CHAPTER 12 METHANE References

Transcription

1 CHAPTER 1 METHANE 1.1. OVERVIEW AND ADDITIONAL PROPERTIES OF METHANE THE RETENTION AND RELEASE OF METHANE IN COAL Ga retention in coal The releae of methane from coal Diffuion and Darcy flow Deorption kinetic Determination of ga content Indirect method (adorption iotherm) Direct meaurement method MIGRATION OF METHANE Fluid Flow through a permeable medium Incompreible flow Compreible flow Radial flow of ga Tranient radial flow The permeability of coal Effect of mechanical tre Effect of ga preure Two phae flow EMISSION PATTERNS INTO MINE WORKINGS Source of methane in coal mine Methane layering Ga outburt In-eam outburt Outburt from roof and floor Prediction of methane emiion into the ventilation ytem METHANE DRAINAGE In-eam drainage Gob drainage by urface borehole Cro-meaure methane drainage Drainage from worked-out area Component of a methane drainage ytem Pipe range Monitor Control and afety device Extractor Pump Planning a methane drainage ytem Data collection and etimation of ga capture Deign of the drainage network Utilization of drained ga Reference

2 1.1. OVERVIEW AND ADDITIONAL PROPERTIES OF METHANE The major propertie and behavioural characteritic of methane were outlined in Section That Section hould be reviewed a an introduction to the more detailed treatment given in thi chapter. There are three primary reaon for giving particular attention to methane. Firt, it i the naturally occurring ga that mot commonly appear in mined underground opening. Secondly, it ha reulted in more exploion and related lo of life than any other caue throughout the recorded hitory of mining. The flammability characteritic of thi ga have been tudied ince, at leat, the time of Agricola in the 16th Century. The number and everity of coal dut exploion initiated by methane ignition declined after the development of electric cap lamp and the replacement of haft bottom furnace with fan (Section 1..). However, the intenity and wider deployment of mechanized rock-breaking equipment reulted in a renewed incidence of frictional ignition of methane following the 1960 (Richmond et al, 1983). Fortunately, modern tandard of ventilation and dut control prevent mot of thee from developing into the larger dut exploion and diatrou lo of life. The third reaon for giving pecial attention to methane concern the continued development of methane drainage technology. Although the primary reaon for extracting methane at high concentration from the trata around mine continue to be the reduction of methane emiion into thee mine, a growing incentive ha been the drainage of methane to provide a fuel ource in it own right. Methane drainage may now be undertaken for thi purpoe from trata where there i no known intention of ubequent mining. In thi chapter we hall conider four broad area; firt, the manner in which methane i retained within the trata and the mechanim of it releae when the rock i diturbed by mine working or borehole. We hall alo outline mean of determining the ga content of carbonaceou trata. Secondly, we hall examine the migration of the ga from it geologic ource toward working or borehole. Thi will encompa an analyi of trata permeability and flow through fracture network. The third conideration i the dynamic pattern of methane emiion into active mine working, varying from normal cyclic variation, through ga layering phenomena, to outburt activitie. Finally, we hall claify the major technique of methane drainage. No one of thee ha univeral application and the election of drainage method mut be made with great care according to the geology of the area, the method and layout of mining (if any) and the natural or induced phyical propertie of the rock. Firt, however, let u lit ome further propertie of methane in addition to thoe given in Table The data are referred, wherever applicable, to tandard temperature and preure of 0 C and kpa. H molecular tructure : H C H H melting point : 90.5 K boiling point : K critical temperature and preure : K and 4.63 MPa latent heat of vaporization (at 111.3K) : 508. kj/kg 1 -

3 latent heat of fuion (at 90.5 K) : 58.8 kj/kg pecific heat C p : 184 J/(kg K) pecific heat C v : 1680 J/(kg K) ientropic index : olubilitv in water : 55.6 litre per m 3 of water (33.1 litre/m 3 at 0 C) upper calorific value : MJ/kg lower calorific value : MJ/kg thermal conductivity : W/(m C) [0.038 W/(m C) at 0 C] dynamic vicoity : ( t) x 10-6 N/m (temperature t = 0 to 100 C) 1.. THE RETENTION AND RELEASE OF METHANE IN COAL In order to undertand the manner in which methane i retained within coal and the mechanim of it releae, it i firt neceary to comprehend the internal tructure of coal. The exitence of a large number of exceptionally mall pore and the correponding high internal urface area of coal ha been recognized for many year and numerou conceptual model have been uggeted. However, electron microcopy ha revealed the actual tructure that exit. Coal i not a ingle material but a complex mix of foilied organic compound and mineral. The compoition and tructure depend upon the nature of the original vegetation from which a given coal eam formed, the timing and turbidity of the water flow involved in the edimentary procee of depoition and the metamorphic effect of preure, temperature and tectonic treing over geological time. Electron micrograph have, indeed, confirmed the exitence of pore, many of which may not be interconnected. An example i hown on Plate. However, other area may indicate amorphou, granular, ponge-like or fibrou tructure, even within the ame eam. Furthermore, the coal ubtance i interected by a fracture network with aperture varying from thoe that are comparable to pore diameter through to ome large enough to be een with the naked eye. In thi Section we hall concentrate on the manner in which methane i-held within the coal, the kinetic of it releae when the geological equilibrium i diturbed by mining or drilling, and how the ga content of a eam may be determined Ga retention in coal Methane exit within coal in two ditinct form, generally referred to a free ga and adorbed ga. The free ga comprie molecule that are, indeed, free to move within the pore and fracture network. Poroitie of coal have been reported from 1 to over 0 percent. However, thoe value depend upon the choen definition of poroity and the manner in which it i meaured. Abolute (or total) poroity i the total internal voidage divided by the bulk volume of a ample and may be difficult to meaure accurately. Effective (or macrocopic) poroity i the ratio of interconnected void pace to the bulk volume. The latter definition i more ueful in the determination of recoverable ga from a eam. Unfortunately, the uual procedure of meauring poroity depend upon aturating the internal voidage with ome permeating fluid. The probability of a ga molecule entering any pore or interconnection rie a it molecular diameter decreae or it mean free path increae. (The mean free path i the tatitical average ditance between colliion of the gaeou molecule.) Furthermore, any adorptive bonding between the fluid and the olid may obtruct the narrower interconnection. The meaured effective poroity i, therefore, dependent upon the permeating fluid. In order to handle the extremely mall pore that exit in coal, the maximum value of effective poroity i obtained by uing helium a the permeating fluid. Helium ha a mall molecular diameter (ome 0.7 x 10-9 m), a relatively large mean free path (about 70 x 10-9 m at 0 C and atmopheric preure) and i non-adorptive with repect to coal. For comparion, pore diameter of eatern U.S. coal vary from over 30x10-9 to le than 1x10-9 m (Gan, 197). 1-3

4 Plate. Electron micrograph of a bituminou coal (after Harpalani). An attractive force exit between the urface of ome olid and a variety of gae. Coal urface attract molecule of methane, carbon dioxide, nitrogen, water vapour and everal other gae. Thoe molecule adhere or are adorbed onto the coal urface. When the adorptive bond exceed the hort-ditance repulive force between ga molecule (Section.1.1.), then the adorbed molecule will become packed together a a monomolecular layer on the urface. At very high ga preure, a econd layer will form with a weaker adorptive bond (Jolly, 1968). Figure 1.1 illutrate the variation of both total and adorbed methane with repect to ga preure and at contant temperature. The curve illutrating adorbed methane i known a the adorption iotherm. Some 95 percent of the total ga will, typically, be in the adorbed form, explaining the vat reerve of methane that are contained within many coal eam. 1-4

5 Ga content cu.m/t at STP dry ah-free 0 C total ga adorbed ga free ga Ga preure MPa Figure 1.1 Example of adorbed, free and total ga iotherm for methane in coal. The mot widely ued mathematical relationhip to decribe adorption iotherm wa developed by I. Langmuir (1916 and 1918) for a monomolecular layer. Thi may be expreed a q q max bp = 1+ bp (1.1) where q = volume of ga adorbed at any given preure (m 3 /t at NTP) q max = the maximum amount of ga that can be adorbed a a monomolecular layer at the prevailing temperature (m 3 /t at NTP) P = ga preure (uually expreed in MPa) b = Langmuir contant (MPa -1 ), a function of the adorptive bond between the ga and the urface (1/b i the preure at which q/q max = 1/) The value of Langmuir contant depend upon the carbon, moiture and ah content of the coal a well a the type of ga and prevailing temperature. Boxho et al (1980) report value of b decreaing from 1. MPa -1 at a volatile content of 5 percent to 0.5 MPa -1 at a volatile content of 40 percent. Figure 1.(a) indicate the increaed adorptive capacitie of the higher rank coal, thee having greater value of carbon content. Value of q max are hown to vary from about 14 to over 30 m 3 /t. The effect of the type of ga i illutrated on Figure 1.(b). Adorption iotherm are normally quoted on a dry, ah-free bai. The amount of methane adorbed decreae markedly at mall initial increae in moiture content of the coal. Mot of thi natural moiture i adorbed on to the coal urface. However, adorptive aturation of water molecule occur at about 5 percent moiture above which there i little further decreae in methane content. A widely ued approximation i given by Ettinger (1958): qmoit 1 = qdry h (1.) where q moit = ga content of moit coal (m 3 /t) q dry = ga content of dried coal (m 3 /t) h = moiture content (percent) in the range 0 to 5 percent. (Aume 5 percent for greater moiture content) 1-5

6 Adorbed methane cu.m/t at STP anthracite low volatile bituminou high volatile bituminou Ga preure MPa Figure 1.(a) Example of adorption iotherm for methane in coal. The amount of ga adorbed increae with the carbon content of the coal. Adorbed ga cu.m/t at STP carbon dioxide methane nitrogen Ga preure MPa Figure 1.(b) Adorption iotherm for carbon dioxide, methane and nitrogen in a bituminou coal at 5 C. 1-6

7 The mineral matter that comprie the ah contituent of coal i eentially non-adorbing. Hence the methane content decreae a the percentage of ah rie (Barker-Read, 1989). In order to expre ga content on an ah-free bai, a imple correction may be applied. ( 1 0. a) qactual = qah free 01 (1.3) where a = ah content (percent 1... The releae of methane from coal In the unditurbed tate, equilibrium exit between free ga and adorbed ga in the pore and fracture network of coal. If, however, the coal eam i interected by a borehole, or diturbed by mining, then the ga preure gradient that i created will reult in flow through natural or treinduced fracture. The reulting reduced ga preure in the pore will promote deorption. The proce will move from right to left along the appropriate adorption iotherm. A glance at Figure 1.1 or 1. indicate that the rate of deorption with repect to ga preure increae a the preure fall. The migration of methane from it original location i retarded by narrow and tortuou interconnection between pore and microfracture, obtruction by adorbed molecule and the reitance offered by the fracture network Diffuion and Darcy flow. Current thinking i that two type of flow take place. Within the micropore tructure, diffuion flow occur while, in the fracture, laminar flow dominate. Let u firt dicu diffuion flow. Thi occur whenever a difference in concentration of molecule of a given ga occur. The implet mean of quantifying diffuion flow i by Fick Law: u x = C D x m/ (1.4) where u x = velocity of diffuion in the x direction (m/) D = coefficient of diffuion (m / but often quoted in cm /) C = concentration of the pecific ga (m 3 /m 3 of coal) (The negative ign i neceary a movement occur in the direction of decreaing concentration.) Bulk diffuion i the normal ga-to-ga diffuion that occur becaue of a concentration gradient in free pace (ee Appendix A15.4). However, within the confine of a micropore tructure, two other form of diffuion may become effective. Surface diffuion arie from lateral movement of the adorbed layer of ga on the coal urface while Knudon diffuion occur due to tranient molecular interaction between the ga and the olid. Laminar flow in the fracture network follow Darcy Law (1856) for permeable media. u k P = µ x m/ (1.5) where u = ga velocity (m/) k = permeability (m ) µ = dynamic vicoity (N/m ) P/ x = preure gradient (Pa/m). (Note the analogy between Darcy Law and Fourier Law of heat conduction, equation (15.4).) Darcy Law i eentially empirical and aume that the ga velocity at the urface i zero (Section.3.3). For narrow paage, lippage at the wall (urface diffuion) may become ignificant and can be taken into account by adjuting the value of permeability (Klinkenberg effect, Section ). 1-7

8 Controvery ha exited on whether diffuion flow or Darcy flow predominate in coal. Diagreement in the finding of differing reearcher are probably a conequence of the wide variation that exit in coal tructure. It would appear that diffuion flow govern the rate of degaing coal of low permeability while Darcy flow in the fracture network i the dominant effect in high permeability coal Deorption kinetic A newly expoed coal urface will emit methane at a rate that decay with time. Figure 1.3(a) illutrate a deorption curve. Thi behaviour i analogou to the emiion of heat (Section 15...). A number of equation have been uggeted depending, primarily, on the conceptual model adopted for the tructure of coal. For a pherical and porou particle with ingle ized and non-connecting capillarie, work by Wheeler (1951) and Carlaw (1959) lead to the erie q q () t max 6 = 1 π n= 1 n 1 4 D n exp d π t (1.6) where q(t) = volume of ga (m 3 ) emitted after time t () q max = total volume of ga in particle D = coefficient of diffuion (m /) d = equivalent diameter (m) of particle (= 6 x volume/urface area) fraction adorbed q(t)/qmax Time () theoretical actual Figure 1.3(a) Example of a methane deorption curve plotted againt time. A more practicable approximation i given by Boxho et al (1980) a follow: q q () t max 4π 1 exp d D t (1.7) 1-8

9 Both of thee equation approximate to: q q () t max 1 d D t π 1 D t d (1.8) while for q(t)/q max < 0.5, the relationhip can be implified further to () t q 1 D t (1.9) q d π max Thi latter equation implie that during the early tage of deorption the emiion rate i proportional to t. Thi i illutrated on Figure 1.3(b). Furthermore, plotting the initial emiion rate againt t allow the correponding value of the coefficient of diffuion, D, to be determined initial lope theoretical actual fraction deorbed q(t)/q max time ½ Figure 1.3(b) A methane deorption curve plotted againt the quare root of time. 1-9

10 Unfortunately, there i a major problem aociated with all of thee equation. The coefficient of diffuion, D, i not a contant for any given ample. It i dependent upon the ize range of individual particle the ditribution of pore ize the number, ize and tortuoity of fracture (thi, in turn, depending upon the tre hitory of the ample) temperature preure and current ga content. The latter factor are particularly troubleome a thee, and hence D, will vary a the degaing proceed. Lama (1987) ha uggeted the relationhip m P D = D (1.10) o P o where P = current value of preure (Pa) and D o = coefficient of diffuion at ome original (eam)preure, P o (m /) The variation in D contribute to the change in apparent permeability of a coal ample when ubjected to mechanical tre or variation in mean ga preure (Section 1.3..). Conidering the enitivity of D to a ignificant number of variable, it i not altogether urprizing that a wide range of value have been reported, from 1 x 10-8 to 1 x m /. In order to take the particle ize into account, it ha become common to expre the diffuivity a the ratio D/a ( -1 ) where a = particle radiu (m). A further weakne i that the fundamental equation (1.6) i baed on uniformly ized capillarie. Thi lead to ignificant deviation between predicted and oberved deorption rate at the longer time (q(t)/q max > 0.6) a indicated on Figure 1.4 (Smith and William, 1984). The difficultie encountered in the derivation and general applicability of analytical deorption equation led E. M. Airey (1968) to propoe an empirical relationhip:. q q () t max = n t 1 exp (1.11) t 0 where t 0 = 'time contant' = time for 63 percent of the ga to deorb () and n = an index that varie from ome 1/3 for bituminou coal to 1/ for anthracite and dependent, alo, upon the degree of fracturing. Both t 0 and n are influenced by the range and magnitude of particle ize and, hence, hould be quoted with reference to a pecific ize pectrum. For example, value of t 0 for Welh anthracite lie, typically, between 1500 and 100 econd for the ize range 0.05 to 0.30 mm at 5 C (Barker-Read, 1989). Smaller particle will give reduced value of both t 0 and n. An improved deorption time parameter, τ, that i independent of the ga capacity of a ample ha been propoed by Radchenko (1981) and Ettinger et al (1986) (ee, alo, Barker-Read and Radchenko (1989) and may be related to Airey' contant: 0 60 n t τ = 1 exp ec (1.1)

11 It can alo be hown that π a τ = ec (1.13) 36 D where a = radiu of the particle (m) The value of τ can be gained from the deorbed fraction v time curve (e. g. Figure 1.3(b)): τ = 1/(initial lope) ec (1.14) Equation (1.13 and 1.14) provide a ready mean of determining D/a from a deorption tet Determination of ga content There are two reaon for meauring the methane content of coal eam and aociated trata. Firt, uch data are required in the aement of methane emiion into mine working and, hence, the airflow required to dilute thoe emiion to concentration that are afe and within mandatory threhold limit value (Section and ). Secondly, the ga content of the trata i a required input for computer model or other computational procedure to determine the ga flow that may be obtained from methane drainage ytem. There are alo two ditinct approache to the evaluation of ga content, the indirect method which employ adorption iotherm and the direct meaurement method which relie upon obervation of ga releae from newly obtained ample. Let u conider each of thee in turn Indirect method (adorption iotherm) In thi technique, repreentative chipping are obtained from the full thickne of the eam, mixed and ground to a powder of known particle ize range. The ample i dried at a temperature of not more than 80 C and evacuated to remove the gae that remain in the pore tructure. Unfortunately, the proce of evacuation may alter the internal configuration of the pore tructure due to the liquefaction of tar (Harpalani, 1984). The phenomenon can be overcome by evacuating at low temperature ubmerging the ample container into liquid nitrogen (-150 C) prior to and during the evacuation. After returning to the deired ambient temperature, methane i admitted in tage and the ga preure within the ample container recorded at each increment. The volume of ga admitted may be monitored at inlet (volumetric method) or by meauring the increae in weight of the ample and container (gravimetric method). After correcting for free pace in the ample container, a plot of cumulative methane admitted againt preure provide the total ga iotherm. If the poroity of the ample ha been determined eparately (Section 1..1.), then the reult can be divided into free ga and adorbed ga a illutrated on Figure 1.1. The iotherm hould, ideally, be determined at, or cloe to, the virgin rock temperature of the actual trata. If neceary, the curve can be corrected to eam temperature. At any given preure, the quantity of ga adorbed fall a the temperature increae. Starting at 6 C, the ga adorbed decreae by ome 0.8 percent per C for bituminou coal and 0.6 percent per C for anthracite (Boxho et al, 1980). The curve hould alo be corrected to the actual moiture content of the eam uing Ettinger' formula, equation (1.). In order to utilize the corrected ga iotherm, a borehole i drilled into the eam, either from the urface or from an underground location. In the latter cae, the hole mut be ufficiently long (10 to 0 m) in order to penetrate beyond the zone of degaing into the mine opening. Seal are 1-11

12 emplaced in the borehole to encapulate a repreentative length within the eam. A tube from the encapulated length of borehole i attached to a preure gauge and the rie in preure i monitored. The rate of preure rie will be greater for coal of higher permeability. The initial rate of preure rie may be ued in conjunction with open hole flowrate to determine in-itu permeability. However, for the purpoe of aeing eam ga content, it i the maximum (or equilibrium) preure that i required. Uing thi preure, the ga content of the eam can be read from the corrected iotherm curve. An advantage of the indirect method i that it give the total ga content of the eam. However, thi i not indicative of the actual ga that may be emitted into mine working or recoverable by methane drainage. Furthermore, the meaured in-itu ga preure will be influenced not only by methane but alo by other gae that may be preent and, particularly, by the preence of water Direct meaurement method. Thi technique involve taking a ample of the eam from a borehole and placing it immediately into a hermetically ealed container. The ga i bled off to atmophere in tage and it volume meaured. The proce i continued until further ga emiion are negligible. An early method of direct meaurement of ga content wa developed in France (Bertrard et al, 1970). Thi utilized ample of mall chipping from the borehole. Further reearch carried out by the U. S. Bureau of Mine during the 1970' led to a procedure that ued complete core (Diamond, 1981). Although developed primarily for urface borehole, the technique i applicable alo to horizontal hole drilled from mine working. The ampling peronnel mut be preent at the time the hole i drilled into the eam. A topwatch i ued to maintain an accurate record of the elaped time between which the ample length of trata i penetrated, tart of core retrieval, arrival of the ample at the mouth of the borehole and confinement within the ealed container. Each container hould be capable of holding about kg of core and ome 35 to 40 cm in length. Longer core hould be ubdivided a a precaution againt major lo of data hould one container uffer from leakage. The eal on container mut be capable of holding a ga preure of 350 kpa without leaking. A preure gauge hould be fitted to each container. Ga i bled off from the container at interval of time commencing at 15 minute (or le if a rapid rie in container preure i oberved). The volume of ga emitted at each tage i meaured, uually by water diplacement, in a burette. The time interval are increaed progreively and the proce allowed to continue until an average of not more than 10 cubic cm per day ha been maintained for one week. There i, inevitably, ome ga lot from the core between the time of eam penetration and it confinement in a ample container. The volume of lot ga may be aeed by plotting the cumulative ga emitted from the container againt time. The initial traight line may be extrapolated backward through the recorded elaped time of core retrieval in order to quantify the lot ga. The retrieval time can be minimized by uing wireline drilling. The technique i illutrated on Figure 1.4. The analytical background to the procedure i embodied in equation (1.9). [Should ubequent tet how that the rate of deorption follow a t n law where n deviate ignificantly from 0.5 (Equation (1.11)), then the appropriate plot may be contructed a ga evolved v t n in order to determine the lot ga.] Another method of etimating the lot ga, developed by Smith and William (1984) make eparate allowance for the elaped time of drilling through the coal, core retrieval and period pent at the mouth of the borehole before containment. Thi more ophiticated technique i baed on an analyi that take account of the lack of uniformity in pore ize (Smith and William (1984a), Cloe and Erwin, (1989). 1-1

13 Ga emitted cubic cm time 1 Figure 1.4 Aement of ga lot during core retrieval. In thi example, elaped time before containment, t = 840 econd (i.e. t = 9 ). Backward extrapolation of the initial traight line give the ga lot a 85 cm 3. 1 The combination of meaured ga and lot ga give an etimate of the maximum amount of ga that may be emitted into mine working or captured by methane drainage. (It may be much le than thi.) However, following the termination of ga evolution meaurement, the core will till contain ome reidual ga. Thi can be quantified, if required, by cruhing the coal and continuing to meaure the additional ga that i then liberated. The cruhing proce may take place in a eparate ealed ball mill within a nitrogen atmophere. However, teel ball in the ample container allow the cruhing to take place without removing the ample from the container. Another method i to activate a teel hammer by electromagnetic mean within the container in order to cruh the ample through a fixed grid. In either cae, the ample and evolved ga hould be cooled to the original ambient temperature before the ga i bled off. A variation of the direct meaurement method of ga content i the deorbmeter (Hucka, 1983) in which deorbing methane from coal chipping in a ample veel puhe a mall plug of fluid along a tranparent piral tube. An electronic verion employing the ame principle ha been developed in Germany (Jana, 1980) MIGRATION OF METHANE Following releae from it long term geologic home, methane will migrate through rock under the influence, primarily, of a ga preure gradient. That movement will occur through the coal eam and if the ga preure gradient i tranvere to the eam, alo through adjoining fractured or permeable trata. We aume that the flow path and velocitie are ufficiently mall that laminar flow exit. Hence, Darcy' Law applie. Thi wa introduced in it implet form a equation (1.5). In thi Section, we hall conider the further ramification of Darcy Law. 1-13

14 Fluid Flow through a permeable medium Incompreible flow Let u conider, firt, an incompreible fluid (liquid) paing through a permeable medium in the direction x a illutrated on Figure 1.5. An elemental volume of the fluid within the medium ha a thickne, dx, and an orthogonal area A. Two force act upon the element. Firt, a force in the direction of flow i exerted becaue of the preure differential, dp, acro it two face. Let u call thi F p, where. F p = -A dp N (1.15) (negative becaue P decreae in the x direction). Secondly, a vertical downward force, F v, exit due to gravitational pull: F v = m g where the ma of the element (denity ρ, x volume Adx) giving m = ρ A dx F v = ρ g A dx P 1 x dx X Ө P Ө F p F v Figure 1.5 Force on an elemental volume of fluid, Adx, within a permeable medium. Total force in the x direction, F t = F p + F v co Ө. 1-14

15 If the direction of flow i at an angle Ө to the vertical then the component of F v in the x direction i F v co Ө = ρg A co Ө dx N (1.16) (Note that Ө i alo the inclination of the permeable medium to the horizontal.) The total force on the element in the direction of flow i then F t = F p + F v co Ө = -AdP + ρg A co Ө dx N (1.17) Thi can be expreed a force per unit volume F t dp N = + ρ g coθ (1.18) Adx dx 3 m Thi i a fuller verion of the preure gradient P / x in Darcy' Law, equation (1.5). Hence, we can rewrite Darcy' Law for an incompreible fluid in a non-horizontal flow field a k dp m u = ρ coθ x g (1.19) µ dx where u x = fluid velocity in the x direction (m/) k = permeability of medium (m ) µ = dynamic vicoity of fluid (N/m ) and dp i negative in the x direction. In the cae of horizontal flow (co Ө = 0) or where the fluid denity i mall, then equation (1.19) revert to the imple form of Darcy' Law. u k dp = m/ (1.0) µ dx A the fluid i incompreible, then u remain contant and we can integrate directly between the bounding wall hown on Figure 1.5, giving u = ( P P ) k ( P P ) k µ X 1 1 = µ X m/ (1.1) or, flowrate, Q, acro a given orthogonal area, A become Q = u A = k A µ ( P P ) 1 X m 3 (1.) 1-15

16 Compreible flow For gae the fluid denity i, indeed, low and the gravitational term may be neglected even for non-horizontal flow. However, the ga will expand a it progree along the flowpath due to the reduction in preure. Hence the flowrate, Q, and ga velocity, u, will both increae. Q k dp = u A = A (1.3) µ dx m 3 where A = given orthogonal area acro which the flow occur (m ) A Q i a variable, we can no longer integrate directly. However, if we write the equation in term of a teady-tate (contant) ma flow, M (kg/), then M But denity k dp kg = Q ρ = A ρ (1.4) µ dx P kg ρ = (General Ga Law, equation 3.11) RT 3 m where R = ga contant (J/(kg K)) and T = abolute temperature (K) giving M k A P dp kg = (1.5) µ RT dx A M i contant, we can integrate acro the full thickne of the medium (Figure 1.5) to give M ( P P ) k A 1 kg = (1.6) µ RT X Thi can be converted to a volume flow, Q, at any given denity. In particular, at the denity, ρ m, correponding to the arithmetic mean preure, the flow become Q m m 3 M = (1.7) ρ m However, equation (1.6) can be written a M ( P + P ) ( P P ) k A 1 1 kg = (1.8) µ RT X ( P P ) k Pm 1 = A µ RT X kg k = A ρ µ m ( P P ) 1 X kg (1.9) 1-16

17 Combining equation (1.7 and 1.9) give Q m ( P P ) k 1 kg = A (1.30) µ X Comparing thi with equation (1.) how that the volume flow of a ga at the poition of mean preure i given by the ame expreion a for an incompreible fluid. At any other denity, ρ, the volume flowrate, Q, i given by ρ m m 3 Q = Q m (1.31) ρ If the flow i iothermal (contant temperature) then thi correction can be expreed in term of preure Q Pm = Qm (1.3) P ( P P ) m m = Q m (1.33) P Radial flow of ga Figure 1.6 illutrate a borehole of radiu r b interecting a ga bearing horizon of thickne h. The ga preure in the borehole at the eam i P b while, at ome greater radiu r into the eam, the ga preure i P. P r P r b P b r dr Figure 1.6 Radial flow into a borehole. 1-17

18 Conider the elemental cylinder of radiu r and thickne dr, at which the ga preure i P. Then, from equation (1.5) M = k µ A RT P dp dr kg where A = π r h m giving or M = dr M r k π h P dp µ RT r dr kg k π h kg = P dp (1.34) µ RT Integrating between r and r b give r M ln r b = ( P Pb ) k π h µ RT kg or M = k π h µ RT ( P + P ) b ( P Pb ) ln r rb kg (1.35) But a ( P + P ) RT b = P m RT = ρ m Q m = M ρ m = k πh µ ( P Pb ) ln r rb 3 m (1.36) Equation (1.31 and 1.33) can again be ued to give the flowrate at any other denity or preure Tranient radial flow The previou Section aumed a teady-tate ditribution in ga preure throughout the medium. Thi i not, of coure, the real ituation in practice. A a given ource bed i drained, the ga preure will decline with time. An analyi analogou to that given for heat flow (Section ) lead to the time tranient equation for a non-adorbing medium: k P P µ φ r + 1 P r r = P t Pa (1.37) where Φ = rock poroity (dimenionle) and t = time () For methane in coal, ga will be generated by deorption a the ga preure fall (Figure 1.1) and, hence, retard the rate of preure decline very ignificantly. The ga deorption equation of Section 1... mut be coupled with equation (1.37) to track the combined effect of drainage and deorption. 1-18

19 1.3.. The permeability of coal In the previou ection, it wa aumed that the permeability, k, of the rock remained contant. Unfortunately, in ome cae including coal, thi no longer hold. The aniotropy of the material caue the natural permeability to vary with direction. Furthermore, the permeability change with repect to mechanical tre, ga preure and the preence of liquid. We hall examine each of thee three effect in turn. Firt, however, let u clarify the concept of permeability and it dimenion in the SI ytem of unit. Permeability, k, may be contrued a the conductance of a given porou medium to a fluid of known vicoity, µ. That conductance mut depend only upon the geometry of the internal flowpath. In a rational ytem of unit, the permeability mut, therefore, be expreed in term of the length dimenion. Thi can be illutrated by re-expreing Darcy' Law, equation (1.5) a x k = u µ P m N m m m = m The unit of permeability in the SI ytem are, therefore, m. However, the older unit of Darcy (or millidarcie, md) remain in common ue. The Darcy wa defined a "the permeability of a medium that pae a ingle-phae fluid of dynamic vicoity 1 centipoie (0.01 N/m ) in laminar flow at a rate of 1 cm 3 / through each cm of cro-ectional area and under a preure gradient of 1 atmophere ( Pa) per cm". Such definition make u grateful for the implicity of the SI ytem. The converion between the unit ytem i given a 1 md = x m (1.38) Effect of mechanical tre Many reearcher have reported tet on the repone of coal permeability to applied loading (e.g. Somerton, 1974; Gawuga, 1979; Harpalani, 1984). Such tet conit of delicate machining of coal into ample cylinder of 30 to 50 mm in diameter and a length/diameter ratio of about (Obert, 1967), then placing a ample into a triaxial permeameter. Thi device allow a liquid or ga to be paed while the ample i ubjected to axial loading (by a tiff compreion machine) and radial treing (by oil preure exerted on a ynthetic rubber leeve around the ample). Such tet on coal have revealed the following phenomena during non-detructive loading. Permeability reduce during loading and recover during unloading. However, there may be a ignificant hyterei effect with lower permeabilitie during unloading, particularly when the axial and radial tree are unequal. Sample left under a contant applied tre exhibit a creep effect. The permeability reduce with time down to a limiting value. Following a loading tet, a ample may not recover it initial permeability. Furthermore, repeated loading tet will give progreively reduced permeabilitie. Thee obervation ugget that coal exhibit a combination of elaticity and non-recoverable train. The latter i thought to be caued by permanent damage to the weaker bridge between pore. 1-19

20 tet coal hale treing detreing tet 1 Figure 1.7 The variation of permeability with hydrotatic tre for two load/unload tet of a bituminou coal, After the completion of tet 1 the ample wa left lightly loaded for 18 hour before commencing tet. Figure 1.7 how the reult of hydrotatic loading (axial and radial tree remaining equal) on the permeability of a bituminou coal. Hydrotatic treing appear to minimize the hyterei effect. Two load/unload cycle are hown. After completion of the firt tet, the ample wa left for 18 hour in the triaxial cell at a holding tre of lightly more than MPa. The creep reduction in permeability before commencement of the econd tet how clearly. It appear from uch tet that the indicated permeability of a coal ample meaured during a laboratory tet depend upon the tre hitory of the ample. That i, the cycle of loading and unloading caued by mining cloe to the original location of the ample, and the method of recovering that ample. However, for any given laboratory tet, the coal permeability fall logarithmically with repect to applied hydrotatic tre, i. e. the relationhip i of the form ( Bσ ) m k = A exp (1.39) where σ = effective tre (MPa) = applied hydrotatic tre - pore (mean ga) preure A = contant (m ) B = contant (1/MPa). The value of A i the theoretical permeability at zero tre and depend not only upon the coal tructure but alo the tre hitory of the ample. Value of B from -1.5 to -.5 (1/MPa) have been reported for bituminou coal. For comparion, Figure 1.7 include a imilar tet on hale. The reduction in permeability with applied tre i much le marked than that for coal. 1-0

21 Effect of ga preure For mot rock, the permeability increae a the pore preure exerted by a aturating ga decreae. The effect wa invetigated by Klinkenberg (1941) who propoed the relationhip 1 a k = k liq + m (1.40) P where k liq = permeability of the medium to a ingle phae liquid, or permeability at a very high ga preure (m ) and a = contant depending upon the ga (Pa) The value of k liq and a may be determined by plotting experimentally determined value of k againt 1/P. It i thought that the Klinkenberg effect i caued by urface diffuion, i. e. lippage of ga molecule along the internal urface of the medium (Section ). However, in the cae of coal, a traight line relationhip i not found when meaured value of k are plotted againt 1/P. Hence, there i no longer any advantage in plotting the variable in that way. Reearcher have reported both riing and falling permeabilitie with repect to mean ga preure during laboratory tet on coal. Figure 1.8 illutrate a typical et of reult and how the effect of ga preure and applied hydrotatic tre. Curve fitting exercie how that uch curve take the form C k = C1 + + exp ( C3 P C4 ) m (1.41) P Klinkenberg effect + dilation of flowpath where C 1, C, C 3 and C 4 are contant for the curve. It will be oberved that (C 1 + C /P) ha the form of the Klinkenberg effect and i dominant at low ga preure. A the ga preure rie through thi phae, urface diffuion reduce and, adorbed molecule begin to obtruct the flowpath. Hence, the permeability fall. However, with further increae in ga preure, the exponential term in equation (1.41) become dominant cauing the permeability to rie again. One hypothei i that a ufficiently high pore preure reult in compreion Figure 1.8 In addition to the effect of mechanical tre, the permeability of coal i alo a function of the internal ga preure. of the coal ubtance and unconnected pore while the flowpath become dilated. 1-1

22 Two phae flow The pore and fracture network of trata are often occupied by a mixture of fluid. In petroleum reervoir, three phae flow may occur with oil, ga and water a the occupying fluid. In mot other cae, including coal, two phae flow take place a a mixture of gae and water. The preence of water greatly inhibit the flow of ga and vice vera and, hence, reduce the permeability of the rock to both phae. The effect i decribed quantitatively in term of the relative permeabilitie k r, g and k r, w for ga and water repectively: k g k r, g = (dimenionle) (1.4) k g and kw k r, w = (dimenionle) (1.43) k w where k g and k w are the effective permeabilitie of the rock to ga and water repectively, m (dependent on degree of aturation) k g = permeability of the rock to ga when aturated by ga, m (no water preent) and k w = permeability of the rock to water when aturated by water, m (no ga preent) In the petroleum indutry, it i often aumed that k g = k w for the andtone and limetone that are typical of petroleum reervoir rock. However, the effect of adorption and ga preure indicate that thi may not hold for coal. Figure 1.9 illutrate a typical behaviour of relative permeabilitie with repect to aturation of water and ga. The actual loci of k r,g k k r,w r,g k r,w ga become mobile Figure 1.9 The relative permeabilitie to ga, k r,g, and water k r,w depend upon the degree of rock aturation by the two fluid. the relative permeability curve depend upon whether the coal ubtance i wetted preferentially by the water or the ga. Thi, in turn, varie with the proportion of coal contituent, vitrain and clarain tending to prefer the ga while durain and fuain are more eaily wetted by water. The curve on Figure 1.9 ugget a net hydrophobic coal, i.e. the ga i the preferred wetting phae. The water will, therefore, tend to reide in the larger opening within the matrix and inhibit 1 -

23 migration of the ga which exit in the maller intertice. Hence, a indicated on Figure 1.9, the ga will not become mobile until the water aturation ha fallen ignificantly below 100 percent. Thi explain why coniderable volume of water may be produced from a borehole before ga flow appear EMISSION PATTERNS INTO MINE WORKINGS The rate at which methane are emitted into mine working vary from near teady-tate, through cycle that mimic rate of mineral production, to the dangerou phenomena of ga outburt or "udden large emiion". In general, the rate of ga emiion into the mine ventilation ytem depend upon: initial ga content of the coal degree of prior degaing by methane drainage or mine working method of mining thickne of the worked eam and proximity of other eam coal production rate panel width (of longwall) and depth below urface conveyor peed the natural permeability of the trata and, in particular, the dynamic variation in permeability caued by mining comminution of the coal Source of methane in coal mine Variation in methane emiion into mine are influenced trongly by the dominant ource of the ga. In room and pillar working, ga will be produced from face, ribide and the pillar of the eam being worked. While expoed pillar may be degaed fairly quickly (dependent upon the coal permeability), ribide ga may continue to be troubleome for coniderable period of time. In uch cae, it i preferable for ribide that border on virgin coal to be ventilated by return air (Figure 4.7a). Peak of ga emiion will, in general, occur at the face of the room due to the high rate of comminution caued by mechanized coal winning. Thi will be moderated by the degree of earlier degaing, either by methane drainage or by ga migration toward the working. The latter i enhanced by high coal permeability and a low rate of advance. In addition to ribide, the major ource of methane in longwall mine are the working face and roof and floor trata. Ribide ga tend to be a low and near contant ource. It i convenient to claify the other ource of methane in a longwall mine into face (or coal front) ga and gob ga. Peak emiion occur at coal-winning machine due to rapid fragmentation of the coal. It i thi emiion that give rie to frictional ignition at the pick point (Section 1.1). The peak emiion move along a longwall face with the machine. However, the frehly expoed coal front will alo emit methane - rapidly at firt and decaying with time until the machine pae that point again. Coal front ga i an immediate and direct load on the ditrict ventilation ytem. Figure 1.10 i an example of a moothed record of methane concentration in air returning from a longwall face. In thi example, coal wa produced on two out of three hift for five day per week. The correlation of methane make with face activity how clearly, decaying down to a background level over the weekend. The ga flow into the gob area behind a longwall face originate from any roof or floor coal that ha not been mined from the worked eam, but more particularly from ource bed within the roof or floor trata. Any coal eam or carbonaceou band within a range of ome 00 m above to 100 m below the working horizon are liable to releae methane that will migrate through the relaxed trata into the gob area. If methane drainage i not practiced, then that methane will ubequently be emitted into the mine ventilation ytem. Figure 1.11 illutrate the variation in tre-induced permeability of roof and floor trata that create enhanced migration path for the ga. 1-3

24 Figure 1.10 Recording of airflow and methane flow in an airway returning from a longwall face. permeability tre microfracturing opening of microfracture and/or fiure recompaction of trata treed zone tranition tre relief zone roof ource bed recompaction zone coal face working horizon floor ource bed Figure 1.11 The migration of ga from roof and flow ource bed toward the working horizon occur becaue of the large increae in permeability of adjacent trata in the tre relief zone immediately behind a longwall face. 1-4

25 A fragmented coal i tranported out of the mine, it will continue to emit methane. The ga make depend upon the degree of fragmentation, ga content of the cut coal on leaving the face, the tonnage and the time pent during tranportation. The deorption equation given in Section 1... allow etimate of the tranport ga to be made. Precaution hould continue to be taken againt accumulation of methane after the coal ha left the mine. Ga exploion have occurred in urface hopper and in the hold of coal tranport hip Methane layering Methane emitted from the trata into a mine opening will often be at concentration in exce of 90 percent. While being diluted down to afe general body concentration, the methane will, inevitably, pa through the 5 to 15 percent range during which time it i exploive. It i, therefore, important that the time and pace in which the exploive mixture exit are kept a mall a poible. Thi can be achieved by good mixing of the methane and air at the point of emiion. Unfortunately, the buoyancy of methane with repect to air (pecific gravity 0.554) produce a tendency for concentrated methane to collect in roof cavitie and to layer along the roof of airway or working face. In level and acentionally ventilated airway with inadequate airflow, the layer will tream along the roof in the direction of airflow, increaing in thickne and decreaing in concentration a it proceed (Figure 1.1(a)),. Multiple feeder of ga will, of coure, tend to maintain the concentration at a high level cloe to the roof. There are two main hazard aociated with methane layer. Firt, they extend greatly the zone within which ignition of the ga can occur. Secondly, when uch an ignition ha taken place, a methane layer act very effectively a a fue along which the flame can propagate -perhap leading to much larger accumulation in roof cavitie or gob area. methane feeder (a) methane feeder (b) Figure 1.1 Methane layering in (a) a level airway (b) a decentionally ventilated airway. 1-5

26 Figure 1.1(b) indicate that in a decentionally ventilated airway, the buoyant methane layer may tream uphill cloe to the roof and againt the direction of the airflow. However, at the fringe between ga and air, vicou drag and eddy action will caue the ga/air mixture to turn in the ame direction a the airflow. The reult i that exploive mixture may be drawn down into the airway uptream from point of emiion. Although the layering phenomenum of methane in mine wa all too obviou during the Indutrial Revolution (Section 1.), ytematic reearch on the topic eem not to have been well organized until the 1930' (Coward, 1937). A combination of analytical and experimental work at the Safety in Mine Reearch Etablihment, England in the early 'ixtie led to a quantification of the important parameter (Bakke and Leach, 196). Thee were velocity of the ventilating airtream, u (m/) rate of ga emiion, Q g (m 3 /) width of airway W (m) inclination of airway relative denitie of the air and ga roughne of the roof above the layer. Although a rough lining will promote better mixing than a mooth one (except for free-treaming layer), the effect of roughne i fairly weak (Raine, 1960). Bakke and Leach found that the characteritic behaviour of a ga layer wa proportional to the dimenionle group u ρ Qg g ρ W 1 3 (1.44) where ρ/ρ i the difference in relative denitie of the two gae. ( =0.446 for air and methane) Uing a value of g = 9.81 m/ give the dimenionle number for methane layer in air to be 1 3 u u W or L = Qg g L = (1.45) ( Q / W ) The dimenionle group, L, i known a the Layering Number and i of fundamental ignificance in the behaviour of methane layer. Examination of equation (1.45) indicate that the air velocity i the mot enitive parameter in governing the Layering Number and, hence, the length and mixing characteritic of the layer. Although u i, theoretically, the air velocity immediately under the layer, the mean value in the upper third of the airway may be ued. For non-rectangular airway, W may be taken a ome three quarter of the roadway width. The power of 1/3 in equation (1.45) reduce the effect of error in etimated value of W and Q g. Experimental data from Bakke and Leach have been employed to produce Figure 1.13 for level airway. For any given ga make and roadway width, the horizontal axi may be caled in term of air velocity. It can be een from thi graph that at low Layering Number (and, hence, low velocity) the layer i affected very little by mall additional increae in velocity. Indeed, in an acentional airway, the layer will actually lengthen a the airpeed increae from zero to the free-treaming velocity of the methane. The mixing proce i primarily due to turbulent eddie. 1-6

27 The efficiency of mixing increae a the relative velocity between the methane and air rie. However, a the air velocity continue to increae giving Layering Number of over 1.5 the layer horten rapidly. On the bai of uch reult, it can be recommended that Layering Number hould be not le than 5 in level airway. Similar experiment in inclined airway have led to the recommended minimum layering number hown in Table 1.1 in order to inhibit the formation of methane layer.. Angle to horizontal (deg.) Acentional Decentional Table 1.1. Recommended minimum Layering Number at variou roadway inclination. Figure 1.13 Variation of layer length with Layering Number for level airway. Methane layer can be detected by taking methanometer reading or iting monitor at roof level. The mot probable location are in bleeder airway or return roadway cloe to a longwall face. On detecting a methane layer, an immediate temporary remedy i to erect a hurdle cloth, that i, a brattice cloth attached to the ide and floor but leaving a gap at the top. The ize of the opening hould be uch that the increaed air velocity near the roof dipere the layer. The hurdle cloth may need to extend ome three quarter of the height of the airway. However, 1-7

28 anemometer reading hould be taken to enure that the overall volume flow of air through the area i not reduced ignificantly. Compreed air venturi or other form of air mover can be ued to dipere methane layer provided that they are earthed againt electrotatic parking. The longer term olution are to (a) increae the airflow and, hence, the air velocitie through the affected panel; (b) reduce the rate of methane emiion, or a combination of the two. Ventilation network analye hould be employed to invetigate mean of increaing the airflow (Chapter 7). Thee may include adjuting the etting of regulator or fan, and controlled partial recirculation (Section 4.5.). The methane emiion can mot effectively be reduced by intalling a ytem of methane drainage (Section 1.5.). While methane layering wa once a common occurrence in gay coal mine, well deigned ytem of ventilation and ga drainage are capable of eliminating thi hazard from modern mine. However, if topping or other ventilation control are dirupted by any incident in gay mine the reduction in airflow can reult in the formation of methane layer and the poibility of ga exploion Ga outburt The mot dramatic mode of ga emiion into mine working i the releae of an abnormally large volume of ga from the trata in a hort period of time. Such incident have caued coniderable lo of life. In many cae, the rate of emiion ha been exploive in it violence, fracturing the trata and ejecting large quantitie of olid material into the working. Ga outburt are quite different from rock burt that are caued by high trata loading. However, the probability of a diruptive ga releae from adjacent trata i enhanced in area of abnormally high tre uch a a pillar edge in overlying or underlying working. There are two ditinct type of ga outburt, in-eam burt and udden large emiion from roof and floor. Each of thee will be dicued in turn In-eam outburt A the name implie, thee are outburt of ga and olid from the eam that i currently being mined. They have occurred in many countrie, particularly in coal and alt (or potah) mine. The geologic condition that lead to in-eam outburt appear to be quite varied. The one common feature i the exitence of mechanically weakened pocket of mineral within the eam and which alo contain ga at high preure. Methane, carbon dioxide and mixture of the two have been reported from in-eam outburt in coal mine while nitrogen may be the major component in potah mine (Robinon et al, 1981). The genei and mechanim of in-eam ga outburt have been a matter of ome controvery. A current hypothei i that the tructure of the material within outburt pocket ha been altered by tectonic treing over geological time. Seam outburt in coal mine have been known to eject up to 5000 t of dut, much of it compried of particle le than 10 µm in diameter (Evan and Brown, 1973). Thi can be accompanied by everal hundred thouand cubic metre of ga (Campoli, 1985). It i thought that pulverization of the coal ha been caued by very high hear force. Laboratory tet have hown that a ample of normal anthracite can be reduced to the fragile "outburt" anthracite by uch mean. Coal mine located in area that have been ubject to thrut faulting are more prone to in-eam outburt. A ample of "outburt" coal i, typically, everely lickenided and friable to the extent that it may crumble to dut particle by queezing it in the hand. The eam i often heavily contorted in the vicinity of the outburt pocket - again, evidence of the exceive tectonic treing to which it ha been ubjected. If the overlying caprock had maintained a low permeability during and ince the pulverization of the coal, then the entrained methane will remain within the zone. However, the mall ize of the particle enure that deorption of the ga can occur very quickly if the preure i relieved (equation (1.9)). 1-8

29 When a heading or face approache an outburt zone that contain highly compreed coal dut and ga, tre increae on the narrowing barrier of normal coal that lie between the free face and the hidden outburt pocket (Sheng and Otuonye, 1988). At ome critical tage, the barrier will begin to fracture. Thi caue audible noie that ha been variouly decribed a "cracking, popping" or "like a two-troke engine". When the force exerted by ga preure in the outburt pocket exceed the reitance of the failing barrier, the coal front burt outward exploively. The blat of expanding ga may initiate hock wave throughout the ventilation ytem of the mine. A wave of decompreion alo pae back through the pulverized coal. The expanion of the ga, reinforced by rapid deorption of large volume of methane expel the ma of dut into the airway. Although the outburt may lat only a few econd, the deorbing ga caue the dut to behave a a fluidized bed. Thi can flow along the airway, almot filling it and engulfing equipment and peronnel for a ditance that can exceed 100 m (William and Morri, 197). The cavitie that remain in the eam following an outburt may, themelve, indicate lickenide. Several cavitie that are interconnected can contribute to a ingle outburt. The volume of dut ubequently removed from the airway or face ha often appeared too large for the cavity from which it wa apparently expelled. The danger aociated with ga outburt in mine are, firt, the aphyxiation of miner by both ga and dut. Compreed air 'lifeline' may be maintained on, or cloe to, face that are prone to in-eam outburt. Thee are rack of flexible tube connected to a compreed air pipe, and with valve that open automatically when picked up. Such device can ave the live of miner who become engulfed in outburt dut. A econd hazard i that the violence of the outburt may damage equipment and caue parking that can ignite the highly flammable ga/dut/air mixture. Spontaneou ignition of methane during outburt in a alt mine have alo been reported (Schatzel and Dunbier, 1989). Thirdly, the udden expanion of a large volume of ga can caue diruption of the ventilation ytem of a mine. Precautionary meaure againt in-eam ga outburt include forward drilling of exploratory borehole. However, pre-drainage of outburt pocket ha met with limited ucce due to rapid blockage of the borehole. Similarly, teting ample of coal for trength or tructure i uncertain a normal coal may exit very cloe to an outburt pocket. Monitoring for unuual micro-eimic activity i a preferred warning technique (Campoli, 1985). When it i upected that a face or heading i approaching an outburt pocket, then machine mining hould be replaced by drill and blat method (volley firing), clearing the mine of peronnel before each blat. Thi hould be continued until an outburt i induced or the dangerou area ha been mined through Outburt from roof and floor Thee are mot likely to occur in longwall mine. A dicued in Section and illutrated on Figure 1.11, methane will migrate from higher or lower ource bed toward the working horizon behind the working face. Strata in the tre relief zone normally exhibit a ubtantial increae in permeability due to relaxation of the fracture network. The migration of ga then proceed through the overlying and underlying trata at a rate that follow cyclic face operation and, under normal condition, i quite controlled. However, a band of trong and low permeability rock (caprock) exiting in the trata equence between the ource bed and the working horizon may inhibit the paage of ga. Thi can reult in a reervoir of preurized ga accumulating beyond the caprock. Any udden failure of thi retention band will then produce a large and rapid inundation of ga into the working horizon (Woltenholme et al, 1969). Figure 1.14 illutrate the development of a potential outburt from the floor. In Figure 1.14(a), relaxation of the trata allow ga to be evolved from the ource bed and migrate upward. A trong intervening bed of ufficiently high natural permeability or which contain induced fracture allow the ga to pa through. 1-9

30 working horizon iobar 00 kpa trong bed 400 (a) ource bed caprock kpa (b) ga reervoir 1000 Figure 1.14 Development of a ga outburt condition under a longwall panel. (a) Safe condition: Iobar ditributed; trong bed ufficiently permeable (or fractured) allowing ga to migrate through in a controlled manner. (b) Potential outburt condition: reduced permeability of the trong bed reulting in an increaed preure gradient acro thi bed and high preure ga accumulation under it. However, Figure 1.14(b) illutrate a condition in which a trong low permeability bed ha failed to fracture. Ga accumulate under thi bed which i then ubjected to an intenified ga preure gradient. Thi i the potential outburt ituation. The rate at which ga accumulate in the reervoir rock depend upon the relative rate of ga make from the ource eam and ga leakage through the caprock. Either an increae in the ga make from the ource eam or a decreae in flow through the caprock can produce the potential outburt condition. Reitance to ga flow through the fracture network of the caprock may occur by increae in it trength or thickne. Furthermore, a porou bed uch a a fine-grained andtone may uffer a large decreae in relative permeability to ga if it become partially aturated by water (Figure 1.9). There are coniderable difference between the udden large emiion that occur from roof and floor trata. Strata above the worked eam are ubjected to larger vertical movement due to ubidence of the rock ma. Thi create greater voidage in which ga can accumulate. Bed 1-30

31 eparation alo promote flowpath parallel to the trata. Very large ga reervoir may, therefore, develop in the overlying rock. However, fracturing of low permeability bed i more probable and high preure are le likely to develop in the ga reervoir rock. For thee reaon, udden emiion from the roof have uually been characterized by large flow that may lat from a few hour to everal month (Morri, 1974). However, the initiation of the emiion i unlikely to be accompanied by violent dilocation of the immediate roof trata. In contrat, ga outburt from the floor are uually of horter duration but more violent - even to the extent of rupturing the floor trata upward with ejection of olid material. The reduced deformation in the floor equence enable a caprock to reit induced enlargement of fracture. Thi lead to high preure accumulation of ga. However, the extent of the ga reervoir i likely to be much le than one in roof trata. Morri (1974) reported methane emiion a large a m 3 from floor outburt, but over 8 x 10 6 m 3 from udden roof emiion. Ga outburt from roof or floor are often preceded by maller increae in general body ga concentration, often intermittent in nature, during the hour before the major flow occur. Thi i probably caued by increaing train within the caprock and interconnection of bed eparation voidage before the main failure. Continuou ga monitor can detect uch warning. Immediately before the burt, evere weighting on the roof upport and caving in the gob may occur. Roof and floor outburt are more likely to occur along plane of maximum hear tre in the trata, i.e. in bleeder or airway bordering gob area, or on the longwall face itelf. Roof outburt may occur due to the initial failure of an overlying caprock within ome two face length of the initial tarting-off line of the longwall. Similarly, paing under or over old pillar edge in other eam can promote udden failure of caprock. However, previou working may have caued partially degaing of ource bed. Mot floor outburt have occurred when no previou mining ha taken place at a lower level. The mot effective mean of preventing roof and floor outburt i regular drilling of methane drainage hole wherever any potential caprock may exit. The hole hould be angled over or under the caved area and hould penetrate beyond the caprock to the ource bed(). The pacing between hole hould be dictated by local condition but may be a little a 10 m. It i particularly important that the drilling pattern be maintained cloe up to the face. All hole hould be connected into a methane drainage ytem (Section ). In outburt prone area, it i eential that drilling take place through a tuffing box in order that the flow can be diverted immediately into the pipe range hould a high preure accumulation be penetrated. It i often the cae that routine floor borehole produce very little ga. However, in area liable to floor outburt, thee hole hould continue to be drilled a a precautionary meaure. Tet hould be made of open-hole flow rate and rate of preure build-up on the cloed hole. When both of thee tet how higher than normal value, then it i probable that an outburt condition i developing (Oldroyd et al, 1971). In the event of any indication that a ga reervoir i accumulating in roof or floor trata, then additional methane drainage hole hould be drilled immediately to relieve that preure and to capture the ga into the drainage pipe ytem Prediction of methane emiion into the ventilation ytem A indicated earlier in Section 1.4 a coniderable number of variable affect the rate at which methane i emitted into the ventilation ytem of coal mine or captured by methane drainage. The non-linear interaction of the variable render it difficult to apply analytical mean to the problem of predicting methane emiion. For thi reaon numerou numerical and empirical model have been developed and utilized in everal countrie (e.g. Durucan et al (199)). Older empirical method of predicting ga emiion into coal mine varied from curve fitting procedure to pocket calculator (e.g. Creedy et al (1988); CEC (1988)). The continued enhancement of computing power on dek and portable machine ha promoted the utilization of 1-31

32 ophiticated data proceing oftware. Thi ha reulted in improved reliability of predicting methane emiion from data that are often difficult to correlate (e.g. Lunarzewki (1998). One uch procedure utilize artificial neural network (Karacan (007)). Thi technique eek pattern and relationhip between group of input data that have appear to have obcure interrelationhip from conventional mathematical or tatitical methodologie. A feature of artificial neural network oftware i that it can learn a more data i added, o increaing it accuracy of prediction. Neverthele, we are reminded that the reliability of all empirical model i dependent on the range, quality and detail of of the meaurement on which the model i baed. Hence, it i the reponibility of the uer to acertain that any given model i applicable to the mine under conideration METHANE DRAINAGE The organized extraction of methane from carboniferou trata may be practiced in order to (a) produce a gaeou fuel, (b) reduce methane emiion into mine working or (c) a combination of the two. If the intent i to provide a fuel for ale or local conumption, then it i important that the drained ga remain within precribed range of purity and flowrate. There i no ingle preferred technique of methane drainage. The major parameter that influence the choice of method include the natural or induced permeability of the ource eam() and aociated trata the reaon for draining the ga the method of mining (if any). In thi Section, we hall outline method of methane drainage, the infratructure of pipe range and ancillary equipment, method of predicting ga flow and conclude with a ummary of the procedure for planning a methane drainage ytem In-eam drainage Drainage of methane by mean of borehole drilled into a coal eam i ucceful only if the coal ha a ufficiently high natural permeability or where a fracture network i induced in the eam by artificial method. Hence, for example, while in-eam drainage can be practiced in ome North American coalfield, it ha met with very limited ucce in the low permeability coal of the United Kingdom or Wetern Europe. A knowledge of coalbed permeability i neceary before ineam drainage can be contemplated (Section 1.3.). Advance in drilling technology have increaed the performance potential of in-eam ga drainage (Schwoebel, 1987). Uing down-the-hole motor and teering mechanim, borehole may be drilled to length exceeding 1600 m within the eam (Brunner (006). Provided that the coal permeability i ufficiently high, methane flow into mine working can be reduced very ignificantly by pre-draining the eam to be worked. Ga capture efficiencie up to 50 percent are not uncommon, where Ga captured by methane drainage Ga capture efficiency = 100 Ga captured + ga emitted into ventilation (1.46) 1-3

33 In gay and permeable eam, ribide bordering on olid coal are prolific ource of ga. Figure 1.15 illutrate flanking borehole ued to drain ga from the coal ahead of heading that are advancing into a virgin area. On the other hand, previouly driven heading or working may have degaed the area to a very coniderable extent. Figure 1.15 In-eam drainage borehole to reduce methane flow into advancing heading, applicable only where the coal i ufficiently permeable. In-eam ga drainage can alo be effective in permeable eam that are worked by the retreating longwall ytem (Mill and Stevenon, 1989; Ely and Bethard, 1989). Figure 1.16 illutrate the layout. Borehole are drilled into the eam from a return airway and connect into the methane drainage pipe ytem. The preferred pacing of the hole depend upon the permeability of the eam and may vary from 10 to over 80 m. The ditance from the end of each borehole and the oppoite airway hould be about half the pacing between hole. The application of uction on the borehole i often unneceary but may be required for coal of marginal permeability or to increae the zone of influence of each borehole. Figure In-eam borehole draining methane from a coal eam in a two-entry retreating longwall. 1-33

34 The time allowed for drainage hould be at leat ix month and, preferably, over one year. Hence, the hole hould be drilled during the development of what will become the tailgate of the longwall. Spalling of coal into the borehole can be a problem, epecially in the more friable coal. Thi may be reduced by employing mooth drill rod. Drill chipping can be removed by mean of a continuou water fluh. Additionally, auger may be ued to remove palled coal from the borehole. The internal diameter of ome ection of finihed borehole may be coniderably larger than the drill bit. Perforated platic liner can be inerted in order to maintain the hole open, ubject to the governing legilation. The firt 5 to 10 m of each borehole are typically drilled at 100 mm diameter. A tandpipe i cemented into place and connected through a tuffing box into the methane drainage pipeline (Section ). The remainder of the hole i drilled through the tandpipe at a diameter of ome 75 mm. The flowrate of ga from a ga drainage borehole will vary with time. Figure 1.17 illutrate a typical life cycle for an in-eam borehole. A high initial flow occur from the expanion and deorption of ga in the immediate vicinity of the hole. Thi may diminih fairly rapidly but then increae again a the zone of influence i dewatered, hence, increaing the relative permeability of the coal to ga (Figure 1.9). Thi, in turn, i followed by a decay a the zone of influence i depleted of ga. In-eam borehole drilled into outcrop or from urface mine may produce little methane. The ga content of coal eam tend to increae with depth. It i probable that eam near the urface have lot mot of their mobile ga. However, multiple borehole drilled with directional control from a urface drill rig can be guided to follow a coal eam for ditance that produce acceptable flowrate. Vertical hole drilled from the urface to interect coal eam are likely to produce very little ga becaue of the hort length of hole expoed to any given eam. However, hydraulic timulation or hydrofracturinq can be ued to enhance the flowrate. Thi involve injecting water or foam containing and particle into the eam. Other ection of the borehole are caed. The objective i to dilate the fracture network of the eam by hydraulic preure. The and particle are intended to maintain the flowpath open when injection ceae. The ucce of hydrofracturing depend upon the natural fracture network Figure 1.17 Typical life cycle of a ga drainage borehole in coal. that exit within the eam and the abence of clay that well when wetted. Friable coal are more likely to repond well to hydrofracturing. On the other hand, in tronger coal, induced fracture may be concentrated along dicrete bedding plane 1-34

35 and give a poor recovery of ga. In the majority of cae, water mut be pumped from urface borehole before ga flow can be attained. Another mean of in-eam ga drainage i to ink a mall diameter haft to interect the coal bed (U. S. Bureau of Mine, 1980). Long multiple borehole are drilled radially outward from the haft into the eam and connected into a methane drainage line that rie to the urface. Thi method may be conidered when a haft i to be unk at a later date at that location for mine ventilation or ervice acce. Thi enable degaing of an area for everal year prior to mining Gob drainage by urface borehole The relaxation of trata above and below the caved zone in a longwall panel create voidage within which methane can accumulate at high concentration, particularly when other coal bed exit within thoe trata. If thi ga i not removed, then it will migrate toward the working horizon and become a load on the ventilation ytem of the mine (Figure 1.11). Capture of thi "gob ga" may be accomplihed either underground by cro-meaure drainage (Section ) or by drilling borehole from the urface. to ga pump Figure 1.18 Gob drainage of a longwall panel. Each hole become productive after paage of the faceline. Figure 1.18 depict methane drainage from the gob of a longwall panel by urface borehole. Thi i a method that i favoured in the United State. Typically, three or four hole are drilled from urface rig at interval of 500 to 600 m along the centreline of the panel and ahead of the coal face. The hole may be 00 to 50 mm in diameter and drilled to within ome 8 to 10 m of the top of the coal eam (Mill and Stevenon, 1989). The hole hould be caed from the urface to a depth that i dictated by the local geology and, in particular, to extend below any bed that are likely to act a bridging caprock. A perforated liner can be employed in the ret of the hole to inhibit cloure from lateral hear. 1-35