Characteristics of fracture development and gas extraction of a lower protected seam

Journal of Chongqing University(English Edition)[ISSN 1671-82241December 2009Article ID:16718224(2009)0402390Tocte this artic: WANG Hai-feng, CHENG Yuan-ping, WANG Liang, YUAN Liang. Characteristics of fracture development and gas extraction of lower protected seam J]J Chongqing Univ: Eng Ed [ssN 1671-82241, 2009, 8(4 239-244Characteristics of fracture development and gas extraction ofa lower protected seamWANG Hai-feng., CHENG Yuan-ping, WANG Liang1, YUAN LiangNational Engineering Research Center for Coal Gas control, China University of Mining Technology, Xuzhou 221008, Jiangsu, P. R. China2 Postdoctoral Science Research Workstation, Huainan Mining industry( Group)Ltd, Huainan 232046, Anhui P.R. ChinaReceived 17 September 2009: received in revised form 19 October 2009Abstract: To ensure the mining safety of working face in the protective seam and meanwhile extract pressure relief gas of thelower protected seam and eliminate its outburst risk, the present study researched into fracture development of floor coal-rockmass of the protective seam and migration rule of pressure-relief gas from a protected seam so as to obtain an effective pressure-relief gas extraction method. The results show that after the upper protective seam was mined, mining-induced fracturing floorcoal-rock mass fell into two zones: a floor heave fracture zone and a floor heave deformation zone. Lower protected seams locatedat different fracture subzones were different in the degree and characteristics of fracture development, and penetration boreholedrilling from the floor roadway effectively extracted pressure-relief gas. The engineering practice of the Hongling coal mine inShenyang indicated that for a lower protected seam in the floor heave fracture zone, taking the one time seam spacing(16 m)asthe spacing between penetration boreholes ensured the mining safety of the No 321 working face in the protective seam andcompletely eliminated the outburst risk of the lower protected seam, the No 12 coal seamKeywords: short distance protective seam; gas emission; penetration borehole; gas extractionCLC number: TD713Document code: A1 Introductioncontrol experience of many years [(4-8]. The techniqueuses two types of coal seams, protective seams andwith the increase of mining depth in coalmines, gaprotected seams, a protective seam is the one firstlyressure increases greatly, gas content becomes higher, exploited for eliminating the outburst risk of itsand coal and gas outburst disasters in exploiting seams adjacent coal seams; and protected seams refers toare more serious. China has already been the country adjacent coal seams of outburst risk [9-10suffering the most serious coal and gInterlayer spacing between protective and protecteddisasters in the world [1-3]. The technique of protective seams has great influences on the pressure-rreliet anseam mining is considered as the most effective protecting effects for protected seams [11-13]. Changeprevention method against gas outburst based on gas of the spacing makes distinct difference in the fracturedevelopment of a protected seam, and in pressure-reliefgas flow and gas extraction effect. For upper protectedT WANG Hai-feng(王海锋): baizhizhuang@163cmseam mining, the smaller the interlayer spacing is, theTe:+86-1391347411better protecting effect on the lower protected seamFunded by the Major State Basic Research Developmentwill be. However, a smaller interlayer spacing bringsProgram of China(No, 2005CB221503)and the Key Program ofmore threat of pressure-relief gas emission from athe Natural Science Foundation of China(No. 70533050lower protected seam, which will cause potential safety50904068and50674089)hazard for the working face of a protective seam. In中国煤化工 characteristics ofCNMHGH. F WANG, et alFracture development and gas extractionfracture development and the migration rule of and oblique beddings with rock collapsing. The lowerpressure-relief gas in lower protected seams, and limit of floor heave deformation zone is 50 m to 60 mdetermined reasonable technical parameters of gas under the floor. Fractures in this zone mainly areextraction to ensure the mining safety of working face bedding ones, and penetration fractures developand eliminate coal and gas outburst risk of protected incompletely. The position of a lower protected seamseams under the condition of short distance protective determines the degree and characteristics of theseam exploitationfracture development in the seam2 Characteristics of fracture development in 3 Migration rule of pressure- relief gas in thefloor coal-rock mass after mining of upperlower protected seamprotective seamThe degree and characteristics of fractureAfter the upper protective seam is mined, movement development in rocks of the floor and the lowerand deformation occur in the floor coal-rock mass, protected seam decide the migration rule of pressure-nd pressure decreases and fractures develop [relief gas in the lower protected seam. When the lower14]. With the increase of depth to the floor of the gob, protected seam is in the floor heave fracture zonefracture development and pressure-relief effect become fractures develop sufficiently, absorbing gas desorbedweaker and weaker. Mining-induced fracturing floor from the protected seam and thus producing a pressurecoal-rock mass falls into many categories of fracture relief flow. Penetration fractures between seamssubzones. Based on similarity simulation test and field connect the lower protected seam with the workingtest,we divided the mining-induced fracturing floor face of the upper protective seam. The protected seamcoal-rock mass into two zones: a floor heave fracture is usually the outburst coal seam with high gas pressurezone and a floor heave deformation zone, as shown in and high gas content. Because of this gas pressure andFig. 1.the ventilation negative pressure in the mining tunnelCompared with the fracture development in roof there lies a risk of pressure-relief gas from thecoal-rock mass, that in floor coal-rock mass is less protected seam to flow along penetration fractures toinfluenced by the mining height of the protective seam. the working face of a protected seam, increasing safetyUnder general geological conditions, the lower limit of hazard in the working face of the protective seam.floor heave fracture zone is 15 m to 25 m under the Using penetration boreholes effectively avoidsfloor, and the mining-induced influence of protective pressure-relief gas swarming into the working face ofseam is large on the zone. In this zone, fractures fully the protective seam, improves mining safety, reducesdevelop, which mainly are bedding fractures formed by pressure-relief gas content of the protected seam andseparation and penetration fractures formed by vertical eliminates its outburst risk [11, 15].Protective seamFloor heave fracture zoneProtected scamFloor heave deformation zoneFig. 1 Fracture distribution and zonation of underlying strata中国煤化工CNMHGJ Chongqing Univ. Eng. Ed [ISSN 1671-82241. 2009, 8(4): 239-244yang, et aL.Fracture development and gas extractionA lower protected seam in the floor heave working face of No. 1l coal seam was chosen as thedeformation zone also relieves pressure and increases initial mining face in the experiment, which was 16 mpermeability. However, the interlayer spacing is too from the No 12 coal seam of protected seam and waslarge, it limits the movement and deformation of rocks, called a short distance upper protective seam. Themaking fractures in the spacing and lower protected strike of No. 321 working face was designed as 300 mseam mainly bedding type that absorb gas desorbed long and the trend was 220 m. The mining thicknessfrom the protected seam. A large amount of pressure- was 1.4 m, through 1.0 m thick roof-floor rock and arelief gas accumulates in the bedding fractures of coal 0. 4 m thick coal seam.seams. The gas inside the seams can not exhaust andhas to be drawn out. When the pressure-relief gas in the 4.2 Analysis of gas emission and overrun in No. 321lower protected seam is extracted through penetrationworking faceboreholes, it flows from bedding fractures to theboreholes, and the goal of eliminating the outburst riskThe interlayer spacing between the No 1l coal seamin coal seams can be reachedand the No. 12 coal seam was small. The No. 12 coalseam was in the floor heave fracture zone. fractures4 Engineering applicationbetween seams developed sufficiently, and penetrationfractures connected the No 12 coal seam and the4.1 ExperimeNo321 working face together. By theoretical analysis,the gas drainage rate of the lower protected seamHongLing Coalmine, located in the area of Sujiatun reached 65% without gas extraction. with a 22. 5m/in Shenyang, P. R. China is a outburst-prone coal mine, original gas content in the No 12 coal seam, a largewith coal output of 2. 1 Mt. The No. 7 and No 12 coal amount of pressure- relief gas would swarm into theseams are the primary coal seams with high gas No 321 working face during protective seam mining,aspressure, high content and low permeability. They both shown in Fig. 2. To avoid gas emission, reduce the gasare coal seams of high outburst risk. Their dip angles content and eliminate outburst risk of the No. 12 coalare between 4 and 45, with some deep parts close to seam, we adopted penetration boreholes to extract0. Based on thorough investigation, the protective pressure-relief gas of the protected seam. Nevertheless,seam mining technique has been applied with the insufficient number, improper position, and weakextremely thin No 1 l coal seam as the upper protective extraction ability of boreholes would also cause gasseam of the No. 12 coal seam whose thickness is 4.0 m, emission, even gas overrun, and jeopardize the mininggas pressure is 6.5 MPa, and gas content is 22.5 m/t.According to production distribution, the No 321Protective seamFig 2 Schem中国煤化工CNMHGJ Chongqing Univ. Eng. Ed [sSN 1671-8224] 2009, 8(4): 239-244H. F WANG, et al.Fracture development and gas extractionDuring the initial mining time of No 321 working penetration boreholes were drilled uniformly from theface, we drilled penetration boreholes for extracting gasfloor roadway to the protected seam, with a netlikeof the No 12 coal seam from the waist roadway as distribution in the protected seam. Two floor rockshown in Fig. 3. Due to improper disposal of boreholes, roadways had to be constructed to support penetrationthe insufficient number of boreholes and the limited boreholes seeing that the trend of No 321 working faceextraction ability, a large amount of pressure-relief gas was too long, as shown in Fig. 3.of protected seam swarmed into the mining face, gasoverrun occurred in the No 321 working face, and the 4.4 Effect analysisgas concentration in the upper comer reached 1. 5%,which brought extreme risks to miningThe production safety was greatly improved with theplication of the amended4.3 Scheme improvement and parameter design of Fig 4 shows the variation curves of gas concentrationand gas extraction. The amount of gas extractedthrough penetration boreholes was more than 8 m/minThe gas control in the initial mining time as in average, and 15.8 m/min maximally. with effectivedescribed above failed to satisfy the safe mining extraction through penetration boreholes, the gasdemand. An improved scheme was developed by using concentration in the upper comer of the working facenet-like upward penetration boreholes at floor roadway decreased from 1. 5%(at the initial mining time)towith the spacing of penetration boreholes carefully below 0. 7%, that in the return roadway reduced fromdesigned0.6% to below 0.4%. The air displacement of gas inThe No 12 coal seam of protected seam was in the working face reduced from 6 m /min to 3 m/min, andfloor heave fracture zone. Pressure-relief gas herein the ratio of air displacement to the total gas amounttended to flow into penetration boreholes along reduced from 80% to 30%. All above shows that thebedding fractures, and part of it swarmed into the gob outburst risk of protected seams was eliminated.of the protective seam along penetration fracturesThe relative swelling deformation of the protectedunder the action of gas pressure. Frictional resistance seam reached 7. 2%0, the permeability coefficient wasformed by gas flow in fractures controlled the flow increased by 1 010 times, the gas extraction ratedirection of pressure-relief gas. To reduce frictional reached 77.5%, the remaining gas content was reducedresistance, the spacing between penetration boreholes to 5.06 m/t, and the remaining gas pressure waad to be shortened Based on the results of theoretical reduced to 0.25 MPa. the outburst risk of No 12 coalanalysis and field investigation, we took the one time seam was eliminated, which provided the condition ofseam spacing as the penetration-borehole spacing in the safety and high efficiency mining for the No 12 coalfloor heave fracture zone which was 16 m. TheseamNo lI coal seamRetum roadwayWaist roadwayFloor roadwayIntake airflow roadwayFig 3 Arrangement of net-like upward penetrating boreholes from floor roadway in Hongling mine of Shenyang mining area.中国煤化工CNMHGJ Chongqing Univ. Eng. Ed [ISSN 1671-82241. 2009, 8(4): 239-244H. F. wang et alFracture development and gas extractionGas extraction from air ventilationGas concentration in the upper comer 1.2Gas concentration in the return roadway0.8妨惜一00.2Distance along the length of working face/mFig. 4 Variation curves of gas concentration(in terms of volume fraction) and extraction amount5 Conclusionsextraction technology in coal mines of China [Joumal of Mining Safety Engineering, 2009, 26(2)1)After the upper protective seam is mined, mining-127-139.(In Chinese)duced fracturing floor coal-rock mass is divided into程远平,付建华俞启香中国煤矿瓦斯抽采技术的发two zones: a floor heave fracture zone and a floor展.矿与安全工程学报2009,26(2):127-139heave deformation zone. Lower protected seams [2] Cheng YP, Yu QX. Development of regional gaslocated at different fracture subzones are different incontrol technology for Chinese coalmines [J]. Joumalthe degree and characteristics of fracture development.of Mining Safety Engineering, 2007, 24(4): 383-392)The method of penetration borehole drilling from(In Chinese)floor roadway should be employed to extract pressure-程远平,俞启香中国煤矿区域性瓦斯治理技术的发relief gas during mining a protective seam.展门.矿与安全工程学报200724(4):383390.3)The engineering practice of Hongling coal mine in [3] Lu TK, Yu H, Zhou TY, et al, Improvement ofShenyang indicated that for a lower protected seam inmethane drainage in high gassy coal seam usingthe floor heave fracture zone, taking the one time seamwaterjet technique [] International Journal of Coalspacing(16 m)as the spacing between penetrationGeology,2009791-2):40-48boreholes ensured the mining safety of the No 321 [4] Cheng YP, Yu QX. Application of safe and highworking face in the protective seam and completelyefficient exploitation system of coal and gas in coaleliminated the outburst risk of the lower protected seamseams ] Journal of China University of Miningthe No, 12 coal seamTechnology, 2003, 32(5): 471-475(in Chinese)程远平,俞启香煤层群煤与瓦斯安全高效共采体系Acknowledgements及应用门J中国矿业大学学报2003,32(5):471-475.[5] Hu GZ, Wang HT, Fan XG, et al. Gas pressureThe authors are grateful to the Major State BasicResearch Development Program of China and the Keylayer of pitching oblique mining J]. Jourmal of ChinaProgram of the National Natural Science Foundation ofUniversity of Mining and Technology, 2008, 37(3)China for financial support328-332.(in Chinese)胡国忠,王宏图,范晓刚,等俯伪斜上保护层保护范围References的瓦斯压力研究[中国矿业大学学报,200837(3)328-3321] Cheng YP, Fu JH, Yu QX. Development of gas [6中国煤化工Mine Safety. SafetyCNMHGJ Chongqing Univ. Eng. Ed. [ISSN 1671-82241, 2009, 8(4): 239-244H. 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