Archives of Mining Sciences 51, Issue 4 (2006)

Archives of Mining Sciences 51, Issue 4 (2006) 577 588 577 MIROSŁAW WIERZBICKI*, MARIUSZ MŁYNARCZUK* MICROSCOPIC ANALYSIS OF STRUCTURE OF COAL SAMPLES COLLECTED AFTER AN GAS AND COAL OUTBURSTS IN THE GALLERY D-6, COAL SEAM 409/4 IN THE ZOFIÓWKA COAL MINE (UPPER SILESIAN COAL BASIN) STRUKTURALNE BADANIA MIKROSKOPOWE PRÓB POBRANYCH Z MAS POWYRZUTOWYCH W CHODNIKU TRANSPORTOWYM D-6 W POKŁADZIE 409/4 KWK ZOFIÓWKA One of the possible causes of rock and gas outbursts in collieries is structural changes in coal occurring in geologically disturbed areas. This paper summarises the results of measurements of rock fracturing as well as vitrinite and mylonite content after an outburst in the gallery D-6 in the Zofiówka coal mine. Measurement data show that coal in this region contains vast amounts of structurally deformed coal, with a thick network of internal fracturing. Such structure of the coal matter may facilitate gas accumulation in coal s internal structure and gas release whenever pressure changes should occur. Keywords: gas and coal outburst, coal structure, stereology W dniu 22 listopada 2005 r. w chodniku transportowym D-6 KWK Zofiówka (rys. 1) miał miejsce wyrzut metanu i skał. W jego wyniku chodnik został zasypany masami wyrzutowymi na długości ok. 35 m od czoła przodku a do atmosfery kopalnianej w czasie jednej godziny wydzieliło się ponad 8000 m 3 metanu. Po wyrzucie, w rejonie kawerny wyrzutowej, widoczne były dwa uskoki. Ich przebieg wskazywał na zawiasowy charakter tych nieciągłości (rys. 2). Jedną z przyczyn występowania wyrzutów węgla i gazu w kopalniach węgla kamiennego mogą być zmiany strukturalne węgla występujące w rejonach zaburzeń geologicznych pokładów. W pracy zaprezentowano wyniki pomiarów stereologicznych prób węglowych pobranych z mas powyrzutowych w omawianym wyrobisku. Masę tą w dominującym stopniu stanowił miał węglowy. Próby ziarnowe przeznaczone do badań pobrano w sposób krzyżowy z górnej (2 m od spągu), środkowej (1 m od spągu) i dolnej (20 cm od spągu) części mas próbki oznaczone jako OM/U, OM/M, OM/D oraz na wysokości 2 m od spągu chodnika, w rejonie prawego ociosu, środkowej części wyrobiska oraz przy lewym jego ociosie próbki oznaczone jako OM/UR, OM/UM, OM/UL. Z poszczególnych próbek wykonano zgłady a podczas ich obserwacji szczególną uwagę zwrócono na zawartość witrynitu, udział substancji odmienionej strukturalnie oraz na udział spękań występujących na poszczególnych składnikach struktury. * INSTYTUT MECHANIKI GÓROTWORU PAN, UL. REYMONTA 27, 30-059 KRAKÓW; POLAND

578 Próbki obserwowano przy powiększeniu 200. Zliczano następujące obiekty: klej, (nie był przedmiotem analizy), witrynit nieodmieniony (V), struktura odmieniona typu mylonit (MY), inertynit + liptynit (nieodmienione) (I+L), spękania na witrynicie (Cr(V)), spękania na strukturze odmienionej (Cr(MY)), spękania na inertynicie i liptynicie (Cr(I+L)), substancje nieorganiczne (IS). Przykłady analizowanych struktur widoczne na zgładach pokazano na rys. 3. W tabeli 1 podano udziały procentowe poszczególnych struktur widocznych na zgładach oraz wartości odchyleń standardowych dla poszczególnych zliczeń, obliczone zgodnie z Polską Normą. We wszystkich próbkach występował węgiel odmieniony strukturalnie mylonit. Średnia jego zawartość w masach powyrzutowych, liczona wraz ze spękaniami na tej strukturze, wynosiła 15.2%. Badany węgiel posiadał dużą zawartość witrynitu, wynoszącą średnio 83.7±2.7% ostatnia kolumna tabeli. Niezwykle ważną cechą strukturalną węgla, wpływającą na gazopojemność węgla i kinetykę odgazowania, jest stopień jego spękania. W tablicy 2 podano udziały spękań na poszczególnych, wydzielonych wcześniej grupach strukturalnych węgla obliczone ze wzorów (1-3). Struktura węgla zmylotynizowanego charakteryzuje się bardzo gęstą siecią spękań, podczas obserwacji mikroskopowych, osiągającą kilkaset pęknięć na milimetr. Gęstość spękań na mylonicie, (kol. 2, tab. 2) zawierała się w przedziale od 22.8 do 40.2[%] średnio 31.6%. Średni udział spękań na węglu nieodmienionym strukturalnie wynosi natomiast 5.1% (kol. 3 tab. 2). Różnica w istniejącej sieci spękań pokazuje, że przy takim samym ciśnieniu gazu mylonit posiadać może wielokrotnie większą pojemność gazową (dla gazu nie związanego sorpcją) niż węgiel nieodmieniony strukturalnie. Dodatkowo, gaz ten może zostać szybciej uwolniony poprzez niezwykle rozbudowaną sieć spękań wewnętrznych, co przejawia się wyższą wartością współczynnika dyfuzji stwierdzoną innymi badaniami (Cybulski et al., 2006). Czynniki te niewątpliwie wpływają na wzrost zagrożenia wyrzutowego. Brak wcześniejszych objawów sygnalizujących wzrost zagrożenia wyrzutem węgla i gazu bądź zagrożenia metanowego w przodku chodnika transportowego D-6, wysoki wskaźnik odgazowania węgla oraz istnienie w masach powyrzutowych znacznej ilości węgla odmienionego strukturalnie, mogącego gromadzić znaczne ilości gazu oraz oddawać go ze zwiększoną kinetyką w przypadku wystąpienia gradientu ciśnienia, pozwalają na stwierdzenie, że wyrzut w wyrobisku nastąpił w wyniku zbliżenia się czoła przodku do tzw. kieszeni gazowej. Słowa kluczowe: wyrzuty węgla i gazu, struktura węgla, analiza ilościowa, masy powyrzutowe 1. Introduction On 22 nd November 2005 an outburst of methane and coal occurred in the gallery D-6 in the Zofiówka colliery. The gallery was filled with coal and rock mass over the distance of 35 m from the front face and during one hour over 8000 m 3 of methane were released to the atmosphere in the mine (Dziurzyński et al., 2006a). The amount of rock mass was evaluated to be 320 m 3 (Report, 2006). That was the second outburst of coal and gas which occurred recently in the Rybnik Coal District of the Upper Silesian Coal Basin (USCB). The previous even took place on 23 rd August 2002 in the Pniówek coal mine, at the level 1000 m (Jakubów et al., 2003). It appears that outburst hazards in USCB collieries will be gradually becoming worse. According to Krause (Krause, 2005), this situation is not restricted to the previously mentioned Rybnik Coal District. As coal saturation with methane tends to increase with depth, other collieries in the USCB, for example Szczygłowice, Biełszowice, Knurów, Halemba and Sośnica should be regarded as outburst-prone as well.

579 High values of the coal degasification index were reported in the outburst zones in the collieries Pniówek and Zofiówka. The coal degasification index is expressed as the ratio of the volume of methane released during the outburst to the mass of burst rock. In the light of the fact that the two coal seams did not previously reveal any methane-bearing and outburst-prone behaviour, it is reasonable to suppose that this time gas pockets were encountered that often accompany geological disturbances. The term gas pocket implies that this spot is difficult to detect. It contains large amounts of gas at elevated pressure and physical and mechanical and structural parameters of coal in this area are different than in the adjacent regions. Variations of coal properties might occur over very short distances in the driven heading. Outbursts in the collieries Pniówek and Zofiówka occurred nearby hitherto undetected faulted regions and were closely associated with those geological features. In the inlet of the inclined drift in the outburst zone in the colliery Pniówek, there was a fault with the thrust 0.7 m ahead of the face front (Jakubów et al., 2003). There were some faulted regions ahead of the gallery D-6 in the colliery Zofiówka with two hinged faults, filled with clay material hardly permeable to gases (Dziurzyński et al., 2006b). 2. Coal structure and outburst phenomena state of art The research on the phenomenon of sudden gas and coal outburst is being conducted in laboratory conditions as well as in coal mines conditions. (Topolicki et al., 2004). Spacious work on the subject of the nature of gas and coal outbursts was published by (Lama, Bodziony, 1996). It is generally assumed that outburst prone areas are those adjacent to faulted or geologically upset regions where additional stresses lead to modifications of the coal structure: formation of new cracks or of mylonite structures. In the consequence, mechanical strength of coal is reduced, which in turns may leads to coal and gas outbursts (Suchodolski, 1977). According to many authors, geometric structure of coal is a major determinant of outburst-proneness of coals. In the middle of 20 th century Skoczyński (1954) observed that coal structure determines its cohesion and coal seam resistance to rock strata pressure which prepares coal seam for an outburst. Furthermore, it controls gas desorption rate and work performed by gas while it is released from coal. Cybulski and Bloch (1964) investigated the crack structure in coals and showed that it might be one of the indicators of increased outburst risk. Bodziony (Bodziony et al., 1990) suggest that proper investigation of microcracks on the polished sections of coal might give us a valuable parameter enabling us to identify local outburst hazard in individual seam sections. But this method was time-consuming and has not been well developed so far. It seems, that in the future there will be possibe to improve the measrments by using numerical methods, like e.g. of automatic image analysis (Młynarczuk, 2002).

580 Li (Li et al., 2003) observed that coals with mylonite structure exhibit 3-5 times larger porosity and significantly larger methane contents. According to their report, during one of the outburst in China 92 m 3 of methane were released per one ton of coal, whilst normally methane content in the seam prior to the event would not exceed 10 m 3 /t. Similar conditions were reported in the colliery Pniówek. Li (Li et al., 2003) are advocates of the gas pocket theory whereby an outburst occurs when the front section of the heading approaches the zone of structurally modified coal and gas under high pressure. High gas contents in outburst coal is associated with a larger effective surface, characteristic of mylonite, granulate or cataclastic coals. Basing on the experience of the Chinese, Yunxing and Cao et al. (2001) claim that nearly all gas and rock outbursts occur in regions characterised by major structural changes as deformed coals (with the granulate or mylonite structure) are unstable due to deterioration of their mechanical strength and high gas bearing capacity. They formulated their hypothesis stating that particular outburst hazard is encountered when the thickness of faulted strata should exceed 0.8 m. Williams and Weissmann (1995) showed a schematic diagram of gas pressure and stress variations around a mine heading approaching coals with mylonitic structure. They are of the opinion that when the distance between mylonitic coal and the front section of the heading is reduced, the stresses tend to increase, coal permeability decreases and a pressure gradient is produced near the heading s front section. Breaking of this protective barrier becomes the direct cause of an outburst. Basing on measurement data Li (Li et al., 2003) suggests that coal with modified structure will not always release gas at a faster rate than ordinary coal. They are of the opinion that outburst-prone zones occur in structurally disturbed regions. Researchers point out the differences in the behaviours of particular coal macerals with respect to gas. Because of well developed micropore space, vitrinite coals tend to sorb larger amounts of gases than the remaining maceral groups and release it at a slower rate (Lamberson and Bustin, 1993). Beamish and Crosdale (1998) claim that the differences in kinetics of desorption of individual maceral components are sufficient to generate the gas contents gradient, in consequence leading to an outburst. 3. Sample collection point An outburst of gas and rock in the gallery D-6 in the colliery Zofiówka occurred when the coalface was at the distance of 111.4 m from the ramp D-4. A fragment of map with indicated outburst location is shown in Fig. 1. After an outburst, two faults became well evident in the region of a cavern formed after an outburst. These proved to be hinged faults. Fault fissures were 30 cm wide. The first fault had an inclination of 30-40 in the NW direction, the other had 75-85 in the NW. The diagram of the geology of the outburst region is shown in Fig. 2.

581 Fig. 1. Fragment of map of the coal seam 409/4 with an indicated location of an outburst Rys. 1. Fragment mapy pokładu 409/4 z naniesionym miejscem wystąpienia wyrzutu 1 8-10 m 2 3m 3 0.9 2.9 m 4 8 6 0.9 3.3 m 5 1.6 m 7 0.5 m 2.2 m Fig 2. Geology of the front section of the gallery in the coal seam 409/4 after an gas and coal outburst (author: D. Janik Zofiówka Coliery, updated by T. Ratajczak AGH, Cracow). Legend: 1, 6 sandstone, 2 arenaceous shale, 3, 5 mudstone, 4 coal, 7 fault, 8 rubble Rys. 2. Sytuacja geologiczna w przodku chodnika transportowego w pokładzie 409/4 po wyrzucie metanu (autor D. Janik KWK Zofiófka, aktualizacja T. Ratajczak AGH Kraków). Objaśnienia: 1 i 6 piaskowiec, 2 łupek piaszczysty, 3 i 5 łupek ilasty, 4 węgiel, 7 uskok, 8 rumosz skalny

582 The rock mass after an outburst contained chiefly powder coal. Samples for grain analysis were collected by the crossover method. Samples designated as OM/U, OM/M, OM/D were taken from the upper (2 m from the floor), middle (1m from the floor) and down (20 cm from the floor) section, at the distance 111.4 m from the gate inlet. Samples designated as OM/UR, OM/UM, OM/UL were collected at the elevation of 2 m from the floor, at the distance of 111 m from the gate inlet in the region of the side wall on the right, in the middle part of the working and near the left side wall. 4. Microscopic analysis of coal samples Coal grains used in the quantitative tests were 0.5-1.0 [mm] in size. The sample material (about 30 g per sample) was first evacuated in the vacuum conditions, then covered with methyl methacrylate and autoclaved for 48 hours under the pressure 7.5 MPa at the temperature 65 C. Samples were then ground and polished to form polished sections to be used in further analyses. While planning the measurement procedures, the guidelines were applied that are set forth in the standard PN-ISO 7404-3. Measurements were taken with an AXIOPLAN microscope (ZEISS) and a computer-controlled stage XYZ. The image from the optical microscope would pass to a monitor via a CCD camera. On the monitor the crosshairs were indicated. Samples were observed at the magnification of 200. The analysed sample was placed on the stage and shifted with the step of 320 micrometers in the direction X and Y. Unless specified otherwise, 50 lines were measured on each sample, each line having 50 points. Accordingly, 2500 measurement points per one polished section were obtained. The analysed section area was 15.7 15.7 [mm]. The following objects on the polished sections were subject to analysis: glue (not analysed) not modified vitrinite V modified structure of the mylonite type MY inertinite + liptinite (not modified) I + L cracks on vitrinite Cr(V) cracks on modified structures Cr(MY) cracks on inertinite and liptinite Cr(I + L) inorganic substances IS Selected structures evident on the polished sections are depicted in Fig. 3. The percentage fraction of vitrinite group in the total coal volume was obtained accordingly. A preliminary inspection and analysis of polished sections reveals low liptinite content in the tested samples. That confirms the earlier findings of Gabzdyl (1969) who noticed relative low vitrinite content in the seam 409/4.

583 a) b) c) d) Fig. 3. Selected coal structures: a) vitrinite (V), b) cracks on vitrinite Cr(V), c) inertinite (I), d) strongly modified structure (mylonite MY) Rys. 3. Przykłady analizowanych struktur węgla: a witrynit (V), b spękania na witrynicie Cr(V), c inertynit (I), d silnie odmieniona (zmylotynizowana) struktura (MY) The percentage fractions of rock mass after an outburst (excluding the glue) as well as the standard deviations calculated according to the ISO standard PN-ISO 7404-3 are listed in table 1. The analyses reveal small amounts of inorganic substances (IS) in samples. 2.8% on the average. The largest amounts of inorganic matter are contained in samples collected from the middle section of the outburst mass. Some portion of the material at this point probably comes from fault fissures filled with clay. All samples contain structurally modified coal mylonite. 15.2% on the average. including the cracks on rock structures. Fluctuations in the amounts of mylonite are rather minor. except the sample collected near the left side wall (OM/UL) which has the lowest mylonite content. The column 9 in table 1 shows percentage fractions of not modified vitrinite (V) content in structurally unmodified coal (V + I + L). well evident on the polished section. The analysis of unmodified portion of coal reveals that it contains huge amounts of

584 Results of local analysis for samples from outburst mass TABLE 1 TABLICA 1 Zestawienie wyników analizy punktowej dla badanych prób ziarnowych mas powyrzutowych Sample percentage fraction [%] standard deviation [%] 100*V V+(I+L) V Cr(V) I+L Cr(I+L) MY Cr(MY) IS 1 2 3 4 5 6 7 8 9 OM/D 67.2 3.4 11.8 0.36 11.4 3.4 2.5 (1.26) (0.49) (0.86) (0.16) (0.85) (0.48) (0.42) 85.1 OM/M 59.8 2.4 14.0 0.5 12.0 4.3 7.0 (1.34) (0.42) (0.95) (0.20) (0.89) (0.55) (0.69) 81.0 OM/U 60.9 5.9 10.9 0.4 10.3 6.3 5.4 (1.30) (0.62) (0.83) (0.16) (0.81) (0.64) (0.60) 84.8 OM/UL 73.5 4.30 13.1 0.20 6.0 2.8 0.2 (1.91) (0.88) (1.46) (0.19) (1.02) (0.71) (0.19) 84.9 OM/UM 63.2 4.1 13.1 0.4 11.3 7.6 0.4 82.8 (2.07) (0.85) (1.45) (0.26) (1.36) (1.14) (0.26) 66.9 3.0 13.3 0.0 10.8 4.7 1.3 OM/OR 83.5 (2.04) (0.74) (1.47) (0.00) (1.34) (0.91) (0.49) Average 3.9 12.7 0.31 10.3 4.9 2.8 83.7 vitrinite. approaching 83.7±2.7%. This high vitrinite content is a distinctive feature of bright. high rank coals in the colliery Zofiówka. Fluctuations of vitrinite content shown in column 2 are mostly due to variable proportions of modified substance in particular samples (column 6 and 7, table 1). For that reason no average value is provided. Crack density (CD) is an important structural parameter of coal. which has influence on gas bearing capacity and kinetics of gas release. Table 2 provides the crack density parameters on particular. pre-selected structural groups of coal evident on polished sections. derived from the formulas 1-3. Cr( MY ) CD ( MY ) 100% (1) MY Cr( MY ) Cr( V ) Cr( I L) CD ( V I L) 100% (2) V ( I L) Cr( V ) Cr( I L) Cr( V ) Cr( I L) Cr( MY ) CD ( Coal) 100% (3) V ( I L) MY Cr( V ) Cr( I L) Cr( MY )

585 Crack density on coal structures derived from formulas (1)-(3) Gęstości spękań na strukturach węgla wyznaczone ze wzorów (1)-(3) TABLE 2 TABLICA 2 Crack Density [%] Sample CD(MY) CD(V+I+L) CD(Coal) 1 2 3 4 OM/D 22.8 4.5 7.33 OM/M 26.4 3.8 7.74 OM/U 37.9 8.1 13.20 OM/UL 31.9 4.9 7.29 OM/UM 40.2 5.6 12.06 OM/UR 30.1 3.6 7.77 Average 31.6 5.1 9.23 The parameter CD(MY) falls in the range 22.8-42[%]. yielding 31.6% on the average. The average proportion of cracking on structurally unmodified coal is 5.1%. This major difference indicates that under the same gas pressure the gas bearing capacity of mylonite (for gases not bound by sorption) could be decidedly larger than that of structurally unmodified coal. Crack measurement data for modified and unmodified coal types are summarised in the last column in Table 2. The largest proportion of cracks on coals was found on samples OM/U and OM/UM. These samples were collected near the gate axis. at the height of 2 m from the floor. at the distance of 0.4 m from one another. The network of cracks is much denser on this portion of sample material. both on structurally modified and unmodified coals. Assuming that some cracks originated in the process of destruction and when rock mass was thrown out. it is reasonable to suppose that intensity of processes occurring on this material shall be rather high. Locations where the samples OM/U and OM/UM were collected. at the inlet to a cavern formed after an outburst. suggest that the material comes from the final phases of the outburst. It appears that the outburst finally stopped not because the reserves of compressed gas. supplying the energy for an outburst. were exhausted. but because the space for removing the rock mass after the outburst was blocked. The crack density on coal. particularly on mylonite. is a major determinant of gasbearing capacity of coal and of process kinetics in the coal-gas systems. Sorption properties of coal samples collected from the rock mass after an outburst in the gallery D-6. coal seam 409/4 were investigated in the Laboratory of the Central Mining Institute in Katowice (CMI). Samples for sorption tests were collected at the same spots as those designated as OM/D. OM/M. OM/U. described in previous sections. The results reveal (Cybulski et al. 2006) that gas release from coal proceeded at a fast rate. the process

586 kinetics being expressed by the diffusion coefficient De. The average value of this coefficient for the tested samples was 1.01 10 8 cm 2 /s. while the value higher than De = 0.15 10 8 cm 2 /s is considered to be a danger. Results obtained in the CMI are consistent with the research data presented in this study. 5. Conclusions Microscopic examination of coal samples collected from the mass after an outburst in the gallery D-6 in the colliery Zofiówka confirm the presence of structurally modified substance- mylonite. There is a dense network of cracks on mylonitic coal. which is its distinctive feature. Microscopic examination reveals even several hundred cracks per one millimetre. The amount of free gas contained in cracks of mylonite coal might be several times higher than that of free gas contained in structurally unmodified coal. Furthermore. this gas is not bound by sorption so it can be easily released through an extended network of internal cracks. which is demonstrated by a higher value of the diffusion coefficient. These factors are responsible for an increased gas and coal outburst risk. The absence of earlier indication of an outburst hazard or methane hazard in the front section of the gallery D-6. the high value of gas release indicators and the presence of structurally modified coals in rock mass after an outburst. which might accumulate vast amounts of gas that can be next released at a fast rate when a pressure gradient should occur. lead us to the conclusion that a coal and gas outburst have occurred as the face front came near the gas pocket. The proportion of mylonite in the rock mass after an outburst would amount to 15%. Assuming that the entire outburst mass has the same proportions of structural components. the amount of pure mylonite thrown out during an outburst could estimate to be 50 m 3. According to the authors. finding exactly how structural parameters of coal affect the behaviours of a coal-gas system might give better tools to recognise outburst hazard in underground mines.

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