Effect of magma intrusion on the occurrence of coal gas in the Wolonghu coalfield

Mining Science and Technology( china)21(2011)737-741Contents lists available at SciVerse Science DirectMining Science and Technology( China)ELSEVIERjournalhomepagewww.elsevier.com/locate/mstcEffect of magma intrusion on the occurrence of coal gas in the Wolonghu coalfieldJiang Jingyu", Cheng Yuanping, Wang Lei, An Fenghua, Jiang HainaNational Engineering Research Center for Coal 8 Gas Control, China University of Mining G Technology, Xuzhou 221008, ChinaARTICLE INFOABSTRACTReceived iChemical analysis, methane isothermal adsorption studies, and mercury porosimetry were performed on4 January 2011ten samples taken from the magma intrusion boundary in the wolonghu coalfield. the physico-chemicaln revised form 11 February 2011Accepted 2 March 2011properties of coals from the magma intrusion region are compared to those from the normal regions. TheAvailable online 9 November 2011results show that the volatile content( Vad). the limiting adsorption constant(a), and the initial methanediffusion rate of samples from the magma intrusion region are generally smaller than those values fromsamples from the normal region. The number three coal sample from the magma intrusion region has aKeywords:large vitrinite reflectance, well developed macropores, a small surface area, and weak methane adsorp-ion capacity the number ten coal sample from the normal region has a small vitrinite reflectance, wellMaceraldeveloped micropores, a large surface area, and a strong methane adsorption capacity. The maceral of theVitrinite reflectancecoal samples from the magma intrusion region and the normal region are similar. The coal in the areaSurface areanear the magma intrusion boundary is rich in methane and is an area where coal and gas outbursts oftenoccure 2011 Published by Elsevier B V on behalf of China University of Mining Technology.1 Introductionwidely used in China [14. pores having laminar gas flow can beseen as visible pores and fissures Seepage pores for the gas canThe occurrence of coal and gas outburst is due to the combined be considered intermediate to laminar flow and absorption: theyeffects of methane, ground pressure, and the structure of the coal. are both macropore and mesopore in size. Adsorption pores areGenerally, outburst occurs at the soft layer of the coal seam, which small pinhole and micropores.is the geological structure where stress concentration occurs at theThe Wolonghu coalfield is used herein as a source of field datamining face and is the dominant area for outbursts [1]. Many of the Annalysis of multiple physical parameters existing in magmanumerous coal and gas outburst accidents in China happenedintrusion and normal regions is done and a discussion of how mag-because of magma intrusion into the coal mines. Examples include ma intrusion influences the coal, and gas occurrence, is presentedthe Anlin Coal Mine in Anyang, the Daxing Coal Mine in Tiefa, theHaizi Coal Mine in Huaibei, and the Wolonghu Coal Mine in 2. Background coalfield geologyWanbei. Considerable research has been done on magma intrusiono coal seams 2-102.1. Coalfield characteristicsPhysical properties, such as coal structure or the metamorphismof the coal in the intrusion region, differ because of different space-The huaibei coalfield includes the three mineral fields of Suxiaotime evolution processes for the igneous rock in the different min- Suxian, and Linhuan. Wolonghu coalfield is located at the south-eral fields. this leads to different amounts of gas in different mines. west of the suxiao mineral field within the counties of Tiefo andYang and Tang pointed out that thermal metamorphism of the re- Qiuji, in Suixi, Anhui province. The stratum layering observed bygional magma helps increase gas content and coal seam permeabil- drilling boreholes shows the carboniferous Taiyuan formation, theity [11 They did preliminary research on the mechanism of Shanxi formation, the Xiashihezi formation, the Shangshihezi forthermal metamorphism. Studying gas occurrence and flows in a mation, and the Permian, tertiary, and quaternary Shiganfeng for-coal seam located within the magma intrusion region requires clas- mation. The Carboniferous and permian layers are the main coalsification of the pore structure as it relates to the coal porosity. bearing strata. The mineable coal seams are the numbers 6, 7, 8,generalized an appropriate, representative scheme [12, 131. The the working face 103 in the number 10 coal seam is 4.5 Mpa e atNumerous classification schemes exist and Qin and Zhang have and 10 coal seams. The actual measured maximum pressdecimal classification scheme of Khodot is presently the most2.2. Regional magmationorresponding author. Tel89521664Magma moved vigorously during the Yanshan periodmail address: jiangjngyu(205-135 Ma)in several cynd in alternate directions [11].A中国煤化工1674-5264/s- see front matter o 2011 Published by Elsevier B V. on behalf of China University of Mining& Tedoi:10.l016/mstc20110002CNMHG38J Jiang et aL /Mining Science and Technology ( China)21(2011)737-741using an HCA(gas adsorption set and a high-capacity method ) pro-duced by the Chongqing Coal Science Research Institute. The initialmethane diffusion rate was obtained from a Wr-I rate tester pro-duced by the Fushun Coal Science Research Institute. Porosimetrywas performed with an automatic mercury porosimeter producedMagma intrusion boundaryin the United States. vitrinite reflectance tests and the quantitative∴, ocation and number of coal samanalysis of the maceral were performed with a microscopic pho-tometer produced in Germany by Zeiss.Intake airflow roadways of 02 face4. Results and discussionRetum airway of 102 face 0 15 30 m4.1. Proximate analysis and gas adsorption isothermsThe test results in Table 1 show that the volatile content of coalsamples from the magma intrusion region is typically less than thevolatile content of samples from the normal region. The minimumFig. 1. Magma intrusion and sampling locations in the Wolonghu mineVad, associated with the number two sample, is 4.2 while themaximum Vad, sample number nine, is 8.85. Magma intrusion intoseries of NNE fold structures, and a faulted structure dominated by the coal seam would bring about contact metamorphism and in-compression, were formed in the Yanshan period, at Huaibei coalcrease the coal rank. The limiting adsorption constant and the ini-field, accompanied by massive magma intrusion. The coal structure tial methane diffusion rate of the coal samples subjected to magmawas destroyed by active tectogenesis and tectonic coals were subintrusion are generally less than the values from the normal samsequently formed [15,16). Research indicates that the hanging wall ples. The value of a for the number two(MIR)coal sample isand the foot wall suffered magma intrusion through the EW Subei 16. m/t while the number eight sample(normal region)hasfault during the Yanshan period. At this time magma invaded the an a of 59.0229 m /t. The value of ap for the number two(MIR)nhuan coalfield in the south and the Suixiao coalfield in the sample is 1.3 mmHg while the number eight coal sample has anorth. The southern part of Wolonghu coalfield adjoins the Ew Ap of 40 mmHg. Fig. 2 shows the volatile content, the initial gasSubei fault. Regional geological information shows that coal was diffusion rate, and the limiting adsorption constant of the ten coalabundantly distributed in the south but thinly distributed in the samplesThe elevation of the intrusion in the number ten coal seams is atnorth. All coal seams were invaded by magma in the south but only _506 m: above this -506 m elevation is the magma intrusionweak magma intrusion is seen in the north. these facts indicatethat magma intruded the Wolonghu coalfield from the south to region and below it is the normal, unaffected region. The curvesthe north through the Subei fault.in Fig. 2 show that Vad, the adsorption constant, and the initialmethane diffusion rate are all generally smaller for samples from3. Coal samples and experimental methodsthe magma intrusion region than they are for samples from thenormal region. As the burial depth increases and the region movefrom the intrusion to the normal region the values of these threeThe experimental samples were taken from the 102 working parameters increase. The trend of the three curves shows consisface nearby a magma intrusion boundary in the Wolonghu coal- tent changes between samplesfield. A sequence of ten coal samples was collected along theintake roadway of face 102: five of these came from the magma 4.2. Porosity of the coalsintrusion region(MIR) and the other five came from a normalregion. Fig. 1 shows the sampling locations. Proximate analysisThe mercury injection method was used to obtain the porend gas adsorption constants are shown in Table 1tructures shown in Figs. 3 and 4. Fig. 3 shows the results of analysisProximate analysis was carried out using a 5E-MAG6600 auto- of the number three coal sample from the magma intrusion regionmatic industrial analyzer Gas adsorption isotherms were obtained Fig 3a shows the accumulated Hg-injection and Hg-ejection atTable 1Proximate analysis and gas adsorption constantsSample number locationElevationInitial methane diffusion rate Ap Adsorption constant a Adsorption constant b Proximate analysis( %)I# 80 m within the MIR -49420.15720251334526844.7864572# 60 m within the MIR -497 1.60.19582494242420508928.13760.12154# 20 m within the MIR(3.2036855.72542324.47532.53794.97378744052.76away from the48.42892.2699363355278253037# 60 m away from the45.72301.51683.7326.106.8763.309o m away from the-52040.05902291.56744.253263823548120 m away from the-5255197811.38694.1725.52885614610# 150 m away from the -53030.047.0957YH中国煤化工174690CNMHGJ. Jiang et al Mining Science and Technology( China)21(2011)737-74110r c r- Limiting adsorption constantthe coal bed is rich. a permanent ground drainage system has been- Initial methane diffubuilt to generate electricity. This can provide evidence for the magvolatilesma intrusion induced"superposition of hydrocarbonse4.3. Measurement of coal organic maceral and vitrinite reflectanceOrganic macerals in coal can be divided into vitrinite inertiniteand exinite. the order of priority for hydrocarbon productioncapacity is exinite > vitrinite >inertinite 1 Table 2 shows themaceral content and average vitrinite reflectance of the numbersthree and ten coal samples.Generally, as the volatiles in coal decrease or the metamor-500510-520530phism of the coal increases, the coal surface area is observed toElevation(m)increase substantially (1. the number three coal sample fromthe region of magma intrusion has a higher metamorphism degreele content, initial methane diffusion rate and adsorption constant for than the number ten coal sample from the normal region. Howeverthey differ little in porosity. The surface area of the number threesample is 6.930 m/g, which is far lower than the value of15.055m' /g found for sample number ten(Table 2).The structuraldifferent pressures. Fig. 3b shows the relationship between Hg- unit of high rank coal exhibits a higher degree of aromatization andejection amount and pore size. Fig 3c shows the accumulated pore the number of side chains and functional groups drops signifi-surface area obtained through the Hg-injection and Hg-ejection cantly. Molecular radii become smaller and the macro-molecularexperiments In these three figures the ejection curves are all plot- stack becomes more compact. Simultaneously, ordering of theted above the injection curves.structural units is strengthened and the degree of carbonificationFig 3b shows that macropores with a diameter from 1000 to increases. The adsorption pores become smaller. Some absorption10000 nm are well developed in sample number three. Micropores type pores change to porous type adsorption and a lower specifiwith a diameter from 1 to 10 nm are less developed in this sample. surface area of the high rank coal, compared to low rank coal, isAs a result the accumulated pore surface area for sample number observed. But at the same time the absorption pore surface isthree is smallgreater than in low rank coalsFig. 4 shows the experimental results from mercury porosime-When vitrinite reflectance exceeds 4.5% crystallization hastry on sample number ten, located in the normal region. Figs. 3 occurred in the macromolecular structural units because of theand 4 are presented in the same way, all the ejection curves are folding. a drop in porosity and reduced sharp surfaces result frorplotted above the injection curves. Fig. 4b shows that micropores the reduction in absorption type pore structure [12 The numbewith a diameter from 1 to 10 nm are well developed in sample three sample has a vitrinite reflectance of 4.7050%, which is largernumber ten. The volume of macropores larger than 10000 nm in than 4.5%diameter is small. As a result, the accumulated aperture surfaceCompared to the number ten coal sample the surface area of thearea of sample number ten is great.number three sample is significantly reducedThe background coal rank is low because of the period of theThe surface area of sample three is 6.930 m2/g and the limitingYanshan magma intrusion into the seam. High temperatures from adsorption constant, a, is 28.1376 the surface area of sample ten israpid warming necessarily cause gas to be generated again. Thus, 15.055 m?/g and the limiting adsorption constant is 470957.Thehe gas generated between the plutonic metamorphism stages is coal near the magma intrusion region has a higher methanemixed with coalbed methane formed by regional magmatic theadsorption capacity than coal from within the intrusion regionmal metamorphism. Thissuperposition of hydrocarbons"has Methane adsorptioMethane adsorption capacity of coals has a positive correlationnot only produced a large quantity of gas but also has rapidly with the coal surface area. Coal near the intrusion region has high-generated hydrocarbons. During the superposition of hydrocar- er initial methane diffusion rate and limiting adsorption constantbons considerable stomatal pyrolysis occurred. The apertures are This area is gas rich and is an area with a high incidence of coalthen larger than the apertures seen after plutonic metamorphism mine gas accidents There have been three coal- and gas-outburst[11 This can best explain the phenomenon of well developed accidents in the Wolonghu coal field. Two of these happened inmacropores with a diameter from 1000 to 10000 nm in the number the number ten coal seam and one happened in the number eightthree coal sample, see Fig 3. the absolute emission rate of meth- coal scoal seam. All the accidents occurred at the boundary of the magane from the Wolonghu coalfield is 56.70 m /min and the relative ma intrusion region, which verifies the validity of the view putabundance of methane there is 70 m/t. The methane content of forth above0002004- Injectionc444‘4a00015ection彐0.03002Injection与o0010▲ InjectionEjection001002001000XX10X0I00010X10Pressure (MPa)Aperture(nm)中国煤化工n)Flg. 3. Mercury intrusion results: coal sample number three(MICNMHGI Jiang et aL/Mining Science and Technology(China)21(2011)737-7470020b0.04Injection0.0015EjectionInjection0.03Eiection三§0020.0010Injection100200000001000010001001010000010000100010010Pressure (MPa)Aperture(nm)Fig. 4. Mercury intrusion results: sample number ten(normal region).Table 2Vitrinite average reflectance and quantitative microscopic components of samples three and ten.CoalVitrinite(% bynertinite(% byExinite (% by Mineral (% bySurfacePorosity Vitrinitevolume)reflectance3.505.726.9305.67774.705068.617.7415.0555.353027477VitriniteClay filling on cellInertiniteevilFig. 5. Photo micrographs of coal samples three(a-c) and ten(d-f)Fig 5 shows a set of maceral photos from coal samples three components results in different degrees of development of theand ten: the scale is 100 um. Fig 5a-c is from the number three pores. Thus, the adsorption capacity of the maceral variescoal sample(Rm=4705%). Fig. 5d-f is from sample number ten(Rm=2.7477%). Notice the vitrinite and fusinite( Fig 5a). the vitri- 5. Conclusionsnite and inertinite(fig. 5b), and the clay filling the cell cavities( Fig. 5c). Vitrinite, inertinite, and clay( Fig. 5d): inertinite and clay()By the Yanshan period(205-135 Ma)magmatic activityFig. 5e): and exinite( Fig. 5f) are also observed. Because both sam-affected the coal in the Wolonghu Mine so that magmaples are the high rank anthracite, the maceral from the two regionsintrusion areas had less volatiles than normal areas. the limhas little difference(see Table 2 and Fig. 5). The vitrinite, inertinite.iting gas adsorption constant, a, and the initial gas diffusionand exinite mineral groups in the two coal samples are similar inrate Ap, of samples from the intrusion region are smallerproportion but vitrinite accounts for 67.00% and 68.61% of thethan those from the normal region. the trends of volatiles,acral in samples three and ten, respectively. Likewise, the inert-adsorption constants, and initial methane diffusion ratesinite is 26.21% and 23. 58% and exinite is a small proportion, aboutfrom sample ten( the magma intrusion region )show consis-3.30%and4.28%tency(see Fig. 2)In the anthracite stage the adsorption capacity of the coal falls(2) Pores with a diameter from 1000 to 10,000 nm are commonin the order vitrinite inertinite. This is because in highly meta-in the nummorphic stages vitrinite has more volatiles that cause an increasediameter fr中国煤化工 n. the cumulativein micropores [17]. During coal metamorphism the amount ofpore sunachydrocarbons and other volatile substances produced by these6930m2/gCNMHG coal samplehas many microJ. Jiang et aL Mining Science and Technology(China)21(2011)737-741pores with a diameter from 1 to 10 nm but relatively fewerenriched area located within a coal seam. Min Sci Technoltive pore surface area of this sample is 15.055 m2/g, which is )Wos: 19(4):0457-62.big holes having a diameter from 1000 nm up. The cumula-ang L Cheng YP, Li FR, Wang HF. Liu HB. Fracture evolution and pressureelief gas drainage from distant protected coal seams under an extremely thicrelatively large. The vitrinite reflectance of the number threekey stratum. Min Sci Technol 2008: 18(2): 182-6coal sample(Rm-4705%)is higher than that of the number[4] Stewart AK, Massey M. Padgett PL Rimmer SM, Hower JC. Influence of basicintrusion on the vitrinite reflectten coal sample(Rm-2.7477%).nd chemistry of the Springfield(No 5)al. Harrisburg, Illinois. Int J Coal Geol 2005: 63: 58-67.(3)The methane adsorption capacity of a coal is positively cor-[51 Saghafi A, Pinetown KL Grobler PG, Van Heerden JHP. COz storage potential ofelated with the total specific pore surface area of that coalSouth African coals and gasment enhancementdue to igneousThe initial methane diffusion rate, and the limiting adsorp-tion constant, a, of the Wolonghu coal samples near the 16] Rimmer SM. Yoksoulian LE, Hower JC. Anatomy of an intruded coal.[:Effect ofry, Springfield(No. 5)magma intrusion region are larger where the gas resourcesPennsylvanian)coal, Illinois Basin. Int J Coal Geol 2009: 79: 74-82are rich. this is also the area with a high incidence of coalAnlin Coal Mine.uo Inst Technol 2004: 23(6): 423-6.and gas accidents. Vitrinite inertinite, exinite, and mineral [8] Pi YC. Gong B]. Yang Z Relation of igneous rock move and gas outburst ingroups are similar in proportion between the magma intru-Daxing well field. Coal Sci Technol 2007: 26(5):71-3.sion region and the normal region.[91 Shi XW, Zhang YG, Zhang ZM, Structure control analysis on coal and gasurst in Maoyingzi Mine. Coal Science and Technology 2007: 35(2)[10] Lu FS. Xu Pw. Zhang RH, Rui LD, Magmatic intrusion and its Impact toAcknowledgmentsoal seam and soal quality in Hanxing coalfield. Coal Geol China2007:15(9):22-4[11] Yang Q Tang DZ. North China coal metamorphism and permeability of coal gasThe authors are grateful to the National Basic Research Programthe influence. Earth Sci 2000: 25(3): 273-7.of China( No. 2011CB201204) the National Youth Science Founda- [12] Qin Y. China's high rank coal petrologic characteristic and thetion of China(No. 50904068), the Youth Foundation of China Unitructure evolution. Xuzhou: China University of Mining and Technologyversity of Mining Technology( No OY091223)for their support of [13] Zhang XM. Zhang SA, Zhong Lw. China's coal methane. Xi'an: Science andthis project.echnology Press of Shanxi: 1991[14] Huoduote BB, Song SZ Coal and gas outburst. Beijing: China Industry Press;References[15] Zhang ZM. Zhang YG. Gas geology rule and gas outburstrediction. Beijing: China Coal Industry Press: 1998.[11 Yu Qx. Mine methane prevention. Xuzhou: China University of Mining and [16] Zhang ZM. Lin Yl Lu SL China,'s coal seam gas distribution. Beijing: China Coaldustry Press: 1998.[21 Xu YZ, Cui RF Huang WC, Chen T]. Reflectivity forward modeling and a cSSI [17] Zhong Lw. Adsorptive capacity of coals and its affecting factors. Earth Sci jmethod seismic inversion study of igneous intrusive area, coked area, andChina Univ Geosci 2004: 29(3): 77-82.中国煤化工CNMHG