Research on forced gas draining from coal seams by surface well drilling

Mining Science and Technology( China)21(2011)229-232Contents lists available at Science DirectMining Science and Technology( China)ELSEVIERjournalhomepageswww.elsevier.com/locate/mstcResearch on forced gas draining from coal seams by surface well drillingWu Dongmei, Wang Haifeng, Ge Chungui, An FenghuaEngineering Research Cemter of Coal Gas ControL China Untversity of Mining G Technology. Xuzhou 221008, ChinaSafery Engineering China Universty of Mining G Technology, Xuzhou, Jiangsu 221008, ChinaMining Group, Huaibei 234113, ChinaARTICLE INFOA BSTRACTSurface drilling was performed at the Luling Coal Mine in Huaibei, to shorten the period required for gasReceived 12 August 2010draining. The experimental study was designed to reduce the cost of gas control by efficiently draininggas from the upper protected layer. The structural arraignment and technical principles of pressure reliefccepted 23 October 2010ia surface drilling are discussed. Results from the trial showed that gas drained from the surface systemver a period of 10 months. The total amount of collected gas was 248. 4 million m. The gas drainingarmed in three stages: a growth period; a period of maximum gas production; and an attenuationperiod. The period of maximum gas production lasted for 4 months. During this time the methanencentration ranged from 60 to 9o% and the average draining rate was 10other methods of draining it was possible to drain 70.6 of the gas from middle coal seam groups. Theount of residual gas dropped to 5.2 m /ton, and the pressure of the residual gas fell to 0.53 MPa.reby eliminating the outburst danger in the middle coal seam groups. The factors affecting pressurerelief gas draining by surface drilling were analysed.Copyright o 2011, China University of Mining Technology. All rights reserved.1. Introductionlayout of the drilling wells, and the effects of the drainage are dis-The permeability of coal seams in China is in general low, witha permeate coefficient less than 0.1 m2/MPa?d). The permeability 2. Experimental areabelongs to the class of hard-to-drain coal seams[ 1-3 Improving thepal gas drainage, eliminating the dangerof outburst in the coal seam,and safeThe Luling Coal Mine in Huaibei has experienced several coalexploitation in coal seams subject to outburst and gas outburst incidents since beginning mining. The mine fieldrequire measures to improve coal seam permeability and coal seam is divided into three levels: level 1 is 400 m underground; level 2 isgas drainage. Adopting protective seam exploitation technology can590 m underground; and level 3 is 800 m underground. Levels 1Efficiently draining gas requires the engineering of flow conditions million tons per year. The mine is a vertical-shaft central transportCommon methods for the pressure relief and gas drainage fromm.?tunnel design with block-division crosscuts. The coal seams withinprotected layer are the high drainage roadway or the net like pethe mine are divided into a middle group and a lower group ofseams. The middle group includes coal seam 7, 8, and 9, where coaltrating boreholes established through bed drillings from rock road- seam 8 and 9 are the main exploitation seams. Coal seam 8 and 9ways in the roof and floor. Current drainage practice allows the have gas pressures from 2.59 to 4.43 MPa and the gas content isnclusion that the effect of drainage is good but that the engineering from 18.95 to 22.67 m/ton. This level of gas presents significantinvolved is extensive and the construction period is long: sothe costishigh [8,9) Therefore, a method of pressure relief gas drainage fromdangers, with one occurrence per ten thousand tons mined. In thea protected layer by surface drilling has been investigated. The basic lower group coal 10,without danger, is exploited as the protectivetheory of surface drilling to create pressure reliefby gas drainage, the layer of the middle-group coal The layer spacing is 80-100 m.The working face( ll 1048)of the protection experimental lies tothe west of the ll 104 mining face and has an upper level of-548 to-566to-582 m a length of 310 m, a lengthof the中国煤化工 stopping line. The planeCNMHG1674-5264/5- see front matter Copyright o 2011, China University of Mining Technology. Alldot0o6ms20102003W. Dongmei et aL/ Mining Science and Technology( China)21(2011)229-232Cement reinforcingSurfaceSurface drillingdrilling1048High levelsuction roadwayCoal seams 7.89sectionFlg 1. Location of the protected working face relative to the surface drilling.Fig. 3. Schematic of the surface well.o 2.6 m. The roof of the coal seam is mainly a medium grainndstone, while the working face and ground drilling well arproviding ground drilling drainage to the protected layers an effearranged as shown in Fig. 1. The gas pressure in protected coal tive decline in coal gas content can be obtained. Thus, the danger ofseams 8 and 9 in the corresponding middle-group coal is 3. 2 MPaoutburst in the coal seams is eliminated and the safe and efficientThe quantity of gas there is about 18.2 m /ton and the total thick-exploitation of the seam may be done. the surface drilling drainageobjective is in part to provide gas reserves. The pressure relief ganess of the middle-group coal seams with surface drilling drainage drainage of the protected layer can provide a high concentration ofis 13 m. There are abundant reserves of coal gas in this areagas that can be directly used to generate electricity for civilian use3. Principle of pressure relief gas drainage by surface drilling4. Arrangement and construction of the surface wellSurface drilling affects only the gas of the upper protected layer.ot the lower protected layer [10, 11 After mining the protected The mining location of the working face in the protected coalseamlayer the roof and floor coal and rock seams move to cause defor- determines the drilling coordinates. We used the surface drillingmation as the stress and fracture fields re-equilibrate. As fractures plane coordinate of(X=371278, Y=39514959)and drilled to a depthdevelop in the roof and floor the ground stress drops. But as these of 587 m. the surface drilling structure is shown in Fig. 3. As shown, itfractures develop deeply within the roof and floor their influence is divided into three sections. the first section is the surface soil, thatgradually decreases. Roof coal and rock seams are divided by type is, drilling through the topsoil to the hard rock below and the instal-of fracture development into a caving zone, a fracture zone, and lation of casings for fixation. The second section is the bedrocka bend zone from below [ 12 ]. Coal seams in the caving zone cannot section, that is, drilling into the roof of the target layer(the pressurebe mined due to the damage. But the outburst coal seams located relief gas drainage coal seam or seam group)for an additionabetween the fracture zone and the bend zone get a noticeable 20-40 m followed by installation of casings. at this point the casingpressure relief and permeability improvement. within the protec- length is the sum of the first section and the second section, cementedtion scope of the protected layer it is seen that the ground stress of up to the ground. the third section is the target segment, that is,the outburst coal seam decreases and fractures develop to provide drilling to the roof of the protected layer for 5-10 m( this dependsflow conditions favorable to the pressure relief of coal gathe mining thickness of the protected layer )and installation of a nons The principle of pressure relief gas drainage by surface drilling is cemented screen. After drilling, the routine procedures are followedown in Fig. 2. Coal seam gas flows through the cracks and fissures a survey, well logging, washing the well, and so on.to the surface drillings and is then drawn out by the negative pres-sure. The drainage radius may be as large as 100-150 m. By 1. The surface soil segment: A 311 roller bit was used to drilldown to the bedrock in the quaternary system topsoil layer. TheSurface drillingdrilling depth was 256.09 m. After coring and verifyingD273 9-mm threaded casings were set into the bore. Two orBendine bandProtected searthree 50 x 300 x 6-mm plates were welded outside to preventthe breaking of casings at the threads when shifting of theformation by mining induced settlement occurs. The gapbetween the outer casing and the well wall was425# high-strength Portland cement until the mufrom the wellhead. The cementing mixture consisted(10 tons), triethanolamine(50 kg), and salt(50 kg)k section: The original design called for a o273-mm中国煤化工 f at a depth of425mof coal 7 was a 29 m thickC N MH Which could not supportFi 2 Schematic showing gas drainage in a pressure release area through a surfacechanged to 436. 28 m. A petroleum type casing D1778 x 9.17w. Dongmei et aL/ Mining Science and Technology( China) 21(2011)229-232average of 30. 474 m per day occurred on the 177th day when theAmount of drainage gasworking face was located 120 m past the surface drilling. The dailyproduction then reached 46, 656 m, which is the largest of all. On28the 31st of May the working face was completedThe third phase is an attenuation period from June 1November 8 or from the 134th day to the 246th day. This was百the gas production phase that occurred after the completion of theW平working face. The daily gas production tended to drop as did thedensity of the mixed gas. This period which lasted 5 months or 158days, includes two subdivisions with an accumulated gas production of 469,500m or a daily rate of 2972m' per dayTime(d)1. Gas production gradually declined from June 1 to August 14.OverFig. 4 Quantity and composition of drained gas from the surface well versus time.these 73 days the accumulated gas production was 377, 500 m.rage daily production of 5171 m. However, the gaswas then set in the well the material of construction is N80extraction system was still producing 3600-4320 m per day onand it has external threaded coupling-type connections. TheAugust 13th and 14th. However, when drainage was restarted 4annular gap between the casing, the wall, and the 273 innerdays later the gas production had dropped sharplypipe was filled with a 42.5# silicate high-strength cement2. During the 81 days from August 12 to November 8, asharpslurry until the mud returned from the wellhead. The compo-gas production was observed. A total of 92, 000 m was collectedsition of the cement slurry was cement (12 tons), triethanol-in production for a daily gas production average of 1136m3amine(6 kg), and salt(60 kg)during this time. On the last day of this phase the daily methane3. The target segment: In the screen layer a drilling diameter ofgas production was only 576 m. The total gas flow was 2 m/minp152 mm was used to sink the hole to the coal 10 seam roof atand the methane concentration in this gas was only 20. 46 Thea depth of 574.65 m. A sieve tube, D139.7 x 9.17 mm, was set inreason why the gas recovery was shut down was thatthe drainagethe hole for a total length of 142. 16 mconcentration did not reach the standards for practical use.5. observations and analysis of the gas drainage efifect5.2. An analysis of the gas drainage from the middle- group coal5. 1. An analysis of the pressure reliefDuring the mining of the 2-1048 working face both surface drillingand underground drilling through coal beds, from the roadway floorThe surface drilling gas drainage period lasted for a period of 10 were adopted. Pressure relief of gas from the middle-group coal wasmonths, from January 21 to December 8. The total amount of pure achieved this way. The amount of gas obtained by surface drilling wasdvance the periodic breaking and caving of the main roof, the bed was 2, 784,000 m.The total gas recovery was 5. 268, 000 m. Fromorking face fault structure, the relative position of the drilling and the mining area and the angle of the gas pressure relief area at the 2orking faces, and the suction pressure of the drainage system 1048 working face the gas reserves of the middle-group coal can beinfluenced the drainage quantity from the ground drilling. The estimated as 7, 459,000 m. Thus, the ratio of drainage from therelationship between the methane concentration and the amount middle-group coal was 70.6% The remnant gas content declined togas drainage is illustrated in Fig. 4. it appears that gas drainage by 5.2 m/ton with a pressure of 0.53 MPa; so the outburst danger in coal8 and 9 was drastically reduced. This helps ensure safe conditions inThe first 15 days was a period of growth in gas drainage. During the coal layers and efficient exhis time the working face passed the drilling on February 6. At thevery beginning there was a sound of leaking gas when the working 6. Conclusionsface was still 18 m from the drilling. Negative pressure drainagewas then adopted a total of 2000-4000 m of gas was produced1. The favorable effect of gas drainage can be obtained by extractingwith a daily average production of 336.0 m.A16.to 18-kPagas in the protective layer Surface drilling allows gas extractionnegative pressure was applied to the drain system and then the firstunder the conditions of several coal layers and large gas reserves.maximum in gas evolution appeared. The methane concentrationcontrast with other ways of drainage surface drilling costs lessrose from 36 to 84.8% and the flux rose from 17.6 to 21 m/miand is quicker. The surface drilled extraction system has a longerPure methane production rose from 7.5 to 18 m/min and, finally,toexploitation span and, thus, an obvious economic advantage. Thea production of 257-717m min on February 5. This was theurface drilling contains three structural subdivisions: the surfaceinitial effect of the gas pressure relief effort.oil, the bedrock, and the target sections.The second phase is a stage of pressure relief gas drainage with 2. The observed gas drainage can be divided into three phasesa high methane concentration, and large amount, of drained gas ford of maximum gas production; and an18 days, about 4 months, from February 6 to May 30, or from theattenuation period where gas recovery declines. The maximum16th day to the 133rd day of operation, a maximum in gas drainingoccurred. the total accumulation of methane was 180 150.000 mthanerontton lasted 4 months during which time themethane concentration in the gas drainage was between 60%produced at a rate of 400.,000-550.,000 mper month. The dailyand 90%, From 400,000 to 550,000 m of gas can be producedaverage gas production was at a rate of 10.6 m / min for a methaneproduction of 15, 267 m per day. during this period there were four中国煤化工age drainage rate wasres, the draining frommaxima in gas production. The most productive of these was fromMay 9 to May 19. The largest drainage flow was 32.2 m/min andCNMHGniddle-group coal was335, 200 m of gas was accumulated during this period. A 1-day70.6%. Finally the remaining gas content declined to 5.2 m/tonDongmei et aL/ Mining Science and Technology(China)21 (2011)229-232and the pressure dropped to o53 mPa. This reduced the 2 d high- acient exploit tion of coal an sres uxe relief gas in seng distanaceconditions in the coal layer and highly efficient mining.(31 Yuan L3. The effect of drainage by surface drilling is affected by severalfactors. These include the forward speed of the[4] Yu Qx Protection and control of coal gas. Xuzhou, China: China University ofthrough the protected layer, the periodic breaking(5) State Administration of Work Safety, State Administration of Coal Miningthe main roof, the geological structure of the mine,positions of the drilling and the working face inSateris, fg lose oet y reginasteon. Beijing China: China Coal industrylayer, and the negative pressure employed during16)u SG, ui SC, Lin HF, Cheng LH Technique of drawing relieved methane andainage of coal and coal bed methane.JXi'an Univ Scxu儿Yu趴LoAcknowledgementsThe project was financially supported by the Key Project of the 8 cheng Y. Yu x Application f safe and high -efficient010oNB02)1o0ugm:】hmining area. J Mining Saf Eng 2007: 24(4): 409-13 In Chinese.111) Wang HF. Cheng YP, Shen YT, Liu HB Gas drainage technologou DY, Yu Qx Zhou HX Wang HF. Research on diEng 2008: 25(2) 168-71. In Chinesgh production and efficiency. ]Mining Sacoal seams. Mining Saf (121 Qian MG, Liu TCH. Mining pressure and controlling Beijing. China: China Coal2006:23(1}12-8 In Chinesendustry Publishing House: 1984 In Chines中国煤化工CNMHG