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; ytguo@whrsm.ac.cn

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

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

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)

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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

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

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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

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

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Figure 4: Schematic of the in situ stress loading direction forhydraulic fracturing.

5Geofluids

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

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6 Geofluids

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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

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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