Research into comprehensive gas extraction technology of single coal seams with low permeability in

Mining Science and Technology( China)21(2011)483-489Contents lists available at Science Directand TechrflagMining Science and Technology( China)ELSEVIERjournalhomepagewww.elsevier.com/locate/mstcResearch into comprehensive gas extraction technology of single coal seamswith low permeability in the jiaozuo coal mining areaFu jiangwei.. * Fu Xuehai Hu Xiao Chen Li Ou Jianchun dSchool of Resource and Earth Science, China University of Mining G Technology. Xuzhou 221008, ChinaHenan Provincial Coal Seam Gas Development and Unilization Co. LTD, Zhengzhou 450016, China"Department of industry and Business Administration, Henan Engineering College, Xinzheng 451191, chinad Faculty of Safety Engineering China University of Mining 8 Technology, Xuzhou 221008, ChinaARTICLE IN FOABSTRACTArticle history:For a low permeability single coal seam prone to gas outbursts, pre-drainage of gas is difficult and inef-Received 12 December 2010Received in revised form 1 January 2011ficient, serously restricting the safety and efficiency of production. Radical measures of increasing gascepted 20 January 2011extraction efficiency are pressure relief and infrared antireflection. we have analyzed the effect of miningAvailable online 6 July 2011conditions and the regularity of mine pressure distribution in front of the working face of a major coalmine of the Jiaozuo Industrial Group)Co, as our test area, studied the width of the depressurization zonein slice mining and analyzed gas efficiency and fast drainage in the advanced stress relaxation zone. OnSingle coal seam with low permeabilityhat basis, we further investigated and practiced the exploitation technology of shallow drilling, fan dril-igh risk gas and outburst coal seamling and grid shape drilling at the working face. practice and our results show that the stress relaxationStress relaxation zonezone is the ideal region for quick and efficient extraction of gas. By means of an integrated extractionComprehensive gas extractiontechnology, the amount of gas emitted into the zone was greatly reduced, while the risk of dangerousaozuo coal mining areaoutbursts of coal and gas was lowered markedly. This exploration provides a new way to control forGas emissiongas in working faces of coal mines with low permeability and risk of gas outbursts of single coal seamsin the Jiaozuo mining areae 2011 Published by Elsevier B V on behalf of China University of Mining Technology1 Introductionrange of the stope, whose flow- increasing effect appears as antireflection [7-9 According to flow theory the occurrence of coalAntireflection pressure relief is an effective method to improve seam gas, given the conditions of similar basic gas-geology, antire-gas extraction rates [1-3]. Based on mining pressure relief theory, flection pressure relief is the most important factor controlling theprotecting the exploitation of coal seams has become an effective amount of gas draining from drill holes [10, 11). Based on this the-way of controlling gas in multi-seam mining in China. Protecting ory, we have used the example of the jiaozuo mining area as a typ-the exploitation of seams not only reacts to antireflection pressure ically developed single low permeability and high risk gas and coalrelief of coal outbursts, but can also effectively release gas and bed outburst area to study the technical foundation of gas extrac-ground stress and hence has become an effective method of pre- tion, the distribution of pressure release zones and the increasingvention of coal and gas outbursts 14-6). Lacking adequate condi effect of permeation at its working face, affected by mining activ-tions for protecting the exploitation of a single seam with low ity. Given the theoretical basis for gas extraction in pressure re-permeability and a high risk of gas outbursts, pre- mining drainage lease zones, we carried out studies and applications of integratedof coal seams that have been worked is the only method to control gas extraction technology in single coal seams with low permeabil-regional gas controlling. However, high gas outburst seams, with ity. Practice has shown that pressure release zones are ideal re-low permeability and poor drilling conditions, require long pre- gions for highly efficient gas exploitation, with marked effects ofdrainage times and the existence of blank tapes at working faces. its utility model that suppresses the risks of emission and coalNot only does this threaten the conditions for safe production, and gas outburstsbut also seriously restrict the safety and productivity of coal minesTheoretical research and practice have shown that coal is in a stateof pressure relief affected by mining activity within a specified 2. Strata behavior regularity of excavation working face andwidth observation of stress relaxation zones Corresponding author Tel: +86 15093203187.Because of the effect of fnal mining activity, support conditionschange In a se中国煤化工 mined zone, followed74-5264)s-see front matter o 2011 Published by Elsevier B.V.CNMHGdoi:10.1016 mstc.2011.06004f Fu et aL/ Mining Science and Technology(China)21 (2011)483-489by the formation of the caving, the fracture- and bending-subsi- underground pressure at the working face and the amount of gasdence zones above the gob area, roof strata movements change flowing from boreholes.[12]. Roof strata stress of different coal seams in front of the work-From our investigation of mine pressure distribution regularitying face may change along the seams to the front followed by the and the variation of gas flows in the various stress zones( Figs. 2formation of a pressure relief zone, a stress concentration zone and and 3), we see that the advanced work concentrates stress in frontan original stress zone(see Fig. 1)[13. Because of top coal caving of the working face and exceeds the constant Beyond 20 m,thefrom a false leaf trace roof, the immediate roof caves as mining coal seams gradually become areas of stress concentration, whereprogresses and in the end the cantilevered roof will collapse roof pressure increases, coal fractures close, permeability reduces,resulting in a wide range of overlying strata relief. Given the de- gas flows from drilling decreases to an average gas flow of onlyrease of the support pressure of the coal in the pressure relief 0.03 m min. within 10-20 m from the face, the coal forces therange a stress relaxation zone will be formed in first instance with stress concentration zone into the transition zone and further intethe release of the coal stress, where original fractures open or ex the stress relaxation zone where permeability and gas flow frompand. New fractures will be generated and regional permeability drilling gradually increase to12 m/min. Less than 10 m fromof coal increases rapidly. Part of the adsorbed gas is analyzed and the face and as the distance to the working face decreases, thechis adsorbed gas migrates rapidly to effective extraction length of drilling is reduced, resulting in athe free space showing a"flow effect by relief, which is the ideal declining gas flow. because the roof lets go and collapses in thisarea to improve the effect of pumping gas. within a specified range area, the pressure in the coal seam is fully released and the frac-below the stress relaxation zone, due to the cyclical destruction tures in the coal seam increase substantially. The volume extractedand instability of the rock wall. coal pressure is transferred down- by drilling is still increasing. Given our data on mining pressure atmass have not yet been affected by mining activity. This zone bears under normal condition 630award to form a stress concentration zone, where stress will in- the working face and our investigation of gas flows, we considercrease, fractures and large pores will close, decreasing the the range of the full stress relaxation zone in front of a slice ofpermeability of the coal seam and hence the conditions for desorp. the mining working face to be 20-30 m, where a distance oftion and seepage of coal gas. Below this level, coal and its rock 20 m or less is clearly within the range of a stress relaxation zonenormal stress, called the original stress zone, where the gas flow isnot affected by miningTheoretical studies suggest that the width of the stress relaxa- 3. Analysis of basic technology of gas extracton in preact stresstion zone is proportional to the thickness of the coal seam, while relaxation zonethe side pressure coefficient and the depth of coal mining are in-versely proportional to the friction coefficient of the interfaceMethane drainage is not only related to drainage time, negativeand the tensile strength of coal [14]. Because differences among drainage pressure, the diameter of the drill hole, drilling depth butome factors in the same mine are not large, we can analyze the also to other parameters depends on the permeability of coalvariation in underground pressure and amount of gas flow from seams[ 17. when underground coal is not affected within an influ-the boreholes at the working face to investigate the width of the ence radius of drilling during the initial stage of drainage, coal fracstress relaxation zone [15, 16 the Jiaozuo coalfield is a typical tures are connected adsorbed gas is gradually desorbed and thelow permeability and high gas outburst single coal seam zone in extraction results are quite good. when the drainage time is ex-China. the main seam is the #21 coal seam of Shanxi formation, tended, the gas flow from drilling attenuates quickly. As miningwith an average thickness of approximately 6 m, a high coal gas activities are carried out, the stress balance in the coal is broken;content (generally 20-34 m/t dan), and high pressure (0.7- stress redistribution takes place in front of the working face to2.42 MPa) The measured seam permeability coefficient ranges be- reach a new balance. As the working face advances, the front oftween 0.11 and 5.86 m2/(MPa2d) Pre-pumping the coal gas is the the coal is unloaded within a specific pressure range and coal frac-most important technology to remove the risk of an outburst at the tures become further connected, forming a stress relaxation zone.working face. Currently, the major mining methods at Jiaozuo are meanwhile, within a specified range of deep coal, a higher stresstop-coal mining and slice mining, with mine technologies such as appears, holes and fractures close, forming a stress concentrationinclined longwall mining and fully-mechanized mining. During zone in the area of mining activities. In this area, not only is con-our tests, we selected from the Jiaozuo Industrial (Group) Co., struction difficult, but poor permeability is also a problem, givenLTD, the working faces of the #14101 Jiulishan seam and of the the original stress area in this deep area. In stress relaxation zones,#22041 Yanmazhuang seam. Given the variation in mine pressure the permeability of coal seams increase, the conversion rate of thedistribution in different stress zones and gas flow patterns, we adsorbed gas to free gas increases, a new rise in the amount of drillinvestigated the width of the advanced stress relaxation zone at gas inevitably occurs, thus making the area an ideal region forboth of working faces with the help of two conditions, i.e the drawing gas fast and efficiently. According to a study by Zhouand Lin, an application of this technology to draw gas into thestress relaxation zone, shrinkage of coal will certainly occur, givingrise to two benefits[ 10]. On the one hand, coal strength will be increased; on the other hand the permeability of coal seams is fur-ther improved, thus reducing the methane pressure gradient andeffectively preventing coal and gas outbursts. As the coal workingface advances continuously, there will be always stress relaxedcoal in certain areas of the working face. The determination ofthe head relief pressure area of a coal mining face is the basis fora suitable arrangement of gas drill holes and for strengtheningmethane drainage measures and management in a stress relaxa-tion zone. Theret中国煤化工 sions for thepose of safe mstrengthening theFl 1 Schematic diagram of the front of a coal bed showing support pressuremanagement of FC N MH Grainage holes suit-distribution at the workingably, optimizing drainage parameters. Such achients shouldJ Fu et aL/ Mining Science and Technology(China)21( 2011)483-4896010203040506070Distance to working face(m)Distance to working face(m)(a)The #14101 working face, Jiulishan Coal Mine(b)The w22041 working face, Yanmazhuang Coal Mine0.1020102030405060708090100110)The #1410l working face, Jiulishan Coal Mine(b)The w22041 working face, Yanmazhuang Coal MineFIg 3. Amounts of gas flows from pre-extraction boreholes.be applied at the front of coal mining areas, as well as at other parts obtained from continuous monitoring of the shallow drainageof comprehensive drainage areas.holes(Fig 4)show that over a specified time period the drainagegas attenuation coefficient decreased to 0.0002-0.0021/d, indicat4. Study and application of comprehensive gas extraction at a ing a significant improvement in coal seam permeability in theworking faceholes or advanced emissions is effective measures to reduce gas4.1. Technology of gas extraction from shallow boreholesemission during the mining at working faces.When drainage holes are drilled into the working face of a coal 4.2. Technology of gas extraction from fan boreholesseam, gas will flow under the effect of negative pressure in thedrainage area and methane from the coal wall parallel to this flowIn the Weicun Coal Mine of the Jiaozuo industrial Group)Cochanges to a radial flow and reduces gas emission at the working Ltd the measured coal-bed gas content of the *14101 workingface. When the gas is extracted, the coal undergoes shrinkage, face is 18.46 m/tr a top grade mechanized mining technique ofcausing pressure relief to some extent which helps to reduce the a long-arm strike way is applied, with a fully subsident methodrisk of coal and gas outbursts [18, 19]. The mode of arrangement for coal roof management. Before the extraction of the workingof shallow drainage boreholes has some advantages, requiring only face, pre-drainage of coal bed gas at the working face is carriedsmall efforts in technical ingenuity, easy construction and high out through parallel borehole bedding in the upper and lowerefficiency For complex structures, soft coal development and long ducts at the working face. Given a low extraction rate of the work-working faces, coal and gas outbursts can be prevented by rational ing face and in order to reduce gas extraction at the working facecontrol of the density and depth of shallow boreholes, and reduce effectively and guarantee safe production and simultaneously con-gas emissions preventing accidents due to methane overrunstructing drainage boreholes along the strike of cutting holes, oneIn the #15021 working face of jiulishan of the Jiaozuo Industrial drilling field was allocated per 100 m along the slope of the(Group)Co, Ltd, the coal seam gas content is 30 m/tr, the coalam is 5.3 m thick, on top of which there is soft a coal lay0.3-1.0m thick Prior to drilling 523 pre-drainage holes, the◆ Borchole1· borehole2+ Borehole3amount of gas drained within 100 m of the working face was1.0942ex0.0023 m min hm and a large amount of gas was released intothe initial mining area, of which the largest concentration of meth-ane, after roasting, reached 1. 42%. In order to reduce gas emissionsduring mining, drainage drilling proceeded along the outwarddirection of the working face, to depths of 12-15 m, the holes werey=07863cxy=0.8383espaced 1.5 m apart. Immediately after their construction, eachdrainageholecontinuouslypumpedgaslastingatleast4h.compared to pre-drainage face holes, the methane scalar of a singlehole used a shallow hole drainage method with a range of 0.020. 1 m /min. The average amount of gas drained per 100 m was FlgH中国煤化工CNMHGthe #15.021 face of the0.68 m min hm, which improved efficiency 296 times. Data Jiulishan Coal Mine.J Fu et aL/Mining Science and Technology(China )21(2011)483-489transport roadway, 20 m deep Drainage boreholes are fanned outinside(see Fig. 5). 50-60 m long. the #1 drilling field is 85 m re-Row from group amoved from the cutting hole and boreholes in groups of four arelaid out in the drilling field. Group a consists of 12 boreholes athe left working slope; Group b consists of 9 boreholes at the rightfront: Group c consists of 13 boreholes at the right working slopeand Group d consists of 14 boreholes at the right working slope口0.5During our experiment, we traced the amount of gas flow for each量是是是一of the four groups of boreholes203040506070Drilling the #1 field was carried out on time and the boreholeswere linked. The total amount of gas drained per group of bore-Distance to working face(oholes ranged from 0.3477-0.671 m/min to 0.029-0.072 m/minFig. 6. Amount of gas flow from group a boreholes.per single borehole per 100 m, which is not a clear change compared with pre-drainage conditions. In order to evaluate the pro-cess of coal extraction at working faces with fan boreholes, we average space between boreholes of 2 m and an average aperturedrew a trend map of the amount of gas extracted per group a bore- of 94 mmholes as the working face advances (see Fig. 6). In Fig. 6, about 60 mGas extraction started when the dip boreholes in the upper ductaway from the cutting hole, the effective drainage length decreases of the #22041 working face were finished. Owing to better perme-as the working face becomes shorter, while the total amount of gas ability of coal seam in this working face, the original amount of gasdrained continued to increase. about 60 m from the cutting hole, extracted is greater than 0.02 m/min, with an attenuation coeffi-ered, with the attended effect of sharply shortening their length to investigate the change in the amount of gas extracted in grid-and a gradual decline in the total amount of gas drained. Howeverd ithe boreholes were affected by an advanced pressure releasehe working face, successive ion tracking was carried out in gridwith a clear increase in the amount of gas drainage per 100 m. Ow- shaped boreholes. The average amount of gas extracted from theseity of the coal rock clearly increased. The amount of gas flow from per 100 m at the #22041 working face is seen to increase gradu dBeing to mining activity in the pressure release range, the permeabil. boreholes is shown in Fig. 7b, where the amount of gas drainagethe fan boreholes has increased 5-22 times in this pressure release within a distance of 100 m from the working face. The amount ofzone. To make the most of the antireflection effect of the pressure gas extracted per 100 m has increased by more than 3 times. Espe-release zone, as the working face constantly advances, drilling cially within 40 m of the working face, gas extraction has improvedfields should be designed in a timely fashion in airflow and drilling markedly, implying that the region is within range of antireflectionroadways and fan boreholes should be allocated suitably based on pressure relief.If we only take the mechanism of borehole extraction into ac-and wide-aperture boreholes should be laid out following the cut- count because of the space across the gridding boreholes, a mutu-ting hole. This should improve the rate of gas drainage quickly and ally affecting zone and a sufficiently affecting zone are formed indecrease the amount of gas drainage at the working face.the destructive region of the boreholes, which can improve fractureconnectivity between boreholes and the surrounding coal rock.This is of benefit for enlarging the range of gas release of the pres-4.3. Technology of gas extraction from grid shaped boreholessure-relief coal rock and improves the speed of gas desorption, aswell as the rate of gas extraction. Considering the effect of gasGrid-drainage of working faces, middle roadways joining uppeextraction on the #22041 working face in the Yanmazhuang CoalMine, the antireflection pressure relief is the controlling factor toand lower ducts is laid at a specified distances from working faces. improve the rate of gas extraction within a range of 80 m fromIn roadways, strike drainage boreholes are allocated along workinghe working face, the amount of gas flows from the boreholesface in the Yanmazhuang Coal Mine has a measured permeability tion can be quickly and effectively improved before stoping of theboreholes are laid out in the transport roadway of the working face, mining activity Given the results of our tests, a suitable middlewith average space between them of 0.75 m, of which 252 are laid roadway should be located 100-200 m from the cutting holeout in the return airway. The length of the cutting holes and tra- within this range, gridding extraction techniques can take fullverse are 97 m, each of them with 35 boreholes laid out, with an advantage of the effect of pressure-relief of mining activities inimproving gas extraction efficiency.5. Comparison and analysis of effect of gas extraction in a stressThe 14101 retum airwayrelaxation zoneAccording to a large amount of practice and preliminary theo-retical analysis, the technology of shallow drilling has a featurethat for the most part is effective for drilling within the stresselaxation zone, where the fracture of coal body develops andThe 14101 transport tunnhe gas permeability coefficient increases sharply. Hence, givenextraction under negative pressure, the flow directionchanged fromwhich is中国煤化工 w in theand reducingCNMHGa significFig- 5. Schematic diagram of fan boreholes in the #14, 101 working face.on gas extractseen llUll riK. I, along the advancingJ. Fu et aL/ Mining Science and Technology( China)21 (2011)483-489Mixing flowy=00807e2xGas pure flo0026080100120140020406080100120140160180Distance to working face(m)(a)BefFig. 7. Amount of gas flow from grid shape boreholes in the #15 021 working face.direction of the coal face the stress concentration zone is below rise again as the result of abandoning and stopping the cross head-pressure release zone. Based on a theory of coal and gas outbursts, ings borehole in the stress relaxation zonethis is the predominant zone for coal and gas outbursts to occurDuring the experiment, we also investigated the gas emissionThrough shallow drilling. the amount of gas and gas pressure are from the Weicun mine which uses fan drilling at the #14101 work-low within the range of controlled drilling and the stress concen- ing face(see Fig 9). It is seen from this figure that in the area oftration zone will migrate to the inside of the coal seam in order stoping footage between 450 and 690 m, absolute amount of gushto eliminate or lower the danger of outbursts. Shallow drilling is ing gas ranges from 8 to 10 m/min. As the stopping process pro-a very efficient way in the high gas and outburst mine of the Jiao- ceeds, the frequently high gas density occurring after blasting,zuo Industrial( Group) Co, Ltd Gas flows in measured in hectome- will seriously constrains safe production at the coal face. Afterter boreholes are more than thirty times greater than those in using the drainage technology in the stress relaxation zone fromforepumping boreholes, where the gas density of the return air the fan boreholes along the cut-hole in the control area(stoppingfrom the coal face is reduced to less than 0.5%. in general, the phe- footage between 280 and 450 m) the absolute amount of gushingnomenon of methane overrun is eliminated and the extremely high gas decreased to 4-6 m /min, a decrease of 40-50% The phenomrisk of outbursts is reduced from the previous 17% to less than O.6%. enon of extremely high gas density after blasting has essentiallyShallow drilling is in part a common supplementary approach for been eliminated with the help of this technology.regional measures that is quick and efficient. Given the conditionof only limited space, we do not provide a detailed introduction.Detailed test results about the technology of gas pressure relief5.2. Efect of test index of coal and gas outburst hazard duringextraction from fan boreholes and grid shaped pressure relief drilextractionling will be provided in the near future.The application of pressure relief and gas extraction technologycan effectively reduce the absolute amount of gas emission andeliminate the hazard of coal and gas outbursts during actual min-5.1. Efects of the amount of gas emitted during extractioning. According to the requirements for outburst prevention at theJiaozuo Industrial Group)Co, Ltd for every 2.5 m advance of theDuring extraction with grid shaped boreholes at the #22041 working face towards the coal bed strike, one row of 42 mm diamworking face, we measured gas density and the amount of air from eter drill testing holes should be arranged 1 m from the coal seamthe upper air duct during normal production and calculated the roof. The depth of these holes is 4.5 m and the distance betweenrate of gas emission(see Fig. 8). As can be seen from Fig. 8, both holes 5 m. The initial speed of gas emission q(L/(min- m))is takengas density and the amount of air at the upper air duct change at as the index of inspection for outburst hazards During our tests,different rates within the range of grid shaped drilling along the the effects on each working face were observed and analyzedadvancing coal face stopping near the bottom of the trending bore-.( Fig. 10). As seen from Fig. 10a, beyond the effective exhaust areahole. At the bottom of the trend borehole about 100 m from the of strike drilling on pressure relief and gas extraction at the workworking face, gas density and gas emission are low for some time. ing face, the value of q(L/ ( min-m))is relatively large and unstableFurther along the advancing coal face, both gas density and the otherwise, the situation is the opposite. At the same time,continuous outflow from the stress relaxation zone show a gentle Fig. 10b-d shows that, due to fan gas extraction boreholes, atincline. When the trend borehole is around 25 m from the coal ranged in the haulage roadway, control of the top part of theface, i. e, the place where the coal face reaches 175 m, the trend #14101 working face is poor in this Weicun Coal Mine. Althoughfor both variables, i.e. gas density and the amount of gas emitted the inspection index q clearly decreases within the control area,声Working faceARetum airwayolute outflow of gas020406080100120140160180200中国煤化工Distance to working face (m)CNMHGation-exploitation in theFig 8. Absolute outflow of gas during normal mining in the #22. 041 working face. #14, 101 working face.J Fu et aL/Mining Science and Technology( China)21(2011 )483-489020406080100120140160180200200250300350400450500550600650Distance(m)Distance(m)(a)Then22041 working face. Y anmazhuang Coal Min(b)Under part ofthe #1l04l working face, Weicun Coal Mine-- BorcholeBorehole 9200250300350400450500550600650200250300350400450500550600650Distance(m)Distance(m)(c)Middle part of the #1 1O41 working face. Weicun Coal Mine(d) Top part ofthe 1104l working face, Weicun Coal MineFig. 10. Test index of coal and gas outburst hazard at a working face.ts average value at the top part of the working face is still larger(4)Pre-drainage of gas is a necessary measure to realize safean the value at the middle and lower parts: therefore it showsand highly efficient production in a single and high risklat a grid shaped arrangement of gas extraction boreholes is helpgas and outburst coal seam. applying this integrated extrac-ful in drilling and control, while fan boreholes may still retain sometion technology is the key point to increase gas extractionuncontrollable areas due to its restriction in drilling.efficiency. Making the most of the effect of pressure reliefand infrared antireflection in mining activities, especially6. Conclusionsfor low permeability coal seams, the various extraction tech-nologies described for destressed gas extraction with shal(1)Radical measures of increasing gas extraction efficiency arelow drilling, fan drilling and grid shape drilling at workingpressure relief and infrared antireflection. We investigatedfaces can be used. these gas extraction technologies shoulda technology featuring efficient gas extraction in a stressbe carried out according mining slices and in stages, whichrelaxation zone of a single and low permeability and highshould increase gas pre-drainage efficiency. the methodrisk gas and outburst coal seam without protective miningeffectively solves the problem of extremely high gas densityconditions, using mining activity processes of dischargingin high risk coal and gas seams and insures safety at workingpressure and flow effect. the technology is of practicalfaces in minesimportance for improving gas extraction rates, reducingthe amount of gas emission, eliminating the risk of coaland gas outbursts and realizing safety in highly efficient Acknowledgmentsmine exploitation.(2)The difference of regular mine pressure distribution and var-The authors would like to thank the Jiaozuo Industrial Co, Ltd.iation in gas flows is a marked characteristic of stress distri- for providing reams of field data. the authors also thank the Chinabution in different stress zones and in crack changes. We University of Mining Technology for its intellectual support andused a method of investigating underground pressure at a the Major State Basic Research Program of China which providedworking face and the amount of gas flows from boreholes for our financial support( No. 2005CB221501).in order to evaluate the suitable width of an advanced stressReferences(3) Various practical extraction technologies employed at majoroal mine of the jiaozuo Industrial (Group) Co, Ltd show [11 Yu BFUndeliberated seam. Beijing: Chinathat in discharging a pressure range, destressed gas extracCoal Industtion with either shallow drilling, fan drilling or grid shape[2 Cheng YP.drilling at a working face has a positive effect on gas extraction and improves the rate of gas extraction. The experiment 13] oda MT Takemura A Aoki t. Damage growth and permeability change inaxial compression tests of Inada granite. 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