A numerical simulator of outbursts of coal and gas

Transcription

1 Uiversity of Wolloo Research Olie Coal Operators' Coferece Faculty of Eieeri ad Iformatio Scieces 2010 A umerical simulator of outbursts of coal ad as She Xue CSIRO Earth Sciece ad Resource Eieeri Ga Wa CSIRO Earth Sciece ad Resource Eieeri; Collee of Resource ad Evirometal Eieeri, Shado Uiversity of Sciece ad Techoloy, Chia Yuca Wa CSIRO Earth Sciece ad Resource Eieeri Ju Xie CSIRO Earth Sciece ad Resource Eieeri Publicatio Details This coferece paper as oriially published as Xue, S, Wa, G, Wa, Y ad Xie, J, A umerical simulator of outbursts of coal ad as, i Aziz, N (ed), 10th Uderroud Coal Operators' Coferece, Uiversity of Wolloo & the Australasia Istitute of Mii ad Metallury, 2010, Research Olie is the ope access istitutioal repository for the Uiversity of Wolloo. For further iformatio cotact the UOW Library: research-pubs@uo.edu.au

2 2010 Uderroud Coal Operators Coferece The AusIMM Illaarra Brach A NUMERICAL SIMULATOR OF OUTBURSTS OF COAL AND GAS She Xue 1, Ga Wa 1,2, Yuca Wa 1, ad Ju Xie 1 ABSTRACT: A outburst of coal ad as i uderroud coal mies may occur he stress coditio ad coal failure combie ith rapid as desorptio. A mechaical ad fluid coupled umerical simulator, SimBurst, has bee developed to simulate the iitiatio process of the outburst, as a first step to model the hole process of the outburst. This paper describes the simulator ad a simple model set up ith the simulator to model the iitiatio of a outburst i roaday excavatio to illustrate the methodoloy ad approach ith the SimBurst. The model simulated the basic features of a outburst iitiatio process, icludi coal deformatio, pore pressure ad pricipal stress vector redistributio, ad yield ad tesile failure zoe of coal. INTRODUCTION A outburst of coal ad as is a major hazard i uderroud coal mii. It could cause daers of asphyxiatio due to oxye deficiecy, of poisoi y oxious ases, ad explosio by iadvertet iitio of the resultat explosive mixtures ad of ijury due to violece. A umber of theories have bee proposed to explai the mechaism of the outburst. These iclude; cavity theory ad the pocket theory (Shepherd et al, 1981), the dyamic theory (Farmer ad Pooley, 1967; Shepherd et al, 1981), ad the spherical shell destabilizatio theory (Jia, 1995). It is eerally accepted that mii-iduced chaes i strata stress ad as pressure cause deformatio ad failure of coal ad the iteractio betee as ad coal results to a dyamic type of failure or outbursti. More specifically, there are to coditios hich must be met for a outburst to occur: coal must be failed uder a effective stress; as must be able to desorp rapidly from the coal ad the as pressure must be lare eouh to push the failed coal ito the mii opei istataeously. I other ords, the occurrece of a outburst is the result of combied effects of stress redistributio, as desorptio, coal property ad time. Despite the eeralized uderstadi of the outburst mechaism, quatitative predictio of the outburst is still a challei issue as curret outburst predictio methods are larely based o past practical experieces ad empirical data. Some attempts have bee made to umerically model the process of the outburst. These iclude maily a phase trasformatio model (Litiiszy, 1985), a as desorptio ad flo model (Paterso,1986), a boudary elemet model (Barro ad Kullma, 1990), a airay as flo model (Otuoye ad She, 1994), a fracture mechaics model (Oditsev s,1997), a simple fiite elemet model (Xu et al., 2006) ad a plasticity model (Wold et al., 2008). Despite these reat efforts there is still o sile umerical model to simulate the hole process of the outburst. It has prove to be difficult because the outburst is i fact a to-step process (iitiatio ad developmet) ad each step has its o characteristics ad requires differet approaches. As a first step, a umerical simulator, SimBurst, is developed to model the iitiatio process of the outburst. SIMBURST DESCRIPTIONS SimBurst is a three-dimesioal umerical simulator developed by CSIRO Earth Sciece ad Resource Eieeri. With this simulator, the process of mii-iduced stress redistributio, chaes of coal/rock permeability ad pore pressure, ad coal/rock failure (iitiatio of the outburst) ca be simulated to ai a better uderstadi of outburst mechaism. It ca also be used to uderstad the relative importace of the key cotributi factors i the outburst iitiatio throuh parametric studies ad help determie the threshold values for outburst risk. 1 CSIRO Earth Sciece ad Resource Eieeri, PO Box 883, Kemore, QLD, Australia Collee of Resource ad Evirometal Eieeri, Shado Uiversity of Sciece ad Techoloy, Qidao, Shado, Chia February

3 2010 Uderroud Coal Operators Coferece The AusIMM Illaarra Brach There are to importat processes i outburst iitiatio: coal deformatio ad failure, ad as desorptio ad flo. The deformatio ad failure of coal are modelled ith a idely used commercial code for eotechical aalysis of rocks (FLAC3D), ad the as desorptio ad flo are simulated ith a ell-established code for fluid flo ad as desorptio modelli (COMET3). These to codes are coupled to model the outburst iitiatio. Modeli of the deformatio ad failure of coal Coal is treated as a cotiuous medium ad its mechaical behaviour is umerically modelled as it reaches equilibrium or steady plastic flo. Applicatio of the cotiuum form of the mometum priciple yields the folloi equatios of motio: ij, j b i dvi dt (1) Where is mass per uit volume of the medium; b i is body force per uit mass; ad material derivative of the velocity. dv i / dt is Equatio (1) is solved ith a stress-strai la. The icremetal stress ad strai duri a time step is overed by various elastic or elasto-plastic costitutive las, hich ca be ritte i a eeral form as follos: ' ' H (, t) (2) Where H is a ive material fuctios; ad t is a time icremet. ' is the effective stress; is ifiitesimal strai-rate tesor The effective stress i Equatio (2) is related to the total stress i Equatio (1) by ' I P (3) Where is Biot s effective stress parameter; I is the uit tesor; ad P is pore pressure. Simulati as desorptio ad flo Coal is cosidered as dual-porosity/sile-permeability system. Gas diffuses from the discotiuous coal matrix blocks ito the cotiuous cleat system i coal. The basic equatios overi fluid flo i the coal cleats (fractures) are mass coservatio for as ad ater: b M p Z R b M p Z q q d dt b S R b M (4) b M p Z q d dt b S s (5) Where is radiet operator; is diverece operator; subscript ad stad for as phase ad ater phase respectively; M kk r is phase mobility, here k, k r, are absolute permeability, relative permeability ad viscosity respectively, is viscosity; is as or ater ravity radiet, here is phase mass desity ad is ravitatioal acceleratio; S is deree of saturatio; b 1 B is as or ater shrikae factor, here B is formatio volume factor; t is time; Z is elevatio; is effective fracture porosity; q ad q are the ormal ell source terms; R s is as solubility i ater. Gas phase pressure P ad ater phase pressure m P are related by capillary pressure P : s c February 2010

4 2010 Uderroud Coal Operators Coferece The AusIMM Illaarra Brach P c p p (6) Water ad as saturatio satisfy: S S 1 (7) Equatios (4)-(7) make up four equatios ad cotai four uko variables P, P, S ad S, hece it is a solvable system. The volume of adsorbed as i the coal matrix is described by the Lamuir adsorptio isotherms: V V L (8) P L p p Where V is volume of as adsorbed at pressure P ; V L is Lamuir volume; P L is Lamuir pressure. The as flo (rate) throuh the matrix is described mathematically by Fick s first la of diffusio expressed i the form: q m Vm C C( p) (9) Where C is averae matrix as cocetratio; V m is bulk volume of a matrix elemet; p is as pressure; ad is as sorptio time. Coupli Because FLAC3D ad COMET3 are to separate codes, he coupled toether, the resulted equatios caot be solved simultaeously; istead they are solved sequetially ith coupli parameters passed to each equatio at specific itervals. I the SimBurst, the FLAC3D ad COMET3 codes are executed sequetially o compatible umerical rids ad coupled throuh user-defied modules hich serve to pass relevat iformatio betee the equatios that are solved i the respective codes. The basic ideas of the coupli process are sho i Fiure 1. k k e 0 J 1 P p S p S Fiure 1 - Basic process of coupli mechaical model ith fluid model AN EXAMPLE WITH SIMBURST A simple model is set up ith the SimBurst to simulate the iitiatio of a outburst i roaday developmet to illustrate the methodoloy ad approach (Fiure 2). The model is 150m lo i x- directio, 105m ide i y-directio ad 4m i z-directio (Fiure 3) February

5 2010 Uderroud Coal Operators Coferece The AusIMM Illaarra Brach xx zz yy rock coal rock yy zz xx Fiure 2 - A computatioal model i roaday developmet y Fiure 3 - A plai vie of the model The roaday is 5 m i idth ad a total of five excavatio steps (from 1 to 5) ith 5m i each step are simulated ith the model. The developmet rate is 13.75m/shift. Table 1 lists other major parameters used i the model. Table 1 - Key parameters used i the model Parameter Value Uit Mii depth 500 m Coal UCS 10 MPa Permeability i x-directio 0.4 md Permeability i y-directio 1.3 md Permeability i z-directio 0.2 md Lamuir costat V L 21 m 3 /t Lamuir costat P L 900 kpa x The modelled results preseted here iclude mesh deformatios, pore pressure cotours, pricipal stress vectors ad yielded ad tesile failure zoes aroud the excavated roaday February 2010

6 2010 Uderroud Coal Operators Coferece The AusIMM Illaarra Brach The mesh deformatio for each exactio step is sho i Fiure 5. It idicates that marial deformatio occurs i the 4 th step, ad a lare deformatio occurs at the 5 th step, idicati a iitiatio of a outburst. It should be oted that the deformatio at the 5th step as sho i the Fiure is take at a istat he the outburst is iitiated. S1 S2 S3 S4 S5 Fiure 5 - Mesh deformatios ith roaday developmet Fiure 6 shos cotours of the pore pressure distributio at the 5 th step. The elliptical shape of the pore pressure distributio is caused by aisotropic permeability. The maximum pore pressure radiet occurs i the directio of miimum permeability at the face. Fiure 6 - Pore pressure chaes ith mii advace Fiure 7 shos the pricipal stress distributio he a outburst occurs. It idicates that a stress arch is formed about 4 to 5 m ahead of the face. Due to yield ad tesile failure of coal failure, a destressed zoe is formed aroud the excavated roaday, as sho i Fiure February

7 2010 Uderroud Coal Operators Coferece The AusIMM Illaarra Brach S1 S2 S3 S4 S5 Fiure 7 - Pricipal stress distributios Yield failure Tesio failure Fiure 8 - Coal failure zoes aroud the excavated roaday SUMMARY A umerical simulator, SimBurst, has bee developed by coupli a eotechical aalysis code ad a fluid flo ad as desorptio model to simulate the iitiatio process of the outburst, as a first step to model the hole process of the outburst. A simple model is set up ith the SimBurst to simulate the iitiatio of a outburst i roaday excavatio to illustrate the methodoloy ad approach of the SimBurst. The model simulated the basic features of a outburst iitiatio process, icludi coal deformatio, pore pressure ad pricipal stress vectors redistributio, ad yield ad tesile failure zoe of coal. It should be oted that the curret versio of the SimBurst is limited to simulate the process of the outburst iitiatio maily because it is based o cotiuum media ad o explicit mechaisms of fracture ad frametatio of coal is icluded. The CSIRO outburst research team is curretly developi a e versio of SimBurst hich ill overcome some of the limitatios of the curret SimBurst. For example, coal ill be cosidered as a o-cotiuum medium ad to-ay coupli betee as ad fractured coal ill also be take ito cosideratios February 2010

8 2010 Uderroud Coal Operators Coferece The AusIMM Illaarra Brach ACKNOWLEDGEMENTS The research as joitly fuded by CSIRO ad Huaia Coal Mii Group of Chia. REFERENCES Barro, K, ad Kullma D, Modelli of outburst at #26 Colliery, Glace Bay, Nova Scotia, Part 2, proposed outburst mechaism ad model. Mii Sciece ad Techoloy, 2, Farmer, I W ad Pooley F D, A hypothesis to explai the occurrece of outbursts i coal, based o a study of est Wales outburst coal. Iteratioal Joural of Rock Mechaics & Mii Scieces, 4, Litiiszy, J, A model for iitiatio of as outburst. Iteratioal Joural of Rock Mechaics, Mii Scieces & Geomechaics Abstract, 22, Otuoye, F ad She, J, A umerical simulatio of as flo duri coal/as outbursts. Geotechical ad Geoloical Eieeri, 12, Oditsev, V N, Sudde outburst of coal ad as failure of atural coal as a solutio of methae i a solid substace. Joural of Mii Sciece, 33, Paterso L, A model for outbursts i coal. Iteratioal Joural of Rock Mechaics & Mii Scieces & Geomechaics Abstract, 23, Shepherd, J, Rixo, L K. ad Griffiths, L, Outbursts ad eoloical structures i coal mies: a revie. Iteratioal Joural of Rock Mechaics & Mii Scieces & Geomechaics Abstract, 18, Wold M B, Coell, L D ad Choi, S K, The role of spatial variability i coal seam parameters o as outburst behaviour duri coal mii. Iteratioal Joural of Coal Geoloy, 75, 1 14 Xu, T, Ta, C A Ya, T H, Zhu, W C ad Liu, J, Numerical ivestiatio of coal ad as outbursts i uderroud collieries. Iteratioal Joural of Rock Mechaics & Mii Scieces, 43, February