陳四樓煤礦240萬噸新井設(shè)計【含CAD圖紙+文檔】,含CAD圖紙+文檔,陳四樓,煤礦,萬噸新井,設(shè)計,cad,圖紙,文檔 翻 譯 部 分 英文原文 Hydraulic fracturing after water pressure control blasting for increased fracturing Bingxiang Huang,Changyou Liu,Junhui Fu,Hui Guan School of Mines,China University of Mining and Technology,South 3rd Ring Road,Xuzhou,Jiangsu 221116,China Abstract:Traditional hydraulic fracturing techniques generally form main hydraulic cracks and airfoil branch fissures,but main hydraulic cracks are relatively few in number.Hydraulic fracturing after water pressure control blasting can transform the structure of coal and rock mass.Experiments prove that it is an effective method for increasing the number and range of hydraulic cracks,as well as for improving the permeability of coal seams.The technical principle is as follows.First,a hole is drilled in the coal seam and is injected with a gel explosive(a mining water-proof explosive).Then,water is injected into the hole to seal it,at low enough pressure to prevent cracks from forming.Third,water pressure blasting is done by detonating the explosive.The water shock waves and bubble pulsations produced by the explosion cause a high strain rate in the rock wall surrounding the hole.When the stress imposed on the rock wall surrounding the hole exceeds its dynamic critical fracture strength,the surrounding rock breaks and numerous circumferential and radial fractures propagate outward.Lastly,water injection processes,such as general injection,pulse injection,and/or cyclic injection,are carried out to promote hydraulic fracturing.Depending on the fissure water pressure,detonation fissures continue to expand and additional hydraulic fractures with a wider range are formed.Under the effect of detonation pressure,joints and fissures in the coal mass open and propagate,leading to reduced adhesive forces on structural surfaces and thereby enhancing coal cutting.Therefore,this method improves the permeability of the coal seam,effectively weakens the strength of the coal and rock mass,and reduces the surrounding rock stress of the weakened area,effectively solving the problem of having a small number of big cracks.It is a useful technical approach for improving top coal caving,preventing rock burst,preventing coal and gas outbursts,and raising the gas extraction efficiency in colliery. Key words:Hydraulic fracturing;Water pressure blasting;Crack propagation 1 Introduction Low-permeability coal-seam gas extraction;hard,thick coal-seam fully mechanized top coal caving;and rock burst control are technical challenges in colliery at present.Hydraulic fracturing is an effective technical approach to resolve these challenges[1].The structure of coal and rock mass is altered through hydraulic fracturing,which can increase cracks in coal and rock mass improve permeability,and weaken strength to reduce any rock bursting liability.After decades of development,more study on hydraulic fracturing has been conducted both in China and else-where[2–14].Simulation experiments and field investigations of hydraulic fracturing show that the traditional hydraulic fracturingin number.In the case of homogeneous rock,a single hydraulic main crack is generally generated and cracks are mainly concen-trated in a band around the hydraulic main crack,whose extent is small.However,to improve hard,thick top coal cavability,handle hard roof,prevent rock burst,increase permeability of gassy coal seams,and prevent coal and gas outbursts,full re-formation of the structure of coal and rock mass by hydraulic fracturing is needed.This requires that hydraulic fracturing produce more hydraulic cracks,i.e.,increase the number of hydraulic cracks.Therefore,there is an urgent need to study hydraulic control fracturing technology to increase the number of hydraulic cracks,which has important theoretical and practical significance in guaranteeing efficient and safe colliery production.Common explosives blasting for gassy coal seams has safety risks,so they are not suitable.Water pressure blasting,developed in the past century as a kind of controlled blasting method,can effectively control the generation of blasting flying rocks,air shock waves,blasting tremors,and detonation toxic gases[15–18].Water pressure blasting is a gun-hole blasting technology that uses water as a coupling medium between the cartridge and the charge hole to techniques mainly form water pressure main cracks and airfoil branch fissures,but water pressure main cracks are relatively fewin number.In the case of homogeneous rock,a single hydraulic main crack is generally generated and cracks are mainly concern-trated in a band around the hydraulic main crack,whose extent is small.However,to improve hard,thick top coal cavability,handle hard roof,prevent rock burst,increase permeability of gassy coal seams,and prevent coal and gas outbursts,full re-formation of the structure of coal and rock mass by hydraulic fracturing is needed.This requires that hydraulic fracturing produce more hydraulic cracks,i.e.,increase the number of hydraulic cracks.Therefore,there is an urgent need to study hydraulic control fracturing technology to increase the number of hydraulic cracks,which has important theoretical and practical significance in guaranteeing efficient and safe colliery production. Common explosives blasting for gassy coal seams has safety risks,so they are not suitable.Water pressure blasting,developed in the past century as a kind of controlled blasting method,can effectively control the generation of blasting flying rocks,air shock waves,blasting tremors,and detonation toxic gases[15–18].Water pressure blasting is a gun-hole blasting technology that uses water as a coupling medium between the cartridge and the charge hole to transfer the explosion pressure and energy at the moment of the explosion to break up rock.The principal characteristics of water are exploited as follows.Since water is difficult to compress, deformation energy losses are low and energy transmission efficiency becomes high.Water acts to deliver uniform pressure, making the pressure on the surrounding medium relatively smooth and evenly distributed,leading to even breaking of the surrounding rock and greatly reducing the harmful effects of blasting.However,the compression ratio of water exceeds that of rock under high pressure and water also acts as the buffer layer between the explosive products and the rock mass.Not only does this buffer layer extend the interaction time of the shock wave on the rock, but it also can reduce or eliminate the energy loss in the plastic deformation zone generated in the rock mass.Water pressure blasting is currently a more mature technology in fields such as tunnel excavation and project demolition.In recent years,the application of water pressure blasting to colliery has started in China and elsewhere[19,20].In the former Soviet Union,coal-seam pre-injection internal explosions were conducted by using an 8-m-deep hole of 40 mm diameter to prevent coal and gas outbursts in a gently inclined thin coal seam and a medium thick coal seam.In China attempts were made to create cracks by water pressure blasting to improve the coal-seam gas drainage rate[21]. In view of the problems of existing technology,a preliminary test has been conducted to exploit the advantages of water pressure blasting and hydraulic fracturing.The test results show that hydraulic fracturing after water pressure blasting can increase the number and range of hydraulic cracks efficiently. Based on preliminary studies and test results,the author has proposed the use of water pressure control blasting for increasing permeability and weakening strength as a result of hydraulic fracturing. 2.Using water pressure control blasting to increase permeability through hydraulic fracturing Water pressure control blasting induces hydraulic fracturing in the borehole of a coal-rock seam,which changes the structure of the coal-rock mass and increases the number and range of hydraulic cracks,thereby increasing permeability and weakening strength.The technique involves the following steps: (a)Drill a borehole for hydraulic fracturing weakening with a drilling rig,inject an adequate amount of gel explosive(a water-proof mine explosive),and pull the lead wire out of the borehole. (b)After sealing up the borehole orifice with hole packer or cement mortar,inject water into the hole until it fills the hole or reaches a pressure value below that which would generate water pressure cracks.At this moment,the initial water pressure in the borehole must be less than the orifice rupture water pressure: where is the minimum principal stress of the crustal stress field around the borehole, is the maximum one,and is the tensile strength of the borehole rock. (c)Detonate the explosive to carry out water pressure blasting.The water shock waves and bubble pulsations produced by the explosion will cause a high strain rate in the rock wall surround-ing the hole.When the stress imposed on the surrounding rock wall exceeds its dynamic critical fracture strength,the rock ruptures and generates abundant circumferential and radial fractures surrounding the borehole.Meanwhile,because of the rock’s elasticity,the hole’s influence on the surrounding rockstress distribution is about 3–5 times the borehole diameter.Under the effect of subsequent water pressure,cracks are initiated in the wall of the hole when the effective tangential tensile stress of the wall exceeds the rock tensile strength. However,for a given crustal stress field,the position of the maximum effective tangential tensile stress of the borehole wall is a constant.Therefore,to increase the difference of hydraulic crack initiation between the follow-up borehole hall and the blasting cracks and make the blasting cracks craze preferentially,the length of blasting cracks must be greater than 3–5 times the borehole diameter. (d)Then,perform water injection processes such as general injection,pulse injection,and cycle injection to carry out hydraulic fracturing.Depending on the fissure water pressure,blasting cracks continue to expand and more water pressure fractures with a wider range are formed. The surrounding rock loosing zone of colliery roadway or grotto for constructing a borehole is generally 1.5–2.0 m.Because the water pressure induced by water pressure blasting is great,the sealing length in the complete surrounding rock section of the borehole must be greater than 2 m.The borehole length for installing the gel explosive must exceed 1 m.Thus,the underground fracturing borehole depth in colliery should not be less than 5 m. The structure of coal and rock mass is re-formed by hydraulic blasting control fracturing,leading to an increased number of hydraulic cracks,an increase in the permeability of the coal seam, an efficient weakening of the strength of coal and rock mass,and a reduction in the surrounding rock stress of the weakened area.This effectively solves the problem of having a small number of big cracks.There are a number of beneficial effects from this process.The weakening of the hard coal can improve top coal capability,reduce the risk of rock burst,increase the range of coal- seam fracturing cracks,make gas extraction easier,and prevent coal and gas outbursts,all of which are important in guaranteeing efficient and safe colliery production. 3.Experimental scheme 3.1.Experimental system We developed a true triaxial hydraulic fracturing experimental system.The system consists of an experiment-bench framework,a loading system,and a monitoring system.The main technical indicators are as follows:(1)the true triaxial stress is loaded on cubic samples to simulate crustal stress;the pressure from the loading plate in three directions can reach 4000 kN.(2)The size of the cubic specimen is or .(3)The water pressure for borehole fracturing can reach 70 MPa. During borehole fracturing,parameters such as water(liquid) pressure and flow are monitored by an Intelligent Vortex Flow-meter connected to the computer,using established procedures for data collection and storage.During the fracturing simulation, the crack propagation process and geometric morphology are monitored by a Disp-type 24-channel acoustic emission instrument,an RSM acoustic instrument,and a TDS-6 Micro-seismic acquisition system. 3.2.Experimental method The simulation experiment adopts a side length of 500 mm for the cubic specimen mixed with coal and briquette.(The original coal size is about .)A parameter test of the mechanical properties of both coal and briquette of different ratios has been conducted to ensure that the stiffness,strength,and other properties of the briquette are as similar to coal’s as far as possible.The quality ratio of the simulated sample is determined as coal powder:cement:plaster:water?0.5:1:1:0.8 and its mechanical properties are shown in Table 1.After the specimen has naturally dried,a borehole of 30 cm in length is drilled at the center of the upper surface of the specimen and then SHZ bar glue is used to bond the device bond to the borehole wall to complete the sealing while the sealing depth reaches 20 cm. We originally planned to use electric detonators to carry out the simulation experiment of hydraulic blasting control fracturing.The explosive amount(1 g)in each electric detonator is modest and the detonators can be detonated in water to achieve the purpose of blasting after sealing under water pressure.Thus, an electric detonator is the ideal blasting equipment for the simulation experiment.However,because the public security sector strictly controls electric detonators,it is hard to obtain blasting electric detonators.Therefore,the large firecracker shown in Fig.1b was used as blasting equipment for the simulation experiment.The hydraulic blasting control fracturing is simplified into two stages to simulate(1)blasting in the borehole and(2)hydraulic fracturing. The simulated stress field condition is ,,and and the stress direction is shown in Fig.1c.Red poster dye is added to the water tank to make it easier to observe the hydraulic fracture morphology. During the experiment,a microseismic instrument is used to monitor microseismic information of the specimen;the trigger threshold(STA/LTA ratio)of a microseismic event is 1.2,and the amplitude range reaches 500 mA with an STA/LTA time window of (0.1 s)/(1 s).At the same time,acoustic emission and electromagnetic radiation are monitored during the experiment.An acoustic emission probe(R.45)placed in the experimental framework closely sticks to the specimen and an electromagnetic radiation probe stays close to the outer steel ring of the experimental framework.An acoustic emission instrument uses the acoustic emission probe and the electromagnetic radiation probe to take samples at the same time.The frequency domain f of the electro-magnetic radiation probe is 30 kHz.The sampling frequency both the pre-amplifier(BP-SYS)and the acoustic emission probe is 5 MHz;the trigger threshold of the electromagnetic radiation probe is 20 dB,the trigger threshold of the acoustic emission probe is 39 dB,and the pre-amp gain is 60 dB for both.The high-pass filter of the electromagnetic radiation probe is set to 20 kHz and the high-pass filter of the acoustic emission probe is 1 kHz.The low-pass filters for both are set to 400 kHz. To compare with the results of common hydraulic fracturing in coal and rock mass,one common hydraulic fracturing simulation experiment of fissured coal and rock mass under the same simulated crustal stress and quality ratio of sample has been conducted. 4.Analysis of results 4.1.Crack propagation process of hydraulic fracturing after water pressure control blasting 4.1.1.Blasting After the large firecracker shown in Fig.1b is lit,it is put at the bottom of the drillhole and then the square iron pad of 70.2 kg containing the experiment framework is set to cover the orifice area of the specimen.The typical microquakes monitored during the experiment are shown in Fig.2,where the abscissa plots time, every small division stands for 0.1 s,and the total time shown is 5 s.In the figure,the first event is the quake caused by the square iron pad after lighting the firecracker;the second event is the microquake event caused by blasting;the third event marks the upward jump of the iron pad caused by the detonation gas after the explosion. After six blasts at the bottom of the drillhole,the specimen surface shows no visible cracks and is still integrated.The specimen is then placed on the test desk for the hydraulic fracturing experiment after sealing. 4.1.2.Hydraulic fracturing The water pressure and acoustic–electric effect during hydraulic fracturing after blasting are shown in Fig.4.A total of seven water injection fracturing experiments were conducted.For the first two,the pressure was controlled manually;for the last five,a high hydraulic pressure was pre-set by a stabilizer and water injection fracturing with high flow was carried out by pressure output switches.When the hydraulic pressure of the first water injection fracturing reaches 1.1775 MPa,a turning point in the hydraulic pressure curve appears(Fig.3b).At this moment,both the pulse number and the amplitude of the electromagnetic radiation show a small peak(Fig.3e and f),indicating that the drillhole wall ruptures(or the original blasting cracks open and burst),meaning that the hydraulic pressure of rupture is 1.1775 MPa.After the hydraulic pressure reaches 1.4775 MPa,it then decreases,showing that the hydraulic fracture propagates at this time.After the hydraulic pressure reaches a maximum of 1.5375 MPa,it falls to 1.4075 MPa with a relatively high speed,meaning that the hydraulic fracture propagates with a large scale. When the hydraulic pressure becomes about 1.40775 MPa,it remains constant for 9 s and then sharply declines.Meanwhile,both the pulse number and the amplitude of electromagnetic radiation have significant peaks and the deformation and failure of coal and rock mass are exacerbated.In the second water injection by manual control,when the hydraulic pressure reaches 1.2575 MPa,the same situation as with the first injection fracturing appears.The hydraulic pressure exhibits a turning point,which indicates renewed opening of the hydraulic crack.Afterward,the hydraulic pressure rises to 1.4075 MPa and it declines stably in only 3 s.The hydraulic crack perforates through the specimen surface fully;water comes out of the specimen orifice surface(upper surface)and the hydraulic pressure decreases sharply. During the subsequent fracturing of multiple injections,the ratio of water filtration decreases relatively because of high flow.So the hydraulic pressure reaches a maximum of 1.6775 MPa,which is greater than the maximum pressure obtained by manual control.Thus,when the filtration rate of the coal and rock seam is large,a high flow of water injection fracturing should be used to ensure higher water pressure on the crack tip to cause the hydraulic fracture to propagate. During the whole process of hydraulic fracturing,seven microquake events were monitored;a typical example is shown in Fig.4.In comparison to blasting quakes,microquakes induced by hydraulic fracture propagation are much weaker.Under laboratory conditions,because the layout space of the probes is small(2 m or less),the difference in time at which each probe receives the microquake events is very small,leading to difficulty in locating microquake events. Just as microquakes induced by hydraulic fracturing in a laboratory specimen can be monitored,large-scale microquakes induced by hydraulic fracturing in the field also can be monitored.And because the on-site monitoring region is large,the micro-quake source(hydraulic fracturing point)can be located at the same time.Therefore,microquake events induced by hydraulic fracturing can be monitored by a microseismograph during hydraulic fracturing in the field,le