Variable frequency of pulse hydraulic fracturing for improving permeability in coal seam

International Journal of Mining Science and Technology 23(2013)847-853Contents lists available at Science DirectInternational Journal of Mining Science and TechnologyELSEVIERurnalhomepagewww.elsevier.com/locate/ijmstVariable frequency of pulse hydraulic fracturing for improvingpermeability in coal seamLi Quangui, Lin Baiquan, Zhai Cheng, Ni Guanhua, Peng Shen, Sun Chen, Cheng YanyingSchool of Safety Engineering, China University of Mining 8 Technology, Xuzhou 221116, ChinaARTICLE INFOABSTRACTArticle historariable frequency, a new pattern of pulse hydraulic fracturing, is presented for improving permeabilitReceived 25 March 2013in coal seam. a variable frequency pulse hydraulic fracturing testing system was built, the mould withReceived in revised form 22 April 2013Accepted 11 May 2013triaxial loading was developed. Based on the monitor methods of pressure sensor and acoustic emission,Available onlNovember 2013the trials of two patterns of pulse hydraulic fracturing of single frequency and variable frequency wererried out, and at last fracturing mechanism was analyzed. The results show that the effect of variablefrequency on fracture extension is better than that of single frequency based on the analysis of macroscopic figures and AE And the shortage of single frequency is somewhat remedied when the frequencyPulse hydraulic fracturingis variable. Under variable frequency, the pressure process can be divided into three stages: low freuency band, pressure stability band and high frequency band, and rupture pressure of the sample issmaller than that of the condition of single frequency Based on the miner fatigue theory, the effect ofdifferent loading sequences on sample rupture is discussed and the results show that it is better to selectthe sequence of low frequency at first and then high frequency. Our achievements can give a basis for theimprovement and optimization of the pulse hydraulic fracturing technologyo 2013 Published by Elsevier B V on behalf of China University of Mining Technology1 Introductionits self-properties besides the results above. PHF is a technologythat water with a certain frequency is pumped into boreholescontrol of gas disaster becomes more difficult, and it is difficult fracture network is formed inside the coal seam the relationshipfor original technologies to meet the demands of safe mining between pulsating parameters and fracture extension in coal is1-4 Several field tests of pulse hydraulic fracturing(PHF) have one research emphasis on PHF. As one of pulsating parameters.been conducted as a measure of regionalrol. The results frequency is in direct relation with the effect and efficiency ofshow that the effect of improving permeability and gas drainage PHF. Two aspects should be taken into consideration when fre-is increased markedly after the implementation of PHF. The Phf quency is selected: (i)resonant frequency, the probability of coaltechnology provides a new mentality for improving permeability resonance destroy is increased when the frequency is close toand controlling gas in coal seam.inherent frequency of coal; (ii) frequency provided by equipmentAs one of major stimulation measures, hydraulic fracturing haA research showed that the inherent frequency of coal varied inbeen widely used in oil industry, and correlational researcha large range from 70 to 400 HZ, and furthermore it was more com-relatively comprehensive. Research results have great referential plicated when the coal was drilled or injected with water, so inherignificance on coal seam hydraulic fracturing such as basal princi- ent frequency of coal is just a guideline for frequency selectionple of fracture development and propagation and rule of fracture [10 At present, the plunger pulse injection pump is commonlyoriented extension [5-7]. In the last few years, the studies on coal used for PHF, the frequency provided by which is from 0 toseam hydraulic fracturing have made some achievements, the qua- 25 HZsingle frequency, such as 20 and 25 HZ, is usedsi three-axis hydraulic fracturing tests were carried out to study throughout the process for the limitation of the equipment.the coal hydraulic fracturing mechanism thoroughly, and crackill the effect of phf be enhanced if different frequencies arepropagation law of hydraulic fracture in coal-rock mass had been used in the same procedure and will it make up for the limitationstudied [8, 9 However, as an extension technology of coal seam offrequency using the adjustment and combination ofhydraulic fracturing, PHF should be studied in connection with frequency? Variable frequency of phf is presented in this paper,ial PHFquencies are cond中国煤化工sts at different frequency on fracCorresponding author. Tel. +86 18086792556ture developmentE-mailaddress:15062196192@163.com(BLin).CNMHGinvestigated. Theh t: /d. do, oree o m imes.2013. 1.oDi hed Dy Elsevier Bv, on ehalt of China University ot Mining technolOgy848Q Li et aL. International Journal of Mining Science and Technology 23(2013) 847-853ents will provide a theoretical basis for the improvement Fracturing space, whose diameter is 10 mm and height is 2 cm, isand optimization of Phf technology.formed in the center of sample after a 9 cm long hole sealing byaccelerated cement. The designed and completed sample is shownin Figs. 2a, b, respectively2. ExperimentalAE is an experimental method to research internal state andmechanical strength characteristics of rock, through which the2.1. Schemechange procedure in sample under water pressure can be obtained,especially sample rupture and fracture extension [11-13mentioned in the introduction, the single frequency wasSome red dye powder is put into the fracturing pipe beforemostly chosen when using the PHF technology, and variable fre- starting the experiment, and it will get into the fractures in samplequency of PHF was not used in the past Two types of experiments under water pressure[14]. Then the range of fracturing influence iswill be conducted, one is single-frequency PHFe estimated roughly by red zonesother is variable-frequency PHF (VFPHF) The influence of variablefrequency on the effect of PHF will be obtained from comparative 2.2. Methodsanalysis on parameter changes such as macroscopic figures, pres-sure and acoustic emission(AE),etc.Four experiments are conducted two of which are SFPHF andA triaxial PHF test system, as shown in Fig. 1, consists of a pulse the others are VFPHE. The value of single frequency is 6 andinjection pump, an overflow valve, a pressure and frequency15 Hz, and the range of variable frequency is 6-15 and 15-6 Hzcontrol console, a mould, pressure sensors, and aE detector, etc. Triaxial stress gap of each experiment is about 0.5 MPaThe highest pressure of pulse injection pump is 12 MPa, andThe method of SFPHF is as follows: (1)Open overflow valve tothe frequency adjustment range is from 0 to 20 Hz. Thethe biggest, and turn on pulse injection pump, and then adjust200 x 200 x 200 mm mould has a function of triaxial loading, the frequency to 6 Hz(15 Hz)steadily; (2)Adjust overflow valvewhich consists of steel plates, bolts and screws, stress and strain to raise the pressure; (3)Finish the experiment when water oozesensors, etc. The seal of mould joints is rubber blanket.For it is a tentative exploration for law study, there is no need to maximum pressure Pa(Pb) during the whole process and the tinpursuit similar mechanical strength characteristics between Ta(Tb)samples used in this experiment and raw coal. Compositions ofThe method of VFPHF is as follows: ( 1)Open overflow valve tosamples are cement and coal ash, and a little of gypsum plaster the opening at the pressure P(Pc- min(Pa, Pb)), and turn on pulseis added to regulate setting time. The mix proportion of samples injection pump: ( 2) Adjust the frequency to 6 Hz, and then changeis determined as cement: coal ash: plaster: water=2: 10: 1: 10 after it to 15 Hz after T(T-T/2nish the experiment whenmany trials and 4 samples is made. After the sample setting, an water oozes out of the sample or pressure falls back sharply. In11 cm deep hole is drilled at the center of top face by a 10 mm the similar way we conduct the experiment of 15-6 Hzdiameter drill, in which an 8 mm diameter fracturing pipe is putThe main information and results of the four experiments areshown in table 13. Results and discussion3. 1. Macroscopic figures analysisof the sample or pressure falls back sharply is observed in samplesa, b and c except d. After removing the mould the sample is shown□口Ain Fig. 2c. fractures and their trends are clearly detected on thefaces of samples a, b, and C, and the red zone can be seen on theplane of fractures shown in Fig 3 when the sample is divided intotwo parts along the fracture trendFractures and their trends are shown in Fig. 3a-C, indicatingthat fractures on samples a, b, and c perpendicularly develop toFig. 1. Experimental system of variable frequency pulse hydraulic fracturing,fracturing pipe. There is a relationship between fracture extensiondirection of hydraulic fracturing and three-dimensional stress Due中国煤化工CNMHG(a)Design drawing(b)Sample before fracturing(c)Sample after fracturingFig. 2. Sample design drawing and photos before and after fracturingQ Li et aL /Intemational Journal of Mining Science and Technology 23(2013)847-853Table 1Sample information of the four experiments.pe Name Frequency(Hz) Stress(MPa)Time(s) Maximum pressure(MPa) ResultSFHPF aressure falls back shar0.510VFHPF C365back sharplyPerfectFracturingFractureFrac(a)6 Hz(b)15 Hz(c)6-15Hz(d)6H(e)15 H(f)6-15HzFig 3. Fracture trend on sample faces and boundary influenced by fracturing.to the limitation of the equipment, there is no loading along frac- 3. 2. Pressure change analysisturing pipe, i. e, 03=0 in all experiments, so the plane of fracturesbasically perpendicularly develops to the fracturing pipe which ac-The changes of pressure can reflect the progress of fracturingcords with rock failure criterion that fractures always perpendicu- The reason of pressure increasing could be that amount of waterlarly initialize and propagate to the direction of minimum principal pumped exceeds the amount of the sample can absorb, or therestress [15]. However, the fracture trend of VFHPF shown in Fig 3c is no fracture development and extension in the sample. the readoesn't exactly follow the rock failure criterion while that of SFHPF son of pressure decreasing is fracture extension, and the pressuredoes. It is worthy of future work whether the phenomenon is re- will get back to initial state when the extension is big enoughlated to the change of frequency.The pressure changes during SFPHF and VFPHF are shown inRed zones shown in Fig. 3d-f mean the areas that pressure Fig. 4, defining Pmax is the pressure high limit, Pmin the pressurewater reached, partly stating the influencing range of the water. low limit and AP is the amplitude of pressure(Ap=Pmax-Pmin)Of the two single frequencies, the range of fracturing influence fillThe pressure changes of SFPHF are shown in Fig 4a, b. Seen fromthe whole plane of fractures(Fig. 3d)when the single frequencoverall trend the two both have a similarity that Pmax gradually6 Hz, while the range is mainly at one side of the fracturing pipe grows with the increasing time till the end When single frequencyind is about half the whole plane( fig. 3e) when the frequency is is 6 Hz, the amount of water pumped into the sample is small due to15 Hz. The red zone shown in Fig. 3f also fills the whole plane of the adjustment of overflow valve in the beginning, so pressure in-fractures as 6 Hz when the fracturing type is variable frequency creases slowly but rapidly to the maximum by 7. 693 MPa at 68sbut misdistribution. The distribution of redshows that the when we quicken overflow valve. The state of high pressure lastsrange of fracturing influence is different at different frequencies. about 15 s till the sample completely broken, and pressure fluctu-When the fracturing type is variable frequency red zone is discon- ates at 7.5 MPa and中国煤化工 ingle frequency istinuous, stating that pressure water doesnt permeate to the same 15 Hz pressurem atplane of fractures and fractures do not extend in a single direction. 42 S. Unlike the 6CN Gel throughout theTherefore, the fracture extension of VFPHF is more complex than process, which keeps at 4 MPa at 20 s and reaches 7 MPa when thethat of sfphFsample broken, and Pmin is close to 1 MPa from beginning to endQ Li et al. International Journal of Mining Science and Technology 23(2013) 847-8538000600060000102030405060708090z6000400020002000l000(c)6-15Hd)15-6HzFig. 4. Pressure changes under different frequencies.It is a process of plastic accumulative deformation for the (6-15 Hz) has outstanding features in effectiveness and efficiency,sample, and fractures in the sample open and close with the action which can make up for the shortage of single frequencyof pulse pressure and confining pressure. In turn the opening andThe pressure change of VFPHF(15-6 Hz) is shown in Fig. 4dclosure also impact the rise and fall of pressure. When the single Due to the pressure controlled to be lower than 7. 179 MPa thefrequency is 15 Hz, AP maintains high level and fractures open sample doesnt rupture when the frequency is 15 Hz. When the fre-and close rapidly. The probability of opening and closure is equal quency is turned into 6 Hz, the pressure decreases as amount ofand it is difficult for the sample to fracture equably which causes water per unit time reducesnis reason the sample can,t bethe red zone to distribute only at one side of the fracturing pipe. brokenHowever, when single frequency is 6 Hz, AP is small and theprobability of fracture opening is bigger than closure, thus fracture 3.3. AE analysisinitiation is relatively equal. The analysis of AP shows that it maybe the reason for causing different extension of fractures, theaE technology can realize the location of sound using time lag.bigger of Ap the faster fracture extension and the smaller of Ap in which the location is determined by parameters such as time lagthe more obvious plastic deformation of the sampleof signals to different sensors, wave velocity of signal and relativeThe pressure change of VFPHF (6-15 Hz)is shown in Fig. 4C. position of sensors. ae detecting can be applied for fatigue crackDifferent from SFPHF, the pressure of VFHPF (6-15 Hz)presents initiation and extension using the kaiser effect Feature parameterstwo characteristics: (1) The process of pressure change is divided of acoustic emission are divided into process parameters and stateinto three parts: low frequency band, pressure stability band andhigh frequency band;(2)The maximum pressure (5.342 MPa)is parameters [12 Process parameters during fracturing suchlower distinctly than SFPHF (7.693 and 7.719 MPa). In low frecounts, strikes and events are wrote down in this experimentquency band pressure reaches two peaks and the drop of pressure and events and counts are analyzedafter peaks indicates that the pressure can make fractures inThe locations of events inside samples under four frequenciessample extend but it is not big enough to rupture the sample. are shown in Fig. 5, in which the green and red dots stand for AE sen-The pressure reaches 3.6 MPa at the second peak and then turn sor and event, respectively. Under single frequency the total ofinto steady band, in which it's a balance state in the sample and events is nearly equshown in Fig. 5a, b. But there are differ-it needs to be higher to make the fractures extend continuously. ences between the two that for 6 Hz events are distributed relaIn high frequency band the amount of water per unit time is added tively around the fracturing pipe which is produced from inside towhich causes pressure to increase rapidly. Under high pressure outside successively under dynamic observation, while events arestate fractures keep on extending until the sample ruptures when mainly distributed at one side of the fracturing pipe when the fre-pressure falls sharply. All over the process the effect of low quency is 15 Hz. That goes with the macroscopic figures. Thefrequency band is to make fractures in the sample develop and amount of events of tiextend and accelerate the sample rupture in high frequency band, different Under varial中国煤化工g5 C,d)is quiteof eventand in pressure steady band, it indicates that there is a balance is far more than otherCNMH Gly around waterafter fracture extension under low frequency. Therefore, combin- outlet of fracturing pipe in the direction ot X, y and Z When variableing the advantages of low and high frequency sufficiently, VFPHF frequency (15-6 Hz), the amount of events is littleQ Li et al Intemational Journal of Mining Science and Technology 23(2013)847-853(a)6Hz(b)15 Hz(c)6-15Hz(d)15-6HzFig. 5. Events location of AE in the sample under different frequencies.Combining with the macroscopic figures analysis, the location (Fig. 7). Defining omax, Omin, Ao are for the upper limit stress, theand amount of ae eventsllustrate that it is the most lower limit stress and the stress amplitude, respectively.complicated in fracture extension under variable frequencyThe water with a certain frequency is injected into boreholes(6-15Hz)when we conduct a PHF, and fatigue failure occurs under repeatedCount, as one of process parameters of AE, records the times of loading of pulse water. When coal seam is burdened with a stresscillation exceeding threshold signal, which is used for activity exceeding itslimit stress, each loading cycle can causeevaluation of AE. The change trends of counts under different fre- some damages, and at the moment that cumulative damages exquencies are similar to that of pressure. However, the rupture pres- ceed the threshold level, material breakage occurs, which is the fasure(5.342 MPa) of variable frequency(6-15 Hz) is lower than tigue accumulative damage theory [16]. According to Miner'srupture pressure(7.693 and 7. 179 MPa)of single frequency, while linear accumulative damage law, load tests are conducted at stressthe amounts of counts under the conditions of 6, 15 and 6-15 Hz amplitudes of Ao and Ao2, the fatigue life corresponding to theare nearly equal. The amounts of counts of variable frequency stress amplitudes are NI, N2, and the actual loading cycle numbers(15-6 Hz) that can, t rupture sample are the least in the four condi- are Ni, N2 [17]. If accumulative damage leads to rupture of thetions. Therefore, the amount of counts can be criteria of sample sample, the result can be written asrupture( Fig. 6).+N2/N23. 4. DiscussionThere is no difference on the surface of the above equation if exchange the position of items on the left of the equation, howeverThe experimental results indicate huge differences in speed, the experiments aiming at Miner's law show a significant differencedirection and quantity of fracture extension between SFPHF and with different loading sequences. If a lower stress is firstly applied,VFPHF. From macroscopic figure analysis, variable frequency(6- and then a higher stress, the material is likely to generate"training15 Hz) can rupture the sample on some conditions, with more effect, which requires longer time to make fractures extend On thecomplicated fracture distribution. From pressure change analysis, contrary, a higher stress is applied at first, the fractures are easilvariable frequency(6-15 Hz) requires a lower pressure to rupture formed and N1/Nn N2/N could be less than 1the sample. The differences are verified by AE locations and events, extension at a lower stress in later period. It seems that applying awhich show that the effects in the process of SFPHF and VFPHF higher stress at first is more likely to develop fractures. However,have great differences. VFPHF shows its advantages in fracture for the coal or sample there are initial fractures or fractures causedby drilling. Therefore, the crux of fracturing coal is the extension ofTwo different kinds of VFPHF (6-15 and 15-6 Hz)are carried initial fractures orout,and the sample rupture when variable frequency is 6-15 Hz, sideration, if a higl中国煤化工 -ctures extend rathus different kinds of variable frequency account for different re- idly and theC N MH Facture surface issults Stress parameters are substitute for the pressure parameters produced only at dheg. e). It I> ulificult for fracturesf VFPHF (6-15 Hz) fracturing and the process is simplified, so that to extend if we apply lower stress later(Fig. 4d ). On the contrary,stress variation along with life of the sample n can be obtained if we apply lower stress at first, fractures around fracturing pipe52Q Li et aL. International Journal of Mining Science and Technology 23(2013) 847-853≌250000≌25000015H15000050000010203040506070300006-15k15-6Hz0102030405060Fig. 6. Counts of AE changes under different frequencies.The above discussion of the experimental results is based onsteady equipments, similar samples and uniform testing methodsHowever, the analysis of the results is somewhat influenced because of the inherent differences in equipments and testing methds, such as pressure and flow control, and parameters setting ofE. In addition, although parts of the fracturing process, e.g., holesealing, flow control and loading order are similar to field test,Nng pressure and mechanics chteristics of samples according to actual application. Consequently,It is oneemphasis in future to minimize the above differencesand provide a better guiding for parameter design and optimiza7. Relation of fatigue parameters of variable frequency pulse hydraulic tion of field applicationhave enough time to extend under cyclic loading of lower stress. Acknowledgmentsand the sample ruptures when a higher stress is applied. Thereforea loading sequence of low stress and high stress is better thanFinancial support for this work, provided by the National basicsequence of high stress and low stress in effect and efficiency ForResearch Program of China(No. 2011CB201205), the Natural SciVFPHF, frequency determines flowing and the rate of flowing deter- ence Foundation of Jiangsu Province(No BK2012571), the Programmines the level of pressure. So it is better for VFPhF to select thefor New Century Excellent talents in University(No. NCET-12quence of low frequency first and high frequency second0959), the"Qing-Lan Project"and Collegial Graduate Researchand Innovation Program of Jiangsu Province(No. CXZZ13_0955).is gratefully acknowledged4. ConclusionsReferences(1) The analysis of pressure and ae in pulse hydraulic fracturingprocess and macroscopic figures after fracturing shows that1] Lin BQ Meng FW, Zhang HB. 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