Principle and engineering application of pressure relief gas drainage in low permeability outburst c

Availableonlineatwww.sciencedMININGScience DirectSCIENCE ANDTECHNOLOGYELSEVIERMining Science and Technology 19(2009)0342-0345www.elsevier.com/locate/jcurPrinciple and engineering application of pressure relief gasdrainage in low permeability outburst coal seamLIU lin, CHENG Yuan-ping, WANG Hai-feng, WANG Liang, MA Xian-qinNational Engineering Research Center for Coal& Gas Control, China University of Mining Technology,Xuzhou, Jiangsu 221008, ChinacHongqing Institute of China Coal Research Institute, Chongqing 400037, chinaAbstract: With the increase in mining depth, the danger of coal and gas outbursts increases. In order to drain coal gas effectivelyand to eliminate the risk of coal and gas outbursts, we used a specific number of penetration boreholes for draining of pressure re-lief gas. Based on the principle of overlying strata movement, deformation and pressure relief, a good effect of gas drainage wasobtained. The practice in the Panyi coal mine has shown that, after mining the Cl lcoal seam as the protective layer, the relativeexpansion deformation value of the protected layer C13 reached 2.63%, The permeability coefficient increased 2880 times, the gasainage rate of the C13 coal seam increased to more than 60%, the amount of gas was reduced from 13.0 to 5.2 m /t and the gaspressure declined from 4.4 to 0. 4 MPa, which caused the danger the outbursts in the coal seams to be eliminated. The result washat we achieved a safe and highly efficient mining operation of the C13 coal seam.Keywords: protective layer mining; technology principle; drainage of pressure relief gas; engineering application1 Introductionmine, we adopted a protective layer in our miningtechnology to drain pressure relief gas from protectedWith an increase in mining depth, the amount and seams, using RC net penetration boreholes in floorpressure of gas increases, gas disasters are becoming roadways, effectively reducing its gas content, com-more serious and several shallow and non-outburst pletely eliminating the danger of outbursts andcoal seams are gradually becoming outburst coal achieved safe and highly efficient mining condiseams. Today, China has become the country with the tionsmost serious coal and gas outburst disasters in theentire world. Globally, our frequency and intensity of 2 General situation of test areaoutbursts are the most severe. In contrast. coal andgas outbursts coal mines had been stopped productionThe Panyi coal mine is a large mine designed for 3in the USA, Australia, Russia and other countries. Mt production capacity. The C13 coal seam is one ofAccording to a statistical analysis by expert consult- its primary and most productive coal seams, whichants in 2005, among 415 coal mines of the 45 major contained a large amount of gas under high pressuremonitored state owned coal enterprises, 234 mines A large number of outburst accidents have formerlyre high gas and outburst coal mines, accounting for occurred during mining, including a major gas explo-56.4%of the major coal mines and 142 mines are sion, so control over coal gas was very urgent beforesubjected to coal and gas outbursts, accounting for we could exploit this coal seam34.2%The test area was located between the east -1 andUsually, the permeability coefficient of outbursrst East-2 mining areas, with the Bll coal seam ascoal seams is low. For example, on average, this co- protective layer. We planned to combine mining theefficient in the C13 coal seam of the Huainan ore area 2151(1) working face in East-l and the 2352(1)inwas only 0.011 m/(MPad), which presented great East-2 and called the combination 2352(1) insteaddifficulties in pre-drainagend in gas controlThe strike of the face is 1640 m and the trend 190 mto prevent outbursts in coal6 2I. Based on the The thickness of the coal seam is between 1.5-2. 4 m,current condition of coal seams in the Panyi coal 2.0e and the dip angle6°-13°,9°on中国煤化工Received o9 August 2008: accepted 20 November 2008rojects 2005CB221503 supported by the National Basic Research Program of China,CNMHGNNatural Science FoundationCorrespondingauthorTel+86-13913474114:E-mailaddressbaizhizhuang@@163.comLIU lin et aliple and engineeringtion of pressure relief gas drainageaverage. The amount of gas of the Bll coal seam is protective layer mining technology. When we mine4-7.5 m/t and it is a non-outburst coal seam. The coal seams in outburst coal mines we should firstcoal seam is stable and its geological structure simple. mine the non-outburst or low risk outburst coal seamsFully mechanized coal mining technology had been as protective layers 5-6, where the outburst coal seamadopted for the working face, designed for 2000 t/dis then called the protected layer. With protectiveThe C13 coal seam is a protected layer, located 70 layer mining, the coal and rock mass among the roofm above the bll coal seam. The corresponding test and floor moves and deforms within certain limits,working face of the protected layer consists of the which causes stress changing. Fracture fields are re-2121(3)working face in East-1 and the 2322(3)in distributed, ground stress is reduced, the coal seamEast-2(called 2121(3)2322(3)combined). The strike expands, the coal seam permeability coefficient in-is 1680 m(two faces in all) and the trend is 160 m. creases and drainage condition are created for pres-The coal seam is 5.57-6.25 m thick, 6.0 m on average, sure relief gas in the protected coal seams In the casethe dip angle 6-13 and 9 on average. The original of the Panyi mine, we drilled penetration boreholes ormeasured gas pressure was 4.4 MPa, the amount of carried out surface well drilling to drain the pressuregas 13.0 m/t and the original permeability coefficient relief gas, which caused the amount of gas and pres-only 0.011 m7(MPa. d). The coal seam was also sure to decrease significantly and the coal massstable and its geological structure simple. By draining turned hard. Naturally, gas pressure reduced to belowthe pressure relief gas, the danger of outbursts in the 0.5 MPa, the amount of gas reduced to below 6 m/tC13 coal seam could be eliminated and the amount of and the coal consistency coefficient rose 48%-gas was effectively reduced. A comprehensive, mec- 100%. At the end, the danger of outburstshanized cover caving coal mining technology had completely eliminated from the protected layersdbeen adopted for this working face, which was de- conditions for safe and highly efficient mining hadsigned for 5000 t/dbeen achieved. Drainage indices of pressure relief gas3 Drainage principle of pressure relief gasin the different ore areas are shown in table 1Draining of pressure relief gas is referred to asTable 1 Drainage indices of pressure relief gas in differenNamespace(m) deformation(%) permeability coefficient rate(%) ual gas(m/)pressure (MPaLng categoryUpper protectiveHuainan0.50.78468.85.76Underside4 Drainage methods of pressure relief gasgic condditions, the floor roadway was located10-20 m below the C13 coal seam in a good litholDrainage methods of pressure relief gas consist ogy rock bed and 46-56 m from the protective layerlargely of draining gas from the protected layers af- On the one hand, this position assured safe excavafected by mining. The space between the protective tion, avoided gas inrush from the C13 coal seam andworking faces 2352(1)and 2121(3)/2322(3)was prevented the excavation of another coal seam byabout 70 m, which is really a long distance below the mistake. On the other hand, this location did not af-protective layer mining. We have often used surface fect the mining of the B11 coal seam and assuredell drilling and upper penetration boreholes located normal functions for gas draining. The floor roadwayfloor roadways to drain pressure relief gas from the was located in the middle of the working face of theunderside of the protective mining layer. Surface well protected layer on the strike where, in principle, onedrilling is unreliable and we often used penetration should insist on avoiding forming down holes. Theboreholes. The design of penetration boreholes in- proportion of the section of the floor roadway wascludes the construction of a floor roadway, a suction designed for 6 mdrilling field and upper penetration boreholes/.Allconstruction activities should be completed befor4.2 Design and construction of the suction drill-starting mining the protective layer and we shoulding fieldensure that the pressure relief gas has been drainedrV凵中国煤化工 ields had been consimultaneously with mining the protective layerstrufrom thlineCNMHGe C13 coal seam In4.1 Design and construction of the floor roadwaythe pressure relief area, a field was set every 40 m, soBased on the occurrence of coal strata and concrete the pressure relief area of face 2121(3)/2322(3)in theMining Science and Technolool. 19 No. 3protected layer needed 39 fields in all. According to seam, the C13 coal seam was first compressed, andthe pressure relief angle, non pressure relief areas then expanded, the maximum compression deformawere present in the protected layer, with a field every tion was 27 mm and the maximum expansion defor-10 m, so that the non pressure relief areas of face mation 210.44 mm,soe maximum relative2322(3)in the protected layer needed only three compression deformation was 0.337%, and thefields and nine in face 2121(3). The drilling field was maximum relative expanding deformation 2.63%perpendicular to the floor roadway, with a length of 5 The bigger the expansion deformation, the better them and a net section proportion of 6.16 meffect of pressure relief.Bolt-shotcrete support technology had been adopted4.3 Design and construction of upper penetrationboreholesthe ressure rnie area The space be ween h oles was40 m, which was oriented on the middle of thethick-surface of the coal seam. The position of thehole opening was located at the top of the drillingfield and the final hole position ended 0.5 m into thehole and working face(m)roof of the C13 coal seam. Its design is shown in Fig1. The total length of the draining holes was 8879 m,Fig 2 Deformation curve of the protected layerof which 1460 m was drilled through the cl3 coal 5.2 Variation of permeability coefficientseam. The space between holes was 10 m in the nonpressure relief area.The original coal seam permeability coefficientwas only 0.011 m (MPa d). After mining theBllcoal seam, the pressure was relieved from the160mC13 coal seam and the permeability coefficientclearly increased. By using the amounts of residualgas and the borehole gas inrush, we calculated thatthe permeability coefficient had increased to 32.687m(MPa2-d), an increase of almost 2880 times,whichindicated that the c13 coal seam was now in the conFloor roadway a Suction drilling fielddition where gas has been drainedFig. 1 Draining hole dispersal in pressure relief area6 Drainage effect of pressure relief gas5 Pressure relief level6.1 Amount and rate of gas drainageAfter mining the protective layer, this layer moved1)Analysis of drainage of pressure relief gasand deformed, which caused ground stress to reduceWhen the protective working face advanced 40 mthe permeability coefficient to increase and pressure over the suction drilling field, the amount of gasrelief occurred. Pressure relief levels are related to the drained rapidly increased. Fig 3 shows the variationspace of the layer, rock lithology, mining height and in the amount of gas drainage vs. time in the drillingso on. The levels are higher and the pressure relief yard of the Huainan coal mine. The original 20 dayseffect is better in the protected layer. The change of was the period of increased gas drainage, enhancedground stress can visibly reflect the pressure relief pressure releasing activity and the amount of gaslevel, but the stress is hard to measure in the field, so drainage increased. The period between 20-80 dayswe adopted the extent of coal deformation and the was the active period of gas drainage, the pressurecoal seam permeability coefficient to reflect the level release activity became regular and permeabilityindirectlyreached a maximum. Gas drainage was regular and5.1 Relative deformationthe amount of gas drained from a single hole wasover 1.0 m/min After day 80 the attenuation periodWe adopted a base point method to investigate the set in, where the coal seam became gradually com-deformation of the C13 coal seam. To start, we drilled pacted, permeability reduced, the residual pressurea deep hole, and then installed test points in the roofand floor of the C13 coal seam, where the deforma- expe中国煤化工8 is drainage reduceds research, a highlytion was determined by the relative displacement of effeCNMHGnonths, the length ofthe two test points. The measured results are shown in the anrecwas Iou m ana the number of activeFig. 2. We can see that, during mining of the Bll coal drainage holes was 16. On average the amount of gasLIU lin et aleening application of345drained from a single hole was about 1.0 m/min Af- m/t for the amount of gas and 0. 74 MPa(pressureter 4 months of continuous gas drainage, the rate of gauge) for gas pressuregas drainage reached over 60%oC13 coal seam had changed from a high gas andoutburst coal seam to a low non-outburst coal seamand C13 became a safe and highly effective coalseam where mining conditions became considerablybetter, as proven by its current mining practice.7 Conclusions1)With the advancement of a working face, coal100120140and rock masses among the roof and floor move andbecome deformed within certain limits, ground stressis reduced, coal seams expand and the coal seamFig. 3 Variation in the amount of gas drainage vsdrilling yardpermeability coefficient increases in outburst coalseams. After mining of the Bllcoal seam, the maxi2)Total amount of gas drainage and amount of re- coal seam was 0.337% and the maximum relativeThe total amount of gas drainage includes the expansion deformation 2.63%. The permeability co-sidual gasefficient increased almost 2880 times and favourableamount drained from penetration boreholes and windemission in the floor roadway. The period of workingconditions for drainage of pressure relief gas wereface 2121(3) for which statistics were obtained wasobtained in the protected coal seam C13from Feb. 1, 2000 to Jan. 31, 2001, 365 days in all2)The most effective gas drainage method is theThe accumulated amount of gas drainage by penetra- roadway. For an optimum pressure relief level, thetion boreholes was 7.99 Mm and the amount bywind emission 0.74 Mmspace between holes should be 40 m in the pressureThe reserves of the protected layer were 14.68relief area and 10 m in the non pressure relief areaMm and the total amount of gas drainage 8.73 Mm3)After gas draining of the C13 coal seam, theso that the rate of gas drainage reached 60%. The creased, from 13.0 m/t to 5.2 m /t and the rate of gaswork-out areas of the protective layer were not in- drainage reached above 60%. The gas pressure wasreduced from 4.4 MPa to 0.4 MPa, In the end, theluded in our calculation, so that the actual rate of gas danger of outbursts had been completely eliminateddrainage was larger than the calculated value, i.e,arate over 60%. Since the original amount of gas of the in the protected layer and safe and highly efficientmining conditions had been achievedC13 coal seam was 13 m/t, the residual gas contentwas 5.2 m/t, given our calculationsAcknowledgements6.2 Gas pressureWith the advancement of the working face, theThe authors are grateful to the National basicpressure gauge value gradually decreased from 4.4 search Program of China, and the National NatMPa, when the working face was 100 m away from Science Foundation of China for their support.the pressure measured hole; when 80 m away, thepressure sharply decreased, and at 62 m the pressureReferencesgauge finger pointed to zero. When the protectiveworking face advanced 400 m past the pressure [l] Cheng Y P, Yu Q x Development of regionalmeasured hole, the pressure value increased fromtechnology for Chinese coalmines. Journalzero to 0. 4 MPa and remained stable, which indicatedand Safery Engineering, 2007, 24(4): 383-390 (Irthat the residual gas pressure was 0. 4 MPa[2] Cheng Y P, Yu Q x. Application of safe and high-effi6.3 Outburst danger eliminated analysis of thecient exploitation system of coal and gas in coal seams.protected layerJournal of China University of Mining Technology2003,32(5):471-475.( In Chinese)The amount of gas in the protected layer is deH[3Cheng Y P, Yu Q x, Zhou Z Y, Zhou H X,earch on the amount of safecreased by draining the pressure relief gas. Theamount of residual gas of the C13 coal seam was re中国煤化工8duced to 5.2 m /t and the residual gas preessureCNMHGdeclined to 0. 4 MPa. The amount of gas and pressurevalues were lower than the critical value, which was 8Continued on page 351)ZHU Qi-hu et alPrevention of rockburst by guide holes based on numerical simulations2000,19(2):167-172.( In Chineserockburst and stress state based on energy principles.[7 Yan Y H. 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