Response of coal reservoir porosity to magma intrusion in the Shandong Qiwu Mine, China

Mining Science and Technology( China)21(2011)185-190Contents lists available at Science DirectMining Science and Technology( China)ELSEVIERjournalhomepagewww.elsevier.com/locate/mstcResponse of coal reservoir porosity to magma intrusion in the shandong QjwuMine. chinaLi Wu.., Zhu Yanming.b, Chen Shangbin,b, Wang HuicSchool of Mineral Resources and Geoscience, China Universiry of Mining G Technology. Xuzhou 221008, ChinaKey Laboratory of CBM Resource and Reservoir-Generaning Process Ministry of Education, Xuzhou 221008. ChinaShunhe Mine of Etermal Coal Group Co, Ltd, Yongcheng 476600, ChinaARTICLE INFOA BSTRACTArticle history:The Qiwu Mine is located in the Ten Xian coal field of Shandong province. It experienced repeatedvolcanic activity. after the coal beds formed, where magma intrusion was significant. The effect of coalty after magma intrusion was studied by analysis of regional and mine structure and3 0ctober 2010magmatic activity. Experimental methods including maceral measurement under the microscope andetry were used for testing the pore structure. The authors believe that magma intrusioninto low-rank bituminous coal causes reservoir porosity to gradually increase: the closer to the magmaticrock a sample is, the less the porosity. The pore size distribution also changes. In the natural coal bed thepore size is mainly in the transitive and middle pore range. However, theto anthracite nebituminous coalto the magmatic rock and larger pores dominate Regional magma thermal evolution caused coal dose tomagmatic rock to be roasted, which reduced the volatile matter, developed larger holes, and destroyedplant tissue holes. The primary reason for a porosity decrease in the vicinity of magmatic rock is thatBituminite resulting from the roasting fills the holes that were present initiallyCopyright o 2011, China University of Mining Technology. All rights reserved.1. Introductionin magma intrusion zones and also did some research on thefeatures of pore structure and the impact of magma intrusion onA coal seam is a reservoir and it may produce gas. The degree of gas storage and out-gassing [12]. Zhang et al. studied factorsmetamorphism of the coal bed, the porosity, permeation, adsorp. influencing CBM generation in magma coal reservoirs. Theytivity, and the type of maceral are the main factors influencing coal determined that magma baking volatilized some organic matter inbed methane( CBM)generation, migration, and concentration( 1). the coal, which left many circular and tubular pores and improvedowever, magma intrusion changes many of the original charac- the porosity of the coal reservoir [13]. Fang et al analysed thehigh pressure squeezing Pores in the coal, and the character of the morphism with CBM formation, adsorption value, porosity develpores(quantity size, proportion of different type pores, and extent opment, and coal seam characteristics. They thought that theof connection between pores) can influence collection and trans. impact of coal metamorphism on CBM accumulation was reflectedmission of the gas. This is an important physical property of coal by the development of a coal reservoir porosity-fracture systemreservoirs having great influence on the occurrence of coal bed [14. Wang analysed the litho-structure of coal-bearing strata andethane[2].studied the magma intrusion mechanism with a scanning electronMuch work has been done studying the characteristicsreservoirs and the impact of magma intrusion on the coalrank, and coal bed methane [3-11. Experimental measurengas content, gas components, and the coal pore capacitythe safmicroscope. He summarized the impact of lamprophyre intrusionon coal seams, coal rank, and gas accumulation [15] So it is verysignificant to analyse coal reservoir characteristics in magmaamples were made by Lu et al. They analysed the gas components2. Geological introductionCorresponding author. School of Mineral Resources and CeoscieThe Qiwu Mine is located southwest of Shandong province, inUniversity of Mining Technology Xuzhou 221008, China. TeL: +86 138153E-mailaddress:Iiwucumt@126.com(Lwu中国煤化工 ession in the chengwu674-5264'S-see front matter Copyright c 2011, China University of Mining TechnologyCNMHGdoi:101016mstc201102008L wu et aL/ Mining Science and Technology( China )21(2011 )185-190region of the Zhong-Chao Para platform(Fig. 1)and has a coal- working pressure ranges from 0 to 207 MPa and the low pressurebearing area of 46 km?. It contains low-rank bituminous coal of the resolution ratio is 0.001 MPa, the high pressure resolution ratio isCarbonic and Permian strata where the main coal-bearing strata are 0.01 MPa, the contact angle between the mercury and the sample isfrom the Taiyuan formation of the upper Carbonic series and the140, the surface tension of mercury is 480 dyn/cm, and theShanxi formation of the upper Permian series. There are four diameter of the pores ranges from 60 to 3,600,000A.Amineral seams: the upper coal seam, Number 3: the lower of coal 1.5 cm x 1.5 cm x 2.0 cm coal sample was tested by first drying it atseam, Number 3 the lower coal seam, Number 12; and coal seam 60-70 for 24 h. It was then enclosed in an evacuated chamber toNumber 16. The average seam thickness is 8.15 m, and the average remove all the gas. the mercury was added to the chamber whentotal thickness of the upper and lower Number 3 seams is 6.18 m. the vacuum was below 50 um. Mercury was then forced into theThe latter accounts for 76% of the total commercial seam thickness. sample pores by increasing the pressure, mercury enters the biggerMagma intrusion is extensive but mainly occurs in the southen holes first followed by the smaller ones as the pressurepart of the coal field. It is much less in the middle part of the The mercury volume in the pores is obtained by capacitance varinorthem area. In the southern area intrusion occurs from explo- ations in a micro-bore capillary containing the mercury. The sampleration line 8. in the northem apex, to the southern boundary. the porosity is calculated from the maximum mercury volume dividedintrusion is a sill 9.51 km2 in area. In the mid-north the intrusion by the sample volume. Since the pore volume is obtained byarea spreads from exploration line 5 to exploration line 2. Here it is mercury infiltration into holes, isolated unconnected holes andmainly sill with little dike formation and it trends to the northwest holes smaller than 60 A cannot be measured. The porosity value isand has an area of 2.68 km( Fig. 2). There are three kinds of magma an effective porosity [16]intrusion rocks: lamprophyre, diorite-porphyries, and diabase. Theintrusion time was during the later period of the Yanshanian 4 Results and discussionmovement(cretaceous). Magmatic rock intrusion leads to thechange of the coal reservoir and part of the coal seam turns toThe"porosity properties "of a coal reserve refers to the total coalcinder coal near the faultporosity, the cleat porosity, the volume compressibility, the coalpermeability, and the structural features of the pores [17]. These3. Sample collection and testsproperties are very important for estimating gas reserves. Theimpact of magma invasion on coal porosity properties will be dis-cussed below. Table 1 shows the results of a mercury porosimetryThe research involves a vertical and horizontal contrast. Twodikes were chosen having a downward cross heading in the 313 4.1. Analysis of porositymining area in coal seam Number 3 as reference points, Vertical andhorizontal coal samples were collected. Here dike A has an angle ofThe porosity is the ratio of the total volume in the micro-holeso and a thickness of 28 m Dike B has a 70 angle and it is 30 m and fissures in the coal to the total volume of the coal; it showsthick. When collecting samples the position of the coal seam was different coal ranks. It can be seen in Table 1 that the porosity firstdecreases rapidly and then increases a little. The normal value is(Fig 3).doubled near the magmatic rock. Fig. 4 shows how porosity ranges32. TestsThe specific area, porosity, and distribution of pomintrusion were studied by running mercuron the four samples. The measuringvas a 9310 mercury injection pore tester m.01如mJungsu province迪[中国煤化CNMHFig. 1. Schematic showing regional tectonics.Flg. 2 a diagram showing the intrusion range.L Wu et aL/ Mining Science and Technology(China)21(2011)1&5-19Fig 3. Vertical sampling position for samples Mi through M18Table 1The distribution of pore sizes can be obtained from stagedmercury porosimetry. Fig. 6 shows staged curves from 18 samples.Sample nu"The different kinds of distribution may be seen in Table 2. It is22540obvious that staged mercury porosimetry of samples M2, M3, andPorosity (3) 8.82 3.68 441 418 1921 1299 350 404 3.47 M4 gives similar results(Fig. 6). and that they belong to one branchof a hyperbola. The main pore diameter is 0. 1-0.01 um, whichM1MlM12M13M14M15M16M17M18Distance( m)represents transition holes that arise mostly from fusinite andPorosity(t67445311389382341323339323anthraxylon plant pores. But in sample Mi the pore diameter ismostly larger than 1 um or between 0. 1 and 0.01 um. the transitionwith distance. In Fig 4 it can be seen that the curve drops in sample pores between 0.1 and 0.01 um are fusinite and semi-fusinite plantpores. The big pores larger than 1 um are air holes resulting fromnumbers M2 and M6, which is due to Bituminite developing as the heating by magma: this is also evident in the horizontal direction.and, owing to the influence of high temperatures from themagmatic rock, the fugitive constituents have vaporized leaving the aperture distribution is between 0.1 and 8.0 um and the main porespores to form gas. Hence, the porosity increases [191Fig. 7 shows the hole distribution of the different samples. Thisshows directly the characteristic hole type distribution in a normal42. Analysis of the pore diameteroal seam. The hole distributions in samples M2 through M4 and M6through M18 resemble each other. M1 and M5 are obviously differenthe Xonot classification scheme is used to classify the pores. from the others due to magma intrusion. It may be concluded thatThis method considers the width ofat radius. The classes magma intrusion has a limited effect on coal seam pore distributionare: micro-pores(throat radius <0.01ansition holes(throat and pore structure. this study also shows that the influence dikeradius between 0.01and 0.1 um):width on coal porosity extends over 1.5 m. This metamorphic beltbetween 0. 1 and 10 um): big holes(throat radius >1 um)1201from the dyke is three times larger in width than the dike itself.100一MMM"Mn2M1M0051.0152025303.5012502505Distance to magmatic rock(m)8163264Distance to magmatic rock(m)g 4 Ratio of porosity to distance from the magmatic rock.中国煤化工CNMHGFig. 5. Bituminite from samples M2 and M6.L Wu et aL/ Mining Science and Technology(China)21(2011)185-190b00400080006叫b010010.11101001000000l0010.1l10100100000010010.1101001000d0004ef003I000300020.0020.0010001001010000010.1110000l001010100I000Pore diameter(umPore diameter (um)Pore diameter (um)h0002110100100000010010.I1101001000Fig 6. Intrusion volume versus pore diameteConsider the contrast in pore distributions of the MI and M5 The pore diameter in coal samples unaffected by magma inva-vertical and horizontal coal samples, which are influenced by sion is always smaller than in samples influenced by magma Poresmagma heating Note that Mi developed big pores but M5 devel- are well developed in vitrinite, where they are mainly air holes,oped middle sized and big pores. this suggests that the influence of inter-granular holes, or molded cast holes. the diameter of the airting from magma is greater in the vertical samples than in the holes is between 0.1 and 10. 4 um. They are differentiated fromhorizontal samples. This is because magmatic rock appears as a sill inter-granular holes by their smooth margin. the higher thend the velocity of heat exchange is larger in the vertical amount of inertinite. the larger the amount of coal porosity. Thevalue of coal porosity depends on the structured and unpackedent by other[21] Fig. 8b shows tissue4.3. Analysis of pore characteristicsholes (anthraxylon If) photographed at 200 magnificareflected light. Fig 8d shows semi-fusinite photographed aMi was roasted by magma so the coal metamorphism increasedThe coal seam changes because of regional magmaover a short time and fugitive constituents escaped. The epigenetic evolution within the invasive areas. Coal porosity is closely relatedpores developing from primary structural pores are always larger, to the degree of thermal evolution. The coal types are magmatiegularly shaped, and well connected. Pore diameters range from10 to 200 um and are mainly in the range from 50 to 100 um Fig 8at MIshows holes from roasting by magma: the expansive tissue holeswere photographed at 200 power magnification with reflectedlight: Fig. 8c shows the holes at 100 powerTable 2Distribution of pore diameters(x)ore size MI M2 M3 M4 M5 M6 M7 M8 M95605807.1034.76665787%8.2135905300476056002897585549.056105585723.5012310M10 MI1 M12 M13 M14 M15 MI6 MI7 M18中国煤化工M-M100.16058946237866375347875853.52CNMHG010014894634588248576035573590561535352Fi 7. Distribution of pore types in coal samples.L Wu et aL/ Mining Scince and Technology(china)21(2011)185-190& Micrographs showing hole charactenstics.Thickness (28 m)4。,,,,Hi 9. Coal seam meiamo phasisrocks, anthraciteoke, meager coal, blind coal, coking coal, Referencesand gas fat coalRo( %)is the reflectivity of vitrinite). Thedex function within the magma invaded Ill u Q ou dH, Xv L Zhou aL Our country coal crag reservoir hole-crevasseinvaded section because the coal seam temperature goes up2】because of magma roasting, which leads to high-grade anthracite,or even natural coke 22 As the components in the coal changnal of China University of Miningdoes the porosity change.The magma heating contributes to formation of groups of round 14] Sang S Qin Y Fan BH. Jiang B, Fu xH, Zhang H, et al Features of low rankTechnogenic and exogenous cracking. The power extrusion when magma (5)Jiang BQin Y Fan BH. FuXH.deposition.The hydrocarbon gases resulting from metamorphism 16 Wang T. intrusive features and its influence of l mprop)yre on icoaongxin mine field. Coal Geologydrops and the coal bed gas migrates to low pressure areas and002: 30(5): 11-3 in Chinesedeposits in spaces between magmatic rocks, gathering in the holes [7 Nong HC. 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