Theories and techniques of coal bed methane control in China

Jourmal of Rock Mechanics and Geotechnical Engineering. 2011, 3(4): 343-351Journal of Rock Mechanics and Geotechnical EngineeringJournalonlinewww.rockgeotech.orgCsRMETheories and techniques of coal bed methane control in ChinaLiang yuanNational Engineering and Research Center for Coal Gas Control, Huainan, 232001, ChinaReceived in 1 March 2011; received in revised form 13 July 2011; accepted 14 September 2011Abstract: Coal bed methane control with low permeability is a hot issue at present. The current status of coal bed methanecontrol in China is introduced. The govermment-support policies on coal bed methane control are presented. This paper proposethe theories of methane control in depressurized mining, including methane extraction in depressurized mining, simultaneousmining technique of coal and methane without coal pillar, and circular overlying zone for high-efficiency methane extraction incoal seams with low permeability. The techniques of methane control and related instruments and equipments in China areintroduced On this basis, the problems related to coal bed methane control are addressed and further studies are pointed out.Key words: coal bed methane control; depressurized mining; low permeability; coal seams; simultaneous mining technique ofcoal and methane without coal pillar; circular overlying zonecoal field investigations, the total reserve of coal1 Introductionresources in China is approximately 5.57x10t, inwhich the predicted reserve of coal resources at depthsof 0-600 m accounts for 26.8%, 600-1 000 m for 20%,China is one of the largest coal producers in the1000-1500mfor25.1%,and15002000mforworld,accounting for 37% of the global coal 28. 1%[3]. Meanwhile, the abundant coal bed methaneproduction. As the major energy in China, coalin China proves that geological reserve of coal bedaccounts for 76% and 69% of primary energymethane is approximately 1.023x10"m, recoverableproduction and consumption, respectively []. Thereserve is about 4.7x10 m. and the coal bed methaneState Energy Long-term Development Plan states that resources within the depth of 2 000 m is 3.68x109China will uphold the energy strategy that coal and ranking the third in the world [4]. In 2009, China'selectricity are used as the main resources; oil, gas and coal production reached 3.05x10%t, accounting fornew energy are developed in all-round way. Ine 45.6%of the global coal production. In 2010, China'sNational Energy Development Strategy 2030-2050 coal production reached 3. 25x10%t, accounting foissued by Chinese Academy of Engineering(CAE) 45% of the global coal production. The amount ofpoints out that the annual demands of coal will be up to methane extraction is 9.1x10%m. and the amount of3.8x10 t( ton), which still accounts for 50% of the methane utilization is 3.610%m, with a death rate perenergy-consuming structure in China. Therefore, the million tons(DRPMT)of 0.725[5, 6]. Most coalpresent status and prospect of energy development and mines in China are rich in methane and more thanthe coal resources sustainable supply dominate the 70% of state-owned major coal mines are methane-richimportant situation and prominent role of the coal or coal and methane outburst-prone mines.Theindustry in economic and social development in China permeability of most coal seams is relatively low,[2]. It is evident that the coal as one of the dominant which is basically less than 1 mD, averagingenergy resources in China cannot be replaced in a long 0.002-16.17 mD. The coal seams with the permeabilityof timeless than 0. 1 mn.for 350 1-1 mD for 37According to prediction results of the 2nd national larger than 1中国煤化工han10mDisscarce. EvenCNMHG PermeabilityDoi;10.3724SPJ235.2011.00343such as mines of Shuicheng, Fengcheng, HuogangCorresponding author. Tel: +86-554-7624616E-mail:xuejunhua2003@163.comKailuan and Liulin, permeability is only 1-18 mD,Liang Yuan/J Rock Mech Geotech Eng 2011, 3 (4): 343-351about two or three orders of magnitude lower than safe situation of coal mines in China is stillthose in the United States and Australia [7, 8]. Coal discouragingBed Methane Drainage Engineering DesignDuring the 1lth Five-yearof the methaneSpecification (GB50471-2008)indicates that the outburst disasters and deathwere graduallymethane in coal seams with permeability less than 1 reduced. Compared with 2006, the methane outburstmD is difficult to be extracted. The geological disasters in 2010 were reduced by 182(accounting forconditions in Huainan mining district are very 55.7%)while the death toll was reduced by 696complicated, roughly characterized by (1)large (accounting for 52.8%). But the particularly severeoverburden depth of 400-1 500 m, (2)8-15 layers of methane outburst disasters occurred time and time, andcoal seams, (3)methane-rich coal seams with methane the severe methane outburst disasters with life losscontent of 12-36 m/t,(4)very soft coal (robustness over 50 cannot be avoided, as shown in Table 1 [9-13].coefficient f=0.2-0.8), (5) low coal seam permeability Therefore, the coal bed methane control is still the keyof 0.001 mD,(6)high gas pressure(up to 6.2 MPa), issue for mining safetyand(7)complex geological structures [7]With further development in coal mining, there is a 3 Key issues and techniques in coaltrend to mine at depth. The coal mining depth in China bed methane controlincreases 10-20 m per year on average, coal bedmethane pressure increases by 0. 1-0.3 MPa per year, 3. 1 Key issuesand methane emission quantity increases by aboutAs the mining depth increases, the methane outburst1.5x10 m per year. As the mining depth increases, the accidents occur frequently. At present, 95% of coalgeological structures are complex or extremely mines in China are underground mines. The averagecomplex. The threat of natural hazards in coal mine mining depth of middle- and large-scale mines is aboutgradually increases. Most key state-owned coal mines 456 m, and the deepest mining depth is 1 365 m. There turned to be methane-rich mines, and treatment of coal production of mines at mining depth over 600 mmine-out coal seams becomes more and more difficult accounts for 28.5% of total coal production at present.[4,7,8The amount of mines with depth larger than 1 000 mAt present, China is one of the countries with the are up to 20. The mining depth is gradually increasedseverest coal and methane outburst disasters in the As the mining intensity increases, the content ofworld. Since the problem was not properly solved for a methane in coal seams increases by 1 m/t per yearlong period of time in the past, the methane outburst Gas pressure and coal-methane outburst incidents indisasters occurred frequently, and the production coal mines also increase. Accordingly, coal bedefficiency was considerably low. Consequently, the methane control becomes more difficultsafe and efficient mining could not be achieved.In order to improve the safety level of coal mines inChina, coal resources integration and massive mergers2 Summary of coaal bed methane and acquisitions in coal mines are carried out. However.disasters in Chinathe mining information of small coal mines is notsufficient, thus the coal resources integration programsAccording to the statistics [9-13], the dRPMt of may bring risks to mining safety.China s coal mines reduced from 2.811 during the 10thThe key techniques and equipments for coal bedFive-year to 0.749 during the 1lth Five-year, and the methane control in deep gassy multi-seam with lowoverall operation was going well. However, due to the permeability are urgently needed. The multi-couplingtechnical and management bottleneck problems, the issues of stress, fracture and gas field during miningTable 1 Summary of methane outburst disasters in the 1lth Five-year [9-13].Methane outburst incidentsLarge methane outburst incidents Severe methane outburst incidentsParticularly severe methaneoutburst incidentDeath tollNumberDeath tollNumberDeath tolll3910841917163中国煤化工157755577CNMHGTotal389728Liang Yuan/J Rock Mech Geotech Eng. 2011, 3(4): 343-351need to be solved.with low saturation, low permeability, low reservoir3.2 Key techniquesWith the technical advance in coal industry the pressure and high deterioration degree. Recently,aseries of key techniques for methane control ofmethane extraction technique in China was quickly different geological conditions were developed throughdeveloped. From the 1980s, methane emission in paneltechnical innovation [7, 8, 14-16faces increased dramatically with introduction of 3.2. 1 Ground boreholeslongwall mining, in particular of longwall top coal 3.2. 1 Ground boreholesTo extract methane in overlying depressurized coalcaving(LTCC) mining. To control the methane seams. the methane extraction boreholes could beemission,integrated methane extraction methods have arranged in depressurized expansion crackbeen useddevelopment area or fully depressurized crackIn the late 1990s, some new methane pre-extrdevelopment area to get a higher gas extraction rate intechniques such as methane extraction in coal seamsa long range. In Huainan mining district, the methanewith 200-500 m long horizontal boreholes and extraction amount of single borehole in depressurizedcontrolled pre-split blasting, were used in zone is up to 22 190 m /d, averaging 14 94] m/d.ThePingdingshan mining district. The methane extraction extracted methane concentration is 95%.and the totalratio was increased by 20%methane extraction is 3x10 m annually. In the miningIn the early 2000s, extensive researches indistrict, the extraction radius of single borehole isprediction, prevention, monitoring and control ofnormally 300 m, and the largest one is 500 m. Thehazards associated with methane and coal dust werecarried out in the Huainan mining district. Various ground borehole can be used to extract methane inand goaftechniques were developed to extract methane fromcoal seams with low permeability. During the 11th 3.2.2 Drilling techniques in roadway.Five-year, Huainan mining district continuedto1) Long boreholes or methane extraction roadwaysconduct extensive researches on safety technology andn ring-shaped crack circle in roof of mined seamsequipment development in deep mining. High-efficientHigh-elevation roadways or boreholes should becoal bed methane exploitation and utilization techniquesrranged in vertically wedge-shaped crack circle inwere developed, and the key techniques and theoriesroof of mined seams, and the opening position offor simultaneous mining of coal and methane withoutboreholes should be arranged in verticallycoal pillar were proposed, where multi-seam withwedge-shaped crack circle formed by depressurizedabundant methane and low permeability was observed. mining. In order to reduce the engineering quantity,theThe coal seams in China are basically featured with pre-arranged boreholes should be drilled as long ascomplex geological structures, low permeability, high possible, normally above 100 m. The number ofmethane content and high outburst risks. 70% of extraction boreholes is 6-8 in a drilling site, and themining districts in China have the same geological optimally negative extraction pressure is 16-20 kPaconditions as those in Huainan mining district, and According to the investigations of methane extractionthese situations indicate that coal seams have to be from 39 panel faces, all the methane extraction ratesdepressurized before methane extraction. Thus, theexceed 50%emphasis of methane extraction should be put on the2)Methane extraction by grid-type upward crossingunderground. Furthermore, the three-dimensional boreholes in floor of depressurized coal seamsextraction pattern from surface to underground shouldIn the rock strata of floor of protected coal seams,be built to realize methane extraction by depressurized the roadways are arranged, and the crossing boreholesminIng.are drilled at proper intervals in the drilling fieldsIn the meantime, there are two major pattems in towards the protected area of outburst-prone coalmethane control according to the difference in seams, as shown in Fig. l. After the mining of lowerpermeability of coal seams in different mining districts. protected coal seams, the stress of protected coal seamOne pattern is coal mining after methane extraction, is depressurized and the permeability is increased. Anamed Jincheng pattern, which is suitable for mining larrge amoun中国煤化工is extracted bydistricts with high permeability. The other patten ispre-arrangedCNMHGsimultaneous mining of coal and methane, named the depress.bility of coalHuainan pattern, which is suitable for mining districts seam is increased by hundreds or thousands of times,Liang Yuan/J Rock Mech Geotech Eng. 2011, 3(4): 343-351Depressurizedtop of goaf by mining-induced cracks in mined seamand depressurized seam. The depressurized andvertically banded crack zones are located in crackInitiallyzones of separated strata above the caving zone of goafdepressurizedroof, as shown in Fig. IaDepressurizedlti(2)Depressurized methane extraction of the upperpart of remote coal seamsAfter mining of the first coal seam, the range ofdepressurized seam in dip direction develops towardsZone of elevatedZone of elevatroof with a height of 130-150 m, and the gastractioncoefficient of permeability of the depressurized coalseam at the upper part of the protected area will bem“入multiplied by thousand times. Then the crossingboreholes for methane extraction, arranged with spacing(a)Drilling boreholes in roadwaysof 20 mx20 m, are drilled towards the depressurizedcoal seayzones of remote coal seams in the retained roadwaysThe extraction boreholes are directly drilled throughthe upper remote coal seams, and the drilling angle isless than mining-induced depressurized angle. For签选Mmgently inclined coal seam, the drilling angle is notcoal seamgreater than 80; and for steeply inclined coal seamthe drilling angle is not greater than 75, usually50°-65°()Depressurized methane extraction in floorAfter mining of the first coal seam, the range ofUnderlyingdepressurized seam in dip direction develops towardsfloor with a depth of 80-100 m, and the gas coefficient(b) Drilling boreholes in retained roadwaysof permeability of the depressurized coal seam at theFig. 1 The simultaneous mining of coal and methane in lower part of the protected area will be multiplied bydepressurized coal seams.hundred times. The downward boreholes for methaneextraction, arranged with spacing of 20 mx20 m, areand the methane flow resistance is decreased, thus a drilled through the lower depressurized coal seam fromlarge amount of depressurized methane is extracted retained roadways to extract the desorbed methaneand the outburst risk is reduced. Due to the long and then the continuous and high-efficiency extractiondistance, the roadways and crossing boreholes will not of high-concentration methane can be realized. Thebe destroyed, and the extraction of depressurized angle of downward boreholes for methane extractionmethane in protected area will further reduce the generally varies from-50 to-80methane concentration. The safe and high-efficiencyBased on the above discussions methane outburstmining can be achieved.disasters of coal mine in China have been effectively3.2.3 Drilling technique in retained roadwayscontrolled in the past few years. The amounts of(1) Methane extraction of goaf in roof of mined methane extraction and utilization are increasedseamssignificantly with a remarkable decrease in DRPMT2-3 extraction boreholes in dip direction are and coal production continues to grow. In 2010arranged at an interval of 20 m in retained roadways, China's coal production reached 3. 25x10%t, and theand the final boreholes are arranged in the amounts of methane extraction and utilizatiodepressurized and vertically banded crack zone at the 8.8x10andtop of goaf. The desorbed free methane in goaf is of 0.725. HowH任邮extracted,which includes desorbed free methane in the control in low-permeadinity coal seams is a hot issuedepressurized and vertically banded crack zones at the Large numbers of experiments on safe mining underLiang Yuan/J Rock Mech Geotech Eng 2011, 3(4): 343-35134such conditions have been conducted in the major coal and the overlap degree of the stress increasing zonesmining countries worldwide, and various effective induced by repeated mining will be controllable for amethane control methods and techniques were smooth mining [2, 3developed. However, it is still difficult to prevent the 4.1 The depressurized extraction technique of coalmethane outburst in coal mines under such conditions. seams with low permeabilityIn recent years, a lot of effective efforts on coal bedTo solve the technical issues in safe mining of coalmethane control have been made in China, especially seams with low permeability and abundant methane,the scientific concept of simultaneous mining the depressurized mining scheme of loosing coal masstechnique of coal and methane without coal pillar, can be considered through improving the traditionalmaking a breakthrough in the methane control top-to-bottom coal mining program, and the methanetechnology. However, there are many types of disastersextraction principle to increase the gas permeability ofdistributed widely in China' s coal mines witlcoal seams by depressurized mining is put forward inunfavorable mining conditions. So it is difficult to this study. The traditionally natural methane drainagerealize the safe mining policy of"methane extractionis changed to concentrative extraction, whichprior to mining "and energy strategy of""simultaneoushighlights the scientific concept of simultaneousmining of coal and methane"in a short period of time. mining of coal and methane. As shown in Fig.2, a largeMeanwhile, the theory and technique of simultaneousquantity of desorbed methane is released in radialmining of coal and methane still need furtherdirection through the depressurized tensile cracksunder negative pressures after the mining of firstimprovementmined coal seam. In the coal seams, the safe andreliable coal seam should be mined first, resulting in4 Methane extraction theory in swelling deformation, looseness and increase in gasdepressurized miningpermeability of the upper and lower coal seams.Meanwhile, the extraction roadways and boreholes arearranged at the roof and floor of depressurized coalMining practices in low-permeability coal mining seam. The theoretical findings are listed as followsareas in China indicate that the key issue of highfficiency methane extraction is how to improve theHigh-elevation extraction Remote depressurizedpermeability of coal seam effectively. The highefficiency methane extraction should meet two basicowelevation extractionconditions, i. e. steady high flow and high methaneconcentration. To achieve the high-efficiency methaneextraction, the extraction boreholes should be arrangedin the areas with high degree of methane desorption,high permeability and high methane concentrationRemote depressurized 3According to strata movement theory,during thecoal seam mining, its roof and floor will fall, move andcrack, resulting in the in-situ stress relief of adjacentborehole in floorseam. Moreover, coal dilatancy, gas permeability, I-caving zone: II-fracture zone; I-curve subsidence zone; A-coal wallmethane desorption and methane extraction capacitypporting-affected zone; B--bed separation zone; C-re-compaction zone;1-fracture zone in upper goaf; 2-vertically fracture-rich zone: 3-bedwill increase, and methane pressure will decline, thus separation-rich zone in remote depressurized coal seamsthe risk of outburst will be reduced. Mining practicesFig 2 Zoning model of mining-induced fracturesalso show that, once rock movement induced by coalmining occurs, the permeability will be increased by (1)According to the laws of methaneseveral hundred times, even for the coal seams with flowing towards the ring-shaped fractured zone, andvery low permeability, which is helpful for methane methane in depressurized zones of roof and floorextraction.Therefore, the high stress in mining area flowing towar中国煤化工the reasonablecan be transferred to surrounding areas through certain positions ofadways weredepressurized measures. The stress in the areas will be determinedCNMHGdecreased, the stress distribution state will be improved, (2)Under the condition of coal mining, the key348Liang Yuan/J Rock Mech Geotech Eng 2011, 3(4): 343-351depressurized coal seam was mined firstly. It increased upward( downward) and high-elevation (low-elevation)the gas permeability of top and bottom coal seams rich boreholes were arranged in the retained roadways toin methane and the coal mining could be achieved after extract the methane from coal seam floor(roof) andmethane extraction. The methane extraction rate goaf, and the methane extraction pipes were buried inreached 65% or more. The methane-rich coal seams the working face to prevent a large amount of methanewere pumped to a low-methane state to adopt safe and emission in goaf By using retained roadways insteadhigh-efficiency mining, coming up to the international of excavating various roadways to extract methane, theadvanced level in generalnumbers of roadways and drilling projects can be()To efficiently solve the problem that the substantially reduced, and the safe and high-efficiencyaffected the safe mining, the methane extraction achieved, as shown in Fig, al and methane will bemethane-rich coal seam in the first mined coal seam simultaneous mining of cotechnique was developed to drain the methane in theroof of methane rich working face. It made the dailyproduction of coal in fully mechanized coal face bimproved from 3 000 to 10 000 t.(4)To reduce the methane outburst and avoidover-limit methane concentration during tunneling,new pattern of drilling in methane extraction roadways,e. methane extraction in heading face and regionalmethane extraction, was established. The monthlydrilling footage in methane-rich coal seams was raised1-haulage roadway: 2--material roadway: 3--backfill body: workingfrom 70 to 150 mface: 5-methane extraction pipe; 6-Iow-elevation methane extraction()To achieve the methane extraction of surfaceboreholes and reduce the quantity of underground Fig-3 Sketch of simultaneous mining technique of coal andmethane without coal pillar.methane projects effectively, a three-dimensionalmethane extraction pattern from surface to The research findings have been used in more thanunderground was proposed. The field tests were 40 mining districts, nearly 200 mines, such as Huainan,successfully conducted in more than a hundred Huaibei, Tiefa, Songzao, Jincheng, and Fenxi,working faces in Huainan, Jincheng, Xishan, Shaqu accounting for 60% of the methane-rich coal mines inand huaibei mining districtsChina, The safe and high-efficiency mining of low-4.2 Simultaneous mining technique of coal and permeability and methane-rich coal mines is achievedmethane without coal pillarThe technical issues of simultaneous mining of coalAlthough the methane extraction technique in and methane are adequately solved under complexdepressurized mining is successful, there still exist geological conditions in deep coal mines withsome shortcomings such as the large number of abundant methane, low permeability and high in-situmethane extraction roadways and boreholes. For this stress, and the economic benefit and safety effect arereason, Huainan mining district proposed and very significant. The research findings are summarizedimplemented the simultaneous mining technique of as followscoal and methane without coal pillar in 2004(1) The simultaneous mining technique of coal andaccording to the oces of coal seams, the key methane without coal pillar is set up for coal seamsdepressurized coal seam was mined firstly, and then with low permeability and abundant methane.the non-pillar continuous mining was implemented Non-pillar continuous mining along retained roadwaysalong the goaf edge. The mining roadways were with Y-shaped ventilation is achieved and the methaneretained to replace the methane extraction roadways in accumulation in the comer is prevented. The extractedroof and floor through building a high-strength support methane concentration is as high as 60%100%, andbody by rapid mechanization, changing the traditional the methane中国煤化工ressurized coaU-shaped ventilation to Y-shaped ventilation. Of seams reacheized by a longcourse, the retained roadway and the fully mechanized period of timCNMHGextractioncoal face should be advanced simultaneously. The(2)The number of roadways is reduced by 2-3, andLiang Yuan/J Rock Mech Geotech Eng 2011, 3(4): 343-351the temperature in working face is lowered. When the with a certain width still retain a higher depressurizedmining without coal pillar is implemented, a great degree, so the mining-induced fractures are notamount of coal resources is saveddisturbed and the permeability of the middle zone is4.3 The circular overlying zone theory for high- the same as that of the front zone. Both zones connectefficiency methane extraction in coal seamswell with each otherAccording to the simultaneous mining characters of (3)due to the deformation characteristics ofcoal and methane, the high-efficiency methane overlying strata, coal rocks near the open-off cuts stillextraction should be conducted in the areas with high remain a certain depressurized degree and a highermethane desorption, high horizontal permeability and permeability during mining process, and can connecthigh methane concentration. Accordingly, based on the with the front zone through the middle zone but themodel of pressure-relief of surrounding rocks, the whole depressurized degree and permeability of thepermeability distribution, the dynamic migration of back zone are less than those of the front zoneextracted methane and the basically geometricalIt can be seen from Fig 4 that the circular overlyingfeatures of the high-efficiency methane extraction zone with a certain thickness is distributed at a certainrange can be plotted in Fig4. It is of a circular level above goaf, which is determined by requirementsstructure in shape, overlying the goaf with certain of the high-flow and high-concentration methanewidth, and extending upward at a certainextraction. The methane of overlying coal seamscan be called as circular overlying zone for hiefficiency methane extraction [16]flowing horizontally in high-permeability zone shouldbe extracted efficiently. It is easy to drain the stopeBased on the differences in structural characteristicsof circular overlying zone in the strike direction, theout when extracting methane beneath the mining area,circular overlying zone can be divided into three partsmaking the methane concentration do. e. front zone, middle zone and back zone, described permeability of coal rocks is low, and the velocity andas followsflow of methane desorption are slow. Thus the(1)The front zone, in the rear of working face extraction flow quantity is small when extractingwithin a certain distance, is located in effectively methane at a certain level above the mining area. Thedepressurized range, in which coal mass has high circular overlying zone extends upwards at a certainpermeability, thus the adsorbed methane can be fast angle. The extermal extension angles at two sides ofdesorbed and gushed out. The front zone is the main front zone are the similar(section I-I). Besides thearea for desorption and emission of methaneexternal extension angle, it also has two intemal( 2)As the overlying strata sink gradually, the extension angles, which are totally different from theoverburden pressure in the middle of mining area with extermal ones and influenced by the compactiona certain width recovers gradually to the initial characteristics of the mining area( section II-lm)pressure, making the mining- induced fractures beDue to the large area with highre-compacted and permeability be decreased the mining area, the circular overlying zone issignificantly. However, both sides of the mining area distributed at a certain level above goaf. The extensionLength of working faceFront zonege roadwayGoaf levelBack zoneTrack roadway中国煤化工Goaf levelCNMHGFig 4 Circular overlying zone for high-efficiency methane extractionLiang Yuan/J Rock Mech Geotech Eng 2011, 3(4): 343-351pattern and mechanism in strike direction are the same mining variation characteristics should be studiedas those in dip direction. It extends upward at a certain according to different geological and operativeangle, and the external and internal extension angles conditions of coal rocks containing methane which isare different in strike direction(section III-III). In helpful for understanding the evolution mechanism ofterms of deformation time of overlying rocks, the mining-induced stress and fracture fields of first minedexternal and internal extension angles in the back zone, coal seam, and for constructing the hydro-mechanicawhich are different from those in the front zone, are coupling and methane flow models of coal rocks undersimilar to those in the middle zonemining. Finally, the theoretical system of simultaneousAccording to the structural characteristics of the mining of coal and methane can be improved andcircular overlying zone, its range should be determined developed.by the following geometrical parameters shown in 5.2 Relationship between geological structure andFig 4: (1)the heights of upper and lower bounds of the outburst of coal and methanecircular overlying zone, hm and hmin ;(2)theThere is a close relationship between geologicalprojection length I of the circular overlying zone in structure and outburst of coal and methane. Thestrike direction on the goaf level, and the extension outburst accidents causing injuries and deaths areangles a, a, and their directions; (3)the projection basically concerned with faults and folds. In deepwidth w of the circular overlying zone in dip direction mining, it is clear that none-outburst coal seam willon the goaf level, and the extension angles B, B, and turn into outburst-prone coal seam gradually. There aretheir directions; and (4)the symmetrical characteristics no regular and conclusive cognitions about theof both sides of the circular overlying zone.elationship between geological structure and outburstThe theory of the circular overlying zone is applied of coal and methane. The geological investigation isto working face 1115(1)of Guqiao mine in Huainan limited, and small structure detection is still inmining district, Anhui Province, China. It clearly exploration stage due to the lack of effectivereflects the high-efficiency extraction range of techniques and equipments. The outburst regularityhigh-flow and high-concentration methane [16]. It and sensitive index should be further studiedprovides important significance for the simultaneous 5.3 Prediction of methane contentmining of coal and methane and the controlThe methane content of coal seam is a veryprevention and utilization of methaneimportant index for predicting the methane emissionThe high-efficiency extraction of depressurized quantity and outburst of coal and methane. Since themethane can ensure the safety of the first mined coal outburst mechanism of strong outburst-prone coalseam, reduce the outburst risks of nearby coal seams seams in deep mining is significantly different fromand convert them into low-methane coal seams, which that in shallow mining, the new outburst risk predictioncan guarantee the mining safety of adjacent coal seams technique such as prediction methods, sensitive indicesnd realize the simultaneous mining of coal and and critical values should be studied. Meanwhile, withmethane. Therefore the simultaneous mining of coal the development of coal mining mechanization leveland methane is an important technical way for further and increase in mining intensity in China, coal minesexploitation of deep coal resources in China.with high-efficiency and intensive mining activities areincreasing gradually, and the previous outburst5 Prospect of coal bed methaneprediction methods cannot satisfy the requirements ofcontrol techniques in Chinahigh-efficiency mining at present. Therefore, newoutburst prediction methods and techniques should bedeveloped5.1 Theoretical development5.4 Drilling machines and techniquesThe geological conditions of coal fields in China areSoft coal seams are of large proportion in the totalvery complicated because of several times of strong coal seams in China, and drilling in soft andgeological transformations. Therefore, the tectonic outburst-prone coal seams is a challenging problemfields, evolution of stress and fracture fields in mining (1)The key drilling techniques and advancedprocess, methane occurrence and methane flow fields drilling equip中国煤化工 d, among whichare still not clear at present, and the researches onthe long- lifeCNMH Gasuring systemmulti-field coupling mechanism is few. Therefore,etc. are veryIL. eanwnmle. the crawlerstructural features dynamic mechanical behaviorsdrilling machine with automatic walk ability should beLiang Yuan/J Rock Mech Geotech Eng. 2011, 3(4): 343-351developed urgently to solve the problem of frequent(4)The theory of circular overlying zone for optimalmovement of drilling machines and to improve work methane extraction can reflect the distribution andefficiency.evolution rules of stress fracture and methane fields in(2) The key techniques of methane extraction and overlying rock strata, which provides a scientific anddrilling equipments suitable for soft and outburst-prone theoretical basis for methane controL.coal seams should be developed(5) The technology of coal and methane outburst(3)The safe, reliable and effective techniques of prediction by methane content method was presented,remote outburst prediction and high-performance and the development direction of coal bed methaneremote control system should be developedcontrol was put forward5.5 Methane extraction techniquesIn deep mines, as the methane emission quantityReferencesstrong outburst-prone coal seams is getting larger andlarger, the outburst risk increases gradually. Therefore,[1] National Development and Reform Commission. The IIth Five YPlanofCoalinDustry.http://www.china.com.cn/policy/txt/2007-0technical innovation should be carried out to achieve22/content 9252617htm(in Chinese).safe and high-efficiency mining in China[2] Wang Xianzheng Coal mine safety and sustainable development of5.6 Monitoring of methane extractioncoalindustryhttp://www.china.com.cn/market/nengyuan/438599.htmThe methane monitoring system has been installedin all major state-owned coal mines. But the [3] China Coal Price. China coal resources distribution characteristics.http://www.coalprice.cn/html/2009-07-22/2009072200671597.htmlinvestment and construction levels differ considerably.In the near future, the digital, automatic and intelligent(4 China Coal Resources, The present situation of CBM industry inmine monitoring system should be fully used to realizeChinahttp://www.sxcoalcom/gas/725750/articlenew.html(inChinese).5data sharing.http://xmece.xmsme.gov.cn/2011-1/20111783916.htm(incHinese).5.7 Methane utilization[6] Guo Lifang. The extraction coal gas amount in last year is 8.8 billionAs a kind of clean energy, coal bed methane shouldm. China Energy Newspaper, 2011(in Chinese).be developed and utilized. However, due to the lack ofof integrated pillarless coalproduction and methane extraction in multi-seam of low permeability.safety management system and standards for qualityBeijing: China Coal Industry Publishing House, 2008( in Chinesecontrol, methane utilization rates in different mining [8] Yuan Liang. Theory and technology of gas drainage and capture indistricts are diverse. In some mining districts, thesoft multiple coal seams of low permeability. Beijing: China CoalIndustry Publishing House, 2004 (in Chinese)methane utilization rate is below 30%. Furthermore, 9 State Administration of Coal Mine Safety. The 2006 national coalsometimes the low-concentration methane ismine accident analysis report. Beijing: State Administration of Coaldischarged directly into atmosphere. It is a challengingMine Safety. 2007(in Chinese).problem.[10] State Administration of Coal Mine Safety. The 2007 national coalmine accident analysis repor Beijing: State Administration of CoalMine Safety. 2008(in Chinese).6 Conclusions[11] State Administration of Coal Mine Safety. The 2008 national coalmine accident analysis report. Beijing: State Administration of Coal(1)The simultaneous mining of coal and methane isMine Safety. 2009(in Chinesean effective way for coal bed methane control, which is [12] State Administration of Coal Mine Safety. The 2009 national coalthe most important mining method in China.mine accident analysis report. Beijing: State Administration of CoalMine Safety. 2010(in Chinese)(2)Coal must be mined after methane extraction[13] State Administration of Coal Mine Safety. The 2010 national coalunder complicated conditions. The ground coal bedmine accident analysis report. Beijing: State Administration of Coalmethane extraction should be carried out in coalMine Safety. 2011(in Chinese).mining districts with high permeability, while the [14] Yuan Liang. The technique of coal mining and gas extraction bysimultaneous mining of coal and methane should beroadway retaining and borehole drilling. Journal of China CoalSociety, 2008, 33 (8): 898-902(in Chinese).fully used in coal mining districts with low[15] Yuan Liang. Theory of pressure relieved gas extraction andsystem of integrated coal production and gas extractionof(3)Management is also a key factor for coal bedChina Coal Society, 2009, 34(1): 5-12(in Chinese).methane control. The techniques of depressurized [16 Yuan Liang, Guo Hua, Shen Baotang, Qu Qingdong. Xue Jumhuamining and the simultaneous miningCircular ove中国煤化工 efficient methanemethane should be developed to achieve safe miningcapture ofCNMHGrability. Journal ofof methane-rich coal seamsChina Coal SocieTy, zull, 36(3): 33/-30(n Lnnese