. Conclusions In this study, an experimental PWP propagation system was constructed with a PWP generator, a parallel-straight fissure model and a data collector. The fissure plugging rate plays a very important role in PWP propagation in coal fissures because the blockage may reflect the pressure wave. Upon reflection, the wave may interact with another incident wave and experience constructive interference. Some of the major conclusions from this work are as follows: (1) In a non-plugging fissure, the PWP peak pressure first decreases and subsequently increases. When the PWP stabilizes, the maximum peak pressure emerges at the fissure tip. The coal around the fissure tip will be fractured first during PHF period. The PWP peak increases rapidly when the PWP enters the end of the fissure, due to the reflection wave generated in the fissure end interacting constructively with the PWP incident wave. (2) In a plugging fissure, the plugging rate has a great effect on PWP propagation of PHF. When the plugging rate is larger than 75%, the maximum peak pressure occurs before the blockage; when the plugging rate is less than 65%, the maximum peak pressure occurs behind the blockage. The results demonstrate that the position of initial fracturing depends on the plugging rate. (3) The results of this work have given a basis for predicting and assessing the positions and directions of initial fracturing. According to this work, PHF parameters can be adjusted by
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