The coal cleat system:A new approach to its study

ournal of Rock Mechanics and Geotechnical Engineering 6(2014)208-218Contents lists available at Science DirectJournal of rock mechanics andGeotechnical EngineeringCSRMEjournalhomepagewww.rockgeotech.orgFull length articleThe coal cleat system: A new approach to its studyCross MarkC F Rodrigues,, C. Laiginhas, M. Fernandes, M. Lemos de Sousa,, M.A. P Dinisssoa University, Porto 4249rugald Lisbon Academy of Sciences, 1249-122 Lisboa, PortugalARTICLE INFOA B STRACTArticle historyAfter a general analysis regarding the concept of coal"cleat system", its genetic origin and practicalReceived 28 February 2014applications to coalbed methane( CBM)commercial production and to CO2 geological sequestrationReceived in revised formprojects, the authors have developed a method to answer, quickly and accurately in accordance with theAccepted 8 April 2014ndustrial practice and needs, the following yet unanswered questions:(1) how to define the spatialne 18 April 2014orientation of the different classes of cleats presented in a coal seam and(2)how to determine thefrequency of their connectivites The new available and presented techniques to answer these questhave a strong computer based tool (geographic information system, GIS), able to build a completeoalbed methane(CBMgeoreferentiated database, which will allow to three-dimensionally locate the laboratory samples in theCoal cleat systemoilfield. It will also allow to better understand the coal cleat system and consequently to recognize thebest pathways to gas flow through the coal seam. Such knowledge is considered crucial for underGeographic information system(GIS)standing what is likely to be the most efficient opening of cleat network, then allowing the injection withthe right spatial orientation, of pressurized fluids in order to directly drain the maximum amount of gasCOz geological sequestrationflow to a CBM exploitation well. The method is also applicable to the COz geological sequestrationtechnologies and operations corresponding to the injection of CO2 sequestered from industrial plants incoal seams of abandoned coal mines or deep coal seams.o 2014 Institute of Rock and Soil Mechanics, Chinese Academy of Sciences. Production and hosting byElsevier B.V. All rights reserved1. IntroductionIn the past, coalbed gas was considered mostly as a hazard(Flores, 1998)due to the effect of both fire-damps and gas outThe coal fracture system has been investigated since the earliest bursts. Many studies were also carried out in the scope of minedays of coal mining operations, and the first descriptions and safety related to these phenomena, i. e, coal fracturing and tectonicsspeculations on fracture origin dated back to the late 19th century, (Alpern, 1963, 1967, 1970). An account of more recent investigationsaiming to determine the design of mine workings(Pattison et al., was given by Cao et al. (2001), Jin et al. (2003 ). Ryan(2003), and1996). Such studies consisted in general descriptions of the Solano-Acosta et al. (2007, 2008), respectivelyappearance of the fractures and measurements confined to theiroalbed gas corresponds nowadays almost to a resource com-orientation, which are considered important issues in designing modity through the commercial exploitation of CBM deposits, andal mines so as to maximize extraction efficiency and to improve the study of coal fracturing is again considered crucial. In fact,safety conditions.stated by several different authors(Gamson, 1994: MacCarthy et al1996: Ayers, 2002: Durucan and Shi, 2009), the prerequisite tomical and technical viable projedgascovery as well as in COz injection is intimately related to coalesponding author. Tel: +351 225071300frodrig@gmail.com(c.f.Rodrigues)permeability which, in turn, depends on coal fracturingesponsibility of Institute of Rock and Soil Mechanics, ChineseMany terms were used over the years to designate the naturalAcademyfracturing of coal. However, the term"cleat, used for the first timein 1925. was the orrS,geengineers as the中国煤化工 riety of fracturescommonly foundProduction and hosting by elsevierprocess and basin rlCNMHG the coalifications in coal have been1674-7755 2014 Institute of Rock and Soil Mechanics, Chinese Academy ofdescribed as equivalent to joints in competent rocks or as closelySciences. Production and hosting by Elsevier B.V. All rights reserved.spaced pervasive fractures originated from an almost impercep-tible movement associated with an extensional opening. After theCE Rodrigues et al. Journal of Rock Mechanics and Geotechnical Engineering 6(2014)208-218work of Macrae and Lawson(1954), Nickelsen and Hough(1967), method, similar to the one used in micro-tectonics which is aTing(1977), Karacan and Okandan(2000), and Wolf et al. (2001), direct response of regional and local tectonic settings(Ting, 1977;the formation of cleat appears to be influenced by shrinkage Close, 1993; Levine, 1993; Pyrak-Nolte et al., 1993; van Krevelen,occurring during the process of coalification, stress release, and 1993: Laubach et al., 1998: Montemagno and Pyrak-Nolte, 1999:extensional strain. It was also documented that, in general terms, a Morris et al., 1999; Mazumder et al, 2006).cleat system is present in coal ranging from lignite to anthracite,In the present work, a new, semi-automatic, fast, accurate, andbeing commonly well developed in low volatile bituminous coals. statistically based optical method, aiming to obtain more reliableThis is justified by the fact that the increases of heat and pressure, results in order to satisfy the current industrial practice and needs.sually associated with metamorphism, produce plastic flow that was developed. In this regard, it should be mentioned that, moredestroys the original cleat structure. This fact was more recently recently, Alpern and Lemos de Sousa(2002) have proposed, toconfirmed by Su et al. (2001)adapt to CBM problems, an alternative mechanical degradation testMany hypotheses exist concerning the origin of cleats in coal. that was developed to study the outburst prediction(Alpern, 1963However, authors like Ting(1977)and Close(1993)believed that through which it has been possible to define a"fracturability indexcleat genesis can be effectively classified in three main processes: in correlation with gas circulationdehydration, devolatilization, and tectonics. The first process conIn fact. it is well known that the natural network of fracturessts of dehydration caused by mechanical compaction of plant presented in coal allows the drainage of CBM from coal seams to thefragments when water is expelled from peat induced by over- production wells through the cleat system. Furthermore, in aburden. This process is easily understandable since coal, at the very classical approach, exploitation methods include additional fracbeginning of its formation, has a high moisture content, which ture opening induced by stimulation with injection of variousprogressively decreases as rank increases. Consequently, coal suf- fluids. However, even when using more advanced technologies thatfers considerable changes in volume that lead to fragments being are applied in several basins, such as open-hole cavity completionrearranged due to inter-granular slippage, compaction, and the method the gas production advantage revealed to be either sucollapse of cellular cavities. As a result, coal fractures tend tocessful or unsuccessful depending on the basins and or the coalcrease as dehydration increases. The devolatilization effect consists seams(see also Ayers(2002). This means that only in very favor-n the loss of volatile matter during the coalification process and able cases it is possible to obtain advantageous economic letafter the loss of moisture has already been completed. This mech- CBM productionanism also produces a decrease in coal volume which, once more,duces fracture formationcase, to know(i) the spatialis in the authors'opinion is, in eachTectorparently controls cleat orientation in coal in a pro- fractures(cleat )and (ii the frequency of their connectivity, in ordercess somewhat similar to jointing observed in other rocks. Itto make possible a right orientated hydraulic fracturing injection ofcommon to relate the strike directions of cleats to major structures fluids(water, gas, or combining both fluids )under pressure to openuch as folds in many basins, although local and lateral distur- the cleat system, thus allowing the highest amount of gas release. Inbances, such as faults, folds, and stresses, induced by differential fact, the cleat families of highest connectivity frequency are thosecompaction and produced by underaterial, tend to that define the gas circulation network to the production well, andomplicate the coal cleat system. Another aspect that must be are therefore the most favorable ones to be opened by fluids,ointed out is that, locally, cleats can be rotated and deviated from although they must be injected in the correct direction. Taking thisthe settings resulting from the stress field. In order to avoid this fact into account, drilling a higher number of holes does not solveeffect, it is necessary to study a set of samples strategically posi per se the problem of gas production from coal seams. The methodtioned, depending on the spatial basin geometry, in the coalfield to must be applied with extreme care otherwise it may lead topermit a real representative stress field studymisleading conclusions. One limitation in this method is relatedThe cleat system, as it is currently understood is theoretically with the availability of the core samples needed to this kind ofharacterized by two main sets of sub-parallel fractures ("face s other options, like the televiewer method, considered as thecleat"and"butt cleat"), both mostly orthogonal to bedding Facecleats are usually dominant, with individual surfaces almost planar, best solution to study in situ the cleat system mostly in terms ofpersistent, laterally extensive, and widely spaced. Butt cleats orientation, do not, in the authors'opinion, allow to study theconstitute a poorly defined set of natural fractures, orthogonal or microfractures, only the meso and macrofractures. Additionally, thenearly orthogonal to face cleats. Face cleats are continuous presented method is able to statistically describe in detail thethroughout the coal seam, while butt cleats tend to be discontin- characteristics of the studied samples, also in terms of spacing.lous, non-planar, commonly ending at the intersection with face r should also be noted that, although the coal cleat system alsorture, height, length, filling, and connectivitycleats. However, in practical terms, detailed cleat characteristics of aoal seam are far more complex than the two main fracture sets as depends on the local and regional tectonics, the cleat networkdescribed above. This fact is on the basis of different detailed cleat cannot be inferred using conventional regional micro-tectonicsclassifications in literature, e.g. Ammosov and Eremin (1963), studies. Indeed, in terms of mechanical properties, coal has a veryTremain et al. (1991), and Gamson et al. (1993). In 1998, Laubach particular rheologic behavior; the deformation threshold is totallyet al. (1998)defined the following detailed cleat characteristics: different from the other rocks presented in the local stratigraphiccrucial indices to classify the cleat system in a coal bati ctivity as column, even considering strata directly contacting with coalorientation, spacing, aperture, height, length, and conneseams,i.e. the roofs and floors. This particular rheologic behavioroccurs due to its microlitotypes composition, i.e. if one is dealing2. The need for a new approach to study coal cleat systemwith a rich liptite coal, one will certainly have difficulties inobserving a pernetwork since liptite has a highSince the very first studies on the coal cleat system process, elasticity behat中国煤化工 be more complexseveral authors have been interested in introducing a correct and when liptite is stadequate methodology to quantitatively characterize coal cleat whose behaviorsCNMHGnally, in most basinsnetworks. However, up to date, it was only possible to obtain that correspond to CBm deposits, the ellipsoid of effective tectonicquantitative results by a rather expensive and time-consuming stress is more or less constant, i.e. there are no changes in amountCE Rodrigues et al./ Journal of Rock Mechanics and Geotechnical Engineering 6(2014)208-218or direction of the effective stress. What really changes with localhe deformation strength of the different rocks fillingthe basin, since coal does not have a standard behavior directlylinked to tectonic stress, due to the above-mentioned aspects. Coalhaving very weak deformation strength is not able to resist minorchanges of the effective tectonic stress. This is the reason that it isnecessary to stimulate gas production by opening cleats andinjecting fluids under pressure during CBM operational issues, andthat statement agrees with the experimentally well-establishedfact that coalbed permeability is highly stress-dependent(Gamson et al., 1993; Ayers, 2002).3. Development of the proposed methodology: the"coal coretectonics"(CCT)methodThe necessity of a methodology able to produce accurate, reli-ble, and statistically significant three-dimensional (3D)data in aneasy and semi-automated way, as well as allowing a correct representation of the stress system of the coal basin, has been con-Fig. 2. Image obtained with high-resolution scannducted by the authors regarding the method described in detail inthis section and entitled"coal core tectonics"(CCT)method, basedon the work initially performed by Rodrigues(2002). A suppleN S W Ementary advantage of the method is its time-saving feature, sinceusing the gis tool is possible to perform a detailed analysis of a coresample in just a few hours. GeoMedia software provides gis withadvanced parameters that include improved display, performance.and spatial analysis. GiS has been adopted in many geologicalection2Sectionstudies since it provides the opportunity to combine layers of information about a geographical area in order to produce a betterunderstanding of different parameters involved which will obviously depend on the purpose of the project.F3 Section4th SectionsEction6thSection中国煤化工CNMHGFig 1. Sample orientation in N-S and w-E planes.Fig 3. Representation of the cleat plunging lineation.CE Roet al. Journal of Rock Mechanics and Geotechnical Engineering 6(2014)208-218ES WA一D+一日0公ig. 4.(a) Cleat elements identified on the sample used in the example given and (b)detail of the image represented in Fig 4a.GIS was chosen for three different reasons: (i)A GIS project work of another and that will also improve organizational intepermits to link data sets by common location of the data, suchgration. (ii)A GIS project is not just an automated decision-makinggeographical position(e. g. detailed coalfield geographical location), system but a tool to query analyze, and map data as a support inwhich helps numerous different institutions to share their data. By the decision-making process. GIS can then be used to decide wherecreating a shared database, one institution can benefit from the is the best location to exploit a new coal deposit--for coal mining orN-S plane(N plunging lineation)N-S plane(S plunging lineation)000000临击-d1020304050607080Plunging lineation(°)Plunging lineation(°)(b)w-E plane(W plunging lineation)W-e plane(E plunging lineation)E000000000Illth1020304050607080Plunging lineation(°)中国煤化工CNMHGFig. 5. Frequency of plunging lineation determined on different planes: (a)N-s plane(N plunging lineation); (b)N-S plane(S plunging lineation); (c)w-E plane(w plunginglineation ) and ( d)w-e plane(e plunging lineation).CE Rodrigues et al. Journal of Rock Mechanics and Geotechnical Engineering 6(2014 ] 208-218Table 1as a reference, preferably an correctly orientated core duringData of the most frequent cleats on each planilling or to measure the borehole direction in small intervalsClasses ofN-S planN-S planW-e planewith accuracy; cleat frequency, taking into account differentnumber of cleat connectivity /intersections; and cleat apertureand number of cleats filled with minerals(3) The use of GIS combined with appropriate software as a tool toquantitatively develop the following items: borehole4.25838587564geographical location; sample orientation; scanning of coresamples: the adopted model; georeferentiation of core sampleimages, cleat digitalization and cleat characterization; statisti2432078 and 82cal analyses from georeferentiatiated data; and connectivity79 and 81 5 and 73. 1. Borehole geographical locationunderground direct utilization such as in the case of CBM pro-duction -in order to minimize the potential environmentalThe first stage comprises registering all pertinent local geologimpact-if it is localized in a low risk area or if it is close to a ical information, and collecting all relevant data, such as carto-population center-and to maximize the economic profit. The in- graphic parameters, as well as the geographical coordinates of theformation can be presented concisely and clearly in the form of a system in use. It is absolutely indispensable to create specific da-map and an accompanying report, allowing the project manager to tabases since all data will be georeferentiated through localfocus on the real issues, rather than trying to understand isolated geographical coordinatesdata. Since gis products can be quickly produced, multiple sce-narios can be evaluated efficiently and effectively That will alloy3. 2. Sample orientationthe making of better decisions. (iii)GIS creates maps from the datacollected from databases. Mapping with this method is much moreflexible than by the traditional manual or automated cartography investigatThe second stage is the most difficult to be systematicallyle to high costs and time-consuming proceduresapproaches. It is also possible to digitalize existing paper maps and needed to obtain orientated samples during drilling. However, theto translate them into the GIS environment. The GIS cartographic sample orientation is considered to be an indispensable tool in CBMdatabase can be both continuous and scale free Map products can prospection, because it is the only way to have an accuratethen be created centered on any location, on any scale, and showing knowledge of sample cleat network in field, in terms of the coalpreviously selected information, effectively symbolized to highlight basin stress field In the absence of orientated data it is alwayspecific characteristicspossible, at least, to obtain the orientation of the borehole axis fromThe new methodology in the paper was developed as follows: the televiewer dataIn the example presented and for simplification, the core length(1)The use of samples that correspond to non-damaged borehole plane as a reference plane was used, in which the orthogonal planesore samples, in which it is possible to macroscopically observe of the core drilled correspond to north-south(N-S)and west-eastthe cleats. Cores are then cut into two orthogonal planes and (W-E) planes, wherein all measurements were made(Fig. 1)he two surfaces are roughly polished to clearly identify thecleat characteristics. Prior to the cut, if the cores have the 3.3. Core samples scanningtendency to break in small pieces, a previous treatment withpolymer resin as a binder becomes necessary. Samples withry difficult to obtain an acceptable optical image to behese characteristics will be considered as cleat representative used for a visual interpretation, particularly in the case of the lack ofsamples of the basin, allowing statistical inference(2)The choice of the coal cleat characteristics indicated in the contrasts in the examined surface as it is always the case in coalliterature(Laubach et al, 1998 )that are considered to be moreHowever, using a high-resolution scanner, with proper scanningdirectly related to gas production (cleat directions, measuredparameter adjustments, it is possible to obtain reliable images inTable 3Cleat lines measured in N-s plane (N plunging lineation) and w-e plane(E Cleat lines measured in N-s plane(S plunging lineation) and W-E plane (wClasses ofin N-S plane(NdCleat lines measuredClasses of cleatplunging lineation)()plunging lineation()plunging lineation()以oder)(‰)88→27087→906.56→1885→906.187→18089→2703→0;83→0;and84→0Ha中国煤化工二20CNMHG80→18080→27082→07→90and5→9079→270and81→270CE Rodrigues et al. Journal of Rock Mechanics and Geotechnical Engineering 6(2014) 208-218Pne:N121°,87Pane:N135°89Plane:N108°891.00Catr.sa aca: B2051Pane:N141°,87“EPLane;N124°,87“EPane:N174°,89°EGEed arde 88169Plann: N 179.88.Ephne:N146°,4吒GPne:N91°82Plane:N135°,89"wlane:N143°87WKPlane: N 1G2,88WPane:N142°,86Wane:N109°,89wMMan Maltam era :1200Pane:N12s°,86°WPlane:N139°,85wPlane:N127°84WQCalcuated. rdr 8$22Plane:N135°,83°wPane:N131°,83W中国煤化工CNMHGe1-10Cleat frequenciesdetermined from N-s plane(S plunging lineation)and W-E plane(w plunging lineation letters k-T indicate cleat frequencies 1-10.CE Rodrigues et al. Journal of Rock Mechanics and Geotechnical Engineering 6(2014)208-218Table 4the cleat aperture(open or closed ) the cleat filling(by secondaryalculated planes by combining N-S plane(N plunging lineation) with W-E plane mineralization, as referred to by Faraj et al.(1996)); and any(E plunging lineation)and N-S plane(S plunging lineation ) with W-E(W plungingeventual additional informationClasses of cleatCleat planes calculatedCleat planesfrequency(infrom combining N-Scalculated fromdecreasing order)(‰)(N)to w-E(E) planescombining N-S(S)3.5. Georeferentiation of core sample images, cleat digitalization,to w-E(W) planes and cleat characterizationThe scanned images are georeferentiated in the Gis program,N121°,87°Ewhich directly allows cleat digitalization and cleat characterizationN141°,87°EN124,87°Eof each interpreted line on the basis of the above described model.N109°89°WN174°,89°EN125°,86°VIt is also pertinent to focus the attention on the adopted scale, sinN139°,85it could influence the digitalization process, as well as its interN146°4N127°,84Wpretation. Since the average cleat length in the example presentedN179°,82EFig 2 is less than 1 mm, the best scale corresponds to enlarging119N131°.83Wthe original image up to 10,000 times. Fig 3 shows the cleats, whichwere digitalized in the selected sample taking into account thefollowing direction of dip: light green lines correspond to thewhich discontinuities can be identified(Fig. 2). With a high quality intersection of horizontal cleat-Wane black lines corre.image it is also possible, in"GIS environment, to improve the spond to intersection of vertical cleat-W-e plane: red linesimage, with specific parameters which allow to introduce changes correspond to w cleat plunging lineation -W-E plane light bluewelllines correspond to e cleat plE plane: dao dge bases s hie a we a mento oner ferent ate create sercedois blue ine kcorreseponid to intersection o horizontal cleaternicathe core sample, the high-resolution scanner also allows to improve cleat-N-S plane: brown lines correspond to cleat n cleat plungingthe captured imageslineation -N-s plane; and purple lines correspond to s cleatplunging lineation-N-S plane3. 4. The adopted modelThe next step consists in creating a database comprising all 3.6. Statistical analyses from georeferentiated datameasured and interpreted data, all related to sample images, suchas the images themselves, the cleat digitalization, and some evenThis stage consists in converting georeferentiated data into thetual complementary information. due to the fact that when one is form of text and statistical parameters in order to allow a correctdealing with apparent measurements, it is indispensable to develop and suitable statistical interpretation as well as optimum stereo-a new modified terminology to characterize the cleat system. Note graphic projection Data must then be processed by specific soft-that the main objective is to relate the cleat system to the gas cir- ware able to produce reliable statistical results and to plot linesinformation will be: the description of the and planes determined in GIS. There are a different number ofplane where the data were collected; the azimuth of the cleat, powerful commercial computerized applications available for thatexpressed in the dip direction; the cleat direction; the cleat length; purposeMean Resultant dirn a 87-037lean Resultant dirn 85-010Mean Resultant length=1, 00Calculated, girdle: 23/110Calculated, girdle 8/1Calculated beta axis: 82-325Mean Plane:N100°,85哐EMoan plane:N127°,87EMean Resultant dirn 86-087Calculated. girdle: 57/355Calculated beta axis: 33-175Mean Plane:N177°,中国煤化工CNMHGFig. 7. (a) Mean plane(sketched line)determined for 90-120 plunging lineation interval in N-S plane(n plunging lineation (b) mean plane(sketched line)determined120-150 plunging lineation interval in N-S plane(n plunging lineation ) and (c)mean plane(sketched line)determined for 150-180 plunging lineation interval in N-s planeCE Rodrigues et al. Journal of Rock Mechanics and Geotechnical Engineering 6(2014)208-218NMean Resultant dirn z 89-199esultant length = 1.00Calculated. girdle: 2/323Calculated beta axis: 88-143Calculated beta axis: 82-17Mean planc:N109°89WMean Plane:N135°,85W(a)NMean Resultant dir'n= 88-25Calculated girdle: 9/149Calculated beta axs: 81-329can planc:N162°,88WFig 8. (a)Mean plane(sketched line)determined for 90-120 plunging lineation interval in N-s plane (s plunging lineation):(b) mean plane(sketched line)determined for0-150 plunging lineation interval in N-S plane(S plunging lineation); and (c)mean plane(sketched line)determined for 150-180 plunging lineation interval in N-S plane(S3.7. Connectivity frequencylines, which will be combined afterwards and used to determineThe connectivity frequency is one of the most important pa-The statistical treatment and interpretation of data were carrierameters to be taken into account due to its relevant role in gas out according to the following three sequential procedurescirculation. Based on the digitalized values and statistical data, it ispossible to establish a global connectivity frequency, which is(1)Initially, the two different planes were analyzed individually, inetermined by calculating the total cleat connection. In order toorder to produce a suitable filtering of the large number of el-Iter the huge set of data(e.g. 5072 cleats ), three intervals of cleatements collected as followsplunging lineation were considered(ie.0°-30°,30°-60°,and(i) To calculate the frequencies of the cleat intersections in N-S and W-E planes, and the plunging lineation of eachelement was taken into account since the measured cleatshave a large variety of lengths, the best option consisted in4. Results and discussiondetermining frequencies on the basis of cleat length standardization classes. As a result the following data wereThe example given before to demonstrate the proposed methainedodology refers to a coal core of approximately 1 m in length that theN-S planeluthors collected It belongs to a coalfield in the exploration stage.North plunging lineation=37.6%After the implementation of all the above-mentioned steps,South plunging lineation =60.0%ecessary for the application of the Gis, a total of 5072 elementsIntersection of vertical cleat= 2, 2%were accounted in the present exercise. Fig. 4a represents a block-Intersection of horizontal cleat=0.2%diagram of the 3d data obtained from the whole core, and Fig. 4bW-E planeshows a detail of the image represented in Fig. 4a. It is possible toWest plunging lineation= 52.9%verify that some cleats can be followed from the n-s plane to theEast plunging lineation=45.0%W-E plane, which is possibly due to the effects induced by theIntersection of vertical cleat= 1.8%regional stress identified on the coalfield. Fig. 4 also allowstersection of horizontal cleat=0.3%concluding that all data obtained with the Gis analysis consist of(ii)To determine, on both N-S and W-E planes, the cleat fre-lencies of each plunging lineation in each direction, histograms were drawn. as the vertical and horizontal cleatsTable 5Mean planes frequencies in N-s plane(n plunging lineation)/w-E plane (Eon both planes correspond to plunges of 90 and 0o.plunging lineation) and N-s plane(S plunging lineation)/w-E plane(w plungirespectively, the statistical analysis has focused on thelineation).north and south plunges of lineations on N-S plane, andPlanePlunging lineation Mean planes Frequency (%west and east plunging lineations on W-E plane Histo-intersection interval ()grams in Fig. 5 show that the plunges of lineation around85°andN-S(N》10.1W-E(E)W-E(E)120-150N127,87°E234N-S(N)22.6W-E(E)中国煤化工 ther measurements0-180N177°,86E121N-5(N)8.6W-E(E)althougN109°,89W42N-S(S)5.2W-E(W)CNMHGVant to defining theW-E(W)120-150N135°,85W30.5N-S(5)40.2W-E(W)(iii) To point out the frequency of the cleat intersection in N-s150-180N162,88W5.6N-5(S)6.5W-E(W)and W-E planes, it is necessary to select at least 50% of theCE Rodrigues et al. Journal of Rock Mechanics and Geotechnical Engineering 6(2014 ] 208-218conclude that any set of lines is capable of producing differentplanes, which implies the need to establish criteria to defineN53,90 / Verticallwhich planes should be considered as the most importantones. The question was bypassed by applying statistical datapresented in term from(1)above. In fact, these statisticalresults will allow to combine data from N-s planeplunging lineation) to data from W-e plane(e plunginglineation), which corresponds to plunging lineation planeswith the lowest cleat frequency, and will also allow toI Horizontalcombine data from N-s plane(s plunging lineation) to datafrom W-E plane (W plunging lineation), corresponding toplunging lineation planes with the highest cleat freN127387EThe results obtained from such combinations are presented inN135,85In this second stage, further procedures are necessary andi)Projecting lines using stereographic projectionIn the example given, Fig. 6 presents the main planes foreach cleat frequency as well as the basic statistical pa-Fig. 9. Schematic representation of the four dominant planes determined in the preameters obtained with stereographic projectionssent case study(ii) Determining the most representative planesStructural principles allow dividing cleat plungingTable 6lineation data into intervals, since it is possible toClasses of plunging lineation defined on sample WTB 5/30consider the calculated cleat planes of theClass designationPlunging lineation interval ()plunging lineation changes between an acceptableon degree. In fact, planes deternlines projection shown in Table 4 allow us to consider>30and<60that the best option is to create intervals of 30.Consequently, itible to calculate three differerplanes from N-s plane(n plunging lineation)combining with W-E plane(e plunging lineation), andeasured cleats in order to obtain a statistical samplingfrom N-S(N plunging lineation) combining with W-Erepresentation. The frequency of cleat plunging lineation(W plunging lineation). Fig. 7 shows the three meanbetween 20 and 60 is too low and it will not produce aplanes in N-s(n plunging lineation)/W-E plane(Emajor effect on gas circulation stage. Table 1 presents theplunging lineation) planes, calculated by combiningmost frequent cleat plunging lineation, which conforms toplanes corresponding to cleat frequencies 2 and 10the minimum of 50% mentioned aboy(Table 4 and Fig. 7a), planes corresponding to cleat fre2) The second stage consists of linking intersections of the twoquencies 1, 3, 4, and 5 (Table 4 and Fig 7b)and planeN-S and W-E planes on the basis of the structural theoreticalfrom cleat frequencies 6, 7, and 9 (Table 4 and Fig. 7c)fundaments which consider that two lines collected from twoFig 8 shows the three other mean planes in N-s planedifferent planes will allow the determination of the plane that(s plunging lineation )/W-E(W plunging lineation)goes through those lines. From Table 1 it is possible toplanes, calculated by combining planes corresponding1,2,4,6,78,9,and10( able 4 and中国煤化工CNMHGClassesFig. 10. Histogram of connectivity frequency from different classes of plunging lineation.CF Rodrigues et al. Journal of Rock Mechanics and Geotechnical Engineering 6(2014) 208-218217Fig. 8b). The two other mean planes correspond to financial support for this work that could have influencedplanes that fall into the other specific interval condi- outcometions, i.e. the mean plane defined in 90-120 plunginglineation interval is represented by the cleat frequency 5(Table 4 and Fig 8a), and the one defined in the 150Acknowledgments180 plunging lineation interval is represented by cleatThe authors are grateful to Fundacao Fernando Pessoa/Fernando(ii) Selecting the dominant planes on the basis of cleat fre- Pessoa University for supporting this investigation in the scope ofquency criterion and on the data presented in item(ii)aboveneering Research UnitTable 5 shows that in the first set of three planes interception,the most frequent plane corresponds to N127,87 E, and in tiReferencessecond set of three planes interception, the most frequentN135°,85°WAlpern B. Fissuration-Fragilite, Documents techniques des Charbonnages de france963:5:223-33( in french)vertical and horizontal planes should also be considered (item Alpern B becton ique et gisement diu gaz dans les bas sins houilerse E ue bib.(i) from(1)above). In what concerns the vertical plane, and duenages de France 1967: 12: 687-93 (in Frenchto the highest frequency detected in N-S plane, it should be Alpern B. Tectonics and gas deposit in coalfields a bibliographical study and exconcluded that this plane strikes at 53 N.amples of application. International Journal of Rock Mechanics and MiningSciences and Geomechanics Abstracts 1970: 7(1): 67-74The four dominant planes are represented in the solid model Alpern B, Lemos de Sousa M). Documented internationaldiagram presented in Fig 9.mentary fossil fuels: coal: definitions, classifications.(3)In what concerns cleat connectivity in direct relation with gas Ammosov l, Eremin IV Fracturing in coal. Washington DC: US Departmelease from the seam, the presented method considers a newparameter, the "global connectivity frequency, Gcf, calculated Ayers WB. Coalbed gas systems, resources, and production and a review of cony the ratio between the total length of cleat intersection andtrasting cases from the San Juan and Powder River Basins. AAPG Bulletinthe total length of cleat detected. In the present example2002:86(11):1853-90.Cao Y, He D, Glick DC. Coal and gasin footwalls of reverse faults. Inter-value of Gcf=85.77% is obtainednational Journal of Coal Geology1-2):47-63Close JC. Natural fractures in coal. InRice DD, editors. Hydrocarbons fromAmerican Association of PetroleFinally, it is also important to study the interception betweenCoal, AAPG Studies in Geology, TGeologists: 1993. pp, 119-32some specific cleat plunging lineation. Taking into account classes Durucan S. Shi J-Q, Improving the CO2 well injectivity and enhanced coalbedof plunging lineation of 30, as well as vertical and horizontal cleatsmethane production performance in coal seam. International Journal of Coal(see Table 6), it is possible to obtain important results. The ten Faraj BSM, Fielding CR MacKinnon IDR. Cleat mineralization of Upper Permianpossible combinations are presented in Fig. 10. The most frequentBaralaba/ Rangal coal measures, Bowen Basin, Australia. London: Geologicalnes consist, in decreasing order, in interceptions between classes 1Society:1996.pp.151-64.and 3, followed by interceptions between classes 2 and 3, and beFlores RM. Coalbed methane: from hazard to resource. International Journal of CoalGeology1998;35(1-4):3-26tween classes 3 and 4. This will allow to conclude that the cleat Gamson PD Sorption behaviour and microstructure of coals, the effect of secondaryplanes included in these classes of plunging lineation are themineralisation and the prospects for its removal. In: Coalbed Methane Extracominant ones in terms of gas release. In fact, this conclusion is alsotion- analysis of UK and European Resources and Potential for Development.London, UK 1994strongly supported by the high frequency determined for the Gamson PD. Beamish BB, Johnson DP Coal microstructure and micropermeabilityplanes N127, 87 E and N135, 85W(see Table 2), also sustained byand their effects on natural gas recovery, Fuel 1993: 72(1): 87-99.sub-horizontal to horizontal planes(item(i) from(1)above)Jin G. Pashin JC, Payton JW. Application of discrete fracture network models to coalbedmethane reservoirs of the Black Warrior basin. In: Proceedings of the 2003 Inter-Kara ational Coalbed Methane Symposium. Tuscaloosa, Alabama; 2003. Paper 0321Karacan CO, Okandan E. Fracture/cleat analysis of coals from Zonguldak Basin(northwestern Turkey relative to the potential of coalbed methane production.The present study allows concluding that whenever the paInternational Journal of Coal Geology 2000: 44(2): 109-25.ubach SE, Marrett RA, Olson JE, Scott AR Characteristics and origins of coal cle.rameters referred to above in (2)and (3 )in Section 4 are defined, ita review. International Journal of Coal Geology 1998: 35(1-2): 175-207is therefore possible to inject, in the right space orientation, presLevine jr. exploring coalbed methane reservoir. Short course. Rueil-Malmaisonsurized fluids in order to open the coal cleat system, and directlyInstitut Francais du Petrole: 1993.MacCarthy F]. Tisdale RM, Ayers r WB. Geological controls on coalbed prospectivitydrain the maximum amount of gas flow to CBM exploitation wellsin part of the North Staffordshire Coalfield, UK, voL. 109. London: Geologicalven to inject CO2, as it is the case of CO2 geological sequestrationty: 1996. pp 27-42. Special Publicationprojects. In the specific case study herein presented, the best diMacrae JC, Lawson W. The incidence of cleat fractures in some Yorkshire coal seamsrection to induce the fluid injection falls into the planes N127 87E Mazumder S, Wolf KHAA, Elewaut K. Ephraim R. Application of X-ray computedind N135, 85 W, which is strongly supported by their high fre-omography for analyzing cleat spacing and cleat aperture in coal samples.quency. the present approach to best define the coal cleat systemInternational Journal of Coal Geology 2006: 68(3-4): 205-22Montemagno CD, Pyrak-Nolte L]. Fracture network versus single fractures: mea-was developed as a contribution to the selection of the completionurement of fracture geometry with X-ray tomography. Physics and Chemistrylethod for coalbed exploration wells in terms of design, andof the Earth, Part A: Solid Earth and Geodesy 1999: 24(7): 575-9therefore cost estimates. Moreover, the method is applicable to the Morris JP. Pyrak- Nolte L), Giordano N). Cheng Tran Lumsdaine A Fracture geprimary investigation steps of any potential CBM reservoir, i.eal. In: International Coalbed Methane Symposium. Tuscaloosa: University ofand exploring phases, thus contributing to bestAlabama; 1999. pp. 377-88mate of its economic potentiality since the very beginningNickelsen RP Hougin the Appalachian plateau of PennsylvaniaGeological SocielPattison Cl, Fielding中国煤化工 d origin of fConflict of interestCNMHGAustralia, vol. 109. LondoWe wish to confirm that there are no known conflicts of interest Pyrak-Nolte LJ. Haley GM, cash Bw. Effective cleat porosity and cleat geometry fromwood's metal porosimetry. In: Proceedings of the 1993 International Coalbedassociated with this publication and there has been no significantMethane Symposium, vol 1. Tuscaloosa: University of Alabama; 1993. pp. 639-47.CE Rodrigues et al. Journal of Rock Mechanics and Geotechnical Engineering 6(2014 ] 208-218Rodrigues CFA. The application of isotherm studies to evaluate the coalbed methane Ting FTC. Origin and spacing of cleats in coal beds. Journal of Pressure Vesselpotential of the Waterberg Basin, South Africa. PhD Thesis. Porto, PortugaTechnology1977:994):624-6.University of Porto: 2002. p. 289Tremain CM, Laubach SE, Whitehead Ill NH Coal fracture (cleat) patterns in UpperRyan B. Cleat development in some British Columbia coals. In: Geological Field-Cretaceous Fruitland formation, San Juan Basin, Colorado and New Mexicowork: 2002. C.P. Ministry of Energy and Mines, British Columbia Geologicalmplications for coalbed methane exploration and development. InSurvey:2003.pp.165-83Schwochow S, Murray DK, Fahy MF, editors. Coalbed Methane of Western NorthSolano-Acosta w Mastalerz M. Schimmelmann A. Cleats and their relation toAmerica. Rocky Mountain Association of Geologists: 1991. pp 49-59.geologic lineaments and coalbed methane potential in Pennsylvanian coals in van Krevelen DW. Coal: typology, physics, chemistry, constitution. 3rd ed Rotter-ndiana. International Journal of Coal Geology 2007: 72(3-4): 187-208dam: Elsevier Science: 199Solano-Acosta W, Schimmelmann A, Mastalerz M, Arango I. Diagenetic mineraliWolf K-Haa, Ephraim R, Bertheux W. Bruining J. Coal cleat classification andzation in Pennsylvanian coals from Indiana, USA: C/C and 80/b0 impliermeability estimation by image analysis on cores and drilling cuttings Intions for cleat origin and coalbed methane generation. International Journal ofoceedings of the International Coalbed Methane Symposium. Tuscaloosa,Coal geology2008:73(3-4):219-36.Alabama: 2001.Su X. Feng Y, Chen J, Pan J. The characteristics and origins of cleat in coal fromWestern North China. International Journal of Coal Geology 2001: 47(1)51-62中国煤化工CNMHG