Tectonically deformed coal types and pore structures in Puhe and Shanchahe coal mines in western Gui

Mining Science and Technology( China)21(2011)353-357燃到腰Contents lists available at Science Directaad IchthyMining Science and Technology( China)ELSEVIERjournalhomepagewww.elsevier.com/locate/mstcTectonically deformed coal types and pore structures in Puhe and shanchahecoal mines in western guizhouLi Minga, b.c., Jiang Bo.b, Lin Shoufa a. Wang Jilin. b, Ji mingjun Qu Zhenghui abSchool of Resource and Earth Science, China Universiry of mining g Technology, Xuzhou 221008, ChinaKey laboratory of Coalbed Methane Resources and Reservoir-Forming of ministry of Education, Xuzhou 221008, ChinadEpartment of Earth and Environmental Sciences, University of waterloo, waterloo, Canada N2L 3GId Jiangsu Hogan Group Co, Ltd, Xuzhou 221137,ChinaARTICLE INFOABSTRACTArticle history:To evaluate the effect of tectonic deformation on coal reservoir properties, we provide an analysis of theeceived 28 September 2010cejved in revised form 12 November 2010types of tectonically deformed coal, macro- and microscopic deformation and discuss pore structuralAccepted 2 December 2010characteristics and connectivity based on samples from the Puhe and Shanchahe coal mines. Our researchAvailable online 11 June 2011hows that the tectonically deformed coal mostly includes cataclastic structural coal, mortar structuralcoal and schistose structural coal of a brittle deformation series. The major pore structures of differenttypes of tectonically deformed coal are transitional pores and micropores. the pore volumes of macrop-KeywordPuhe and shanchahe coal minesores and visible fracture pores produced by structural deformations vary over a large range and increaseTectonically deformed coawith the intensity of tectonic deformation. Mesopores as connecting passages develop well in schistosetructural coal. According to the shapes of intrusive mercury curves, tectonically deformed coal can beoalbed methanedivided into parallel, open and occluded types, the parallel type has poor connectivity and is relativelyclosed; the open type reflects uniformly developed open pores with good connectivity while the occludedtype is good for coalbed methane enrichment, but has poor connectivity between pores.o 2011 Published by Elsevier B V on behalf of China University of Mining Technology1 IntroductionZunyi fault arch[8 Structural traces in this zone trend mainly in anortheasterly direction. The folds are mainly gentle folds. TheCoal is a special kind of rock that is uniquely sensitive to stress faults are well developed and move mainly in a normal sense.and strain. Tectonically deformed coal with various structural The geological structure in the puhe and Shanchahe coal mines ischaracteristics and types form under different stress-strain condi- relatively simple and exhibits several normal faults with a neartions and tectonic stresses[ 1, 2]. The distribution of tectonically de- north-south trend. The Longtan formation of the upper Permianformed coal in a coal seam is relatively limited, but it is widely is a major coal-bearing section in the study area, between 300developed in Chinas coal fields and is the main factor affecting and 490 m thick, containing 10-22 coal seams. Coal seams Nos.he distribution and enrichment of coalbed methane(CBM)[3]. 22, 23 and 34 are the main mineable coal seams in this coal fieldThe development of tectonically deformed coal enhances the het with anthracite rank. The semi-bright and semi-dull coals are oferogeneity and porosity of coal seams and also reduces their per- the dominant lithotypes, with secondary amounts of dull andmeability and mechanical strength. The feasibility of cBm bright coaexploitation and the possibility of coal and gas outbursts are signif-icantly affected by late structural transformations and pore-fracture structural characteristics of coal seams 14-7). This provides 2. Materials and methodsbasis for further discussion of deformation characteristics andpore structures of tectonically deformed coal, necessary for furtherIn the puhe and shanchahe coal mines the coal seams are bur.CBM exploitation, safe mining and productionied to a shallow depth, their geological structure is simple and theThe Puhe and Shanchahe coal mines are located in the west of type of tectonically deformed coal is not complex [ 9]. Based onGuizhou province, near Puding and Anshun cities( Fig. 1). They be. geological surveys of the coal mine within a limited undergroundlong to the Zhina coal field, which is located in the southwest of the mining space. different types of typical tectonic coal samples werecollected, as shown in Table 1Systematic observations and descriptions of the mesoscopicCorresponding author Tel +86 13151981375deformation fe.mailaddress:cummingii@hotmail.com(M.Li)ding, joint, fra中国煤化工 nt were made on th1674-5264/5- see front matter o 2011 Published by Elsevier B V. on behalf of China University of MiningCNMHGdoi:101016msc201105.02M. Li et al/ Mining Science and Technology( china )21(2011)353-357+S2Poore to total volume. V1>100,000 nm: v25 kmAnshunm100-10 nm: Vs 10 nm; Vr as total poreservoir. Transitional pores and micropores mainly consist of pri-mary and metamorphic pores. Stage pore volumes and their contri-bution to the total volumes of coal samples are shown in Fig. 2 andFig 1. Structural outline of Puhe and Shanchahe coal mines.Table 23. Results and discussionTable 1Statistics of tectonically deformed coal samples collected in Puhe and Shanchahe coal3. 1. Types and characteristics of tectonically deformed coalDigging Samples Tectonic coal type Sampling location Coal3.1.1. Primary-cataclastic structural coal (S1 and S6)Structural deformation is mainly characterized by brittle frac-Shanchahe scoalstructural coaldipentry of threetures, with the primary structures of coal. as a rule, well preservedmining areaFractures are sparsely distributed with small fracture apertureCataclastic-mortar Haulage roadway 34Tension joints have rough joint planes and extend unsteadily alongdipentry of threeits trend. Shear joint planes are smooth and straight, shown as twomining areSchistose structural Hanging wall of fault 34joint sets in Fig 3a. Tectonic stress is focussed and many second order fractures form at the transition parts of the joints.Cataclastic-mortstructural coalCataclasticNear the outcrop3.1.2. Cataclastic structural coal (S5)structuralof coal seamThe primary structure of coal remains relatively unchangedonly small scale fractures, distributed sparsely, are formed in brit-Primary-cataclastic Near the outcroptle fracture deformation. The joint plane is rough and extends in anstructural coalof coal seamarcuate shape. Coal deforms along the joints but without apparentPuhe coal P1Schistose structural Ventilation roadway 23displacement and the fractures have a preferred orientationraise of 1410 miningFig. 3b). There is, locally, a slight ductile deformation.P2 Mortar structural Ventilation roadway 23dipentry of 14103. 1.3. Mortar structural coal (S2, S4 and P2mining faceDense joints and fractures develop well in the coal mass andlarge fractures are often accompanied by secondary associatedfractures forming fracture-concentrated belts. Many secondarycollected samples in the laboratory. Microscopic deformation was associated fractures form at the intersections of two or more setsobserved and measured using a polarizing microscope under re-of fractures. Larger scale fracture planes that show brittle fractureflected light. The types and deformation characteristics of tecton. features are smooth and stable, but secondary fractures exhibitcally deformed coal were then analyzed and studied. In addition. arcuate shape with poorly preferred orientations on horizontal surthe pore volume and pore size distribution were tested by Micro- faces. The coal mass is segregated into different sized pieces bymerities 9310 type mercury porosimetry with pore diameter testsfracture sets and appears severely damaged showing a mortar tex-ranging from 7 nm to 0. 23 mm. Based on the standard set by Hototire( Fig. 3c).and considering that the minimum distinguishable width by thenaked eye is 0. 1 mm, the micro-scale structures of pores can be 3.1.4. Schistose structure coal (S3 and P1)classified as visible fracture pores(pore diameter >100,000 nm),Due to the effect of tectonic strain, mainly shearing, and the coalmacropores (1000 to <100,000 nm), mesopores(100 to mass is sliced into thin sections. The dominant dense fracture set is<1000 nm), transitional pores(10 to <100 nm)and micropores well developed at an angle of about 45 from the coal bedding(<10 nm), with pore diameters 100,000, 1000, 100 and 10 nm as (Fig. 3d). There are many obvious sliding signs on smooth joint sur-boundaries [10-12). Visible fracture pores, with pore diameters be- faces, which shows that schistose structural coal is formed bytween 0. 1 and 0. 23 mm, consist largely of visible joints and frac- shear failure along a group of dense array jointstures formed by structural deformation and coalification. TheThe structural deformation of the puhe and shanchahe codegree of development of visible joints and fractures in different mines is not very intense and mainly exhibits brittle fracturingimples is reflected in the visible fracture pore volumes. Macrop- Due to the different intensity and property of stress, to which coalores and mesopores are mainly microfractures, exogenous holes samples were subjected, there are specific differences in the denor mineral pores [13, 14]. Visible fracture pores and macropores, sity of fracturewith pore diameters larger than 1000 nm, are the main flow chan- of joints. The中国煤化工 e geometric shapenels of CBM in coal reservoirs, suggesting that their content and breathability arCNMHGe coal seam permeconnectivity determine the breathability and permeability of a coal ability, which enhances ingi alun dnu issipation of CBMM Li et aL /Mining Science and Technology(China)21(2011)353-357volumes of coal samples.Pore volume(cm/g)Pore volume ratio(%)Vu/V,v2/ve/V,va/ve000170.0015001440.008129.783840000500560011816915.540.002692825.797.5435.9421455.2200008003262.45374219335.06S5000080002100182004231894303219956700040000240015000920034711,2411.536924380.590018000088005943.202407276130.3014828.0007600059001230010700373329728.69Fig 3, Micro-structural features of tectonically deformed coal(magnification: 4 x 10).00020.002i.”52H2h110100100010000100000100000001001000100001000001000000Pore size(nm)Pore size (nmFig 4. Relation between typical pore size and stage pore volumes. (a)Primary-cataclastic structural coal ( S1). (b)schistose structural coal( P1), and(c)mortar structural coal3. 2. Pore structural characteristics of tectonically deformed coalin transitional pores and micropores, on average accounting for36.75% and 24.00% of total pore volume(Figs. 2 and 4c). ThisPrimary-cataclastic structural coal (S1 and S6)was deformed is followed by macropores(22. 72%)and mesopores(1444%),akly by structural stress. Its pore volume is concentrated pri- while the lowest fraction consisted of visible fracture pores,marily between transitional pores and micropores, on average accounting for only 2.09%. Compared with cataclastic structuralaccounting for 42.95% and 29.24%, respectively of the total pore coal, the proportion of macropores is nearly three times greater.volume( Figs. 2 and 4a). The dominant transitional pores and suggesting that dense microfractures, formed through structuralmicropores are mainly primary and metamorphic pores, implying deformation, develop well. there is a close relationship betweenthat the primary structure of coal remained mostly unchanged the increase in the proportion of mesopores and the develop-with less structural deformation. Visible fracture pores and mac- ment of crushed granule pores, i.e., the space between crushedropores account for 17. 15%, while mesopores account for 10.67%. granules formed by structural deformation in coal. The visibleThe latter mainly consists of micro-fractures that connect seepage fracture pore volume of mortar structural coal is only aboutfractures to occurrence pores of CBM.one-quarter that of cataclastic structural coal. This data corre-The pore volume of cataclastic structural coal(S5)consists lar- lates well with the phenomenon observed microscopically show-gely of macropores and transitional pores. this is because the rel- ing that fractures are sparse and wide in cataclastic structuralative strengthening of tectonic stress action leads to the coal; in contrast, fractures are narrow, dense and dispersed inenhancement of joints, fractures and micro-fractures, which in- mortar structural coal The pore volume of visible fracture porescrease total pore volume. The effect of structural deformation on and macropores, on average adding up to 0.0082 cm, is 1.5micropores is small and micropore volume remained relativity times the pore volume found in primary-cataclastic structuralunchanged, which causes the proportion of micropores to total vol- coal. Although the proportion of mesopores increased and thatume to decrease relatively; transitional pores and micropores as of visible fracture pores and macropores decreased, their poreoccurrence space of CBM account for 65.02% of total pore volume. volumes all increased to varying degrees. Thus, intense structuralThrough further deformation, visible fracture pore and macropore deformation not only causes development of joints, fractures andvolumes account for 30.02% of total pore volume, an increase of 2- micro-fractures, but is also responsible for the considerable3 times compared to primary-cataclastic structural coaldevelopment of tectonic micro-pores, such as frictional andMortar structural coal (S2, S4 and P2)deformed more inten- crushed granularsely than cataclastic structural coal from structural strain. Both tion and reforma.porosity and total pore volume increased. As with primary- pore space and pH中国煤化工 uctural deforma-CN Gts dissipation ofcataclastic structural coal, pore volumes remained concentrated CBMM. ui et aL/ Mining Science and Technology( China)21(2011)353-357Schistose structural coal(S3 and P1), a structural deformation mercury curve, testing samples canided into parallelproduct resulting from the action of structural shear strain on coal and occlusion typesseams, has its pore volume concentrated in transitional pores and(1)Parallel type: the mercury injection and ejection curves aretively, of total pore volume( Figs. 2 and 4b). This is followed by thealmost parallel and the difference in value of the two curvesvolumes of micropores, mesopores and visible fracture pores. Poresat the same pressure is low(Fig. 5a). Primary-cataclastiodevelop well in schistose structural coal, where porosity is aboutstructural coal (SI and S6)and cataclastic structural coa1. 4 times that of cataclastic structural or mortar structural coal(S5) belong to this type, with well developed microporesIt is largely represented by significant increases in volumes of vis-and transitional pores and less well-developed macropoible fracture pores and macropores, i. e. 0.0026 and 0.0116 cm/g.respectively. The development of visible fracture pores and mac-(2)Open type: there is an acute angle between the mercuryopores consists mainly of joints and micro-fractures, enhancinginjection and ejection curves. The difference in value.the release and seepage of CBM. Its mesopore volumebetween these two curves at the same pressure, increases(0.0095 cm /g)is about twice that of cataclastic structural or morwith a decrease in pressure( Fig. 5b). Cataclastic-mortartar structural coal as determined from microscopic observationsstructural coal (S2 and S4 )and schistose structural coal ($3and scanning electron microscopy. the mesopores consist largelyand P1)belong to this type and are well developed at eachof interfragment pores, crushed granular pores and micro-fracturestage of pore volume(Fig. 4b). As the stage pore volumespores formed by structural deformation. Therefore, the schistosedevelop, both acute angle and permeability increase.structural coal with good connectivity and high porosity is a reser-(3)Occlusion type: the mercury injection and ejection curvesvoir favorable for the development of CBMnearly intersect locally as an"occlusive throat". The differIn conclusion, the contributions of various stage pore volumesence in value between the two curves at the same pressureto total volume are in the following descending order: transitionalfirst decreases locally and then increases with a decreasepores, micropores, macropores, mesopores and visible fracturein pressure( Fig. 5c). Mortar structural coal (P2)belongs topores. The largest proportion of pore volume of tectonically de-this type. Changes as a function of pressure shown in theformed coal, developed in the study area, is transitional pores, con-two curves at the same pressure reflect the connectivity ofsists mainly of primary and metamorphic pores with small porethe coal mass. Given the "occlusive throat"phenomenon insizes. Micropores, the second highest in proportion, also mainlythe occlusion type intrusive mercury curve, the distributionconsists of primary and metamorphic pores. This micropore stagecurve of stage pore volume is always shaped like the lettervolume, with pore sizes between 7 and 10 nm only, accounts for" v"orU", where the pore volume is smaller at the stage20%-30% of total pore volume. Due to their small pore sizes andof the"occlusive throat"pore size than at both sides. thispoor connectivity. micropores contribute only negligibly to coalphenomenon is largely concentrated at the stage of transipermeability from transitional pores and micropores, althoughtional pores with pore sizes between 10 and 100 nm. Thusthey provide most of the space for CBM in its adsorption state. Poreit can be seen that the not-well-developed transitional poresvolumes of macropores range widely and increase with theat the stage of partial pore sizes and overall well-developedenhancement of structural deformation Mesopores, which mainlytransitional pores and micropores are the leading causes ofact as channels linking micropores and microfractures, in generalocclusion type intrusive mercury curvesdo not develop well, accounting for only a small proportion of totalpore volume. But mesopores develop well in schistose structuralThe shape of the intrusive mercury curve is controlled bycoal, with its excellent functions promoting migration and seepage such factors as pore size structure, pore-fracture properties andof CBM. Although visible fracture pores form the smallest percent- combination mode. According to its morphological characteris-age of total pore volume, possibly because of the narrow test range tics, pore shape and connectivity of tectonically deformed coalof pore diameters, they develop well in intensely deformed re- can be evaluated in a preliminary fashion. Open type intrusivegions. Visible fracture pores'developments are of practical impor- mercury curves represent open pores with uniform developmenttance in CBM seepage.and good connectivity. Parallel types with two parallel andnearly coincident mercury curves suggest bad connectivity and3.3. Pore shape and connectivity of tectonically deformed coalpoor permeability. The occlusion type is mainly caused by notwell-developed transitional pores at the partial pore size stage.The effective pore volume in coal can be calculated directly by Overall well-developed transitional and micropores indicate thein analysis of intrusive mercury curves, from which the degree occurrence and enrichment of CBM, but bad connectivity andof development, type and connectivity of pores can be deduced low permeability restrict migration and seepage of CBM. it isby the combination of the shape of its mercury injection and ejec- hard to exploit CBM in areas of this type of tectonically detion curves [15-17]. According to the shapes of the intrusive formed coaL.0.06+ Mercury injection curve0.06500201000100101110100100000010010.1110100100000010010.11101001000Pressure (MPaPressure(MPa)Fig 5. Three kinds of intrusive mercury curves (a)Parallel type(S1)(b)He (MPa)M Li et aL/ Mining Science and Technology(China)21(2011)353-357References[1]Jiang B Qin Y Ju YW. Wang JL uM. 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