High-Efficiency Red Organic Light-Emitting Diodes with External Quantum Efficiency Close to 30% Based on a Novel Thermally Activated Delayed Fluorescence Emitter

Article abstract of DOI:10.1002/adma.201902368

Researchers have spared no effort to design new thermally activated delayed fluorescence (TADF) emitters for high-efficiency organic light-emitting diodes (OLEDs). However, efficient long-wavelength TADF emitters are rarely reported. Herein, a red TADF em

Full text of DOI:10.1002/adma.201902368

Geotech Geol Eng
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ORIGINAL PAPER
Research on Application of Transient Electromagnetic
Method in Hydraulic Fracturing
.
Dong-ming Zhang Han Yang Zi Rao Zhong-yu Ou Ping Tang
.
.
.
Received: 14 April 2019 / Accepted: 23 August 2019
Ó Springer Nature Switzerland AG 2019
Abstract Coal is China’s dominant energy at pre-
sent,but gas outburst occurs frequently in gassy coal
mines, and it causes significant losses to people’s lives
and properties. Hydraulic fracturing is a valuable
technology for gas permeability improvement in gassy
coal mines. In this paper, the field experiment of
hydraulic fracturing was conducted in a coal mine
located in Yibin City, Sichuan Province. The transient
electromagnetic instrument was applied before and
after hydraulic fracturing respectively, combined with
numerical simulation analysis of FLAC3D, aimed to
estimate the influence scope of hydraulic fracturing.
The numerical simulation results denote that the
plastic zone principally distributes along the interface
of the coal seam and roof due to the existence of
stratification. Transient electromagnetic method anal-
ysis demonstrates that the coal-rock mass firstly cracks
at 35 m and 41 m of borehole depth, the dominant
fissure distribution range is in the interval of 33–50 m.
And the influence area covers the range of 28–64 m,
which means the influence scope is up to 36 m. It also
reveals that it is feasible to determine the influence
scope of hydraulic fracturing through transient elec-
tromagnetic method.
Keywords Hydraulic fracturing Á Transient
electromagnetic method Á Numerical simulation Á
Influence scope
1 Introduction
China’s economy is in the period of sustained and
rapid development at present, the demand for energies
also increases substantially. Nowdays coal is still
China’s dominant energy (Yuan 2016). Coal resource
exploitation is a comprehensive and complicated
process, lots of coal mine disasters occurred during
mining process due to complex geological environ-
ments and tough working conditions (Fan et al. 2017;
Black 2019). Gas outburst is one of the most
catastrophic disasters in coal mine, The total propor-
tion of gassy mines and gas outburst incidents in China
accounts for approximate 33% of that in the world
(Huang et al. 2017). Gas outburst is a sophisticated
coal mine gas dynamic phenomenon accompanied
with a large amount of gas released from coal body in
very short time, sometimes even coal would be
brought out (Wang et al. 2018, 2019). Coal mine gas
D. Zhang (&) Á H. Yang
State Key Laboratory of Coal Mine Disaster Dynamics
and Control, Chongqing University, Chongqing 400044,
China
e-mail: zhangdm@cqu.edu.cn
D. Zhang Á H. Yang
School of Resources and Safety Engineering, Chongqing
University, Chongqing 400044, China
Z. Rao Á Z. Ou Á P. Tang
Shanmushu Coal Mine, Sichuan Furong Group Industrial
Co., Ltd, Yibin 644500, China
123

Geotech Geol Eng
outburst has caused great losses to people’s lives and
properties in the past few decades, it’s a severe issue
on coal resource exploitation and needs to be handle
urgently (He et al. 2009; Fisne and Esen 2014;
Hudecek 2008). Moreover, methane is non-renewable
clean energy, therefore it’s critical to facilitate gas
extraction ratio and and promote gas utilization ratio
to improve production status of coal mine and reduce
gas outburst disasters (Wu et al. 2004; Xie et al. 2014).
Up till now, the commonly used technologies to
improve gas permeability include pre-splitting blast-
ing, hydraulic flushing and hydraulic cutting seam, etc.
Domestic and foreign researchers had contributed a
lot to study HF technology and illustrated that it’s a
valid method for gas permeability improvement in
coal mines. Hitherto, the main methods to determine
influence scope are to analyze the water content of
coal, the gas extraction area after HF (Wang 2019),
tracer gas may also be used sometimes. While there
are few literatures focus on the application of transient
electromagnetic method (TEM) in HF realm, thus we
concentrate on the appliance of TEM in HF field
experiment in our study. In this paper, HF field
experiment was conducted in a coal mine located in
Yibin City, Sichuan Province. The transient electro-
magnetic instrument was put in use before and after
HF respectively, the coal-rock mass main cracking
area and influence scope of HF was finally determined
combined with numerical simulation analysis. The
objective of this research is to provide a new approach
to gauge the influence scope of HF and figure out
whether it’s viable to measure HF influence area
through TEM.
(
Wang et al. 2014; Nesterova 2017; Shen et al.2018).
Then hydraulic fracturing (HF) technology was intro-
duced into coal mine field (Morgenstern and Sepehr
1
991). In HF, the fracturing fluid (usually water) is
continuously injected into coal seam through high-
pressure pump. The coal seam would be crushed and
develope lots of new fissures when the pressure of
fracturing fluid exceeds coal seam strength. These
fissures provide more channels for gas flow, thus the
permeability of coal seam increases.
Some scholars have studied the application of HF
technique on natural gases, petroleum and shale gas
industry and found that it’s effective for improving
production (Wanniarachchi et al. 2019; Vishkai and
Gates 2019; Nasriani and Jamiolahmady 2019).
Moradi et al. (2017) investigated HF propagation
process and its associated parameters in different
conditions. Kang et al. (2018) performed field exper-
iment in coal mine and obtained that HF in the main
roof can substantially reduce abutment stresses. Szott
et al. (2018) through numerical studies found that
hydrofracturing can significantly enhance methane
drainage of coal seams under appropriate parameters.
Ni et al. (2018) carried out pulsating HF industrial
experiments to improve the coal seam gas permeabil-
ity and studied some physical properties, they con-
cluded that the coal seam permeability coefficient
increased by 48–217 times after the experiments. Li
et al. (2019) proposed a new method of HF for low
permeability coal seals in China, it can increase the
effective water pressure so that can get better effects.
Zhang et al. (2018) performed HF field experiment in a
coal mine located in Sichuan, China, they acquired
that the methane extraction rate of single fracturing
borehole was approximately ten times larger com-
pared to average extraction borehole.
2 Theories Introduction
2.1 Water Motion Characteristics in Coal Seam
Hydraulic fracturing cracks the stratum by injecting
high-pressure liquid (such as water), then continu-
ously injecting water to extend the fractures further.
The water injection process is actually an unsaturated
permeation process.The general process is that the
liquid firstly flows into preexisting fissures or weak
planes, then facilitates to form secondary induced
weak planes, and finally into micro fissures (Guo et al.
1993). The dominant water movement type in HF
process is the permeation in fissures, diffusion in
micropores and the capillary movement in pores.,
According to the mass conservation and permeability
theory in high-pressure water injection process, it can
be known:
8
divV ¼ ÀoDS=ot
>
>
>
<
divV ¼ k1ðP1 À P2Þ
V ¼ Àk2gradP1
P1 ¼ wðP1; DSÞ
ð1Þ
>
>
>
:
Where V is the permeation velocity of water in
fissures; P and P is the water pressure outside and
1
2
1
23

Geotech Geol Eng
inside the coal respectively; DS is the increment of
water content in coal; k is the permeability coefficient
1
of coal boundary; k2 is the permeability coefficient
related to water pressure in fissures.
Introducing dimensionless quantities u and x:
pffiffiffiffiffiffiffiffiffi
u ¼ a=k3x; x ¼ ðaP3=DSmaxÞt
ð2Þ
where a,a,k3 represents the hydraulic parameter of
coal; P3 represents effective water pressure; DSmax
represents the maximum increment of water content in
coal.
Fig. 1 Diagram of induced electromagnetic field conversion
(Wu et al. 2018)
Once the boundary conditions are determined, the
analytical solution can be obtained by the following
equation:
2
=3
l0 2l SN
0
q ¼ C Â 4pt
¼
Â
pffiffiffiffiffiffiffiffiffiffiffi pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi
5tðV=IÞ
Q ¼ DSmax 2k3=a Á x þ expðÀxÞ À 1
ð3Þ
1
2
2=3
2=3
5=3
C Â 6:32 Â 10 Â ðSNÞ ÂðV=IÞ Ât
ð4Þ
where Q is the flow of water that expands in fractures.
Equations (2) and (3) show that during HF, the
water flow in fissures is directly related to time, pump
pressure, and coal seam geological conditions.
where C is full-space response coefficient; S is the area
of receiving coil; N is the coil turn; t is attenuation
time of secondary magnetic field; V=I is normalized
secondary field potential value.
Due to the difference in conductivity, the apparent
resistivity of fractured coal-rock mass is greater than
that of intact coal-rock mass and water. Furtherly, the
apparent resistivity of intact coal-rock mass is greater
than that of water. Consequently, the coal-rock mass
crushing zone and water-bearing zone can be identi-
fied from TEM detection results before and after HF
respectively, thus the coal-rock mass crushing zone
and influence scope of HF could be determined
eventually.
2
.2 Transient Electromagnetic Method
TEM is also known as time domain electromagnetic
method, it emits electromagnetic field into the ground
through transienting coil, during the pulsed electro-
magnetic field interval (after power failure), it recieves
the secondary eddy current field generated by under-
ground media through an receiving coil or grounding
electrode (Liu et al. 2005).
The basic working principle as follows: applying
current to transmitting coil firstly to create primary
magnetic fields around the coil, then interrupting the
current suddenly, according to Faraday’s law of
electromagnetic induction, the induced electromotive
force would be generated in conductive bodies under
the ground or in coal mine, which in turn generates
secondary induced current. Since the decay law of
secondary magnetic field is related to the conductivity
of underground geological body, the better conduc-
tivity, the lower decay speed (Yu et al. 2007).
Therefore, the underground unfavorable geological
body could be detected by studying the attenuation law
of secondary magnetic field (Fig. 1)
3
Results Analysis
3.1 Geological Overview
The colliery is located in Yibin City, Sichuan
Province. The coal belongs to anthracite from semi-
dark to semi-bright, there are fissures and small faults
in coal seam. The thickness of coal seam varies greatly
in different districts and the gas concentration is high
3
with relative gas emission rate of 10–15 m /t. The
geological structures of coal seam are complex with
dip angle of 4–32°. Coal seam thickness is 3.0–4.2 m
with average thickness of 3.2 m. The hardness coef-
ficient f of coal seam, immediate roof and floor is 2–4,
The calculation formula of apparent resistivity of
full-space transient electromagnetic method in coal
mine is (Wang 2019):
4–6, and 4–6 respectively. The roadways are con-
structed along the roof and there are two faults in this
123

Geotech Geol Eng
area. The hydrology situation is simple, there are no
large water reservoirs on the ground, the water sources
mainly come from meteoric water and the water filling
in roof and fault fissures.
3
.2 Hydraulic Fracturing Scheme
Through comprehensive survey and taking full
account of the topography, attitude of stratum,
construction conditions, obstacles, and other relative
factors, two boreholes (1#, 2#) for HF were designed
with diameters of 120 mm. The depth of 1# and 2#
borehole is 43 m and 32 m respectively as shown in
Fig. 2. The angle between 1# borehole and concen-
trated intake roadway is 3 degrees, so is 2# borehole.
After accomplishing the construction of 1# and 2#
boreholes according to Fig. 2, the boreholes were
sealed by the combination of hole packer and cement
grouting, then high pressure water pump was adopted
to inject water to boreholes to hydraulic fracturing
Fig. 3 Hole packer installation
to figure out whether coal-rock mass was crushed
before and after HF, whether developed extensive
cracks. The water distribution area after HF was also a
crucial problem to be solved. The measuring points
were deployed adjacent to the top of 1# and 2#
boreholes in crosscut for TEM detection, it covered
the boreholes completely and close to the face-rib. A
rectangular coil of 2 m 9 2 m is equiped for transient
electromagnetic instrument, the coil must keep the
pace of measuring line when detecting. It’s important
to avoid the on-site equipments and devices that may
impact the detection results as far as possible to reduce
(
Fig. 3)
3
.3 TEM Detection Results Analysis
The YCS800 transient electromagnetic instrument
was used in this experiment. The main purposes were
1#
Concentrated
Intake Roadway
2#
Seal
(a)
Crosscut
Coal Seam
2#
1
#
Concentrated
Intake Roadway
(b)
Fig. 2 Schematic diagram of borehole layout. a Plan. b Profile
1
23

Geotech Geol Eng
errors. The pressure was 20 MPa and lasted for 4 h
before being relieved in the course of HF. When the
anchor nets,drilling machine, hydraulic supports and
other equipments and devices that may have great
impacts on TEM detection were removed, the YCS800
transient electromagnetic instrument was deployed for
detection. The apparent resistivity before and after HF
is shown in Fig. 4.
channels to facilitate gas extraction. The apparent
resistivity peaked at 35 m and 41 m respectively, it
implied that the initial crack position is at 35 m and
41 m in this HF process, Furthermore, we can
conclude that the coal and rocks were cracked badly
and fissures developed substantially in the interval of
34–35 m and 39–42 m.
As shown in the left part of Fig. 4b, the apparent
resistivity alters obviously from 28 to 50 m and it
becomes smaller, it indicates that the water content of
coal-rock mass increases massively after HF. Also, it
reveals the water was mainly concentrated in the range
of 28–50 m after HF. Between 50 and 64 m, the
apparent resistivity becomes higher compared with
that before HF, but smaller than that of the interval of
33–50 m, it demonstrates that the coal-rock mass gets
wet in this area after HF.
The previous scheme aimed to perform HF in both
boreholes simultaneously. But in the initial stage of
HF, the injected water sprayed out from 1# borehole
due to poor quality of borehole sealing. Thus it could
not proceed in 1# borehole any more but can be carried
out smoothly in 2# borehole. So we ignored 1#
borehole and only conducted HF in 2# borehole.
In the right part of Fig. 4b, the interval of 33–50 m
expresses relative high apparent resistivity, which
indicates that the coal-rock mass was crushed and
fissures developed in this area after HF. It denotes that
the dominant range of fissure distribution is between
Apparent resistivity decreases from 0 to 5 m after
HF. It’s because the injected water sprayed out from
1# borehole during the initial stage of hydralic
fracturing, which made the coal-rock mass wet and
led to decrease in apparent resistivity, it agreed with
the practical situation of field experiment.
3
3 and 50 m. Meanwhile, it also demonstrates that HF
can crack the coal seam and increase gas seepage
In summary, the coal-rock mass is crushed from 33
to 50 m after HF, and the dominant range of fissure
distribution is in the interval of 33–50 m too. The
apparent resistivity alters significantly from 28 to
64 m of borehole depth after HF with influence scope
of 36 m. It also proves that it is feasible to determine
the influence area of hydraulic fracturing through
TEM.
3.4 Numerical Simulation Analysis
Numerical simulation analysis software FLAC3D was
used in this experiment to analyze the effect of HF
under two different borehole layouts. FLAC3D is an
valuable numerical simulation software for geotech-
nical engineering. It can be used to study the
mechanical properties of three-dimensional soil and
rock mass or other materials, especially the plastic and
rheological properties under yield limit. The built-in
modules of FLAC3D can simulate the impacts on
surrounding rock stress, strain and plastic zone, it’s
widely used in construction design, tunnel engineer-
ing, mining engineering and other relative fields.
Numerical simulation physical and mechanical
parameters (as shown in Table 1) were obtained based
Fig. 4 The apparent resistivity. a Before hydraulic fracturing.
b After hydraulic fracturing
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Table 1 Numerical simulation parameters
Types Bulk density Elastic
Bulk
modulus
(Gpa)
Shear
modulus
(Gpa)
Poisson
ratio (–)
Cohesion Friction
angle (°)
Tensile
strength
(Mpa)
3
kg/m )
(
modulus
Gpa)
(Mpa)
(
Sandstone 2487
13.5
5.425
5.97
2.56
6.01
2.36
0.123
0.147
2.06
2.16
40
36
1.13
0.75
Sandy
mudstone
Coal
2510
1420
0.50
0.46
0.19
0.32
0.8
20
0.01
on the geological data provided by colliery
technicians.
in later stage of HF. The roof and floor are also
gradually damaged, and they are mainly characterized
by shear failure and tensile failure respectively.
Besides, the plastic zone has developed to the interface
between coal seam and floor.
According to colliery geological data, the main roof
is sandstone, the immediate roof and floor is sandy
mudstone, the numerical model can be established in
the light of the parameters in Table 1 (Fig. 5).
Numerical calculation was implemented through
FLAC3D according to HF scheme, 1# borehole was
located in coal seam, while 2# borehole was located at
the interface of coal seam and roof.
From all above we can conclude that during the
whole HF process, plastic zone appears surrounding
the borehole at first, then the interface between coal
seam and roof, next the roof, and the floor at last. The
largest area of plastic zone locates at the interface
between coal seam and roof.
(
1) 1# borehole numerical simulation results.
(2) 2# borehole numerical simulation results.
The numerical simulation was carried out accord-
ing to HF scheme of 1# borehole. Figure 6a shows that
in the first stage of HF, the plastic zone appears
surrounding 1# borehole and expresses tensile and
shear failure. Figure 6b reveals that the plastic zone
gradually expands around the borehole as HF pro-
ceeding, and it mainly exhibits shear failure. The
plastic zone begins to develop along the interface
between coal seam and roof in large scales and mainly
exhibits tensile failure. Figure 6d manifests that
plastic zone in the interface expands to larger extents
The numerical simulation was carried out accord-
ing to HF scheme of 2# borehole. Figure 7a manifests
that in the initial stage of HF, the plastic zone appears
around 2# borehole and mainly exhibits shear failure.
But the area of plastic zone is not as large as 1#
borehole, it’s because the strength of roof is higher
than that of coal seam. Figure 7b illustrates that the
plastic zone gradually expands along the interface
between coal seam and roof as HF carrying on, and
tensile stress dominants the plastic zone. As shown in
Fig. 5 Diagram of numerical model. a 1# borehole. b 2# borehole
1
23

Geotech Geol Eng
Fig. 6 Diagram of 1# borehole plastic zone evolution process
Fig. 7d, in the final stage of HF, the major crushing
zone that surrounding 2# borehole is similar oval and
the diameter of which is approximate 2.5 m, Addtion-
ally, plastic zone comes up at the interface between
coal seam and floor. It may result from the stress
redistribution of surrounding coal and rocks caused by
HF, in this situation, the weak planes begin to crush
and crack, then the plastic zone follows.
4 Discussion
The apparent resistivity of fractured coal-rock mass is
greater than that of intact coal-rock mass due to the
difference in conductivity. As shown in the right part
(where 2# borehole located in on site) of Fig. 4b, the
apparent resistivity was significantly larger than that
before HF in the interval of 33–50 m. It denoted that
the coal-rock mass cracked badly in this region.
Combining the numerical simulation results in Fig. 7,
the reason why apparent resistivity increased after HF
may be that the end of 2# borehole located at the
interaface of coal seam and roof, the stratification
developed in the interface and expressed smaller
strength, which led to crush and plentiful cracks in
coal and rocks. Some time later after HF, the injected
water diffused from the cracks, thus it manifests
distinct increase in apparent resistivity.
In summary, numerical simulation results reveal
that during the whole HF process, a small-scale plastic
zone firstly appears around the borehole, then the
interface between coal seam and roof, next the coal
seam, the interface between coal seam and floor at last.
The plastic zone obtains the largest acreage at the
interface between coal seam and roof. The evolution
process of plastic zone is relatively uniform in those
areas with consistent lithology. If there were weak
planes such as stratification, the plastic zone would
expand more easily and get larger.
While the left part (where 1# borehole located in on
site) between 33 and 50 m poses substantially low
apparent resistivity (Fig. 4b), It indicated that the
123

Geotech Geol Eng
Fig. 7 Diagram of 2# borehole plastic zone evolution process
water content of coal-rock mass has greatly increased
compared with that before HF. The reason may be that
the end of 1# and 2# borehole located in coal seam and
the interface between coal seam and roof respectively
the water could be injected into those places anywhere
micro cracks exsited. Thus, the influence area was not
limited to 33–50 m but 28–64 m, which means the
influence scope of this HF experiment was up to 36 m.
Combining the numerical simulation results of both
boreholes (Figs. 6 and 7), we can found that in both
cases HF has enormous impact on the interface of coal
seam and roof, where the acreage of plastic zone is the
largest. The reason for this phenomenon is that there
are weak planes (such as stratification) in the interface
of coal seam and roof, weak plane would shrink the
strength of coal and rocks on account of its geological
structure. When performing HF, it would be the first
part to crush and the fissures propagate mainly along
the weak planes. To some extent, the presence of weak
planes may affect crack propagation even gas extrac-
tion effect.
(
Fig. 2). The relative altitude of 2# borehole was
higher than that of 1# borehole, when the injected
water of 2# borehole flowed along the interface to the
location of 1# borehole, a part of which continued to
flow by the interface, the others entered into 1#
borehole. We didn’t conduct HF in 1# borehole due to
poor quality of hole sealing, thereby the coal-rock
mass intactness was better relative to 2# bore-
hole.When HF finished, the water that gathered in 1#
borehole couldn’t completely diffused instantly, it
turned out the decrease in apparent resistivity.
Figure 4b manifested that in this experiment, the
coal-rock mass was mainly crushed from 33 to 50 m
after HF, but the apparent resistivity varied signifi-
cantly in the interval of 28–64 m with influence scope
of 36 m. It’s because lots of micro cracks would be
generated under continuous high pump pressure, then
The TEM detection results denote that the coal and
rocks cracked intensely in the interval of 39–42 m (the
dark orange section between 39 and 42 m in Fig. 4b),
namely, the scope of coal-rock body major crushing
1
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Geotech Geol Eng
zone is 3 m. The numerical simulation results indicate
that the diameter of major crushing zone surrounding
borehole is approximate 2.5 m (Fig. 7d). The error
between TEM detection and numerical simulation
results attributes to the inconsistency of practical site
conditions and numerical model, in situ geological
conditions is usually complex, it is hard and inacces-
sible to totally keep numerical model conditions the
same as in situ conditions, it needs huge calculation
and is always time-consuming, consequently the
numerical mode is usually simplified and the error
comes out inevitably, as long as the error can meet
engineering requirements, it is acceptable. In this
study, the scope of crushing zone that surrounding
borehole acquired by TEM detection and numerical
simulation is very close (3 m and 2.5 m respectively)
and the error is small, thereby it is acceptable and can
meet the experiment requirements. Meanwhile, the
numerical simulation parameters stems from practical
geological data, it testifies that the numerical simula-
tion results is valid and reasonable. The numerical
simulation results of HF through FLAC3D explained
the analysis of TEM. It denotes that it is feasible to
estimate the influence scope of hydraulic fracturing
through TEM.
Acknowledgements This research was supported by the
National Science and Technology Major Project of China
(2016ZX05045-004) and Fundamental Research Funds for the
Central Universities (No.2018CDYJSY0055).
Author’s Contributions DZ and HY conceived and designed
the experiments; HY and PT performed the field experiments;
HY analyzed the data and wrote the paper; ZR and ZO provided
the experiment site. All authors gave final approval for
publication.
Compliance with Ethical Standards
Conflict of interests The authors declare that they have no
conflict of interests.
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The experiment results illustrate that it is viable to
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