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Research Article
Analysis of Influence of Ultra-High Pressure Water Jet Cutting
Pressure Sequence on Pressure Relief and Reflection
Improvement of Coal Seam
Shoulong Ma
1
,
2
1
School of Civil Engineering and Architecture, Anhui University of Science and Technology, Huainan, Anhui 232001, China
2
China Coal Xinji Energy Co., Ltd., Huainan, Anhui 232001, China
Correspondence should be addressed to Shoulong Ma; 2020100046@aust.edu.cn
Received 15 October 2022; Revised 6 November 2022; Accepted 23 February 2023; Published 1 April 2023
Academic Editor: Zhuo Chen
Copyright ©2023 Shoulong Ma. is is an open access article distributed under the Creative Commons Attribution License,
which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.
In order to explore the pressure relief eect of the two combined pressure relief and antireection technologies of ultra-high
pressure water jet cutting before pressure and pressure before cutting, theoretical analysis, numerical simulation, and eld test
were used to study the main control factors of the combined high-pressure water jet slit cutting and fracturing antireection
technology. is paper introduces the combined technology of ultra-high pressure hydraulic fracturing, and analyses the
mechanism of pressure relief and transparency enhancement of the combination of cutting before pressure and pressure relief
after cutting. e results show that the starting pressure of the coal seam with ultra-high pressure water jet cutting and pressure
relief is 13 MPa, the inuence radius of hydraulic fracturing is 45–55 m, the starting pressure of the coal seam with pressure cutting
and pressure relief is 16 MPa, and the inuence radius of hydraulic fracturing is 35–45 m. Compared with pressure cutting
combined pressure relief and permeability enhancement technology, cutting pressure relief and permeability enhancement
technology can improve the permeability of coal seams more evenly and eectively, and reduce the stress of coal seams near the
hole. e ultra-high pressure cutting and pressure combined technology can make the pressure relief of coal body uniform and
sucient, and the overall permeability coecient of the coal body is greatly improved. e drilling purity is 2.3 times of the
extraction purity of the ordinary single hole drilling, and the extraction inuence range is increased, and the extraction eect is
signicantly improved. At the same time, the stress of coal body is reduced after slitting, and the starting pressure of hydraulic
fracturing is reduced. e research results provide a scientic basis for the coal seam pressure relief and permeability enhancement
under similar conditions in the mining area and have a broad application prospect.
1. Introduction
At present, there are more than 2000 coal and gas outburst,
rock burst, and high gas mines in China, accounting for 30%
of the total number of mines. After entering deep mining,
the problems of high gas and high ground stress are
prominent, the permeability of coal seam is reduced, and the
diculty of disaster management is increased [1–3]. Under
the condition of deep mining, the coal seam is upgraded to
the protruding coal seam. For the protruding coal seam
without protective layer mining or rst mining, the anti-
protruding measures in the prepumping area of dense
conventional borehole are still the main measures. In order
to achieve ecient control of gas disasters in deep mines, the
conventional pressure relief and reection improvement
technologies in coal mines in China at the present stage
mainly include hydraulic punching, hydraulic cutting, hy-
draulic fracturing, and deep-hole presplitting blasting [4–6].
Although the deep-hole presplit blasting technology can
signicantly improve the pressure relief and antireection
eect of coal seam, it is relatively less applied in the pressure
relief and antireection improvement of coal seam, because
the long borehole coal seam is prone to collapse, resulting in
charging diculties, and it is dicult to eliminate the risk of
misring and explosion refusal in blasting operation. In
recent years, the reection improvement technology of
Hindawi
Advances in Civil Engineering
Volume 2023, Article ID 7738042, 15 pages
https://doi.org/10.1155/2023/7738042
hydraulic coal seam in China has entered a stage of rapid
development. e individual technology is constantly im-
proved, and the overall development is in the direction of
integration and diversication. e hydraulic measures such
as low-pressure hydraulic punching, hydraulic cavitation,
hydraulic fracturing, and hydraulic slotting have become the
hot spots of research in scientic research institutions, which
provide support for the control of coal mine gas disasters
and have achieved results under certain conditions [7–10].
e hydraulic technology is often used in the eld, and the
hydraulic punching pressure is generally 5–20 MPa, which
has a certain eect on the pressure relief and reection
improvement of soft coal seam, while the hydraulic
punching eciency of medium and hard coal seam is low.
However, it is dicult to control the shape of punching holes
in soft coal seam, and the amount of slag is not uniform, so
there may be hole collapse, hole injection, or roadway gas
overlimit during operation. e hydraulic fracturing has
a large inuence range and good antireection eect, and is
mostly used in medium and hard coal seams. However, it is
dicult to control the fracture propagation direction in the
coal body [11–13]. Hydraulic cutting seam technology is
based on high-pressure water cutting technology of coal
seam, improve coal seam gas ow state, reduce the stress, can
eectively prevent coal, and gas outburst and the impact of
ground pressure disasters happen, suitable for high ground
stress, gas and low permeability coal seam (seam hardness
f>0.4) bedding face drilling, wear layer drilling, and shimen
uncovering coal unloading antireection and so on [14–16].
e ultra-high pressure water jet cuts the coal body, and
the coal body around the slot produces deformation space so
that the coal body around the slot can be fully depressurized.
At the same time, because a part of the coal body around the
drill hole is transported out of the drill hole by the water jet,
a large number of cracks are generated in the expansion and
deformation of the coal rock, changing the permeability
conditions of the coal rock. Ge et al. established a uid-solid
coupling gas drainage model of slotted borehole. rough
numerical simulation analysis, it is believed that the inu-
ence radius of the borehole drainage after hydraulic slotting
has a power function relationship with slot disc, perme-
ability, drainage time, gas pressure, and other factors.
rough model research, it is determined that the inuence
signicance of each factor from large to small is perme-
ability, drainage time, gas pressure, and slotting depth [17].
Li et al. used a dynamic damage model to study the cutting
process of soft coal rock by water jet. During the cutting
process, with the gradual release of stress around the slot, the
crack continued to expand in some directions. e tension
and shear fractures in coal and rock continue to develop
during the damage accumulation process, in which short
pulses with high peak stress can form relatively short
fractures, and long pulses with low peak stress can form
relatively long fractures. Under the continuous action, the
cracks around the slot cut by the jet gradually develop and
then connect to each other to form a breakthrough failure
[18]. Tang et al. conducted numerical simulation on the
inuence of dierent hydraulic slotting arrangements on the
coal seam pressure relief and outburst prevention, and
analyzed the inuence of parallel, diamond, and staggered
pressure relief. e results show that the coal rock pressure
relief eect above the fracture groove is most obvious. e
results showed that the pressure relief eect of coal and rock
above the slot was the most obvious [19]. rough indirect
measurement of gas ow through similar material test, it is
conrmed that the pressure relief of coal seam has a sig-
nicant impact on permeability, and the permeability co-
ecient of coal and rock increases synchronously with the
degree of pressure relief. By studying the displacement and
stress changes of coal body under dierent slit widths after
high-pressure water jet slotting, the inuence of slit depth on
coal rock disturbance is analyzed. Numerical simulation
shows that the pressure relief range of 1.0 m, 1.5 m, and
2.0 m slotting on coal body reaches 2.6 m, 3.8 m, and 5.0 m,
and the inuence range of slit on coal body increases with
the increase of slit width. e larger the slit depth is, the
more conducive to coal seam pressure relief.
e antireection technology of hydraulic fracturing
was rst used in the exploitation of oil and gas elds, as
a main measure of oil and gas well stimulation. In the 1960s,
the hydraulic fracturing technology began to be used in coal
mines to increase in coal seam permeability, mainly by
drilling deep into the coal body through injection of high-
pressure water, and fracturing the coal body with water as
the energy transmission medium. After high-pressure
water fracturing, the stress of surrounding coal body was
reduced, and the stress concentration was transferred to the
depth of the coal body, thus improving the permeability of
coal body around the borehole, providing a good condition
for drilling gas extraction [12, 20–22]. e research on the
hydraulic fracturing technology in coal mine mainly fo-
cuses on the theoretical research on the initiation and
extension laws of the hydraulic fractures, or the estab-
lishment of hydraulic fracturing mathematical model for
numerical calculation research, or the physical experiment
research of hydraulic fracturing in the laboratory. Hubbert
and Willis described the stress distribution law of hydraulic
fracturing wall and surrounding coal and rock mass based
on classical elastic mechanics, and thus obtained the the-
oretical calculation model of tensile failure fracture pres-
sure of coal and rock mass: P
b
�3σ
min
−σ
max
+f
t
(σ
min
,σ
max
,
and f
t
are the minimum horizontal stress, the maximum
horizontal stress, and the tensile strength of coal rock,
respectively); the comprehensive eects of the tangential
principal stress σ
θ
, straight principal stress σv, and radial
principal stress σ
r
on the wall of the borehole are not fully
considered in this theory [23]. Ma et al. conducted an
experimental study on the inuence of water pressure with
dierent water ow rates on the fracture initiation char-
acteristics of coal. e results show that the increase in
water ow rate makes the fracture morphology more
complex, and the research results have important theo-
retical signicance for revealing the fracture initiation
behavior of boreholes [24]. According to the rst strength
criterion, Lv deduced the critical value calculation formula
of pressure crack initiation pressure and successfully tried it
in Pingmei Ten Mine [25]. Bouteca developed a full 3D
morphological mathematical model of hydraulic fracturing
2Advances in Civil Engineering
by combining the 3D spatial uid ow eld model with the
elliptical fracture deformation model of Shah and Koba-
gashi [26]. Based on the uid-solid interaction theory, Lian
et al. analyzed the problem of hydraulic fracture propa-
gation, took the critical stress as the criterion of the fracture
initiation and extension, deduced the pressure drop
equation expression in the fracture wall, and established the
calculation model. ABAQUS software was used to simulate
the inuence of surrounding rock stress, rock mechanical
properties, fracturing uid seepage characteristics, and
other external factors on hydraulic fracture propagation
[27]. Bjerrum et al. carried out hydraulic fracturing tests by
injecting high-pressure water into a small circular tube at
the bottom of the fracturing sample in a triaxial pressure
test device, and concluded that the propagation direction of
hydraulic fractures was generated along the minimum
principal stress surface [28]. Chen et al. used the true
triaxial test device to conduct AE monitoring on the
fracture of raw coal samples under fracturing. e research
results can reveal the source characteristics of the whole
fracture process of raw coal samples in the true triaxial
hydraulic fracturing process, and evaluate the safety of the
fracture process [29]. Deng et al. studied the control pa-
rameters of hydraulic crack propagation behavior by the
method of hydraulic fracturing under the control of ground
stress eld, and conducted a systematic experimental study
on the relationship between the formation and expansion of
hydraulic crack and the change of coal permeability and the
action of hydraulic pressure. e research results have
practical signicance for improving the design eect of top
coal precracking [30]. Liu et al. studied the internal mi-
crostructure evolution mechanism of dierent coals under
liquid nitrogen cooling. e experimental results show that
the total pore volume and pore surface area of coal are
increased after cold leaching, the heterogeneity of pore
structure is enhanced, the fractal dimension is increased,
and the development of porous structure of coal is pro-
moted by cold leaching [31]. Zhou et al. took Longhu coal
mine in Qitaihe mining area as the research object to study
the stability of roadway oor heave. e new support
scheme is adopted to reduce the oor heave of roadway by
81%. e research results can provide guidance for the
optimization of roadway support [32]. Surrounding rock
control and support stability of super high mining face was
studied by Wang Sheng. e results of this study can
provide guidance for the selection of scaolds and the
adoption of measures to improve the stability of scaolds
when they are used in ultra-high height conditions [33].
Taking Linyi mining area as the research object, Li Xuelong
studied the distribution law of ground stress in deep mines.
It is found that the relationship between principal stresses is
σ(H)>σ(v)>σ(h), which belongs to the strike-slip stress
system. Under this stress condition, the soil lateral pressure
coecients are all greater than 1, and the magnitude of the
three principal stresses increases with the increase in depth.
e research results have certain reference signicance for
mine disaster prevention and safety production [34]. Liu
Haiyan studied the failure mechanism and control tech-
nology of cave-side stoping roadway in close distance coal
seam. It is proposed that U-shaped steel telescopic support
erection and backwall lling are used to control the sur-
rounding rock of goaf mining in the process of roadway
excavation, and the on-site monitoring results also meet the
engineering requirements. e research results can provide
guidance for roadway design of goaf under similar mining
geological conditions [35–51].
e application of ultra-high pressure hydraulic slot-
ting technology can realize the precise seam cutting, rapid
pressure relief, and ecient permeability increase, and
drilling engineering quantity is reduced on the basis of the
extraction standard time to shorten 30% of the eect. With
the application of hydraulic fracturing technology, the
impact area of hydraulic fracturing is more than 50 m, with
a large impact area and obvious antireection eect in the
region. After the application of ultra-high pressure hy-
draulic slit technology and hydraulic fracturing technology,
the antireection eect of coal seam is remarkable.
erefore, in the hydraulic cutting seam technology and
hydraulic fracturing technology in the application process,
there exist the following problems: the super high-pressure
hydraulic cutting seam to improve the unloading antire-
ection eect at the same time, greatly reduce drilling of
quantities, as local antireection measures that it is obvi-
ous, but for large area still needs to undertake a large
number of slotted drilling construction slot unloading
antireection. Although the antireection eect of hy-
draulic fracturing technology has a large inuence range, it
is dicult to control the fracture propagation direction in
the coal body, and the pressure relief and antireection
improvement are not uniform, and there is stress con-
centration phenomenon.
To sum up, to better carry out uniform permeability
improvement in low permeability coal seam, accurately
control the pressure relief and permeability improvement
area, greatly reduce the drilling engineering quantity, and
solve the technical problems of gas extraction and control in
low permeability coal seam. e combined technology of
ultra-high pressure hydraulic slit cutting and hydraulic
fracturing is explored, and the mechanism of pressure relief
and reection improvement by cutting and pressure relief
and cutting is analyzed. e hydraulic cutting pressure
scientic model is established, and the inuence range of
high pressure hydraulic cutting pressure is solved through
theoretical analysis. e PFC software based on the theory of
discontinuous media mechanics was used to simulate the
initiation and expansion characteristics of seam cutting and
fracturing fractures in coal seam, and the distribution rules
of fractures and stresses in coal seam were compared and
analyzed by hydraulic cutting before pressure and pressure
relief after cutting. Combined with the eld research on
ultra-high pressure hydraulic cutting before pressure and
pressure before cutting and combined pressure before
cutting and permeability improvement, the inuence law of
ultra-high pressure water jet cutting pressure sequence on
pressure relief and permeability improvement of coal seam is
further revealed. e research results are of great signicance
to enrich the comprehensive gas control technology of low
permeability coal seam.
Advances in Civil Engineering 3
2. Ultra-High Pressure Hydraulics First Cut and
Then Pressure Combined with the
Principle of Permeation
e concept of hydraulics rst cut and then pressurized joint
mode is to use ultra-high pressure hydraulic cuts to cut the
coal seam in the coal seam rst, and then use hydraulic
fracturing to fracturing the coal seam after the gap is gen-
erated. e groove generated by the hydraulic cutting in the
early stage can guide the hydraulic fracturing so that the
extension direction of the fracture in the plastic zone is
basically the same as the direction of cracking. e crack
expansion is more uniform.
By forming a slot by hydraulic cutting in the coal body in
advance, the eective inuence range of the single hole can
be expanded to a certain extent. e original stress balance of
the coal body can be destroyed. e coal body around the cut
hole is transported to the space of the slot space, and the
pressure relief, deformation, and expansion of the coal seam
can occur, further generating more cracks and expanding the
plastic area of the coal body near the cut hole. Combined
with the empirical formula of plastic theory, it can be seen
that the radius of the plastic zone is about 3 to 5 times the
radius of the cut groove, and the radius of the high-pressure
water groove is determined to be about 2.5 m through eld
tests. It is inferred that the radial plastic zone range outside
the hydraulic groove is about 7.5 to 12.5 m. After the hy-
draulic cut is formed into a crevice, a weak surface is
generated in the drilling hole, and after the fracturing water
enters the crack, it promotes the cracking, expansion, and
extension of the weak side crack, resulting in the full and
uniform development of the coal body fracture near the
borehole. rough the rational arrangement of the cut-
pressure joint hole, a three-dimensional fracture network
of interpenetration is formed between the drilled holes,
which eectively solve the problems of disorderly expansion
of the fracture in the coal body during the ordinary hydraulic
fracturing, local stress concentration, and pressure relief
blind zone after fracturing.
is joint mode not only solves the problems that the
direction of hydraulic fracturing crack is not easy to control,
but also the crack propagation in the fracturing area is
uneven. It is easy to form a high stress concentration area,
and there is a “blind zone” of fracturing, but also increases
the scope of impact of fracturing, which saves a lot of drilling
engineering compared with ordinary drilling holes and
improves the eciency of pressure relief and antiextrusion.
At the same time, the problems of uneven pressure relief and
stress concentration in individual areas are supplemented by
xed-point hydraulic cutting to achieve uniform and e-
cient antipenetration purposes. A schematic diagram of the
rst cut and then press joint is shown in Figure 1.
3. Ultra-High Pressure Hydraulics First Press
and Then Cut and Increase the
Principle of Penetration
Hydraulics rst press and then cut joint mode, that is,
hydraulic fracturing is used to supplement the fracture
within the inuence range of hydraulic fracturing. e gap
fracture is formed in the blank zone of the hydraulic frac-
turing aected area, and the fracture formed by hydraulic
fracturing is conducted, and more fractures are formed.
rough hydraulic fracturing operations to rapidly improve
the permeability of the coal seam in the area and the gas
extraction eect, after the completion of the hydraulic
fracturing construction, the fracturing crack is un-
controllable. Although the cracks in the coal seam are
generated in a large range, the permeability of the coal seam
increases, and the coal body plays a decompression and
permeability eect within the scope of the crack. However,
the inhomogeneity of the physical and mechanical prop-
erties of the coal leads to the uncontrollable weak surface in
the coal seam, resulting in uncontrollable hydraulic frac-
turing cracks. ere is a blank zone aected by hydraulic
fracturing within the scope of inuence of the hydraulic
fracturing, and the area with poor pressure relief eect is
used as a “blind spot” of hydraulic fracturing, and there is
a stress concentration in the uncontrolled area of the crack.
e use of hydraulic cutting joints to accurately increase
penetration and strengthen extraction, under the action of
ground stress, and the fracturing cracks are connected with
the fractured area of the joints, forming an overall pressure
relief area, reducing the stress concentration, and eectively
improving the gas permeability of the coal seam. is mode
eectively combines the advantages of fracturing and
fracture, solves the problem of stress concentration and
uneven fracturing in the fracturing area, and realizes the
accuracy of antiprotrusion. e precise antiprotrusion mode
of pressing rst and cutting is shown in Figure 2.
4. Ultra-High Pressure Hydraulics First Press
and Then Cut and Increase the
Principle of Penetration
Under the action of ultra-high pressure hydraulic force,
when the coal body around the borehole exceeds its own
strength, the hole wall is the plastic softening zone and the
elastic zone are from inside to outside. e drilling me-
chanical model is shown in Figure 3. e model assumes the
following:
①e borehole is subject to the stress P
0
of the original
rock, and the side pressure coecient λ�1 is treated
according to the axial symmetry problem, which is
simplied to planar strain
4Advances in Civil Engineering
Scope of
fracturing Slit plastic zone
Stress distribution curve
roof
Cutting seam
cutting groove Fracturing cracks
Coal
seam
floor
(a)
Cutting
seam slot
drilling
Slit plastic
zone
Fracturing
cracks
(b)
Figure 1: Schematic diagram of high eciency anti-penetration mode of cutting before pressing. (a) Schematic diagram of fracture
propagation prole after cutting. (b) Schematic diagram of the distribution plane of the slit after cutting.
Scope of
fracturing
Slit plastic zone
Stress distribution curve
roof
Cutting
seam slot
Fracturing
cracks
Coal
seam
floor
Pressure in
(a)
Cutting
seam slot
Fracturing
drilling
Cutting seam
drilling
Slit plastic
zone
Fracturing
cracks
(b)
Figure 2: Schematic diagram of high eciency anti-spike mode of pressing before cutting. (a) Schematic diagram of fracture propagation
prole after compression. (b) Schematic diagram of the distribution plane of the crack before pressing and cutting.
Plastic softening zone
The elastic zone
p0
p0
pi
Rp
R0
σθ
σz
σr
Figure 3: Borehole mechanical model.
Advances in Civil Engineering 5
②e coal around the borehole is homogeneous and
isotropic, and the inuence of borehole pressure
relief on the borehole is not considered
③R
0
is the drilling radius; σ
p
is the peak intensity; σ
c
is
the residual strength; the hydraulic fracturing pres-
sure p
i
acts evenly on the wall of the drilled hole
Assuming that the compressive stress is positive and the
tensile stress is negative, the deep borehole is subjected to
ground stress, at this time:σ
1
�σ
θ
,σ
3
�σ
r
, and
σ
2
�σ
z
�(σ
θ
+ σ
r
).e strength characteristics of the elas-
toplastic state of the borehole wall are described by the
unied strength theory, and the expression is as follows:
σθ�Ajσr+Bj,(1)
Aj�(1+b)1+sin j
1−sin j
(1+b) − b1+sin j
Bj�2(1+b)cjcos j
1−sin j
(1+b) − b1+sin j
⎫
⎪
⎪
⎪
⎪
⎪
⎪
⎬
⎪
⎪
⎪
⎪
⎪
⎪
⎭
,(2)
where σ
θ
,σ
r
are the tangential stress and radial stress of the
borehole wall, respectively. Since Aj,Bjare characterizing
the parameters of the coal body, representing the re-
lationship between the maximum principal stress and the
minimum principal stress. e is for Poisson’s ratio; jis the
symbolic parameter; j�erepresents the initial internal
friction angle φ
e
and cohesion force c
e
of the coal body; j�p
represents the friction angle φ
p
and cohesion c
p
of the plastic
softening region; bis the median principal stress coecient,
0≤b≤1.
In the stress-strain curve, failure occurs when the
strength of the coal body exceeds its ultimate strength, and
this paper assumes that the residual friction angle φ
c
and the
residual cohesion c
c
are unchanged. Plastic softening occurs
when the strength of the coal body exceeds its peak strength,
and the values of the friction angle φ
p
and cohesion c
p
in the
plastic region gradually decrease with the increase in plastic
strain, assuming that φ
p
and c
p
are linearly softened with the
initial internal friction angle φ
e
and cohesion force c
e
. e
softening coecients k
φ
and k
c
are introduced, which are as
follows:
p�
e, r ≥Rp
,
ke, r ≤Rpand s
e
≤k≤1
,
⎧
⎪
⎪
⎪
⎪
⎪
⎨
⎪
⎪
⎪
⎪
⎪
⎩
cp�
ce, r ≤Rp
,
kcce, r ≤Rpand cs
ce≤kc≤1
,
⎧
⎪
⎪
⎪
⎪
⎪
⎨
⎪
⎪
⎪
⎪
⎪
⎩
(3)
where k
φ
,k
c
are the internal friction angle and the cohesive
softening coecient. φ
e
,φ
p
, and φ
s
are the initial internal
friction angle, the friction angle of the plastic softening zone,
and the residual internal friction angle of the coal body,
respectively. c
e
,c
p
,c
s
are the initial cohesion of the coal body,
the cohesion, and residual cohesion in the plastic softening
area, MPa. R
p
is the radius of the plastic zone, m.
e drilled coal body is in the linear elastic state, pyis set
as the radial stress at the junction of the elastic zone of the
coal body and the plastic softening zone, and the elastic zone
of the coal body is regarded as a thick-walled cylinder under
the joint action of pyand ground stress p
0
. It can be seen that
the elastic zone stress is as follows:
σre �py
Rp
2
r2+p01−Rp
2
r2
⎛
⎝⎞
⎠,
σθe� −py
Rp
2
r2+p01+Rp
2
r2
⎛
⎝⎞
⎠,
⎧
⎪
⎪
⎪
⎪
⎪
⎪
⎪
⎪
⎨
⎪
⎪
⎪
⎪
⎪
⎪
⎪
⎪
⎩
(4)
where σ
re
is radial stress in the elastic zone, MPa. σ
θe
is the
tangential stress of the elastic region, MPa. ris the distance
from any point in the coal body to the center of the circle, m.
p
0
is the ground stress, MPa. pyis the stress at the elasto-
plastic junction, MPa.
At the elastoplastic junction r�R
p
, formula (5) satises
formula (1) and the radial stress is continuous, and the
nishing can be obtained as follows:
py�2p0−Be
1+Ae
.(5)
Any of the study unit points in the coal body satisfy the
equilibrium dierential equation:
dσr
dr+σr−σθ
r�0.(6)
Substituting equation (1) into equation (6) and in-
tegrating, take σr|r�R0�pias the boundary condition, the
radial and tangential stresses of the plastic region can be
obtained as follows:
σrp �S1+pi−S1
R0
r
1−Ap
,
σθp�S1+Appi−S1
R0
r
1−Ap
.
S1�Bp
1−Ap
.
⎧
⎪
⎪
⎪
⎪
⎪
⎪
⎪
⎪
⎪
⎪
⎪
⎪
⎪
⎨
⎪
⎪
⎪
⎪
⎪
⎪
⎪
⎪
⎪
⎪
⎪
⎪
⎪
⎩
(7)
e radial stress σ
r
is continuous at the elastoplastic
junction, which is σrp|r�Rp�σre |r�Rp.e radius of the
plasticity zone of the rst type (7) and the rst type of (4) of
the rst type of plasticity zone is as follows:
Rp�R0·py−S1
pi−S1
1/1−Ap
.(8)
6Advances in Civil Engineering
5. Numerical Simulation Analysis of Ultra-High
Pressure Hydraulics Combined with
Permeation PFC
Under the action of ultra-high pressure hydraulic force,
when the coal body around the borehole exceeds its own
strength, the hole wall is the plastic softening zone and the
elastic zone from the inside to the outside. e drilling
mechanical model is shown in Figure 3. e model assumes
the following:
e particle ow discrete element method (PFC) is based
on the mechanics of discontinuous media to study the
germination, expansion, and penetration of fractures, which
can truly express the geometric characteristics of jointed
rock masses, facilitate the handling of nonlinear deformation
and destruction, and reect the dierent physical relation-
ships between multiphase media through a variety of con-
nection methods between cells, which can eectively study
noncontinuous phenomena such as cracking and separation.
ere are countless mesoscopic cracks in coal rocks, espe-
cially soft coal bodies, showing obvious inelastic de-
formation characteristics, and these mesoscopic cracks
develop into macroscopic cracks or until they break down
under increased loads. In the particle ow discrete element,
when the contact point node is destroyed, the corresponding
particles will produce cracks, and new cracks will be gen-
erated at the initial crack tip as a sign of hydraulic fracturing.
e nonlinear deformation failure process of the fracture can
be analyzed by direct and indirect methods. e indirect
method uses the constitutive relationship to analyze the
failure process, generally assumes the fractured coal body as
an ideal uniform material, reects the weakening of the
overall strength of the fractured coal body through a certain
constitutive relationship, and expresses the microstructure
failure process in the coal body in this way. e direct
method is a mesoscopic simulation method, which assumes
that the fracture coal material is a collection of various
microstructures, or some particle combinations connected
at the contact point. e failure process of the fractured coal
body can be directly simulated by the microstructure and
particle rupture, and the fractured coal body can be studied
mescologically without simulation through complex con-
stitutive models.
e PFC numerical calculation software is a kind of
software based on particle ow theory, which links the
microstructure of materials with macroscopic mechanical
reactions, and directly simulates material failure from
a mesoscopic perspective, which is suitable for materials that
are dicult to accurately describe their properties through
constitutive relationships based on uniform media, such as
fractured rock masses. e bonding parameters of the
particles determine the location and number of initial
microcracks, so microcracks can only be formed in the
connection contact model. e position and size of the two
particles determine the location and geometry of the cracks,
which can be simplied to a cylindrical surface represented
by the center point position, normal direction, thickness,
and radius parameters.
5.1. First Cut and en Press Combined with Unloading Coal
Seam Fracture and Stress Distribution. e model adopts
a two-dimensional plane model. e direction length is
200 m, the height is 200 m, the drilling diameter is 113 mm,
the cutting pressure is 100 MPa, the test site elevation is
about 550 m, the vertical stress reaches 17.7 MPa, the
pressure measurement coecient is 1, and the horizontal
stress is 17.7 MPa. e model schematic diagram is shown in
Figure 4.
First, hydraulic cutting is used to form a slot, and
a pressure of 25 MPa is applied to the periphery of the groove
for fracturing, and the distribution of fracture, main stress,
and permeability of the coal seam after fracturing is ana-
lyzed. e ssure distribution of coal seam in the process of
drilling construction, hydraulic cutting, and hydraulic
fracturing is shown in Figures 5–7, respectively, and the
maximum main stress distribution curve on the midline of
the test borehole level is monitored in the simulation.
Figure 5 shows the distribution of coal cracks after
drilling construction. It can be seen from the gure that the
ssures in the coal body around the drilling hole are not
obvious after the construction of the borehole, and basically
maintain the original state. e stress of the coal body
around the borehole is evenly distributed and is basically in
a state of stress equilibrium. Only the drilling is excavated,
creating plastic deformation zones and elastic zones around
the borehole, resulting in reduced seam stress. Figure 6
shows the distribution of coal cracks after drilling and
cutting. It can be seen that after drilling and cutting, a cir-
cular gap is formed around the drilling hole, and the coal
body around the gap groove is damaged or plastically de-
formed, forming a crack. e stress of the coal body around
the trough is concentrated and transferred to the deep part of
the coal body, and a stress-reduced pressure relief zone is
formed around the slot area. Figure 7 shows the distribution
of fractures in coal with a fracturing pressure of 25 MPa, as
the water injection pressure increases, when the ssure
expands to a certain extent. e expansion rate begins to
slow down, and the secondary ssures are gradually inter-
connected to form a highly complex fracture network, but
with the continuous development of the ssures in the coal
seam, the degree of stress concentration is becoming more
and more serious.
In the aected area of fracturing, the fractures are mainly
elliptical in distribution. In terms of the degree of damage of
the coal body, the coal body in the area near the fracturing
hole is better than the coal body far from the fracturing hole
area. From the change of water injection pressure during the
fracturing process, it can be seen that the cracking pressure is
about 13 MPa, and the radius of inuence of hydraulic
fracturing can reach 45–55 m.
After hydraulic fracturing of the coal seam, the original
stress balance state of the coal seam is destroyed, resulting in
a decrease in the stress value of the coal seam in the area near
the fracturing hole, forming a pressure relief zone. However,
the stress value of the coal seam around the pressure dis-
charge zone increases, forming a stress concentration area.
erefore, according to the stress distribution of the coal
seam after fracturing, the coal seam around the fracturing
Advances in Civil Engineering 7
hole can be divided into stress reduction zone (pressure
relief area), stress concentration area, stress transition area,
and original stress area from near and far.
5.2. First Press and en Cut Combined with Unloading Coal
Seam Fracture and Stress Distribution. e model adopts
a two-dimensional plane model, the direction length is
200 m, the height is 200 m, and the drilling diameter is
113 mm, which is consistent with the model of Figure 4, as
shown in Figure 8. e central fracturing hole is located in
the center of the coal seam and is set to 113 mm in diameter.
e test site elevation is about 550 m, the vertical stress
reaches 17.7 MPa, the side pressure coecient is 1, and the
horizontal stress is 17.7 MPa. 25 MPa pressure is applied to
the periphery of the borehole for fracturing, and after the
completion of the fracturing, hydraulic fracture drilling is
200 m
200 m
slot
Figure 4: Joint numerical model of coal seam cutting pressure.
(a)
50 100 150 2000
Distance from borehole (m)
0
5
10
15
20
25
Maximum principal stress (MPa)
(b)
Figure 5: Fracture and stress distribution of coal seam after drilling. (a) Distribution of fractures. (b) Stress distribution condition.
(a)
0
5
10
15
20
25
30
Maximum principal stress (MPa)
50 100 150 2000
Distance from borehole (m)
5 MPa
(b)
Figure 6: Fracture and stress distribution of borehole after slit. (a) Distribution of fractures. (b) Stress distribution condition.
8Advances in Civil Engineering
carried out in the blank area of the fracture according to the
fracture and stress distribution. e distribution of fracture,
main stress, and permeability of the coal seam after frac-
turing and cutting is analyzed.
Figures 9–11 are the ssure distribution of the coal seam
and the maximum main stress distribution curve on the
midline of the drilling level of the ordinary borehole hy-
draulic fracturing process, respectively.
Figure 9 shows the distribution of coal cracks after
drilling construction. It can be seen from the gure that after
the construction of the borehole, the fracture of the coal
body around the borehole is not obvious and basically
maintains the original state. e stress of the coal body
around the borehole is evenly distributed and is basically in
a state of stress equilibrium. Only due to the excavation of
the drilled hole, the plastic deformation zone and the elastic
zone are generated around the drilled hole, resulting in
a reduction in the stress of the coal seam.
Figure 10 shows the fracture distribution of coal with
a fracturing pressure of 25 MPa. As can be seen from the
gure, when the ssure expands to a certain extent, the
number of new ssures decreases, and there is a stress-
concentrated area. In the aected area of fracturing, the
distribution of fracture fractures is mainly in the direction of
main stress, and the other regions have less fracture de-
velopment and there are blank areas. e fracture devel-
opment is uneven. After the use of hydraulic cuts, the
number of cracks increases, and the stress concentration
area caused by hydraulic fracturing is signicantly relieved.
From the change of water injection pressure during the
fracturing process, it can be seen that the cracking pressure is
about 16 MPa, and the inuence radius of hydraulic frac-
turing can reach 35∼45 m.
Figure 11 shows the fracture distribution and stress
distribution after hydraulic fracture in the nonformation
area of the coal seam after the fracturing pressure is 25 MPa.
As can be seen from the gure, after the hydraulic fracture is
carried out in the later stage, the fracture blank zone within
the inuence range of the original hydraulic fracturing
generates a crack. e fracture that has been generated can
be further increased. e stress concentration area is
redistributed to play a role in depressurization.
Compared with the same model, there is a big dierence
between the cutting sequence of ultra-high pressure water jet
and the fracture and horizontal direction of the discharge
coal seam:
(1) e cracking pressure of the coal seam with the
combined pressure of cutting rst and then pressing
is 13 MPa, and the radius of inuence of hydraulic
fracturing can reach 45–55 m. e cracking pressure
of the coal seam of rst pressure and then cutting and
unloading coal seam is 16 MPa, and the radius of
inuence of hydraulic fracturing can reach 35–45 m.
(2) e rst cut and then the combined pressure dis-
charge coal seam destroys the original stress balance
state of the coal seam, causing the stress value of the
coal seam in the area near the fracturing hole to
decrease, forming a pressure relief area. e stress
distribution is more uniform, at a low value, forming
a better pressure relief area, and the stress value of
the coal seam around the pressure relief area is in-
creased, forming a stress concentration area. In the
area aected by fracturing, the distribution of frac-
ture fractures is mainly along the direction of main
stress. e fracture development in other areas is less,
and there are blank areas. e development of
fractures is uneven, after the use of hydraulic frac-
tures, the number of fractures increases, and the
stress concentration area caused by hydraulic frac-
turing is obvious. But there are pressure uctuations
and instability in the pressure relief area.
In summary, after the cutting pressure combination, the
pressure relief of the coal body can be uniform and sucient.
e overall gas permeability coecient of the coal body can
be greatly improved. e scope of inuence of extraction can
be increased, and the extraction eect can be signicantly
improved. At the same time, the stress after the coal body is
cut and the cracking pressure during hydraulic fracturing is
reduced.
(a)
0
5
10
15
20
25
30
Maximum principal stress (MPa)
50 2000 100 150
Distance from borehole (m)
(b)
Figure 7: Fracture and stress distribution when fracturing pressure is 25 MPa. (a) Distribution of fractures. (b) Stress distribution condition.
Advances in Civil Engineering 9
200 m
200 m
slot
plot
Figure 8: Numerical model for hydraulic fracturing of coal seam.
Monitori ng line
(a)
50 100 150 2000
Distance from borehole (m)
0
5
10
15
20
25
Maximum principal stress (MPa)
(b)
Figure 9: Fracture and stress distribution of coal seam after drilling. (a) Distribution of fractures. (b) Stress distribution condition.
Monitori ng line
(a)
0
5
10
15
20
25
Maximum principal stress (MPa)
50 100 150 2000
Distance from borehole (m)
(b)
Figure 10: Fracture and stress distribution of 25 MPa pressure fractured coal seam. (a) Distribution of fractures. (b) Stress distribution
condition.
10 Advances in Civil Engineering
6. Engineering Application of Ultra-High
Pressure Hydraulic Cutting Combined
Process Technology
6.1. Application of Combined Pressure Relief and
Antireection after Cutting through Layer Drilling
6.1.1. Project Summary. 220106 working surface is located in
the 2201 mining area, the maximum gas content of 1 coal
seam is 6.8 m
3
/t, and the gas pressure is 1.22 MPa. e rst
coal seam group includes 1 coal and 1 upper coal. e
average upper coal is 3.9 m, the average upper coal is 2.8 m,
and the average gangue lost between 1 coal and 1 upper coal
is 1.0 m. e inclination angle of the coal seam is 2°–12°, and
the average is 6°. e working surface is directly topped with
sandy mudstone with an average thickness of 6.7 m, and the
old top is quartz sandstone with an average thickness of
17.8 m. e application site elevation is about
523.0–−559.6 m, a total of 7 cut pressure combined drilling
holes are constructed, and the oor plan is shown in
Figure 12.
6.1.2. Application of Cutting Pressure Combined with Anti-
reection Technology
(1) Cut-Pressure Combined Drilling Implementation. e
length of the seven cutting pressure combined drilling coal
hole sections is 11–16 m. e maximum pressure during the
cutting period is 90–100 MPa. e cutting gap is 3 m, and the
single knife cutting time is 5–10 min. During the cutting
operation, the drilling and rebating water and slag return is
smooth. e coal output of a single knife is 0.34–0.56 t. e
average single knife output is about 0.43 t, and the equivalent
radius of the average cut is 2.54 m.
After the completion of drilling and cutting, the hole is
sealed immediately. e hydraulic fracturing is carried out
after 48 h. e maximum pressure of fracturing is
22–27 MPa. e number of fractures per drilling hole is 3
times. e fracturing time is 19–28 h, and the total amount
of water injection per borehole is 109.6–132.4 m
3
.
(2) Drilling Quantity Analysis. e spacing between the
layout of the extraction drilling holes in the application area
is 13 m ×13 m, and a total of 183 extraction drilling holes are
constructed, with a drilling volume of about 11800 m.
Compared with the conventional drilling arrangement of the
mine (the layout spacing is 10 m ×10 m, and 297 extraction
drilling holes need to be constructed, about 19200 m), the
drilling volume can be saved by about 38%.
(3) Pumping Volume Analysis, Pumping Pure Volume. e
concentration and extraction scalculus of the drilling hole in
the application area are shown in Figures 13 and 14.
It can be seen from the gure that the extraction con-
centration in the combined cutting pressure area is 10% to
40%. e concentration uctuation is relatively stable. e
attenuation is small. e extraction purity is 5.12–10.13 m
3
/
min, and the average single-hole extraction purity is
0.0245 m
3
/min, which is 2.3 times that of the ordinary
drilling single-hole extraction purity of 0.0108 m
3
/min.
(4) Extraction Standard Time. According to the drilling and
extraction situation in the application area, when the ex-
traction drilling hole extraction is 47 days, the extraction rate
reaches 30%. A total of 14 residual gas content were tested,
and the test result was 3.98–4.75 m
3
/t. e extraction
standard was achieved. e extraction time is 41% lower
than the 80 days of ordinary drilling and extraction.
6.2. Application of Combined Pressure Relief and Anti-
Reection Improvement through Layer Drilling
6.2.1. Project Summary. e application site of the drilling is
220106, the working surface is located in the outer section of
the 2201 mining area (the strike length is about 170m, and
the strike width is about 190 m), a total of 3 fracturing
Slot hole
Monitori ng line
(a)
150 2000 50 100
Distance from borehole (m)
0
5
10
15
20
25
30
Maximum principal stress (MPa)
(b)
Figure 11: Fracture and stress distribution of seam after hydraulic fracturing. (a) Distribution of fractures. (b) Stress distribution condition.
Advances in Civil Engineering 11
drilling holes are constructed, and the spacing is cut
according to the 26 m ×26 m spacing, and the oor plan is
shown in Figure 15.
6.2.2. Application of Compression Cutting Combined with
Antireection Technology
(1) Cut-Pressure Combined Drilling Implementation. e
maximum pressure of fracturing is 22–27 MPa, and the
number of fracturing times for each borehole is 3 times. e
fracturing time is 21–32 h, and the total water injection of
each borehole is 117.8–145.2 m
3
.
After the completion of fracturing, the drilling hole is
drilled according to the construction of 13 m ×13 m spacing
in the control area, and the hydraulic cutting measures are
constructed according to the 20 m ×20 m spacing. e
maximum pressure during the cutting period is 90–100 MPa.
e cutout spacing is 3 m, and the single knife cutting time is
5–10 min. A total of 91 cut holes were implemented, and the
drilling reow water and slag return was smooth during the
seam operation. e coal output of a single knife was
0.31–0.62 t. e average single knife coal output was about
0.41 t, and the equivalent radius of the average cut was
2.52 m.
(2) Drilling Quantity Analysis. e spacing between the
layout of the extraction drilling holes in the application area
is 13 m ×13 m, and a total of 183 extraction drilling holes are
constructed, with a drilling volume of about 11800 m.
Compared with the conventional drilling arrangement of the
mine (the layout spacing is 10 m ×10 m, and 297 extraction
drilling holes need to be constructed, about 19200 m), the
drilling volume can be saved by about 38%.
(3) Pumping Volume Analysis, Pumping Pure Volume. e
concentration and extraction scalarity of drilling in the
application area are shown in Figures 16 and 17. It can be
seen from the gure that the extraction concentration in the
combined cutting pressure area is 30% to 40%. e con-
centration uctuation is relatively stable. e attenuation is
small. e extraction purity is 3.6–6.8 m
3
/min, and the
average single-hole extraction purity is 0.026 m
3
/min, which
is 2.4 times that of the ordinary drilling single-hole ex-
traction purity of 0.0108 m
3
/min.
(4) Extraction Standard Time. According to the drilling and
extraction situation in the application area, when the ex-
traction drilling hole extraction is 43 d, the extraction rate
reaches 30%. A total of 5 residual gas content were tested,
and the test result was 3.76–4.52 m
3
/t, and the extraction
standard was achieved. e extraction time is 35% less than
the ordinary drilling and extraction time of 66 d.
6.3. Economic Benets
6.3.1. Direct Economic Benet Analysis. After the 220106
working surface adopts the measures of cutting pressure
220106 machine lane
220106 working face
220106 wind lane
Extraction from borehole
220106cut
Cut pressure combined measure drilling
Figure 12: Cut pressure combined measures plan.
10 20 30 40 50 600
Extraction time (d)
0
10
20
30
40
50
Extraction concentration (%)
Figure 13: Change curve of extraction concentration.
12.00
10.00
8.00
6.00
4.00
2.00
0.00
010 20 30 40 50 60 70
Extraction time (d)
Extraction concentration (%)
Figure 14: e net volume change curve of extraction.
12 Advances in Civil Engineering
combined pressure relief and removal. e amount of
drilling engineering by 34000 m and the amount of drilling
by 10000 m is reduced by 10000 m. It can reduce the
maintenance of 33 d of drilling holes in the 220106 working
surface (length 860 m) and 36 d of maintenance of the
bottom plate lane (900 m long) extraction drilling, calculated
at 30 yuan/m·d.
Save engineering costs: (34000 + 10000) m ×300 yuan/
m�13.2 million yuan
Saving maintenance investment: 860 m ×2×30 yuan/
m·d×33 d + 900 m ×30 yuan/m·d×36 d�2.6748
million yuan
In summary, a total of 15.8748 million yuan of direct
economic benets have been generated.
6.3.2. Indirect Economic Benet Analysis. 220106 working
surface drilling and extraction time are shortened by 33 and
25 d. e working surface is returned to the production in
advance, and the early recovery of the working surface
produces indirect economic benets:
25 ×6000 t/d ×600 yuan/t �90 million yuan
Xinji No.2 mine generated a total of 90 million yuan in
indirect economic benets.
7. Conclusions
(1) e coal seam initiation pressure of ultra-high
pressure water jet cutting pressure combined with
pressure relief is 13 MPa, and the inuence radius of
hydraulic fracturing is 45–55 m. e initiation
pressure of coal seam is 16 MPa, and the inuence
radius of hydraulic fracturing is 35–45 m. e
combined technology of cutting pressure can make
the pressure relief of coal body uniform and su-
cient, the overall permeability coecient of coal
body is greatly improved, the inuence area of ex-
traction is enlarged, and the extraction eect is
signicantly improved. At the same time, the stress
after the coal seam cutting is reduced, and the
fracturing pressure during hydraulic fracturing is
reduced.
(2) After hydraulic slit, the initiation pressure of coal
seam decreases, and the inuence radius of hydraulic
fracturing increases, with an inuence range of
46–56 m. e permeability within the inuence area
increases by 25 to 30 times. e permeability co-
ecient of coal seam is 0.775 m
2
/MPa
2
·d after the
process of cutting pressure combined with pressure
relief and permeability improvement, which is
23 times of the original coal body. e gas perme-
ability of coal seam is increased signicantly by
hydraulic cutting rst and then fracturing.
(3) e extraction concentration in the combined area of
drilling and cutting pressure is 10%–40%, and the
concentration uctuation is relatively stable and the
attenuation is small. e extraction purity is
220106 machine lane
220106 working face
220106 wind lane
Extraction from borehole
Hydraulic slotted drilling
Hydraulic fracturing hole
220106cut
Figure 15: Pressure before cutting combined measures oor plan.
010 20 30 40 50 60 70 80
Extraction time (d)
Extraction concentration (%)
50.0
40.0
30.0
20.0
10.0
0.0
Figure 16: Change curve of extraction concentration.
0 1020304050607080
Extraction time (d)
8.00
7.00
6.00
5.00
4.00
3.00
2.00
1.00
0.00
Extraction of pure quantity (m3/min)
Figure 17: e net volume change curve of extraction.
Advances in Civil Engineering 13
5.12–10.13 m
3
/min, and the average single hole ex-
traction purity is 0.0245 m
3
/min, which is 2.3 times
of the single hole extraction purity of ordinary
drilling. e time to reach the standard of extraction
is 32.8 d shorter than that of ordinary drilling. e
extraction concentration in the combined area of
borehole and pressure cutting is 30%–40%, the
concentration uctuation is relatively stable and the
attenuation is small. e extraction pure volume is
3.6–6.8 m
3
/min, and the average single hole ex-
traction pure volume is 0.026 m
3
/min, which is
2.4 times of the single hole extraction pure volume is
0.0108 m
3
/min of ordinary drilling. e time to reach
the standard of extraction is 25 d shorter than that of
ordinary drilling.
(4) e pressure cutting combined with pressure relief
and antireection technology has been successfully
applied in Xinji coal mine, and has created good
economic benets. is technology has a wide ap-
plication prospect.
Data Availability
e data presented in this study are available upon request
from the corresponding author.
Conflicts of Interest
e authors declare that they have no conicts of interest.
Acknowledgments
is work was supported by the Open Project of Building
Structure and Underground Engineering Key Laboratory of
Anhui Province (grant no. KLBSUE-2022-03).
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Advances in Civil Engineering 15
