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Research ArticleStudy on Fracture Morphological Characteristics
ofRefracturing for Longmaxi Shale Formation
Yintong Guo , Lei Wang, Xin Chang, Jun Zhou, and Xiaoyu
Zhang
State Key Laboratory of Geomechanics and Geotechnical
Engineering, Institute of Rock and Soil Mechanics, Chinese
Academyof Sciences, Wuhan 430071, China
Correspondence should be addressed to Yintong Guo;
[email protected]
Received 22 August 2019; Revised 17 January 2020; Accepted 6
February 2020; Published 4 March 2020
Academic Editor: Julien Bourdet
Copyright © 2020 Yintong Guo et al. This 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.
Refracturing technology has become an important means for the
regeneration of old wells reconstruction. It is of great
significanceto understand the formation mechanism of hydraulic
fracturing fracture for the design of hydraulic fracturing. In
order toaccurately evaluate and improve fracturing volume after
refracturing, it is necessary to understand the mechanism
ofrefracturing fracture in shale formation. In this paper, a true
triaxial refracturing test method was established. A series of
large-scale true triaxial fracturing experiments were carried out
to characterize the refracturing fracture initiation and
propagation.The results show that for shale reservoirs with weak
bedding planes and natural fractures, hydraulic fracturing can not
onlyform the main fracturing fracture, which is perpendicular to
horizontal minimum principal stress, but it can also open
weakbedding plane or natural fractures. The characteristics of
fracturing pump curve indicated that the evolution of
fracturingfractures, including initiation and propagation and
communication of multiple fractures. The violent fluctuation of
fracturingpump pressure curve indicates that the sample has
undergone multiple fracturing fractures. The result of refracturing
shows thatinitial fracturing fracture channels can be effectively
closed by temporary plugging. The refracturing breakdown pressure
isgenerally slightly higher than that of initial fracturing. After
temporary plugging, under the influence of stress induced by
theinitial fracturing fracture, the propagation path of the
refracturing fracture is deviated. When the new fracturing
fracturecommunicates with the initial fracturing fracture, the
original fracturing fracture can continue to expand and extend,
increasingthe range of the fracturing modifications. The
refracturing test results was shown that for shale reservoir with
simple initialfracturing fractures, the complexity fracturing
fracture can be increased by refracturing after temporary plugging
initialfractures. The effect of refracturing is not obvious for the
reservoir with complex initial fracturing fractures. This research
resultscan provide a reliable basis for optimizing refracturing
design in shale gas reservoir.
1. Introduction
In recent years, hydraulic fracturing technology has beenwidely
used in tight oil and gas reservoir development. Thetypical
production characteristics of shale gas wells is thatthe output
declines greatly and the rate is fast, which is quitedifferent from
conventional oil and gas wells. With the pro-duction of shale gas
fracturing wells, the fractures formedby initial fracturing will be
gradually close and become inef-fective; the production of oil and
gas wells will be decline. Inaddition, many of the early fracture
treatments failed todeliver the expected results, including poor
treatment prac-tices, fracturing techniques, and poor awareness
[1]. Hori-
zontal well refracturing technology is one of the
maintechnologies for the effective development of shale gas
reser-voir. Thousands of shale gas wells in North America arebeing
refractured; as a whole, refracturing can significantlyincrease
shale oil and gas well production and ultimatelyrecoverable
resources. At present, the mechanism of refrac-turing is generally
understood in the following three aspects[2]: (1) reopen existing
fracturing cracks; (2) effectivelyextend the original fracture
system; (3) new fractures areopened in the initial unfractured
zone. In the past decade,shale gas has been industrialized in
Fuling area, Sichuanbasin of China. The first shale gas wells have
entered the stagewhere refracturing is needed to increase
production. At
HindawiGeofluidsVolume 2020, Article ID 1628431, 13
pageshttps://doi.org/10.1155/2020/1628431
https://orcid.org/0000-0001-6392-3644https://creativecommons.org/licenses/by/4.0/https://creativecommons.org/licenses/by/4.0/https://doi.org/10.1155/2020/1628431
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present, researches on physical test and theoretical analysisfor
fracturing fracture propagation of shale reservoir arefocused on
initial fracturing. Various studies have been con-ducted to reveal
fracture morphology characteristics of shalereservoir under
different conditions. It was found that withhigh horizontal stress
difference, low-viscosity fluid can acti-vate discontinuities to
form a complex fracture network, andhigh-viscosity fluid is likely
to produce large fractures [3].The bedding direction of the shale
plays an important rolein gas fracturing [4]. True triaxial
hydraulic fracturing exper-iment was carried out in order to
understand the fracture ini-tiation and vertical propagation
behavior [5]. The influencesof multiple factors (bedding planes,
injection rate, in situstress, fracturing fluid viscosity, etc.) on
fracture propagationpattern were studied based on a series of
hydraulic fracturingexperiments. The hydraulic fracture propagation
rule in ran-dom naturally fractured blocks was investigated. It is
foundthat hydraulic fracture propagation along the natural
fracturesystem with low horizontal stress difference coefficient
isforming a single hydraulic fracture and not connecting withthe
natural fracture [6]. Based on the displacement disconti-nuity
method (DDM), a two-dimensional numerical modelhas been developed
and used to investigate the interactionbetween hydraulic and
pre-existing natural fractures.Research has shown that the cemented
strength of the NFhas an important influence on the interaction
mechanism[7]. In the fracturing experiment, the anisotropy of
shaleplays an important role in the mechanical behavior and
frac-ture propagation. Considering anisotropy, SC-CO2 fractur-ing
effect under different injection rate and stress state wascarried
out [8]. It is found that higher injection rate can leadto higher
breakdown pressure, while higher deviator stresscan lead to the
lower breakdown pressure instead. It is esti-mated that the
prefracture cyclic injection can reduce therock breakdown pressure
and increase the possibility of fail-ure. Compared with
conventional hydraulic fracturing, thereduction of breakdown
pressure and the increase of damageduring cyclic injection are
quantitatively analyzed [9].
There are few studies on refracturing test. The initiationand
propagation of refracturing fractures need furtherstudy. With the
development of fracturing technology,some scholars have begun to
pay attention to issues relatedto refracturing. The properties of
refracturing and the condi-tions limiting the application of
refracturing are discussed[10]. The identification method of
candidate wells is intro-duced, and the treatment design, diagnosis
technology, andeconomic evaluation are discussed. A new method for
calcu-lating the distribution of multicluster coal slurry has
beenapplied in fracturing and refracturing [11]. The main
advan-tages of this approach include (1) simple implementation;
(2)no numerical evaluation of the Jacobian matrix; (3)
computa-tionally efficiency; (4) easy integration of slurry
distribution,proppant transport, stress shadow effects, and
multiple frac-ture propagation. Using a fully coupled
geo-mechanicalmodel and microseismic analysis, the effects of
seismic isola-tion and near-wellbore friction on the refracture
treatment ofa typical Eagle Ford well were studied. The results
show thatthe stress change caused by depletion can enhance the
con-ductivity of the fracture, and the fluid distribution
between
the depleted zone and the undepleted zone will changeduring the
treatment. Different hydraulic fracturing andrefracturing
treatments processes were proposed [12, 13].Considering the
interference from the initial fracturing frac-tures, a new
semianalytical model is proposed to characterizethe transient flow
behavior of a reoriented refracture [14].The fully coupled gas
flows and stress changes of hydraulicfracturing and refracturing
tight gas reservoirs are simulatedby a numerical model. The results
show that the anisotropyof horizontal permeability is a key
parameter in the overalldesign of hydraulic fracturing [15]. The
effects of isolationand near-wellbore friction on refracturing
treatment in typi-cal Eagle Ford Wells were studied by using a
method of fullycoupled geo-mechanical modeling and microseismic
analy-sis. The case studies show that the stress changes inducedby
depletion can enhance stress transfer, and fluid distribu-tion
between fractures in depleted and nondepleted zonescan also change
[16]. Considering the parameters of volumefracturing horizontal
wells, an idealized concept of refractur-ing well is proposed. The
refracturing potential of candidatewells needs to be graded and
optimized, and a numerical sim-ulation method was used to establish
a production predictionmodel of refracturing considering stress
sensitivity [17]. Thetransient flow model of hydraulic fracture
after refracturingis established and the orthogonality of new and
old hydraulicfractures is evaluated. The model shows that the
orthogonalfractures generated during the refracturing process will
resultin orthogonal linear flow in the simulated volume of the
res-ervoir [18]. The viscous zone model (CZM) was establishedbased
on extended finite element method (XFEM) to simu-late the process
of plugging and flow diversion. The modelhas been verified by test
results [19]. Numerical simulationmethod seems to be useful for
identifying the refracturingfracture propagation. However, the
results of these methodswere inaccurate, because the calculation
assumes limitedheight fracture propagation and ignores the
interactionbetween the fractures and discontinuities.
Therefore, it is necessary to study the mechanism ofrefracturing
fracture propagation in shale formations to esti-mate the effect of
refracturing. Laboratory physical tests areeffective because of the
initiation and propagation of refractur-ing fractures can be
clearly observed. In this study, the physicaltest method of true
triaxial refracturing is established. Then, aseries of true
triaxial refracturing experiments were conductedto determine the
mechanism of refracturing fracture propaga-tion for Longmaxi shale
formation. The effect of initial fractur-ing fracture on initiation
and propagation of refracturingfractures was investigated. The
refracturing fracture character-istics of shale samples with
different initial fracturing fracturesare analyzed. The research
results can be helpful to furtherstudy the interaction mechanism
between refracturing frac-tures and initial fracturing
fractures.
2. Experimental Materials and Procedure
2.1. Sample Preparation. The shale samples were obtainedfrom the
outcrops of Longmaxi formation in Sichuan Basinof China. Blocks of
shale were collected from the exposedrock-mass section, as shown in
Figure 1. The average
2 Geofluids
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mineralogical compositions of shale samples are 1.74%
kao-linite, 1.42% Montmo rillonite, 3.15% illite, 58.70%
quartz,2.81% Cristobalit, 12.64% Albite, 5.59% calcite,
5.85%Muscovite, 4.23% Pyrite, and 3.87% Ankerite. The
averagedensity is 2.665 g/cm3 with around 1.24% porosity.
Somephysical properties and mechanical properties in
triaxialfracturing test are listed in Table 1.
After removing the weathered surfaces of irregular shaleblocks,
excavators were used to collect large shale samples.Then, a large
cutting machine is used to process the shalesample into a cube
sample of 300mm × 300mm × 300mm.The horizontal well was simulated
by drilling a hole with adepth of 165mm to ensure an open hole
section (depth of30mm) in the middle of the cubes; the axis of the
boreholeis parallel to the bedding plane, as shown in Figure
1(e).High-strength epoxy glue was used to seal the casing
andwellbore annular space. A 30mm long hole without casingwas used
as the fracture section.
2.2. Experimental Facility. The hydraulic fracturing test
sys-tem employed in this study consists of two parts: a true
triax-ial geotechnical engineering testing machine and
hydraulicfracturing fluid servo pumping control system. Figure 2 is
atechnical route of real triaxial fracturing experimental sys-
tem. Figure 3 shows the main components of the
hydraulicfracturing system. Figure 3(a) is a true triaxial
geotechnicalengineering testing machine. The device has the
function oftrue triaxial test. X (Left and right), Y (Vertical),
and Z (Frontand back) directions are independently pressurized by
axialloading system, which can truly simulate the stress
situationof underground rock strata. The maximum load in
eachdirection can reach 3000 kN. The installation of fracturing
(a) (b)
(c) (d) (e)
Figure 1: Preparation of shale samples for refracturing test.
(a) The location of the shale outcrop; (b) removing the weathered
surfaces of thebroken shale blocks; (c) large samples are cut into
300mm× 300mm× 300mm cubic samples; (d) simulated casing; (e)
prepared standardfracturing samples.
Table 1: Material parameters of shale rock in true triaxial
fracturingtests.
Properties Value
Young’s modulus (GPa) 25.50
Uniaxial compressive strength (MPa) 109.10
Tensile strength (MPa) 9.50
Cohesive force (MPa) 20.35
Friction angle 36.12
Porosity (%) 1.24
Permeability (Darcy) 0.00004
Poisson’s ratio 0.185
The average velocity (m/s) 4860
3Geofluids
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specimens is shown in Figure 3(b). The loading can beapplied
synchronously in three directions according to theset proportion to
ensure no damage to the sample beforehydraulic fracturing.
The hydraulic fracturing pump servo control system isshown in
Figure 3(c). The maximum load pressure is100MPa and the precision
is at level 0.05MPa. The effec-tive volume of the supercharger is
800mL, and the oilinlet and return inlet are equipped with
accumulators toimprove the dynamic response of the system and
ensurethe stability of the servo valve. The pump pressure is
mea-sured by the pressure sensor at the water inlet pipe. Therate
of flow is measured indirectly by using highly preciseelectronic
scale. The precision rate of flow control canreach 0.01mL/s.
2.3. Experiment Procedures. It is difficult to carry out
refrac-turing test in the laboratory, mainly because of how to
effec-tively seal the fractures after initial fracturing. After a
largenumber of laboratory tests, the test scheme of refracturingwas
determined.
First, the natural fractures and micro-open beddingplanes of the
processed 300mm × 300mm × 300mm cubesshale samples were marked. The
sample was placed in thetrue triaxial loading chamber and subjected
to three-dimensional in situ stresses. To avoid the unbalanced
loadingof triaxial stresses, three-dimensional in situ stresses
wereapplied to the setting value with equal proportion loading.The
schematic of the in situ stress loading direction usedfor hydraulic
fracturing test is shown in Figure 4. The in situstresses are set
as follows: vertical stress is 19.30MPa; themaximum horizontal
stress is 21.00MPa; and the minimumhorizontal stress is 16.30MPa.
Then, the slickwater fractur-ing fluid was injected at the selected
pump rate through thehigh-pressure pumping channel into the
wellbore to inducefracturing fractures, and some water-soluble
tracers wereadded into the slickwater fracturing fluid for a better
observa-tion of the fracturing fractures. The flow rate is
0.5mL/s;when the fracturing pump pressure drops suddenly, it
indi-cated that a new fracturing fracture appeared in the
sample,continue pumping fracturing fluid, the pump pressure willnot
to rise or remain constant, and shut down the pumping
True triaxial loadcontroller
True triaxial load hydraulic source
AE system
Electro-hydraulic supercharger
Pump system
Computer
Supercharger
Fracturing fluid container
Fracturing fluid
Hydraulic pump
Sample
Borehole
AE probe
Amplifier Pressure
Computer
Displacement
Electro-hydraulicservo control
Provide liquid
Feedback information
High pressurevalve
Loadingsystem
PressureDisplacement
Figure 2: Technical route of real triaxial fracturing simulation
experimental system.
(a) (b) (c)
Figure 3: The main components of hydraulic fracturing system.
(a) True triaxial loading frame. (b) Installation of fracturing
specimen. (c)Servo control system.
4 Geofluids
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system. The fracturing pressure was recorded automaticallyduring
the whole testing process.
The new fracturing fractures on the surface of the sampleafter
initial fracturing were marked. If the tested sample hasbeen
performed with obvious macrofracture, the refracturingtest is no
longer performed, and the sample is opened forfracture description.
If the tested sample is relatively com-plete, it is used to conduct
a refracturing test. For the sampleto be refractured, the plugging
agent is injected into the well-bore, and a low pumping pressure is
applied on the top of theplugging agent. The plugging agent is
injected into the initialfracturing fracture by using the pressure
difference. After theinjection, the residual plugging agent at the
bottom of thewellbore is removed and left for 2-3 days to fully
solidifythe plugging agent. Then, repeat the process of
hydraulicfracturing test and another water-soluble tracer were
addedinto the slickwater fracturing fluid for a better
observationand distinction of the refracturing fractures. The
characteris-tics of initial fracturing fractures and refracturing
fractureswere analyzed by cutting samples after refracturing test;
therefracturing test parameters are listed in Table 2.
3. Experimental Results and Analysis
3.1. The Characteristics of Fracturing Pump Pressure Curve.The
fracturing pump pressure is monitored during the wholetesting
process. The fracturing pump pressure is on the risebefore the
shale sample cracks, and it increases rapidly afteran initial slow
growth. When the sample cracks, the fractur-ing pump pressure drops
to some extent.
Many researchers have carried out fracture morphologi-cal
comparison under different fracturing parameters [4, 5].However,
there are differences in bedding plane and naturalfractures of
large-size samples. It is difficult to analyze theinfluence of
fracturing parameters on fracturing fracture. Inthis study, under
the condition of the same fracturing param-eters, hydraulic
fracturing test was carried out with shalesamples from the same
stratum. Figure 5 shows the fractur-ing pump pressure curve of
shale samples; it can be seen thatdue to the difference in the
internal structure of shale sample,the characteristics of
fracturing pump pressure curve are dif-ferent. In fracturing
process, the fracturing pump pressure
rose more steeply and went almost straight to the top; beforethe
first breakdown pressure point, the fracturing pump pres-sure
curves of different samples have similar characteristics.
In Figure 5(a), after an initial slow increase, the
fracturingpump pressure increases rapidly, with only one peak
point(31.09MPa). When the fracturing breakdown pressure isreached,
the fracturing pump pressure drops slightly. This indi-cates that
the fracturing process occurs suddenly when the frac-turing fluid
pressure reached the fracturing breakdownpressure. The
instantaneousness of the fracturing decided thatthe flow rate of
fracturing fluid loss wasmuch greater than frac-turing fluid
injection in the moment of fracture propagation,causing the
fracturing pump pressure to drop rapidly. Then,the extension
pressure was maintained above 22MPa withslight fluctuations. After
cutting of H-0-2 sample, the fracturingfracture characteristics
indicate that the primary main hydrau-lic fracture encounters
bedding surface, and it turns andextends, forming the bedding
fracture with certain degree offluctuation. It shows that the
fracture propagation needs toovercome applied stress. In Figure
5(b), the highest fracturingpump pressure (32.01MPa) appeared in
the initial fracturingbreakdown pressure; then, the fracturing pump
pressure fallsto 5.79MPa rapidly. With the continuous injection of
fractur-ing fluid, the fracturing pump pressure keeps increasing,
butit is lower than the initial fracturing breakdown pressure,
andmultiple fracturing breakdown points were formed. The
resultsindicate that the specimen has undergone several initiations
oflocal fracturing fractures. In the comprehensive analysis
offracturing fracture morphology of H-0-7samples, the mainhydraulic
fractures communicate with several bedding frac-tures, forming a
complex fracture network during the processof fracturing fracture
propagation, In Figure 5(c), the highestfracturing pump pressure
(32.93MPa) appeared in the secondfracturing breakdown pressure,
corresponding to the fracturingrupture processes, the fracturing
pump pressure falls rapidly to13.70MPa. After the second rupture,
the fracturing pump pres-sure increased slightly with the injection
of fracturing fluid,then the fracturing pump pressure is stable
between18.85MPa and 19.60MPa, and the fracturing fracture
continueto expand. This phenomenon shows that a stable
fracturingfracture channel has been formed in the sample, and the
frac-turing fluid has stable seepage after overcoming the in
situstress and fracture friction. In Figure 5(d), the fracturing
break-down pressure is highest among the four samples. It
containsone main fracturing breakdown pressure with multiple
lowfracturing breakdown pressure points. The maximum fractur-ing
breakdown pressure is twice that of low fracturing break-down
pressure. This indicates that fracturing fractures ofdifferent
scales have been formed during the fracturing process.
3.2. Impacts of Bedding Plane or Natural Fracture on
FracturePropagation. The activation of pre-existing natural
fractureor bedding plane is the optimum conditions for the
forma-tion of complex fractures. To investigate the interactions
ofpre-existing natural fracture or bedding plane and
fracturingfractures, the samples were opened to compare and
describedthe fracturing fractures after hydraulic fracturing
test.Although the samples used in the study were from the
sameformation, affected by pre-existing natural fracture or
Z2X2
𝜎h
Y2
𝜎V
𝜎H
Figure 4: Schematic of the in situ stress loading direction
forhydraulic fracturing.
5Geofluids
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bedding plane, under the same test parameters, the
fracturemorphology characteristics is different. The fracture
mor-phology of the four samples after initial fracturing is shownin
Figure 6. These four groups of samples have formedmacrofracture
after initial fracturing, which cannot be usedto carry out
refracturing test.
After hydraulic fracturing, the shale samples wereopened along
the hydraulic fractures and opened beddingsurface by geological
hammering, and the fracture morphol-ogy were observed and recorded.
Different modes of fractureinitiation and propagation can be seen
in Figure 6. Most of
the fracturing fractures were initiated from the open
holesection to form a transverse main fracturing fracture that
isperpendicular to the minimum horizontal principal stress.In
Figure 6(a), when the main fracturing fracture expandsto the weak
bedding plane, the main fracturing fracture doesnot continue to
expand, but turns to the direction of bed-ding plane to initiation
and propagation. Affected by shaleanisotropy, the fracture surface
shows fluctuation. Becauseof the tensile failure is the main
failure in fracturing test,corresponding to Figure 5(a), the
extension pressure ofuneven bedding surface is relatively high, and
the fracturing
Table 2: The summary of refracturing test parameters.
Sample numberThree-dimensionalin situ stress. (MPa) σH − σhð
Þ/σh Flow rate(mL/s)
Initial fracturing breakdownpressure (MPa)
Refracturing breakdownpressure (MPa)
σv σH σhH-0-2 19.3 21.0 16.3 0.29 0.5 31.09 /
H-0-7 19.3 21.0 16.3 0.29 0.5 32.01 /
H-0-15 19.3 21.0 16.3 0.29 0.5 31.74 /
H-0-19 19.3 21.0 16.3 0.29 0.5 37.35 /
H-0-4 19.3 21.0 16.3 0.29 0.5 30.90 27.93
H-0-6 19.3 21.0 16.3 0.29 0.5 29.49 38.65
H-0-10 19.3 21.0 16.3 0.29 0.5 29.77 35.29
0
5
10
15
20
25
30
35
0 50 100 150 200 250
Pum
p pr
essu
re (M
Pa)
Time (s)
Pump pressure
(a) H-0-2
0
5
10
15
20
25
30
35
0 50 100 150 200 250
Pum
p pr
essu
re (M
Pa)
Time (s)
Pump pressure
(b) H-0-7
0
5
10
15
20
25
30
35
0 50 100 150 200 250
Pum
p pr
essu
re (M
Pa)
Time (s)
Pump pressure
(c) H-0-15
Time (s)
05
10152025303540
0 50 100 150 200
Pum
p pr
essu
re (M
Pa)
Pump pressure
(d) H-0-19
Figure 5: The injection fracturing pressure curve of shale
samples.
6 Geofluids
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BP HF𝜎H
𝜎h
𝜎V
(a) H-0-2
BP
HF
HF
BP
𝜎H
𝜎h
𝜎V
𝜎H
𝜎h𝜎V
𝜎H
𝜎h
𝜎V
(b) H-0-7
BPHF
𝜎H
𝜎h
𝜎V
(c) H-0-15
BP
HF
HF
𝜎H
𝜎h
𝜎V
(d) H-0-19
Figure 6: The fracture propagations of the samples after initial
fracturing. BP: bedding plane; HF: hydraulic fracture.
7Geofluids
-
fracture complexity is relatively low after hydraulic
fracturing.In Figure 6(b), it can be seen from the description of
the exter-nal surface and opened fracturing fracture surface that
thecomplexity of fracture formed of H-0-7 is higher than that
oftheother samples.Themainhydraulic fracturingcommunicatesmultiple
bedding planes, in both horizontal and vertical direc-tions; the
fracture after hydraulic fracturing is more consistentwith the
description of “complex fracture network” thansimple fracture. In
Figures 6(c) and 6(d), a main hydraulicfracturing fracture is
created along the direction of maxi-mum horizontal stress, then
deviates when the main fractur-ing fracture extension encountered
the pre-existing naturalfractures, and the approximately crisscross
fracture plane isformed. Through fracture morphology analysis after
hydrau-lic fracturing, the fracturing pump pressure curves are
foundto be consistent with fracture propagation patterns. Whenthe
fracturing pump pressure occurred multiple fracturingfluctuation
points, the fracture morphology formed afterfracturing is more
complex; when the fracturing pump pres-sure has only one or two
fracturing fluctuation points, sim-ple fracture characteristics are
formed.
3.3. The Characteristics of Refracturing Pump Pressure
Curve.After hydraulic initial fracturing, shale samples with
fractur-ing fractures but without macrofracture surfaces were
selected for refracturing test. Three samples with
differentinitial fracturing pump pressure characteristics were
carriedout for refracturing tests. The refracturing pump
pressurecurve is shown in Figure 7. Through the refracturing
pumppressure curve, it can be found that the characteristics of
frac-turing pump pressure after initial fracturing affect the
char-acteristics of refracturing pump pressure curve. When
theinitial fracturing pump pressure curve has been occurredmultiple
fracture points, the refracturing pump pressurecurve only has one
fracturing breakdown pressure point,while when the initial
fracturing pump pressure curve hasonly one fracturing breakdown
pressure point, multiple frac-turing fluctuation points appear in
the refracturing process.This indicates that the characteristics of
pump pressure curvecan be used as an index to evaluate the effect
of primary frac-turing and whether it is necessary to carry out
refracturing.
In Figure 7(a), the highest fracturing pump pressure
afterinitial fracturing is not at the first breakdown pressure.
Afterthe initial fracturing fracture is temporarily blocked,
refrac-turing test is carried out. The fracturing pump pressure
risesmore sharply and went almost straight to the breakdownpressure
point, then the fracturing pump pressure dropsslightly, and the
second breakdown pressure point is reachedafter the fracturing
fluid continues to be pumped. Then, therefracturing pump pressure
keeps at a high value for a long
05
10152025303540
0 50 100 150 200 250 300Time (s)
Pum
p pr
essu
re (M
Pa)
Initial fracturingRefracturing
(a) H-0-4
Time (s)
05
10152025303540
0 100 200 300 400 500 600 700 800
Pum
p pr
essu
re (M
Pa)
Initial fracturingRefracturing
(b) H-0-6
Time (s)
05
10152025303540
0 100 200 300 400
Pum
p pr
essu
re (M
Pa)
Initial fracturingRefracturing
(c) H-0-10
Figure 7: Characteristics of the refracturing pump pressure
curve.
8 Geofluids
-
time and enters the stage of fracture expansion. There areonly
two pressure fluctuations in the sample during refrac-turing. This
indicates that the fracture morphology producedduring refracturing
is relatively simple.
As can be seen in Figure 7(b), there are multiple peakpoints of
the initial fracturing pump pressure; the character-istics of
fracturing pump pressure curve is similar to H-0-7sample after
initial fracturing. According to the preliminaryanalysis of the
fracturing fracture surface and fracturingpump pressure curve, the
relatively complex fractures havebeen formed after initial
fracturing. When the initial fractur-ing channel is temporarily
blocked, the fracture initiationpressure of refracturing is higher
than the maximum fractur-ing pump pressure of the initial
fracturing, and only onebreakdown pressure point occurred. The
maximum fractur-ing pump pressure for refracturing reached to
38.65MPa.After that, the refracturing pump pressure is maintained
ata high level in extended fracturing; when the fracturing
fluidexpands to the sample boundary, the refracturing pumppressure
drops rapidly. In Figure 7(c), during the initial frac-turing
process, the fracturing pump pressure is 29.77MPa,and then falls
rapidly. The characteristic of fracturing pumppressure curve is
simple. This indicates that the sample hasformed a fracturing fluid
channel after initial fracturing.Moreover, the fracturing
morphology formed may be rela-tively simple. In the process of
refracturing, the fracturingpump pressure quickly reached to
27.61MPa, and after asmall drop, it rose again to 35.29MPa, higher
than the initialfracturing breakdown pressure. Then, the
refracturing pumppressure curve remained fluctuating at a
relatively high level(16~20MPa). When the servo hydraulic
fracturing pumpwas turned off, the fracturing curve dropped
rapidly. Boththe fracturing pressure and the overall pressure of
the refrac-turing are higher than that of the initial fracturing;
on the onehand, it shows that the initial fracturing fracture has
beeneffectively sealed; on the other hand, it indicates that the
sam-ple has produced new hydraulic fractures. The violent
fluctu-ation of refracturing pump pressure indicates that there
aremultiple initiations and extension of fracturing fracturesand
the fracturing fractures may become more complex.
3.4. The Interference Characteristics of Refracturing andInitial
Fracturing Fractures. Morphological characteristicsof refracturing
fractures after opened the samples are shownin Figure 8. After the
initial fracturing fractures are tempo-rarily blocked, the fracture
morphology formed by refractur-ing is shown in Figure 8(a). As can
be seen from Figure 8(a),the tracer on fracturing fracture surface
is observed; compar-ative analysis of the tracer characteristics,
after initial hydrau-lic fracturing, a complex fracture network
with a mainfracturing fracture perpendicular to the horizontal
minimumprincipal stress, is formed and a single natural fracture
sur-face is approximately along the bedding plane, perpendicularto
the vertical stress. Due to the same in situ stress
conditions,after refracturing, an almost parallel refracturing
fracture isformed the initial fracturing fracture. Disturbed by the
initialfracturing fracture, the propagation path of the
refracturingfracture is deviated. Meanwhile, some weak bedding
surfacesare opened during the refracturing process. During
refractur-
ing fracture propagation, the initial fracturing fracture
wascommunicated after bypassing the temporary plugging point.In the
case of continuous fracturing fluid pumping, the frac-ture extends
to the sample boundary. The refracturing resultof sample H-0-4
shows that, when relatively complex fractur-ing fractures have been
formed in the initial fracturing, therefracturing test can open the
range of the unreconstructedzone effectively.
In Figure 8(b), it was shown that a complex fracturingfracture
network with a large volume of fracture reconstruc-tion includes
one main fracturing fracture and four activatedweak bedding planes,
and the main fracturing fracture com-municates multiple bedding
planes. According to H-0-6 frac-turing sample after opened and
analysis of the fracturingpump pressure curve characteristics, it
indicates that com-plex fracture networks have been formed during
initial frac-turing; the fracturing fluid containing the red
traceroccupied most of the fracturing fractures. This indicates
thatcorresponding to the fluctuation of fracturing pump pres-sure,
the main fracturing fracture opens multiple beddingplanes during
the expansion, and the sample has been fullytransformed after
initial fracturing. For shale samples withmultiple temporarily
blocked fracturing fractures, whenrefracturing is carried out under
the same three-direction insitu stress, the propagation path of
refracturing fracture willbe affected by the initial fracturing
fracture, and the initiationand expansion directions may be
changed. After refracturingsample H-0-6, newmain fracturing
fractures are formed nearthe initial main fracturing fracture, and
the main refracturingfracture was along the direction of the
horizontal minimumstress and intersects with the initial fracturing
fracture. Itcan be observed from the fracturing fracture trace that
thefracturing fluid containing yellow tracer and red tracer
ismixed; this means that after bypassing the temporary plug-ging
point, the new refracturing fracture turns to the initialfracturing
fracture and continues to expand. The results ofrefracturing showed
that the refracturing effect is not obviousfor the samples after
sufficient modification.
In Figure 8(c), after initial fracturing, a slightly
inclinedhydraulic fracturing fracture was formed in the middle
ofthe sample. The fracture is slightly extended in several
adja-cent bedding planes, and the fracture distribution is
approx-imately perpendicular to horizontal minimum principalstress.
It is preliminarily speculated that this is a mainfracturing
fracture surface, with the initiation position atthe bottom of the
wellbore in the open hole section. Afterrefracturing, there is
another yellow tracer overflow at themain fracturing fracture; it
indicates that there may be anew fracturing fracture initiation and
propagation along thisdirection. Meanwhile, new fracturing
fractures containingyellow tracer were found in multiple locations
of the wholeplane. Most of the fractures were distributed along the
bed-ding plane, and there were also a few vertically
distributedfracturing fractures, which communicated the adjacent
bed-ding planes. When the upper left part of the sample was
cutopen, large amounts of red and yellow tracer residues
wereobserved in the bedding surface, indicating that the
beddingplane was opened during the initial fracturing and
continuedto expand during the refracturing process. Three parallel
and
9Geofluids
-
closely adjacent transverse fractures were found. The
fractur-ing fracture was roughly perpendicular to horizontal
mini-mum principal stress, and the fracturing fracture
initiationwas in the open hole section. One of the fracturing
fractureshas a red tracer inside and a yellow tracer outside,
indicatingthat the fracture was formed during the initial
fracturing andexpanded during the refracturing process. The other
twofracturing fractures both with yellow tracer were newlyformed
during the refracturing process. Base on comprehen-sive observation
and analysis, it can be seen that the mainfracturing fracture of a
vertical wellbore is formed after initial
fracturing, but the extent of fracturing fracture and theopening
degree of the bedding surface are inadequate modi-fication, and the
reconstruction is insufficient. During therefracturing process, two
new transverse main fracturingfractures were formed, and the
original transverse main frac-turing fracture continued to expand.
Therefore, the beddingplanes were fully opened, forming a
crisscrossing fracturewith multiple transverse fractures and
bedding planes andthe improvement effect was relatively ideal.
According tothe results of the experimental study, for the
formation withsimple fracturing fracture formed after initial
fracturing,
IF
RF
IF
RF
RF
IFRF
RF
𝜎H
𝜎h
𝜎V 𝜎H
𝜎h𝜎V
𝜎H𝜎h
𝜎V𝜎H𝜎h
𝜎V
𝜎H
𝜎h
𝜎V
(a) H-0-4 (IF: initial fracturing fractures; RF: refracturing
fractures)
IF
RF
IF R F
IF
RF
IF
RF
𝜎h
𝜎V
𝜎H
𝜎H
𝜎h𝜎V
𝜎H𝜎h𝜎V
𝜎H
𝜎h
𝜎V
𝜎H
𝜎h𝜎V
(b) H-0-6 (IF: initial fracturing fractures; RF: refracturing
fractures)
RFIF
RF
IFRF
RF
RF
IF
RF
IF RF
RF
𝜎H
𝜎h
𝜎V
𝜎H
𝜎h𝜎V 𝜎H
𝜎h
𝜎V
𝜎H
𝜎h
𝜎V
(c) H-0-10 (IF: initial fracturing fractures; RF: refracturing
fractures)
Figure 8: Morphological characteristics of refracturing
fractures. BP: bedding plane; 392 HF: hydraulic fracture. The red
tracer is initialfracturing; the yellow tracer is 393
refracturing.
10 Geofluids
-
refracturing can be adopted to increase the fracture complex-ity
of formation through temporary plugging.
3.5. Stress Field Analysis around Initial Fracturing
Fracture.Shale gas reservoirs are extremely low in porosity and
perme-ability; horizontal well staged fracturing is used to
communi-cate natural fractures and weak bedding planes. With
thetechnology of segmental fracturing in horizontal wells,
thegeneration of multiple fractures is in sequence. When theinitial
fracturing crack is formed, it will affect the stressdistribution
in a certain range around the initial fracturingcrack. The induced
stress field is generated around thefracture, and the induced
stress field will have a certaininfluence on the initiation and
extension of the follow-upfracturing fracture. At present, the
calculation of inducedstress field mainly depends on the analytical
formula ofclassical fracture mechanics theory [20].
According to the fracture mechanics theory, a
two-dimensional-induced stress field calculation model is
estab-lished based on the assumption of homogeneity, isotropy,and
plane strain; the calculation schematic diagram of frac-turing
fracture induced stress field as shown in Figure 9.
It is assumed that the fracturing fracture is vertical,
thelongitudinal section is elliptical, the height is H, and
Z-axis
is along the fracture height direction. X-axis is along
thehorizontal well bore and Y-axis is along the direction ofthe
maximum horizontal stress. It is assumed that the ten-sile stress
is positive and the comprehensive stress is neg-ative. The induced
stress at any point (X, Y , and Z) is asfollows [21]:
where c =H/2, σx, σy, and σz are the induced three normalstress
components (MPa); τxz is the shear component(MPa); p is the fluid
pressure (MPa); ν is the Poisson ratio.The relationship of the
parameters is as follows:
r =ffiffiffiffiffiffiffiffiffiffiffiffiffi
x2 + z2p
,
r1
=ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi
x2 + c − zð Þ2q
,
r2
=ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi
x2 + c + zð Þ2q
,
θ = arctan − xc
� �
,
θ1 = arctanx
c − z
� �
,
θ2 = arctan −x
c + z� �
:
8
>
>
>
>
>
>
>
>
>
>
>
>
>
>
>
>
>
<
>
>
>
>
>
>
>
>
>
>
>
>
>
>
>
>
>
:
ð2Þ
In situ stress is determined by σx, σy, andσz . The stressfield
around the later crack should be the sum of the inducedstress of
the initial fractures and ground stress of the previouscrack.
According to the principle of superposition, the stressfield around
the nth fracture is [22, 23].
σH nð Þ′ = σH + ν 〠n−1
i−1σx inð Þ + 〠
n−1
i=1σz inð Þ
!
,
σh nð Þ′ = σh + 〠n−1
i=1σx inð Þ,
σv nð Þ′ = σv + 〠n−1
i=1σz inð Þ:
8
>
>
>
>
>
>
>
>
>
>
>
<
>
>
>
>
>
>
>
>
>
>
>
:
ð3Þ
σHðnÞ′ , σhðnÞ′ , and σvðnÞ′ are the three principal
stressesaround the nth fracture, MPa; σxðinÞ, σyðinÞ, and
σzðinÞ
H/2
H/2x
z
r2𝛽2
𝛽1 𝜎z
𝜎h
𝜎v
𝜎x
r
r1
𝛽
Figure 9: Schematic diagram of fracturing fracture induced
stressfield.
σx = −prc
c2
r1r2
� �3/2sin θ sin 32 θ1 + θ2ð Þ
� �
+ p rr1r2ð Þ1/2
cos θ − 12 θ1 −12 θ2
� �
− 1" #
,
σz = prc
c2
r1r2
� �3/2sin θ sin 32 θ1 + θ2ð Þ
� �
+ p rr1r2ð Þ1/2
cos θ − 12 θ1 −12 θ2
� �
− 1" #
,
σy = ν σx + σzð Þ,
τxz = −prc
c2
r1r2
� �3/2sin θ sin 32 θ1 + θ2ð Þ
� �
,
8
>
>
>
>
>
>
>
>
>
>
>
>
>
>
<
>
>
>
>
>
>
>
>
>
>
>
>
>
>
:
ð1Þ
11Geofluids
-
are the induced stress components around the nth frac-ture
caused by the ith fracture, MPa.
When a new fracture is formed, it produces an inducedstress
field which can be calculated by formula (1). Superim-pose this new
induced stress field into the old one and so on;the final stress
field can be obtained from equation (3). Thismethod only considers
the effect of hydraulic fracture onstress field and ignores the
effect of horizontal well. So, it islimited and cannot provide the
exact value of the stress field.Because the stress field around the
horizontal wellbore ofshale reservoir changes dramatically, the
effect of horizontalwellbore on stress field distribution should be
consider. Theinduced stress field is mainly affected by the
internal pres-sure, fracture length, and fracture spacing. It is
necessary tostudy the effect of initial fracturing fracture on
postfractureduring refracturing by laboratory test. The obtained
fracturecharacteristics can be used to analyze the fracturing
fractureinterference. As can be seen in Figure 8, the initial
fracturingfracture has some interference to the refracturing
fracture,which leads to the deviation of the propagation path
ofrefracturing fracture.
4. Discussion
Refracturing can open new flow channels in oil and gasreservoirs
and communicate with the unused reservoir ofthe old fracture to
increase oil and gas production. Therefracturing treatment design
for shale gas wells is largelybased on the original wellbore
completion and reservoircharacteristics, rock mechanics, geostress
characteristics,and fracture geometry [10]. During the initial
hydraulicfracturing, the fracture morphology formed is of great
sig-nificance to the subsequent refracturing. At present, thereare
few methods to identify and evaluate the fracturingfracture. In
this paper, the indoor hydraulic fracturing testis used to obtain
the morphological characteristics ofrefracturing under different
initial fracture characteristics,which is of certain significance
to study the fracturing tim-ing and location selection of
refracturing wells. The direc-tion of fracture initiation and
extension depends on the insitu stress state. The in situ stress
difference at differentlocations varies with time, and the initial
far-field stressdifference is an important condition to determine
whethera new fracture is generated or not [24]. The induced
stressgenerated by the initial fracturing fracture can be
prelimi-narily evaluated by combining the analytical method
withhydraulic fracturing test. The results showed that for
reser-voir with simple initial fracturing fractures, the
complexityfracturing fracture can be increased by refracturing
aftertemporary plugging. The effect of refracturing is not obvi-ous
for the reservoir with complex initial fracturing frac-tures. Due
to the influence of shale bedding and naturalfracture, fracture
morphology obtained after hydraulicfracturing may be quite
different in the same shale reser-voir. Therefore, comprehensive
evaluation such as pump-ing pressure, initial gas production, and
profile test isneeded to evaluate whether refracturing is
necessary. Theresults can be used to evaluate the timing and
locationof refracturing in shale reservoirs.
5. Conclusions
A series of large-scale true triaxial hydraulic fracturing
andrefracturing tests were conducted on cube shale samples.The
fracturing fracture expansion behavior after refracturingwas
studied and some suggestions on whether refracturingwas required
after initial fracturing were given. The followingconclusions can
be drawn:
(1) The characteristics of bedding plane and natural frac-ture
development in shale samples directly affect theeffect of hydraulic
fracturing reconstruction. Thefracturing pump pressure curve and
fracture mor-phology with the same batch of shale samples
aresignificantly different under the same fracturingparameters
(2) The characteristics of fracturing pump pressure
curveindicated the evolution of hydraulic fractures, includ-ing
initiation and propagation of hydraulic fracturingand communication
of multiple fractures. The vio-lent fluctuation of the fracturing
pump pressurecurve reflects the repeated fracture
characteristics
(3) The fracturing breakdown pressure of refracturing
isgenerally slightly higher than that of the initial frac-turing
breakdown pressure. Under the influence ofstress induced by the
initial fracturing fracture, thepropagation path of the
refracturing fracture isdeviated
(4) Before refracturing test, it is necessary to analyze
thefracturing pump pressure curve characteristics of theinitial
fracturing. In the case of a formation that hasbeen sufficiently
fractured after initial fracturing,there are almost new fractures
after refracturing.Therefore, there is no need for refracturing
(5) In the formation with simple fracture after
initialfracturing, it is necessary to block temporarily
initialfracturing fracture before refracturing
In this study, an effective method of refracturing test
inlaboratory was established. The effect of refracturing
withdifferent initial fracturing fracture morphology was
analyzed.In the refracturing test, the initial fracturing and
refracturinghave the same in situ stress. However, in the actual
fracturingconstruction process, as the induced stress around
fracturesare caused by the initial fracturing fractures, the three
in situstress parameters should be superposition. Further
relevantstudies will be carried out in the future.
Data Availability
The data used to support the findings of this study are
avail-able from the corresponding author upon request.
Conflicts of Interest
The authors declare that there are no conflicts of
interestregarding the publication of paper.
12 Geofluids
-
Authors’ Contributions
Y.G. and L.W. conceived and designed the experiments; J.X.and
X.C. processed and analyzed the experimental data;Y.H. and J.Z.
performed the experiments; X.Z. provided theexperimental support.
All authors have read and approvedthe final manuscript.
Acknowledgments
The research was supported by the National Natural
ScienceFoundation of China (51574218), National Science
andTechnology Major Project of China
(2017ZX05005-004,2017ZX05036-003), and Research on Key Scientific
andChina Postdoctoral Science Foundation (2019M662918)and Technical
Issues in Exploration and Development ofShale Gas (XDB10040200),
CAS pilot project (B). We wouldlike to express our greatest
gratitude for their generoussupport.
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13Geofluids
Study on Fracture Morphological Characteristics of Refracturing
for Longmaxi Shale Formation1. Introduction2. Experimental
Materials and Procedure2.1. Sample Preparation2.2. Experimental
Facility2.3. Experiment Procedures
3. Experimental Results and Analysis3.1. The Characteristics of
Fracturing Pump Pressure Curve3.2. Impacts of Bedding Plane or
Natural Fracture on Fracture Propagation3.3. The Characteristics of
Refracturing Pump Pressure Curve3.4. The Interference
Characteristics of Refracturing and Initial Fracturing
Fractures3.5. Stress Field Analysis around Initial Fracturing
Fracture
4. Discussion5. ConclusionsData AvailabilityConflicts of
InterestAuthors’ ContributionsAcknowledgments