Tracing coalbed gas dynamics and origin of gases in advancement of the working faces at mining areas Preloge and Pesje, Velenje Basin Spremljanje sestave premogovega plina in izvor plinov z napredovanjem čela delovišč na pridobivalnih (rudarskih) območjih jam Preloge in Pesje, Velenjski bazen Tjaša Kanduč1, *, Janja Žula2, Simon Zavšek2 Jožef Stefan Institute, Jamova 39, SI - 1000 Ljubljana, Slovenia 2Velenje Coal mine, Partizanska 78, SI - 3320 Velenje, Slovenia Corresponding author. E-mail: tjasa.kanduc@gmail.com Received: May 9, 2011 Accepted: September 7, 2011 Abstract: During excavation of lignite in the Velenje Coal Mine coalmine, seam problems with gas outbursts occur. Geochemical investigations are designed to help predict, prevent, and manage coal mine gas outbursts and to study their origin and mechanisms. However, geochemical studies of the coalbed gases in the Velenje basin have been initiated since year 2000. Temporal changes in chemical and isotopic composition of "free" seam gases were observed as a function of the advancement of the working face -120/B, G2/C and -50/B within boreholes jpk-28/10, jpk-30/10, jpk-31/10, jpk-22/09 and jpk-23/09. Mass spectrometry and isotope mass spectrometry methods were used to determine gas composition and perform gas characterization. Coalbed gases in the Velenje basin are highly variable in both their concentrations and stable isotope composition. Major gas components are CO2 and methane. Concentrations and isotopic studies revealed several genetic types of coalbed gases: endogenic CO2 (including CO2 originating from dissolution of carbonates), microbial methane and CO2. Izvleček: Med izkopavanjem lignita se lahko pojavljajo plinski izbruhi, zato je pomembno izvajati nadzorne meritve premogovnih plinov na odkopih. Geokemične raziskave premogovnega plina so pomembne 274 Kanduč, T., Zula, J., Zavšek, S. za razumevanje mehanizma nastanka, preprečevanja in napovedovanja plinskih izbruhov. Raziskave premogovnega plina v Velenjskem bazenu potekajo od leta 2000. Časovne spremembe v kemijski in izotopski sestavi "prostih" premogovnih plinov smo spremljali kot funkcijo približevanja čel delovišč 120/B, G2/C in -50/B v vrtinah jpk-28/10, jpk-30/10, jpk-31/10, jpk-22/09 in jpk-23/09. Sestavo in izotopsko sestavo premogovnih plinov smo določili z metodama masne spektrometrije in izotopske masne spektrometrije. Premogovni plini v Velenjskem bazenu se spreminjajo tako po vsebnosti kot tudi po izotopski sestavi. Glavni plinski komponenti sta CO2 in metan. Raziskave vsebnosti in stabilnih izotopov premogovnih plinov kažejo različne izvore plinov: endogeni CO2, (vključno s CO2, ki nastaja zaradi raztapljanja karbonatov) ter mikrobni metan in CO2. Key words: coalbed gas composition, working faces, carbon isotopes, gas origin, Velenje basin Ključne besede: sestava premogovnih plinov, odkopi, ogljikovi izotopi, izvor plinov, Velenjski bazen Introduction An increasing demand for coal, as a result of changes in the availability and cost of other fossil fuels, has re-focused attention on the problem of outbursting in deeper coal mines. When the rate of advance of the cutting face is slow, better opportunities for the slow escape of the gases under high pressure in the virgin coal exist. However, at the faster rates of advance of the coal face now required, less time is available for the equalization of gas pressures and mining induced stresses. Thus, the hazard of instantaneous outbursting of gas and coal is increased, although under favorable circumstances some control may be achieved by gas drainage (Smith & Gould, 1980). It is generally assumed that the pressure and volume of gases held within the virgin coal play a major part in producing outbursts; however, some doubt still exists as to whether outbursts are actually triggered by gas pressure or by stresses induced in the rock itself during mining operations (Flores, 1998). The most trivial definition of coal-bed gas is "gas from coal". Coalbed gas usually consist of hydrocarbons (mainly methane), CO2 in concentrations from 0 to greater than 99 %, and occasionally small percentages of nitrogen (Clayton, 1998). A numbers of models were developed to describe sources of hydrocarbon gases (Shoell, 1983, Kotarba, 1990, Scon, 1993, Kotarba, 2001, Ara-vena, 2003). Stable carbon isotope analyses of methane and CO2 can be applied to identify the origin of coal-bed gases, their migration pathways, and accumulation processes. There are three main sources of hydrocarbon gases and CO2 in sedimentary basins: abiogenic, microbial and thermogenic gas (Scott, 1993). In general, thermo-genic gases are typically associated with high rank coal, whereas microbi-al gases are typically associated with low rank coals and could have been produced throughout the basin history, as long as the coalbeds were never pasteurized. Abiogenic sources of gas are typically found in deep subsurface (Sherwood Lollar et al., 2006). Details of isotopic composition of different lithotypes of lignite and the origin of coalbed CO2 gas based on tectonic events during formation of the basin using cluster analysis and CDMI index (carbon dioxide methane index) of different gases are presented in Kanduc et al. (2005a) and Kanduc & Pezdic (2005b). The study revealed levels of microbial methane and CO2, and endogenic CO2. Unexpected organic arsenic compounds found in Velenje lignite indicated their relation with biogeo-chemical degradation of organic material (Slejkovec & Kanduc, 2005c). The aim of the study is to explain concentrations and origin of gases as a function of the advancement of the working faces -120/B, G2/C and -50/B in mining areas of Preloge South, Pre-loge North and Pesje in Velenje Basin. Materials and methods Sampling procedure Sampling of coalbed gas was performed at working faces -120/B (mining area Preloge South), G2/C (Preloge North) and -50/B (Pesje) from November 2009 till November 2010, in the lignite seam of the Velenje basin with purpose to determine concentrations (CO2, methane, N2 and O2) and 513C of methane and CO2 from pillar coal in advancement with the working face (Figure 1). Boreholes were drilled in pillar coal to a depth of 20 m. After drilling, the capillary tube was inserted in the borehole. Boreholes (jpk-28/10 - Preloge South, jpk-30/10, jpk-31/10 - Preloge North, jpk-22/09 and jpk-23/09 - Pesje) and working faces (-120/B, G2/C, and -50/B) are shown on Figure 1. "Free gas" emitted from the borehole was collected in a 50 mL plastic syringe and then transferred to a 12 mL evacuated ampoule with a septum. After sampling "free gas" from the boreholes, the ampoules were stored at normal atmospheric conditions un- til analysis. Free gas includes both the volatiles filling the pores and cracks within the coal structure and some gas degassed from the coal during drilling and sampling (Kotarba, 2001). Some boreholes were compressed in advancement of the working faces, which caused too much air contamination in boreholes and therefore unable the interpretation. In this study only data of coalbed gases, which contained below 50 % of air were used for interpretation. Analytical procedure Determinations of the concentrations of methane, CO2, nitrogen, oxygen and argon were performed using a homemade mass NIER spectrometer. The method of singular decomposition of matrix was used, to obtain the simultaneous analysis of the gases. The precision of the method was ±3 %. Isotope composition of methane, CO2 was determined using an Europa 20-20 continuous flow isotope ratio Figure 1. Map of sampling locations of coalbed gases from the lignite strata at working faces (-120/B - Preloge South, G2/C - Preloge North and -50/B - Pesje) mass spectrometer with ANCA - TG preparation module. First water was removed and then CO2 was directly analyzed for 13C content. For methane measurements first CO2 was removed and then methane was combusted over hot 10 % platinum CuO (1000 °C). The methane completely converted to CO2 was then directly analyzed for isotopic composition of carbon (513C). Working standards calibrated to IAEA (International Atomic Energy Agency) reference materials were used with value of -3.2 %0 for CO2 and value of -47.5 % for methane relatively to VPDB (Vienna Pee Dee Belemnite). Analytical precision for carbon isotope composition is estimated to be ±0.2 %o. The stable carbon isotopes are presented in the 5 - notation relative to VPDB standards and expressed in parts per million (Co-plen, 1996) as follows (O'Neil, 1979): Where: Rs - ratio 13C/12C in sample Rrm - ratio 13C/12C in reference material Results and discussion "Free gases" accumulated within the lignite coal seam showed a considerable variability in concentrations and isotopic composition. Due to air con- tamination within boreholes and the capillary system samples were recalculated on an air - free basis. Major gas components were CO2 and methane. Only one sample from our study had nitrogen, meaning that it was in excess. Concentrations of CO2 varied from 20.8 % to 86.9 %, methane from 13.1 % to 58.9 %. Geochemical index CDMI ((CO2/ (CO2 + CH4) x 100 %) vary from 26.1 % to 86.9 % and stable isotope ratios varied in the following ranges: 513CCO2 from -12.5 % to 2.3 % and 513CCH4 from -69.1 % to -26.9 % (Table 1). High correlation (r between 0.95 and 1) is obtained between methane and CO2 concentrations (Figure 2) in a lignite seam within boreholes as a function of the advancement of the working faces (-120/B, G2/C and -50/C). Working faces from different mining areas (Pre-loge North, Preloge South and Pesje) have different composition of coalbed gases. At all investigated mining areas Preloge South (-120/B), Preloge North (G2/C) and Pesje (-50/B) CO2 prevails under methane (Figure 2). The highest CO2 concentrations are observed in mining are Preloge South (except 1 sample) where N2 concentrations were in excess of air (Table 1). The results of areas of high and low methane concentrations (Figure 2) are in physico-chemical properties of CO2 and methane (Atkins, 1994). Fissures generated Table 1. Composition of coalbed gases and isotopic composition of carbon (ô13C) of coalbed gases (CO2 and methane) at working faces: -120/B, G2/C and -50/B within boreholes jpk - 28/10, jpk - 30/10, jpk - 31/10, jpk - 22/09 and jpk - 23/09. CDMI index = (CO2/(CO2 + CH4) x100 %) Working face -120/B, borehole jpk-28/10, mining area Preloge South Date of sampling Distance of the working face (m) ch4 (vol. %) c02 (vol %) n2 (vol. %) CDMI index (%) $ c C02 (%„) s cCH4 (%„) March 3, 2010 191 13.1 86.9 0.0 86.9 -5.7 -62.7 March 12, 2010 160 58.9 20.8 20.3 26.1 -7.9 -64.4 March 17, 2010 151 13.8 86.2 0.0 86.2 -7.1 -63.1 April 9, 2010 92 17.5 82.9 0.0 82.6 -4.8 -50.6 May 4, 2010 45 20.5 79.5 0.0 79.5 9.6 -69.1 May 11, 2010 37 19.8 80.2 0.0 80.2 -8.1 -62.7 May 13, 2010 32 23.0 77.0 0.0 77.0 -8.6 -62.9 May 14, 2010 31 20.0 80.0 0.0 80.0 -8.5 -61.9 May 18, 2010 30 24.3 75.7 0.0 75.7 -12.5 -63.3 June 1, 2010 30 29 1 70.9 0.0 70.9 -7.6 -51.1 Working face G2/C, borehole jpk-30/10, mining area Preloge North Date of sampling Distance of the working face (m) ch4 {vol. %) c02 (vol %) n2 (vol. %) CDMI index (%> 5 c C02 <*.) S13CCH4 August 18, 2010 341 42.2 57.8 0.0 57.8 1.8 -31.0 August 31, 2010 316.4 43.5 56.5 0.0 56.5 -1.1 -37.2 September 22, 2010 307.5 46.1 53.9 0 53.9 -6.6 -29.8 November 30, 2010 152.1 42.3 57.7 0.0 57.7 0.9 -50.0 Working face G2/C, borehole jpk-31/10, mining area Preloge North Date of sampling Distance of the working face ch4 vol. % c02 vol % n2 vol. % CDMI index (%) 5 c co2 (%„) ® COM (%o) August 20, 2010 335.3 45.7 54.3 0.0 54.3 -1.7 -45.5 August 27, 2010 316.4 30.7 69.3 0.0 69.3 -9.1 September 22, 2010 307.5 36.6 63.4 0.0 63.4 -3.9 -60.4 November 30, 2010 152.1 43.3 56.7 0.0 56.7 -0.7 -50.3 Working face -50/B, borehole jpk-22/09, mining area Pesje Date of sampling Distance of the working face ch4 vol. % c02 vol % n2 vol. % CDMI index (%) s ° ^ C02 (%„) 0 LjchI November 16,2009 December 14, 2009 January 11, 2010 February 22, 2010 February 24, 2010 March 15, 2010 March 22, 2010 267 214 181 79 76 35 26 41.3 19.9 33.1 24.5 28.0 42.1 27.6 58.7 80.1 66.9 75.5 72.0 57.9 72.4 0.0 0.0 0.0 0.0 0.0 0.0 0.0 58.7 80.1 66.9 75.5 72.0 57.9 72.4 2.28 -4.76 -0.70 -2.1 -4.4 -0.1 1.0 -26.9 -44.4 ^42.6 -58.0 -60.6 -48.3 -56.6 Working face -50/B, borehole jpk-23/09, mining area Pesje Date of sampling Distance of the working face ch4 vol. % co2 vol % N2 vol. % cdmi index (%) s c c02 (%„) 5 cCH4 (%o) November 16,2009 267 38 7 61.3 0.0 61.3 0.44 -49.4 February 22, 2010 79 24.5 755 00 75.5 -2.2 -583 February 24, 2010 76 28.0 72.0 0.0 72.0 -4.4 -60.6 March 8, 2010 48.2 19-2 80-8 00 80.8 -1.3 -54.6 March 15, 2010 35 260 740 00 74.0 -2.5 -59.6 with advance of the working face enable migration of methane through lignite seam in surrounding strata or to a surface, while CO2 remains adsorbed in lignite seam. Figures 3 A, B and C show concentrations of CO2 and methane at researched mining areas (Preloge North, Preloge South and Pesje) in advancement of the working face (-120/B, G2/C and -50/B). Generally, at all working faces it was observed that methane migrates faster than CO2 with advance of the working face, therefore the trend of maximum of methane concentration coincides with minimum CO2 concentrations (Figures 3 A, B and C). Tempo- ral changes of coalbed concentrations (CO2 and methane) fluctuate with the rate of advancement of working faces (Figures 3 A, B and C). At the working face -120/B it was observed that at the distance of the working face 92 m (rate of advancement of working face 2.5 m/d) caused highest concentrations in methane and lowest concentration in CO2 (Figure 3 A). At the distance of the working face 45 m (rate of advancement of the working face cca. 2 m/d) the next maximum of CO2 was detected, coinciding with minimum of methane concentrations. After May 13 the rate of working face approaching to borehole jpk-28/10 gradually slowed down (rate of advancement Figure 2. Concentration of CO2 versus concentration of methane in lignite seam in advancement of the working faces (-120/B Preloge South, G2/C Preloge North, -50/B Pesje) of working face cca. 1 m/d) causing lower fluctuations in concentration of CO2 and methane. At the working face G2/C no trends of increasing/decreasing of CO2 and methane concentrations are observed, probably due to constant approaching of the working face with average rate 1.9 m/d to the boreholes jpk-30/10 and jpk-31/10 (Figure 3 B). The rate of advancement of the working face -50/B was from 0.9 m/d to 2.4 m/d. Maximum methane and minimum methane concentrations were traced within borehole jpk-22/09 at the distance of the working face 35 m (rate of advancement of working face cca. 2.2 m/d) (Figure 3 C). Approaching of the working face also influence on stress situation at working face (Figure 4). During excavation of lignite secondary fissures are generated due to rearrangement of primary stress conditions, enabling releasing of coalbed gas preserved in pores of coalbed reservoir. These stress conditions are presented in Figure 4 modified after William (1999). Results of changing of stresses were investigated by three axis cells build in lignite structure. The research revealed that at the distance of 120 m from the working face vertical stresses start to increase. At the distance of 80 m from the working face shearing and collapsing of lignite structure occur; vertical stresses continue to increase while horizontal and transverse stresses decrease. At the distance of the working face 30-50 m from three axis cell vertical stresses are so high that the lignite structure collapse (Zavsek, 1990). Research of temporal variations of concentrations (CO2 and methane) within borehole 780H IV at working face -80c similar trends as at working faces -120/B, G2/C and-50/B in concentrations of CO2 and methane were observed; at the distance of the working face 177 m and 122 m maximum concentrations of methane were traced and coincide with minimum concentrations of CO2 (Kanduc, 2004). Besides the rate of advancement of the working face concentrations of methane and CO2 measured in lignite seam within boreholes (jpk-28/10, jpk-30/10, jpk-31/10, jpk-22/09 and jpk-23/09) also depends on permeability and porosity of lignite seam. When methane and CO2 reaches strata with high permeability such as fault zones, their concentrations start to increase, therefore also areas with high concentrations of methane and CO2 might be expected. CO2 behaves at certain condition (above its critical temperature 31.1 °C and critical pressure 7.39 MPa, expanding to fill its container like a gas but with a density like that of a liquid) as supercritical fluid (Atkins, 1994), meaning that could be at liquid and gas phases. The conditions of critical temperature and pressure for CO2 are possibly reached at working faces causing its adsorption (liquid phase)/desorption (gas phase). Areas with high CO2 content (CDMI index) might be potentially danger- Figure 3A. Concentration of methane and CO2 versus date of sampling in advancement of the working face -120/B (Preloge South) Figure 3B. Concentration of methane and CO2 versus date of sampling in advancement of the working face G2/C (Preloge North) Figure 3C. Concentration of methane and CO2 versus date of sampling in advancement of the working face -50/B (Pesje) ous for gas outbursts (Clayton, 1998). Anyway, investigation of gas concentrations at each working face should be further related to petrological characteristics of lignite (lithotype type e. g. xylite, detrite), tectonically characteristics (characteristics of fault zones) and geotechnical conditions (pressures, porosity and permeability) to get better insight to coalbed gas migration as well sorption/desorption processes, especially CO2 through lignite. Interpretation of coalbed gases from investigated mining areas of Pesje and Preloge was performed according to previous study of tracing inorganic and organic carbon cycles in different geological media (calcified xylite, carbonate lenses in the lignite seam, lignite and Pliocene, Triassic and Lithotam- nium aquifers) (Kanduc et al., 2010, in review). From the results it was concluded that coalbed gases in the Velenje Basin are mainly microbial (bacterial) origin, while CO2 could be also endogenic. Microbes that generate methane (methanogens) could have either of been deposited with the coal sediments in the geologic past or transported in more recently with active groundwater recharge. To thrive methanogens require an anoxic, aqueous environment with organic carbon substrates and the absence of other free-energy electron acceptors, such as NO3- and SO42-. Fermentative bacteria degrade complex organic matter in coalbeds (e.g. structures of carbohydrates, proteins and lipids that originate in vegetation and sediments) to simpler molecules including acetate (CH3COOH), fatty acids, CO2 and H2 gas. Acetogenic bacteria thrive on fatty acid products to produce acetate, with CO2 and H2 by products (Kotelnikova, 2002). The products of these reactions support a variety of methanogens. Some metha-nogens use an acetate food source to produce CO2 and methane, according to following reactions: ch3cooh ^ ch4 + CO2 Acetate fermentation (2) While other methanogens use the hydrogen gas to reduce CO2: CO2 + 4H2 ^ CH4 + 2H2O CO reduction or HCO3- +4H2 ^ CH4 + 2H2O + OH-CO„ reduction with the 13C isotope and depletion of 12C in generated CO2 (Figure 5). It is known that coalbed lignite strata were formed in the Pliocene in a calm sedimentary environment (Markic & Sachsenhofer, 1997), but the proportion of early stage microbial gas retained in the lignite structure (Smith & Pallaser, 1996) is difficult to estimate. Microbial methane in Velenje basin was generated by microbial reduction and/or by microbial fermentation. Also recent microbial activity (methanogenic and methanotrophic bacteria) might generate microbial (3) (4) We applied diagrams 513CCH4 versus S13CCO2 (Figure 5) to explain the origin of methane and 513CCO2 versus CDMI index (Figure 6) to explain the origin of CO2. 513CCH4 (Figure 4) in the Velenje basin indicate the successive origin of methane: microbial (CO2 reduction) with 513CCH4 values from -40 %o to -50 %, microbial (acetate fermentation) with 513CCH4 less than -50 % and mixed origin between these two (Clayton, 1998). Enrichment with 13C in methane could be also due to microbial oxidation of methane, which results in an enrichment of residual methane Figure 4. Geomechanical conditions and generation of secondary fissures due to longwall method of excavation of lignite, causing a realese of coalbed gases inside of demolished zone (adapted after Williams, 1999) Figure 5. Interpretation of the origin of methane in the Velenje basin using S13CCH4 versus S13CCO2 in a lignite seam at boreholes (jpk - 28/10, jpk - 30/10, jpk - 31/10, jpk - 22/09 and jpk - 23/09) in advancement of the working faces (-120/B, G2/C and -50/B) Figure 6. Interpretation of the origin of CO2 at working faces (-120/B, G2/C in -50/B) using S13CCO2 versus CDMI index in a lignite seam in boreholes (jpk - 28/10, jpk - 30/10, jpk - 31/10, jpk - 22/09 and jpk - 23/09) ahead of the working faces (-120/B, G2/C and -50/B) gas (Kotelnikova, 2002). Methane and CO2 were/are generated by mi-crobial fermentation process while methane was/is generated by CO2 reduction process. CO2 generated by fermentation process is characterized by 513CCO2 similar that organic matter around -25 %o, while microbial reduction process results in an enrichment with the 13C of residual CO2 (Figures 5 and 6). Microbial CO2 reduction is characteristic in mining area Preloge North, while at Preloge South 513CCH4 indicate bacterial origin via fermentation. At mining area Pesje both micro-bial (CO2 reduction and methane fermentation) processes generated methane formation (Figure 5). The origin of CO2 is interesting due to its relation with outbursts in coalmines. High concentrations of carbon dioxide in seam gases in Australian coals occur in regions of igneous activity and associated faulting and this component has been described as being of presumed pneumatolytic origin (Smith & Gould, 1980). In our study CDMI index versus 513CCO2 (Figure 5) was used to explain the origin of CO2 (Kotarba, 2001) and indicate endogenic CO2 (including CO2 originating from carbonates) and CO2 of microbial origin in relation to micro-bial methane (working face Preloge North, jpk-30/10) discussed above. At working face Pesje -50/B (jpk-22/09 and jpk-23/09) 513CCO2 indicate micro- bial and endogenic origin (Figure 6). Typical endogenic values of 513CCO2 are about -7 %o (Kotarba, 2001) and high CDMI index found in our study (at working face Preloge South, jpk-28/10) are probably related to the tectonics of the Šoštanj and Smrekovec faults at the time of formation of the Velenje basin. 513CCO2 derived from carbonates is dependent on 513C of carbonates and the temperature of their degradation. The resulting products are clay minerals and CO2. Supposing that dolomites underwent thermal decomposition and are located below lignite seam along the Šoštanj fault zone values of 513CCO2 from -2 %o to -6 %o might indicate CO2 of thermal decomposition of dolomites (Figure 6). This source of CO2 has similar values as endog-enic CO2 from the fault zones and can not be distinguished with 513CCO2. Also secondary processes should be considered since they might camouflage the origin of gases. Migration of gases as a secondary process caused by reduction of pressure in lignite seam causes enrichment in light 12C isotope in methane and in CO2. During migration of coalbed gases, gases of different origin might be mixed together and complicate interpretation of gas origin. Conclusions Gas outbursts remain for coalminers one of the main problems in the Velenje coalmine, therefore geochemical investigations are one of the important parameters used in case of prediction or prevention gas outbursts. Concentrations of methane and CO2 change in advance of the working face. Areas with high CO2 concentrations accompany with low methane concentrations and could be related with the rate of advancement of the working face. The rate of advancement of the working face is one of the crucial parameters influencing on gas composition at working faces. Concentrations and stable isotope studies of coalbed gases at working faces allow the possible interpretation of the origin of coalbed gas in the Velenje basin. Considering also the results of previous studies, it can be concluded that coalbed CO2 from investigated working faces is endogenic and bacterial origin. Methane is microbial origin (formed through microbial fermentation and/or CO2 reduction). Secondary processes like migration, adsorption/ desorption and mixing of gases of different origin during excavation complicate the interpretation of gas origin and were neglected in interpretation of the origin of coalbed gases. Examination of the origin of nitrogen (at loca- tions with excess of nitrogen) in coal-bed gases from Velenje Basin needs further research. Acknowledgements This study was conducted in the framework of project Z1-2052 funded by the Slovenian Research Agency (ARRS) and the Velenje Coalmine d.d. 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