Archives of Mining Sciences 51, Issue 4 (2006) 577–588

Archives of Mining Sciences 51, Issue 4 (2006) 577–588

Archives of Mining Sciences 51, Issue 4 (2006) 577–588
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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 m3
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 22nd 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 m3 of methane were
released to the atmosphere in the mine (Dziurzyński et al., 2006a). The amount of rock
mass was evaluated to be 320 m3 (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 23rd 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 20th 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).
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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 m3 of methane were released per one ton of
coal, whilst normally methane content in the seam prior to the event would not exceed
10 m3/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.
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Fig. 1. Fragment of map of the coal seam 409/4 with an indicated location of an outburst
8-10 m
Rys. 1. Fragment mapy pokładu 409/4 z naniesionym miejscem wystąpienia wyrzutu
1
2
3m
0.9
2.9 m
3
4
0.9
8
5
6
0.5 m
3.3
7
m
1.6 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.
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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
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TABLE 1
Results of local analysis for samples from outburst mass
TABLICA 1
Zestawienie wyników analizy punktowej dla badanych prób ziarnowych mas powyrzutowych
percentage fraction [%]
standard deviation [%]
Sample
1
OM/D
OM/M
OM/U
OM/UL
OM/UM
OM/OR
V
2
67.2
(1.26)
59.8
(1.34)
60.9
(1.30)
73.5
(1.91)
63.2
(2.07)
66.9
(2.04)
Average
Cr(V)
3
3.4
(0.49)
2.4
(0.42)
5.9
(0.62)
4.30
(0.88)
4.1
(0.85)
3.0
(0.74)
3.9
I+L
4
11.8
(0.86)
14.0
(0.95)
10.9
(0.83)
13.1
(1.46)
13.1
(1.45)
13.3
(1.47)
12.7
Cr(I+L)
5
0.36
(0.16)
0.5
(0.20)
0.4
(0.16)
0.20
(0.19)
0.4
(0.26)
0.0
(0.00)
0.31
MY
6
11.4
(0.85)
12.0
(0.89)
10.3
(0.81)
6.0
(1.02)
11.3
(1.36)
10.8
(1.34)
10.3
Cr(MY)
7
3.4
(0.48)
4.3
(0.55)
6.3
(0.64)
2.8
(0.71)
7.6
(1.14)
4.7
(0.91)
4.9
IS
8
2.5
(0.42)
7.0
(0.69)
5.4
(0.60)
0.2
(0.19)
0.4
(0.26)
1.3
(0.49)
2.8
100*V
———
V+(I+L)
9
85.1
81.0
84.8
84.9
82.8
83.5
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.
CD ( MY ) =
CD (V + I + L) =
CD (Coal ) =
Cr ( MY )
× 100%
MY + Cr ( MY )
Cr (V ) + Cr ( I + L )
× 100%
V + ( I + L ) + Cr (V ) + Cr ( I + L)
Cr (V ) + Cr ( I + L) + Cr ( MY )
× 100%
V + ( I + L) + MY + Cr (V ) + Cr ( I + L) + Cr ( MY )
(1)
(2)
(3)
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TABLE 2
Crack density on coal structures derived from formulas (1)-(3)
TABLICA 2
Gęstości spękań na strukturach węgla wyznaczone ze wzorów (1)-(3)
Crack Density [%]
Sample
1
OM/D
OM/M
OM/U
OM/UL
OM/UM
OM/UR
Average
CD(MY)
2
22.8
26.4
37.9
31.9
40.2
30.1
31.6
CD(V+I+L)
3
4.5
3.8
8.1
4.9
5.6
3.6
5.1
CD(Coal)
4
7.33
7.74
13.20
7.29
12.06
7.77
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 cm2/s. while the value higher than
De = 0.15 × 10–8 cm2/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 m3.
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.
587
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REVIEW BY: PROF. DR HAB. INŻ. WACŁAW DZIURŻYŃSKI, KRAKÓW
Received: 12 Sptember 2006