A novel technology for enhancing coalbed methane extraction: Hydraulic cavitating assisted fracturing

Journal Pre-proof A Novel Technology for Enhancing Coalbed Methane Extraction : Hydraulic Cavitating Assisted Fracturing Congmeng Hao, Yuanping Cheng, Liang Wang, Hongyong Liu, Zheng Shang PII:

S1875-5100(19)30292-6

DOI:

https://doi.org/10.1016/j.jngse.2019.103040

Reference:

JNGSE 103040

To appear in:

Journal of Natural Gas Science and Engineering

Received Date: 14 June 2019 Revised Date:

29 September 2019

Accepted Date: 18 October 2019

Please cite this article as: Hao, C., Cheng, Y., Wang, L., Liu, H., Shang, Z., A Novel Technology for Enhancing Coalbed Methane Extraction : Hydraulic Cavitating Assisted Fracturing, Journal of Natural Gas Science & Engineering, https://doi.org/10.1016/j.jngse.2019.103040. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Elsevier B.V. All rights reserved.

A Novel Technology for Enhancing Coalbed Methane Extraction : Hydraulic Cavitating Assisted Fracturing Congmeng Haoa,b,c, Yuanping Chenga,b,c*, Liang Wanga,b,c, Hongyong Liua,b,c, Zheng Shanga,b,c a

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

Jiangsu 221116, China b

National Engineering Research Center for Coal Gas Control, China University of Mining and Technology, Xuzhou, Jiangsu 221116,

China c

School of Safety Engineering, China University of Mining and Technology, Xuzhou, Jiangsu 221116, China

*Corresponding author at: National Engineering Research Center for Coal and Gas Control, China University of Mining and Technology, Xuzhou, Jiangsu 221116, China. E-mail addresses: [email protected] (Y. Cheng).

Abstract: Low extraction ratio of coalbed methane (CBM) can not only cause the waste of energy resource, inducing environmental pollution, but also make CBM be a potential safety risk for underground coal mining. Increasing the permeability of coal seam is an effective means to solve these problems in low permeable seams. Given this situation, this paper proposes a highly efficient, widely applicable permeability enhancement technology (hydraulic cavitating assisted fracturing (HCAF)). The feasibility of the proposed technology to improve permeability was demonstrated theoretically. Furthermore, the effects of this technology on coal damage, crack development and methane extraction enhancement were simulated with RFPA and COMSOL. The results show that in coal seams, the HCAF technology can reduce the fracture initiating pressure below 15 MPa and increase the effective radius from 8 m to 12 m. Besides, this technology has an impact on controlling the development directions of fracture. Finally, field tests show that the HCAF technology can increase the maximum gas extraction flow rate from 0.56 m3/min to 0.81 m3/min and the gas flow rate is always greater than that of traditional hydraulic fracturing during extraction period. The area of methane extraction by HCAF technology was checked after increasing the borehole spacing from 12m to 24m, which showed that the amount of drilling cuttings s and initial speed of gas emission q were lower than the threshold limit value. Those indicate that the HCAF technology is efficient and

1

reliable to improve methane recovery ratio and reduce methane emissions. Keywords: hydraulic fracturing; cavitating; permeability; methane extraction; emission reduction

1 Introduction CBM emitted into the atmosphere is a kind of greenhouse gases, and its greenhouse effect is equal to 22 times of carbon dioxide (Rodhe, 1990). While the Coal bed methane is a kind of clean energy, its calorific value is 35.9MJ/m3, equivalent to quantity for 1.2kg of standard coal (Dong et al., 2017). Coal bed methane disaster is also a key disaster that affect the coal mine safety in China. At present, coal mines with high methane content account for 50%-70% in China. Along with mining depth increasing, methane content and methane pressure of coal reservoir as well as the risk of coal and methane outburst and other disasters will increase, the methane control will be continuously more difficult (Xiang et al., 2015). Practice shows that CBM extraction is the most effective method to prevent coal and methane accidents. Pre-extraction of methane in outburst coal bed can not only ensure the safety of coal mine production, but also reduce methane emissions and increase the utilization of resources. It is an important measure with triple benefits of safety, energy saving and environmental protection. However, the permeability of coalbed is generally poor in China. The most of the coal mines in China are classified as high methane and coal and methane outburst mines based on statistics. Furthermore, more than 95% coal bed for the high methane and coal and methane outburst mines have low permeability (Lu et al., 2010). Research indicate that the coal bed permeability in China is generally in the range of (0.01-0.001) 10-3 mD, which is 3-4 orders of magnitude lower than that in the United States (Lu et al., 2010; Noack, 1998). Due to the low permeability and methane extraction concentration, as well as the small effective extraction radius, methane control is difficult, which seriously threatens the safety of mine production (Gao et al., 2015; Lin et al., 2018b). In order to establish a kind of efficient technology for low permeability coalbeds in China, the scholars have done a lot of research and put forward a series of plans, such as: hydraulic fracturing, deep hole loose blasting, 2

carbon dioxide phase transition for fracturing rock masses, dense drilling, hydraulic slotting, hydraulic flushing and other permeability enhancement measures (Chen et al., 2017; Karacan et al., 2011; Lin et al., 2019; Zhang et al., 2016). Hydraulic fracturing technology is one of the main permeability enhancement technologies in oil and gas exploitation engineering widely used in petroleum field (Torres et al., 2016). Hydraulic fracturing technology has been deeply studied and developed after introduced into the field of coalbed methane extraction. For example, Baev et al. has studied the permeability enhancement model of hydraulic fracturing for low permeability coalbed (Baev, 2014). Zhai Cheng et al. have raised a joint action of hydraulic fracturing and deep hole loosening blasting to improve permeability (Cheng et al., 2012), but this technology cannot be used for highly gassy coal seam (Fan et al., 2012). Feng Yang et al. have studied the using of viscoelastic surfactant fracturing fluid in coal seam (Feng et al., 2018). Guanhua Ni has studied the effect of pulsating hydraulic fracturing on improving coalbed permeability (Ni et al., 2018). Some scholars have studied the integration of hydraulic slotting and hydraulic fracturing (Yan et al., 2015). This technique was developed on the basis of hydraulic fracturing, and hydraulic slotting is to control the direction of fracturing. However, it is dangerous for production because the hydraulic fracturing can not only cause fractures in coal seam, but also cause local stress concentration (Chun-Zhi et al., 2008; Huang et al., 2011). Moreover, this kind of permeability enhancement method which causes cracks in coal seam through external stress is only suitable for harder coal bed. The effect of this method is not obvious in soft coal bed. Similar permeability enhancement methods include deep holes loose blasting and carbon dioxide phase transition for fracturing (Kang et al., 2018; Zhou et al., 2016). The different methods are pressure relief and permeability enhancement, such as hydraulic cutting, water jet caving and so on. By providing pressure relief space for high stress coal mass, this kind of measures can release the stress of coal mass in a larger range, which leads to obvious increase of coal permeability in the pressure relief range. Lin Baiquan and others have studied the effect of hydraulic cutting on pressure relief and permeability enhancement (Lin et al., 2018a). Zhang Hao et al. have studied the principle of pressure relief and permeability enhancement in hydraulic flushing (Zhang et al., 2017). Zhang Rong et al. have 3

studied the effect of hydraulic cavitating and permeability enhancement in coal seam with soft coal interlayer (Zhang et al., 2019). However, the effective range of this kind of pressure relief and permeability enhancement technology is small due to a large number of holes need to be drilled in order to achieve the purpose of large area permeability enhancement. In this paper, a new technology of HCAF to enhance methane extraction has been raised based on the conditions on site. The novelty of our work is the combination of the idea of enhancement by pressurizing in hydraulic fracturing and the idea of pressure relief in hydraulic cavitating. As studied by other scholars, hydraulic cavitating can rapidly increase the permeability of low-permeability coal seams by pressure relief, but its effective range is not large. The hydraulic fracturing, on the contrary, is to fracture the coal body by high-pressure water, forming a large-scale macro-fracture system in the coal body, but the fracture will be closed again after the water is discharged, thereby reducing the effect of permeability enhancement (Fan et al., 2012; Wang et al., 2009). According to the findings in field application, we find that these two technique have good complementarity to each other. If well combined, they will together achieve better permeability enhancement. Also the technique we proposed for increasing the permeability of coal seams has distinct differences and advantages over the prior techniques. For example, compared with hydraulic fracturing, the FCAF technique can greatly reduce the stress concentration inside the coal body, reduce the risk and improve the permeability. Meanwhile the mechanism of coal body pressure relief caused by hydraulic cavitating makes this technique suitable for soft coal seams where hydraulic fracturing will not have the same good result. And compared with the integration of hydraulic slotting and hydraulic fracturing technique, FCAF’s coal flushing is not meant to control the direction of fracturing, but to remove the stress concentration of the coal body caused by the fracturing. At the same time, the caves formed by the flushing provide space for migration of the coal, thus achieving continuous damage of the coal body. The same as hydraulic fracturing

the integration of hydraulic slotting and hydraulic fracturing technique is often better

applied to harder coal seams. Compared to other techniques, such as SOWJ slotting, HTSPE and so on, this 4

technique has the advantages of wider permeability-enhanced range and better applicability. However, this technique has a relatively complicated process and the workers need to be familiar with the process to better exert its advantages. This article first theoretically analyzed the feasibility of HCAF technology. Secondly, the RFPA simulation software was used to compare and analyze the fracturing process and impact range of common hydraulic fracturing and HCAF on coal. Then the fracturing results obtained in RFPA was imported into COMSOL simulation software to calculate the gas drainage effect of FCAF under different drilling distances. The optimal borehole spacing layout scheme is obtained through numerical simulation and verified on site. The research workflow of this work is shown in Fig. 1. The research shows that the hydraulic cavitating and the hydraulic fracturing are respectively used to relieve pressure of coal and expand the range of permeability enhancement, which complement each other. And it also shows that this technology has obviously improved the permeability of low-permeability coal seam, and expanded the permeability range, and enhanced efficiency methane extraction and utilization. Confirm its scientificity and applicability Put forward technical complementary ideas Summarize engineering application phenomena

Related theory

HCAF Technology for ECBM extraction Related parameter

Theoretical feasibility analysis Analysis of pressure relief effect and crack evolution characteristics by RFPA Result as conditions

Preliminary proof of feasibility

Guide field application Field effect investigation

Analysis of methane drainage effect

Provide the best field application

Fig. 1 Research Roadmap

2 The principle of permeability enhancement of HCAF 2.1 Permeability enhancement principle of Hydraulic cavitating

Hydraulic cavitating is a way to release the stress around the holes by breaking up and discharging the coal

5

with water jet to form a large range of holes in the coal. The holes and fractures in the coal expand and extend, effectively improve the permeability around the caving, as shown in Fig. 2-a. The principle of permeability enhancement of coal by hydraulic cavitating can be mathematically analyzed with the aspect of pressure relief damage. The coal is simplified to isotropic medium in order to facilitate analysis, the geometric equation can be expressed as follows:
 = (, + , )  

(1)

where u is the displacement. The Mohr-Coulomb matching DP yield criterion is selected in this paper to evaluate the failure of the coal mass (Drucker and Prager, 1952): =  +   − 

(2)

where  denotes the first stress invariant and J2 denotes the second stress invariant.  = 

√(±)



,  =

√(±)

Coal shall be a strain softening material which mechanical strength decreases with increasing strain after peak stress (Hoek and Brown, 1997). This is due to the failure of the coal mass after it is suffered from the ultimate strength (Pourhosseini and Shabanimashcool, 2014). According to the cohesive force of coal with damage, the evolution formula can be expressed as following (An et al., 2013):  − ! $%#∗ , ' ( < ' ( = ∗ + , ' ( ≥ ' ( ( " )

%$∗

(3)

where  is cohesion,  is initial cohesion and cr is residual cohesion, ' ( = -2/3(
(
( +
(
( +
(
( ) is equivalent plastic strain of coal mass,
(
( and
( are the principal plastic strains, ' ( is the equivalent ∗

plastic strain value from which the residual stage begins. The influence of shrinkage of coal matrix and pore pressure on coal permeability shall be neglected in order to facilitate calculation. The permeability model of the full stress-strain process of coal mass considering the damage variable of coal mass can be expressed as (An et al., 2013; Zhang et al., 2017): 6

21 + $%∗ 45 6 "789 , 0 ≤ ' ( < ' ( =  1! ∗ (1 + 4)6 "789 , ' ( ≥ ' ( 1



%$∗

(4)

where < is the permeability, < is the initial permeability, ℎ is the compressibility coefficient of fracture volume, 4 is the jump coefficient of permeability, and Θ is the volume stress of coal. The leap coefficient of permeability is an experimental value. It is believed that the permeability of coal mass can be increased several dozen times or even 3-4 orders of magnitude after the peak according to previous studies. Through analysis, it can be concluded that the principle of permeability enhancement by waterjet cavitating is mainly the result of leap increase of permeability after pressure relief damage of coal mass.

Fig. 2 Schematic diagram: (a) effect diagram of hydraulic cavitating, (b) effect diagram of hydraulic fracturing.

2.2 Permeability enhancement principle of hydraulic fracturing

The schematic diagram of hydraulic fracturing application effect is shown in Fig. 2-b. The damage of coal during hydraulic fracturing also accord with the process of full stress and strain. The process of coal damage includes the transition from elastic state to damage appeared and cumulative damage stage, and ultimately damage. According to previous research (Wang et al., 2013; Wang et al., 2009), if the tensile failure criterion is met, the damage parameters can be calculated by equation (5):

0 , CD < ED B FG# ? = 1 − HI! , ED ≤ CD < EDJ A @ 1 , CD ≥ EDJ

5)

where ED is the equivalent strain when the tensile damage surface is satisfied for the first time. K is the

maximum principal stress calculated with the initial modulus of elasticity. LD+ = 'LD and EDJ = MED represent 7

the residual tensile strength and ultimate tensile strain respectively. The parameter ' is a constant taking value less than one to defne the residual strength level, while the constant M with value greater than one determines the

tensile strain at which elements undergo the fully damaged condition. The scope of application of constants is discussed in literature (Zhu and Tang, 2004). It is assumed that the equivalent strain corresponding to the evolution of tensile damage is a combination of principal strain. Its form is as follows: CD = (E ) + (E ) + (E )

(6)

Shear damage criterion is defined with equation (7): ψK − K ≥ L ψ = " O

(7)

where K , K L and P are the maximum and minimum principal stresses, uniaxial compressive strength and internal frictional, respectively. When the failure criterion formula (7) is satisfied, the damage variable can be obtained by formula (8): 0 , E > E ? = Q1 − FS# , E ≤ E 

THI! "HU!



(8)

where L+ = 'L , E are equivalent strains when the element stress satisfies the shear failure criterion for the

first time. K and K are the maximum and minimum principal stresses of the complete element calculated under

the initial Young's modulus, respectively. Scholars established the relation between the damage parameters and permeability with program and the fracturing permeability model considering damage variable, as shown in equation (9): <=Y

\]] ^_%`]a

< 6 [[(

4< 6

U



)]

\]] ^_%`]a

[[(

U

, ? = 0

)]

, ? > 0

(9)

where < is the coal permeability, < is the initial permeability, 4 is the sudden change coefficient of permeability,

and c is the coupling coefficient.

By analyzing, it can be concluded that the principle of hydraulic fracturing to increase permeability of coal mass is also to damage coal and make its permeability change abruptly. Hydraulic fracturing is pressurized damage 8

that is different from hydraulic cavitating that is pressure relief damage.

2.3 Permeability enhancement principle of HCAF

By comparing the permeability enhancement principle of hydraulic cavitating and hydraulic fracturing, it can be concluded that both of them have similarities, that is, the change of permeability is synchronized with the damage variable (as shown in Fig. 3). The difference is that the coal around the cave is in the state of pressure relief after hydraulic cavitating, and there is a pressure relief hole which can provide space for the move of coal. While, the coal is only damaged after hydraulic fracturing. However, the combination of two methods can produce a large pressure relief hole for fractured coal, effectively reduce stress concentration and improve permeability. The regional permeability of residual damage variable generated by hydraulic cavitating will not changed basically during hydraulic fracturing according to the analysis in section 2.1 and 2.2, which is the area after H point in Fig. 2. The area where the damage variable increases will further increase due to the stress generated by fracturing, and the permeability also will continue to increase. The area without damage variable (before F point) will also have some damage variable under the action of fracturing, even to the extent of residual damage variable, where is the permeability enhancement area produced by fracturing. Therefore, the permeability of HCAF can be described as:

< 6 d , ? = 0 < = (4 + 4 )< 6 d ,0 < ? < ? ∗ 4< 6 d , ? ≥ ? ∗

(10)

where < is the permeability, < is the initial permeability, and 4 and 4 are the permeability sudden change

coefficient of the damage variable produced by h HCAF, respectively. 4 is the sudden change coefficient of the

permeability when the residual damage variable of coal mass, and Γ is the coupling factor.

The permeability is generally considered to be controlled by effective stress in the elastic stage of coal, which can be expressed as (Chen et al., 2015): < = < 6 "f (H"H! ) 9

(11)

where K is the effective stress, MPa; K is the initial effective stress, MPa; F is the compression coefficient of coal, which is simplified as a constant in this model.

Fig. 3 Permeability Change of Coal during Full Stress-Strain Process: the black line is the stress change in the process of total stress strain of coal, A point starts to load, A to B is the compaction stage, B to C is the elastic strain stage, C to D is the plastic strain stage, and D point is the broken stage; the red line is the change of permeability in each stage corresponding to the total stress strain process.

3 Numerical simulation and analysis 3.1 Numerical analysis scheme and geometric model

3.1.1 Basic Assumptions The model is based on the following assumptions, which are described in detail elsewhere (Dong et al., 2017; Yan et al., 2015): (1) Methane is the only moving gas in the coal seam (water and other gases are neglected); (2) The system is isothermal; (3) The coal seam is a homogeneous, isotropic, and dual poroelastic medium; (4) The cleat system is saturated by free methane, and the matrix stores free and adsorbed methane; (5) Methane behaves as an ideal gas, and its viscosity is constant; (7) Complete saturation of the rock mass, and the rock is a brittle elastic material with residual strength. In addition, its loading and unloading behaviours are in accordance with the elastic damage mechanics; (8) Adherence of the fluid flow to Biot's consolidation theory; 10

(9) Variation in the permeability of coal as a function of the stress states in elastic deformation. The variation increases dramatically when the element fails. 3.1.2 Numerical analysis scheme The simulation study includes two parts. The effect of hydraulic cavitating on hydraulic fracturing is studied in the first part with Realistic Failure Process Analysis (RFPA) software, including damage change and crack propagation characteristics of coal. The damage change and crack propagation characteristics of coal under the condition of double holes (L=24m) are also discussed. RFPA is used to simulate the real fracture process of rock that is a kind of numerical analysis software developed by LiSoft Technology (Dalian) Co. Ltd (Tang et al., 2002). It can simulate rock rupture process, stress distribution and acoustic emission detection. It can solve the problems that cannot be solved by most simulation software in geotechnical engineering due to the unique calculation and analysis method of RFPA software. The software's built-in formulas and supposed conditions are described in detail elsewhere (Li et al., 2013). So these are not covered much in this article. COMSOL Multiphysics software is used to analyze methane extraction after fracturing in the second part. In order to better guide engineering application and accurately determine the influence of HCAF on enhanced methane extraction, the RFPA simulation results are used as the conditions of COMSOL software simulation, i.e., the stress-fracture nephogram simulated by RFPA is imported into COMSOL software. The simulation results of RFPA are obtained by the image recognition function in the application software and serves as known condition for simulating methane extraction. This simulation method takes advantage from each software. RFPA can perfectly simulate the fracture development and damage of coal mass during fracturing. While COMSOL is a kind of numerical simulation software for methane-solid multi-field coupling based on partial differential equations. It has been widely used in simulation analysis of coal bed methane extraction. The borehole spacing is 16 m if fracturing technology is applied in the field. The borehole spacing distribution under simulated conditions is 16m, 24m and 32m. 11

3.1.3 Establishment of model Numerical models as shown in Fig. 4(a) and Fig. 4(b) are established respectively in order to study the influence of hydraulic fracturing effect before and after hydraulic cavitating. The model is 50m long, 50m wide and 110mm in diameter. The shape caused by hydraulic cavitating is similar to that of the spindle as shown in the Fig. 2(a). The shape of the cave is simplified to a rectangle with a width of 90 mm for calculation. Both the vertical stress Kg and horizontal stress K7 of the model are 12 MPa, and the right and bottom of the model are fixed boundaries. In order to study the influence of adjacent holes formation on fracturing boreholes, a numerical model is established as shown in Fig. 4(c), and the model is 100m long and 50m wide. The distance between two holes is L (24m), and the parameters of drilling and caving are the same as those in model 3-b. While, the model 3-c will be used to simulate methane flow in section 3.3. The effects of different spacing (16m, 24m and 32m) on permeability enhancement are studied during the simulating methane flow. The model parameters are shown in Table 1. The data in table 1 are derived from three aspects: field measurement, laboratory measurement and calculation.

12

Fig. 4 Numerical Model: (a) hydraulic fracturing borehole model; (b) HCAF borehole model (c) borehole model of HCAF with different spacing. Table 1. Table of Numerical Simulation Parameters. Items

Value

Items

Value

Homogeneous /m

3

Elastic model /GPa

11.5

Poisson ratio

0.32

Density /kg/m3

1450

Compressive strength /MPa

15

Pressure ratio

6

Internal friction angle /°

25

Initial permeability /m2

1.2e-17

Porosity

0.1

Initial methane pressure /MPa

1.9

Ash content of coal mass

9.5

Gas constant /J/(mol K)

8.314

Molar volume of methane /L/mol

22.4

Kinematic viscosity /Pa s

1.84×10-5

3.2 Analysis and discussion of pressure relief effect based on RFPA2D-flow

3.2.1 Pore water pressure and crack evolution characteristics of coal during fracturing Fig. 5 shows the pore water pressure distribution and crack propagation characteristics of three fracturing modes. (a) group is hydraulic fracturing, (b) group is HCAF, and (c) group is a two-hole arrangement under HCAF. As it is shown in Group 4 (a), there is not obvious crack around the borehole by hydraulic fracturing when the water pressure is 15 MPa. The damage range gradually increases with increasing of water pressure, and obvious crack propagation occurs around the borehole when the water pressure reaches 18 MPa. The crack propagation increases greatly increases when the water pressure reaches 20 MPa. It can be seen that there are crack initiation pressure and rapid crack propagation pressure in the process of hydraulic fracturing comparing the crack propagation under three kinds of water pressures (15 MPa, 18 MPa and 20 MPa). Group (b) of Fig. 5 shows that cracks occur in fracturing boreholes after hydraulic cavitating when the water pressure reaches 15 MPa. The damage range and crack size gradually increase with increasing water pressure. At the same time, it can be seen that the crack is occurred and extended after caving and fracturing have obvious direction from the crack distribution and the crack mainly develops in the direction of caving, which shows that caving has a significant impact on the crack. It shows that the cracks between two adjacent HCAF boreholes mainly develop toward the middle in group (c) of Fig. 5. As a result, the fissures in the middle area between the two fracturing holes are more developed. This 13

may be because the coal body is more likely to cause damage and broken under the action of water pressure on both sides, which makes the fractures in this area more developed. In addition, the damage and crack development of single fracturing hole are similar to those in group (b) of the Fig. 5. According to the simulation results, it is easier to fracture after hydraulic cavitation, and the resulting fracture channels have a certain directionality, making it easier to form a network of fractures that are connected to each other.

Fig. 5 Water pressure distribution and fracture extension characteristics: (a) hydraulic fracturing, (b) HCAF; (c) double-hole arrangement of HCAF.

3.2.2 The characteristics of coal damage and stress distribution after fracturing The acoustic emission distribution characteristics of three fracturing modes at 20 MPa are shown in Fig. 6. Acoustic emission (AE) is the acoustic wave generated by coal mass damage in the process of fracturing. Therefore, the AE data detected during fracturing can be used as a parameter to characterize coal damage. Fig. 6(a), Fig. 6(b) and Fig. 6(c) are AE nephograms of hydraulic fracturing, HCAF and double-hole layout of HCAF, respectively. Fig. 6 shows that the distribution characteristics of AE in three fracturing modes are basically consistent with the crack propagation in Fig. 5. AE of common hydraulic fracturing (Fig. 6(a)) is distributed around boreholes, but the 14

amount of those at upper and lower parts of boreholes are more than other place. AE of HCAF is mainly distributed on both sides of the caving, it is symmetrically distributed in single hole arrangement, but asymmetrically distributed in double hole arrangement. This may be due to the development of fracturing toward the middle, and it disturbs the regular of symmetrical crack development. The stress distribution of the horizontal axis of borehole in Fig. 6 as shown in Fig.7. It shows that the stress of the latter is significantly less than that of the former compared with Fig. 7(a) and Fig. 7(b), especially the minimum principal stress of the latter as a whole is less than that of the former. In addition, the area at low stress with HCAF is larger, and there are two minimum peaks, should be caused by the caving pressure relief. Therefore, it can be concluded that the effect of pressure relief with HCAF is more obvious by comparing the stress distribution between the two methods. Fig. 7(c) shows that the effect of the stress reduction with double hole arrangement of HCAF is more obvious, especially on both sides of the fracturing hole. However, the stress distribution between the two fracturing holes is small on both sides and large in the middle, but the maximum stress is still far less than the original stress, so the area between the fracturing holes is all relief zone. The pressure relief area on both sides of the two fracturing holes are similar to the stress distribution on both sides of the single hole. The stress away from the caving gradually increases, but there are also a certain pressure relief zones in some areas. Therefore, it can be concluded that the pressure relief range in Fig. 7(c) is relatively large according to the analysis, and the area between the two fracturing holes is the pressure relief zone.

15

Fig. 6 Acoustic emission distribution (water pressure is 20 MPa) characteristics: (a) hydraulic fracturing, (b) HCAF, (c) double-hole arrangement of HCAF.

Fig.7 The stress distribution of the horizontal axis of borehole in Fig. 5.

3.3 Analysis and discussion of methane drainage effect based on COMSOL

3.3.1 Theoretical model of methane drainage

(1) Methane seepage control equation It is assumed that the laminar flow of methane driven by the pressure in the coal fractures and it conforms to Darcy's law. The velocity of methane is linearly related to the methane pressure gradient in the fractures. i = j klF 1

(12

where i is the velocity of methane, m/s; m is the dynamic viscosity of methane, Pa⋅s; < is the permeability, mD;

klF is fracture pressure gradient.

Methane in coal matrix consists of adsorbed state and free state. The methane mass per unit in coal matrix volume can be expressed as follows: 16

q=

op (q (p O(q

•

st ou

+

q t(q vw

(13)

where x is mass of methane per unit volume in coal matrix, kg/m3; y is density of coal, kg/m3; z{ is Langmuir

volume, maximum adsorption capacity of monolayer, L/kg; |{ is Langmuir pressure which is the adsorption equilibrium pressure when the adsorption capacity is half of the maximum adsorption capacity, MPa; }~ is

porosity of coal matrix, %; zt is molar volume of methane in standard condition, m3/mol.

The diffusion of methane in coal matrix is determined by concentration. Its movement conforms to Fick's diffusion law (Liu et al., 2015a), and the mass of methane in coal matrix per unit volume diffusing into cracks in a unit time can be expressed as (Ren et al., 2017):  = −? K vw] (l~ − lF ) t

(14)

Where ? is diffusion coefficient, m2/s; K is shape factor of coal matrix, m-2; l~ is the methane pressure in the coal matrix, Pa; lF is the methane pressure in the coal fractures, Pa; € is the molar mass of methane, kg/mol; 

is an ideal methane constant, J/mol·K; ‚ is the temperature of the coal bed, K.

The continuity equation of methane seepage in coal bed is (Liu et al., 2015a): ƒ t 2 l P 5+ ƒD vw F F

k ∙ 2vw lF i5 = …1 − PF † t

(15)

Combining formula (12) ~ (15), the continuity equation of methane movement in coal fractures can be obtained as follows: PF

ƒ…(f † ƒD

+ lF

ƒ…‡f † ƒD

− j ∇ ∙ …lF ∙ ∇lF † − 1⁄‰ ∙ …1 − }F †…l~ − lF † = 0 1

16

(2) Governing equation of coal deformation Assuming that the coal bed is isotropic, combined with the theory of elasticity and Terzaghi's effective stress law, and considering the strain of coal skeleton caused by free methane pressure, The governing equation of coal deformation field can be expressed as:

G ∑ ƒŽ ] + "‘ ∑ ƒŽ ƒ J a



ƒ  Ja

a ƒ]

−

(’") ƒ( 1_

ƒŽ]

+ “ ƒŽ +  = 0 ƒ(

]

17

where λ and G is Lame constant; α is equivalent porosity; u is the stress component in three directions, N;  is 17

volume force, N/m3. (3) Initial and boundary conditions Generally speaking, for gas flow field, boundary conditions can be defined as gas boundary and flux boundary. These boundaries can also be called Dirichlet and Neumann in same papers and defined as (Liu et al., 2015b):

1

J

l = l(–) —˜ ™š

kl ∙ ˜›œ = (–) —˜ ™š

(18) (19)

where l(–) and (–) are the known constant gas pressure and gas flux on the boundaries. The initial conditions for coal seam can be expressed follows: l = l ˜ ™š

(20)

3.3.2 The characteristics of permeability distribution

The simulation results of 3.2 sections are imported into COMSOL according to the theoretical models of 2.3 and 3.3.1 sections to calculate the permeability distribution after HCAF. The permeability distribution after HCAF with different spacing holes (16m, 24m and 32m) are shown in Fig. 8. The k/k0 nephogram at the end of fracturing is on the left, and the permeability contour map is on the right. The isoline density indicates the disturbance intensity of coal mass, i.e., the ability to improve the permeability of coal mass. It can be seen from the contour map of permeability under the three working conditions that the permeability have largest increases at spacing of 16m. Second is spacing of 24 m and with spacing of 32 has relatively smaller permeability enhancement. This is because the smaller the distance, the more severe the damage of the coal body, and the better the fracture connectivity. That is to say the smaller the distance, the better the permeability enhancement of the coal body. However, the problem is smaller distance means more fracture holes required and excessive far distance does not guarantee the effect of gas drainage. Therefore, it is necessary to have a suitable distance to ensure efficient gas drainage and minimize the number of fracture holes at the same time. So, the permeability nephogram is shown by the ratio of post-fracturing permeability and initial permeability, i.e. k/k0 in order to compare the enhancement 18

extend of coal permeability under different spacing fracturing. It shows that the maximum permeability increases by more than 180 times when the spacing of holes is 16 m, and more than 140 times when the spacing is 25 m, and more than 120 times when the spacing is 32 m. The cracks formed by two fracturing sources have a tendency from high coincidence to end connection and then to basic disconnection along with increasing fracturing distance as shown in the distribution nephogram of the permeability ratio. That is consistent with the above analysis. In other words, the maximum permeability position of two fracturing cracks (main methane migration channel) is connected when the fracturing distance is 16 m. The distance is very close although the main methane migration channel cannot be seen when the fracturing distance is 24 m, the secondary main channel is well developed, and still belongs to the strong permeability region. When the fracturing distance is 32 m, it is obvious that the main methane migration channel is far apart and the less-than-optimal main channel is connected poorly, there may be low permeability area.

Fig. 8 Permeability Distribution of Different Holes Spacing with HCAF.

3.3.3 Analysis on the effect of enhancing methane extraction

The gas pressure nephogram of different borehole spacing during 90 days drainage are shown in Fig. 9. It shows that the gas pressure distribution is very uneven, which is due to the permeability is uneven after HCAF. It is

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easy to extract gas in the area with larger permeability, but it takes more time to extract the same gas in the area with smaller permeability (Aljamaan et al., 2017; Hazra et al., 2018). According to the simulation results, although the gas pressure at the edge varies greatly, the gas pressure in the area between the boreholes is always the smallest. While the gas pressure distribution between two boreholes is obviously different under the three kinds of spacing. The gas pressure between two boreholes gradually increases with increasing spacing, which is also a key factor for determining the optimal spacing of fracturing boreholes. Therefore, it is necessary to study the gas pressure in the middle of two drilling holes to determine the reasonable drilling spacing. This is because the gas pressure in the middle of the boreholes is always the highest in the area between them. Therefore, the change curve of gas pressure at the middle point (point A, B and C) under three kinds of spacing with extraction time are shown in Fig. 9. From Fig. 10, it shows that the methane extraction efficiency is different under the three spacing conditions. The decrease of methane pressure at point A is fastest. It can reach requirement after 62 days of extraction, and point B is the second, it can reach requirement after 85 days of extraction. However, the methane pressure at point C has not reached the requirement after 100 days of extraction. Besides, it can be seen from the spacing of the three curves that the extraction efficiency will be higher and higher along with the spacing of boreholes is reduced. That is, the time to reach the extraction requirements is getting shorter and shorter. According to the design requirements of 90 days of extraction, it is feasible to set the spacing of holes at 24m based on the simulation results, but it needs to be tested on site.

Fig. 9. Gas pressure nephogram of HCAF with different hole spacing after 90 days extraction.

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Fig. 10. Curves of gas pressure at points A, B and C along with extraction time.

4 Field test 4.1 Field situation

Located in the northeast of Qinshui Coalfield, the Xinjing Coal Mine is 11 km away from Yangquan City, Shanxi Province, China. The location and boundary of the mine field are shown in Fig. 11. With production capacity of 750 Mt/a, the mine has 12 layers of coal in the mine field, however, only 3 #, 8 # and 15 # coal seams are the main mining coal seams. The floor roadway of the working face 3218 was selected as the test site and the parameters of coal at the test site are as follow: the average coal thickness is 2.3 m; the sturdiness coefficient of the coal seam is 0.38 - 0.52; the primitive gas content is about 18.17 m3/t; the primitive gas pressure is 1.9 MPa; the permeability coefficient is 0.0188 - 0.1377 (m2/MPa2 d). The outburst risk of coal and gas is greater and it is very difficult for the operation of gas drainage. Experimental boreholes are arranged as shown in Fig. 12, with a spacing of 24m and a pressure relief borehole arranged in the middle. The control range of two fracturing boreholes is more than 35m. Hydraulic cavitating is first carried out on the fracturing boreholes. The caves are made for three times for each hole, and each time is not less than 30 minutes (guaranteeing the depth of caving reach 1m). The fracturing shall be carried out after completion of caving. Finally, connecting pipe and extraction are carried out at the end of fracturing. 21

Fig. 13 is a flow chart of the FCAF technique for enhancing gas drainage. The first step is to construct FCAF boreholes and pressure relief boreholes according to design. The second step is to perform a high-pressure water jet flushing in the FCAF borehole, and through this flushing process a large-scale pressure relief zone is formed around the borehole to provide space for further damage of the surrounding coal body. In the third step, hydraulic fracturing is carried out in the FCAF borehole where the cave is performed already, and the fracture formed by the cave will be extensively extended by fracturing until the fracture system communicates with the pressure relief holes. And in the last step, extraction by joint pipes is carried out. At this stage not only the fracture of the FCAF borehole after water is discharged is hard to close, but new fracture is continuously generated due to creep of the coal body caused by caves formed by the flushing, making it possible for a large amount of gas to be extracted continuously.

Fig. 11 Location of Xinjing Coal Mine (Shanxi Province, China).

Fig. 12 Schematic of the ECBM Extraction Technology Used in the Study.

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Connect the fractures by HF Pressure relief driling

Gas drainage ECBM extraction technology

Driling of HCAF

Implement HC

First step

Second step Pressure relief increases permeability and forms caves

Implement the drillings as designed

Implement HF

Third step

Increase the range of permeability increase

Gas drainage

Fourth step

Monitor the effect of the gas drainage

Fig. 13 Steps in the ECBM extraction by FCAF technology.

4.2 Experimental results and discussions

4.2.1 Fracturing curve

The fracturing curve includes the pressure curve and the flow curve. As shown in Fig. 14, the red solid line and the black solid line are the pressure curves of hydraulic fracturing and HCAF respectively, while the red dotted line and the black dotted line are the flow curves of them. From the fracturing curve we can seen that the starting pressure of hydraulic fracturing is about 16 MPa, while that of HCAF is about 13 MPa. Both flow curves increase rapidly at first, then decrease, and then increase rapidly. This is due to the beginning of fracturing fluid flow into the borehole, when the borehole space is gradually filled with fracturing fluid, the flow rate gradually decreases. At the same time, pressure is rising rapidly. The fracturing fluid increases rapidly and reaches the peak flow rate accompanied by the coal fractured. Then it enters the stable stage, till the fracturing system is balanced. The flow rate of the fracturing fluid gradually reduces to the end of the fracturing. The first peak point of the black dotted line is higher than the red dotted line, which is caused by the increase of space in the borehole after caving. The black dotted line is higher than the red dotted line because more fracturing fluid is injected into the fracturing borehole after caving, that is, the fracturing effect of HCAF is better. By analyzing the fracturing curves of the two, it can be seen that the fracturing difficulty of HCAF is less than that of ordinary fracturing, and the fracturing effect is better than ordinary fracturing.

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Fig. 14 Fracturing curve of borehole with hydraulic fracturing and HCAF during the process of fracturing.

4.2.2 Methane extraction data

Borehole methane flow and methane concentration are used to measure the extraction efficiency of different extraction methods. The methane flow shows the effect of pressure relief and permeability enhancement, in other words, the better pressure relief and permeability enhancement, the larger the methane flow rate. Methane concentration affects the utilization rate of methane extraction, that is, the bigger the concentration of methane extracted is, the methane easier is used. On the contrary, the more methane is discharged into the atmosphere. The difference of methane concentration in boreholes with different extraction technologies is mainly due to the difference of methane scalar quantity per unit time. The more methane scalar quantity is mixed with the infiltrated air, the concentration is the higher. Methane concentration and methane flow of borehole extraction in common borehole, hydraulic fracturing borehole and HCAF borehole are shown in Fig. 15. The Fig. 15(a) shows that the methane flow in boreholes obtained by hydraulic fracturing or HCAF is much larger than that of normal boreholes. The maximum methane flow with hydraulic fracturing is 0.56 m3/min, while that with HCAF is 0.81 m3/min. Besides, the methane flow in the borehole of HCAF is basically bigger than that of hydraulic fracturing in the whole drainage period. By analyzing the change curve of methane flow over time (fig.13 (a)), the following conclusions can be drawn: boreholes with HCAF technology have the best pressure relief effect, 24

followed by normal hydraulic fracturing, while conventional boreholes have the worst gas extraction effect; the gas flow in boreholes with HCAF technology decreased the most slowly and the gas extracted is the most. Similarly, Fig. 15(b) shows that the methane concentration in boreholes obtained by hydraulic fracturing or HCAF is much higher than that of ordinary boreholes, and the attenuation is slower. The both concentration difference is not very obvious. However, the methane concentration fluctuation of HCAF is more obvious, which may be the reason for the permeability fluctuation of fracturing boreholes caused by stage pressure relief because of caving. Those show that the HCAF technology is efficient and reliable to improve methane recovery ratio and to reduce greenhouse methane emissions.

Fig. 15 Methane concentration and methane flow of borehole extraction in common borehole, hydraulic fracturing borehole and HCAF borehole.

4.2.3 Regional calibration index

The regional index shall be tested to inspect the effect of methane extraction. Regional index include quantity of drilling cutting s and initial speed of gas emission q, which comprehensively represent methane extraction effect and coal pressure relief effect. Therefore, the regional calibration index of the area using HCAF technology is test, and the results are shown in Fig. 16. According to the measured results, all s is less than the critical value of 6 kg/m, and all q is less than the critical value of 5 L/min. This proves that the HCAF technology can indeed improve the effective extraction radius, and achieve the purpose of high efficiency gas control.

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Fig. 16. Statistical chart of the regional calibration index.

5 Conclusion In order to solve the problem for poor efficiency of methane extraction and energy utilization we presented the HCAF technology. The advantages of the technology are proved by theoretical analysis and numerical simulation. Finally, a comparative industrial test was performed in Xinjing Mine. The results are as follows: (1) The crack propagation, coal damage and pressure relief of hydraulic fracturing and HCAF has been simulated with RFPA. The results show that in coal seams, the HFAHC technology can reduce the fracture initiating pressure below 15 MPa, and increase the effective radius from 8 m to 12 m. That is to say, the effective radius is increased by 50% when the spacing of holes is increased from 16 m to 24m. The results of numerical simulation are basically consistent with the results of field tests. Besides, this technology has an impact on controlling the fracture direction. This shows that hydraulic cavitating has the function of directional fracturing, which can be used to increase permeability of coal in specific direction. (2) The field test is implemented to verify the results of the numerical simulation. After the HCAF technology the start-up fracturing pressure decreased from 16 MPa to 13 MPa. The maximum methane extraction flow has been increased from 0.56 m3/min to 0.81 m3/min, and the methane flow is greater than that of hydraulic fracturing during the course of extraction period. Boreholes with HCAF technology have the best pressure relief effect, 26

followed by normal hydraulic fracturing, while conventional boreholes have the worst gas extraction effect; the gas flow in boreholes with HCAF technology decreased the most slowly and the gas extracted is the most. (3) The area of methane extraction by HCAF technology was checked after increasing the borehole spacing from 12m to 24m. It finds that the amount of drilling cuttings s and initial speed of gas emission q were obviously lower than the critical threshold limit value. It indicates that HCAF technology can increase the effective extraction radius and permeability of borehole significantly. Those study show that the HCAF technology is efficient, reliable that can ensure the safety of coal mining, improving improve methane utilization recovery ratiorate and to reduceing greenhouse methane emissions.

Acknowledgments This work was supported by the Future Scientists Program of “Double First Class” of China University of Mining and Technology (2019WLKXJ060).

Notes The authors declare that we do not have any commercial or associative interest that represents a conflict of interest in connection with the work submitted.

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To solve the problem of low efficiency of CBM extraction, a novel technology of coupling hydraulic cavitating (relieve pressure to improve permeability) and hydraulic fracturing (expand the range of permeability enhancement) is put forward.




The permeability enhancement mechanism and extraction effect are studied by theoretical analysis and numerical simulation.




The field test is implemented not only verified the results of numerical simulation but also proved the advantages of the new technology in enhancing gas extraction and energy reduction.

The authors declare that we do not have any commercial or associative interest that represents a conflict of interest in connection with the work submitted.