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Article

Geological and Mining Factors Controlling the Current Methane Conditions in the Rydułtowy Coal Mine (Upper Silesian Coal Basin, Poland)

by
Sławomir Kędzior
1 and
Marcin Dreger
1,2,*
1
Institute of Earth Sciences, Faculty of Natural Sciences, University of Silesia in Katowice, Będzińska 60, 41-200 Sosnowiec, Poland
2
CLP-B Laboratory, Rybnicka 6, 44-335 Jastrzębie-Zdrój, Poland
*
Author to whom correspondence should be addressed.
Energies 2022, 15(17), 6364; https://doi.org/10.3390/en15176364
Submission received: 1 August 2022 / Revised: 21 August 2022 / Accepted: 29 August 2022 / Published: 31 August 2022
(This article belongs to the Special Issue Integrated Approaches for Unconventional Oil and Gas Extraction and Exploration)
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Abstract

:
Methane emissions into mine workings and the atmosphere are still a significant environmental and work safety problem. Since 2000, the Rydułtowy coal mine, located in the western part of the Upper Silesian Coal Basin, has been struggling with significant methane emissions compared to the previous period. The distribution of the methane content in coal seams was analysed, and the factors that influenced it were reviewed. Then, the annual variability in methane emissions in mining excavations was investigated, and the depth of coal extraction was linked to methane conditions and the time of mining works. It has been shown that the currently observed distribution of methane in coal seams is the result of, inter alia, the geological development of the western part of the basin, the lithological character of coal-bearing Carboniferous deposits and overburden, and fault tectonics. The sorption capacity of coal seams decreases with increasing temperature and the coal rank. The amount of methane emitted into mine workings depends mainly on the methane content in the coal seams in mining sites and on the sorption capacity of the coal seams. The depth of exploitation, increasing from year to year, leads to an increase in the methane content in coal seams and a simultaneous decrease in the sorption capacity of coal, which will result in higher methane emissions in the future.

1. Introduction

Coal mining in the Upper Silesian Coal Basin (USCB), the largest coal basin in Poland and one of the largest in Europe, is in a constant downward trend. It has decreased threefold since the late 1990s. In 2020, 42 million tonnes of coal were extracted [1]. This decline in production has not diminished the methane hazard in mines. This is manifested by the significant emissions of methane into mine workings, which for many years have not fallen below 700 million m3 of methane annually. The reason for this is the constantly increasing depth of mining and the associated greater gas capacity of the seams, as well as the methane concentration of the coal extracted [2]. Some mines in the basin initially had no or minor problems with methane. However, this has changed over the last two decades, with increasing gas emissions into mine workings. One of these mines is the Rydułtowy mine, with an annual coal production of approximately 1.5 million tonnes. It is located just off the western boundary of the Upper Silesian Coal Basin and is the westernmost working mine in the Polish part of the basin. After 2000, there was a significant increase in the total methane emitted into workings, and since 2002, data on the demethanation of the deposit have been reported [1,3].
The aim of this paper is a spatial analysis of the gas content variability and the identification of factors that led to the currently observed distribution of the methane in the deposit. At the same time, the volume of the annual methane emitted into the mine workings was analysed in order to correlate this with the gas content in the coal beds and geological and mining factors affecting the deposit. Thus far, the analysis of this issue has been carried out in the rest of the USCB and elsewhere, [4,5,6,7,8,9,10,11,12] and it has exposed a significant impact of the natural and mining factors on the changes in methane emissions over time. Therefore, it is important to verify whether such a dependence also occurs in the Rydułtowy mine, which is less frequently studied at this point. The relationships between the gas conditions and the individual elements of the geological structure of the USCB have been described in numerous works [11,13,14,15,16,17,18,19,20,21]. They show that the current distribution of methane content depends on the geological development of the basin and the origins of the gases, the degree of coalification of the seams, the petrographic composition of coal, and the lithology of the surrounding rocks, tectonics, as well as the temperature and pressure in the deposit. The results of these studies are mostly consistent with global outcomes [21,22,23,24]. The subject of the study is the Rydułtowy deposit, located within the folded zone of the USCB near the western boundary of the basin, which is characteristic of the predominance of Paralic coal-bearing series and varied overburden layers with a constant emission of methane into the mining works. The aforementioned analysis was based on the results of gas content tests on coal samples from the mine, data on methane emissions, and the results of coal quality tests. These data were collected in the geological documentation of the deposit and reports on methane emissions.

2. Geological and Mining Features of the Upper Silesian Coal Basin

The Rydułtowy hard coal deposit is located in the western part of the Upper Silesian Coal Basin, which is one of the largest in Europe, with an area of 7250 km2, including 5650 km2 in Poland. The other part is in the Czech Republic [22] (Figure 1).
The basin was developed in the foreland of the Moravian and Silesian Variscan fold zone, filled with molasses-bearing coal deposits [23]. The sedimentation of the coal-bearing formations took place in the Upper Carboniferous period (Mississippian and Pennsylvanian). The carboniferous sediments include interbedded packages of siltstones, mudstones, and sandstones, with numerous coal seams. Due to the variety of sedimentation, the coal-bearing complex is divided into four lithostratigraphic series, which differ in their lithological character, as well as in the number, thickness, and distribution of the coal seams, only two of which are located in the Rydułtowy coal deposit (Figure 1; Table 1).
The basin is complicated in terms of its tectonics. There are three main tectonic zones—(i) the folded zone, a 20 km-wide belt in the western part of the basin, consisting of meridian-oriented overthrusts and troughs; (ii) the disjunctive zone (Bytom Syncline, Main Saddle, and Main Syncline), occupying the largest part of the basin with the large latitudinal dislocations, with displacements of several hundred meters and smaller faults, often dividing the coal-bearing series into smaller tectonic blocks; and (iii) a fold-block zone in the north-east. Faults and tectonic disturbances often make coal mining difficult [23].
There is hard coal of all ranks in the USCB, ranging from low-rank, non-coking coal (subbituminous) and coking coal (high- to low-volatile bituminous) to high-rank coal (low-volatile steam coal) and anthracite [23]. Of these, only the low-rank and coking coals are mined. The remaining ranks are in the minority or lie at a depth inaccessible for mining, >1250 m (e.g., anthracite).
Coal is mined in every part of the Upper Silesian Coal Basin, where geological and mining conditions differ, which can be seen in the methane emissions and coal production quantities [2,10,11]. The methane concentration in coal seams is not homogenous; rather, the rule of increasing methane content with depth is evident in most of the working mines in the USCB today. In general, the Carboniferous strata in the northern and central regions are shallow deposited, mostly devoid of Miocene overburden. The overburden occurs here as local patches and thin layers, which have not prevented methane migrating into the atmosphere in the geological past. In the present day, the shallow coal seams (up to ~400–600 m) are mostly methane-free, but the methane concentration grows with increasing depth. On the other hand, the southern part of the USCB is covered by a thick, sealing Miocene screen. Migrating methane was trapped under the overburden, and the coal seams and surrounding strata were secondarily accumulated. The secondary methane zone consists of microbial gas mixed with thermogenic gas [24]. Coal production, which was focused on seams under Miocene overburden, was impeded by the high methane emissions caused by releasing methane from the coal being extracted and migrated from surrounded, unmined deposits. At greater depths, methane concentrations decrease, but deeper, the primary methane content zone can be observed (G~15 m3/t coal daf).
The western part of the USCB is a mix of two types of vertical methane distributions, as described above. In some areas, the Miocene sealing deposits cover the Carboniferous coal-bearing surface, acting as sealing strata for the gases, but in others, Carboniferous sediments occur as outcrops, which are free of methane.
The average depth of extraction in the Polish part of the USCB increases by ~10 m every year, reaching ~800 m on average in 2020 (~524 m in 1989) and extending to ~1290 m at the deepest in 2020. In the same year, the deepest coal works were conducted at ~1315 m [3]. Coal production in the USCB has been falling since 1997 due to the closure of coal mines or their being merged into larger enterprises. Deeper deposited coal seams need to be mined to maintain production continuity, but operations at greater depths involve higher risks of natural hazards (e.g., temperature, tremors, and methane) [2,12,25]. Therefore, coal production is becoming increasingly burdensome every year, which can be seen in the lower coal production. Operations in more methane-rich seams are connected with intensified methane emissions, reaching ~820 million m3 in 2020 [3]. The increasing trend in methane emissions is evident; despite periodic drops, the annual emission of >700 million m3 has been maintained. The regional large dislocations, lithology, and stratigraphy of the Carboniferous strata, combined with the depth of extraction and mining factors, are the main reasons for CH4 emissions in the USCB coal mines. Previous works have studied methane migration associated with geological and mining conditions [2,10,11,12].

Geological Setting of the Rydułtowy Coal Deposit

According to the geological documentation and the coal mine materials, the Rydułtowy 1 coal deposit is located in the Carpathian Foredeep, in the western part of the USCB, in the upthrow of regional tectonic discontinuity, the Michałkowice-Rybnik overthrust, with throw size ~750–1500 m. This deposit is represented by a fault-folded structure, with the form of an asymmetric trough (Jejkowice trough) inclined to the northeast. The incline of the western wing sediments reaches up to 15°, locally 60°/E close to the boundary, and ranges from 8° to 10°/W in the eastern wing. According to the division of Kotarba et al., the Rydułtowy coal deposit is part of the seventh gas region of the USCB [15]. The coal seams documented in the Rydułtowy area are recognised as Mississippian and Pennsylvanian (Namurian A-C) layers. The main lithological types of the rocks surrounding the coal seams are sandstones, claystones, and mudstones. Their summarised description is presented in Table 1.
The proportions between the rocks depend on the stratigraphic position of the layers and the sedimentary development of the basin. In the study area, the share of sandstones in the profile of the Poruba layers amounts to approximately 35%, and in the lower-lying, Jaklovec layers, it is reduced to approximately 29% [26]. The porosity of the sandstones is around 6% [26]. The others are claystones and mudstones.

3. Materials and Methods

Data comprising the total methane emissions, degasification, ventilation methane emissions (VAM), and annual net coal production were obtained directly from the PGG SA ROW Rydułtowy coal mine. The elements of the geological structure, such as lithology, stratigraphy, tectonics, hydrogeology, and coal parameters (firmness of the coal and the methane desorption intensity), were obtained from the geological documentation of the Rydułtowy coal deposit. Additional parameters regarding other coal mines in the basin and the entire USCB were obtained from annual technical reports published by the Central Mining Institute in Katowice (GIG) [3]. The data were carefully analysed and presented in charts, tables, and figures. The spatial distribution of the methane in the Rydułtowy coal mine was assessed by tests performed systematically in the coal works. In these tests, the sample holes are drilled in the fresh exposed coal face, and ~100 mg coal samples are enclosed in hermetic steel containers, which, in the next phase, are shaken to obtain crushed coal, then transformed into powder. All the methane is released into a pipette, and this released amount is then measured [27]. Knowing the amount of methane released and the mass of the coal sample allows one to calculate the methane content, i.e., the volume of methane per one tonne of coal. Subsequently, this volume is corrected for the moisture and ash content, coal dry ash-free state (daf), and gas losses during sampling.
Data on the methane content were obtained from mining excavations and two surface bore holes with depths of 1300 and 1700 m, located in the northern part of the studied area.
The depths of the sampling measurements of the methane emissions from each year were compared, and a chart displaying the average and the deepest annual works was created.
The specific methane emission values refer to the volume of methane that is emitted with every single tonne of coal (m3/t). These values make it possible to adjust the total measured methane emissions according to the number of tonnes of coal extracted and compare the emissions between mines with different types of coal extracted.
The coal sorption capacity of the methane at a given pressure and temperature was obtained from the literature [28]. Additionally, it was measured using a coal sample taken from a depth of approximately 1000 m in the Rydułtowy mine. The measurement device was a Hiden Isochema sorption apparatus, model IGA 001.

4. Results and Discussion

4.1. Distribution of the Methane Content and the Coal Mine Methane Development

In the discussed deposit, methane is mainly associated with the coal seams and generally presents as adsorbed gas in the coal matter (micropores). Free methane is in the minority and is present in breaks and fractures, as well as in the coal macropores. Thus far, no gas has been found in the non-coal surrounding rocks. The gas occurring in the study field consists mainly of methane (80–90% in the methane zone). The share of ethane increases with depth, and below the level of 1000 m, it can reach several per cent, while propane and butane do not exceed trace levels. The other gas components are nitrogen, carbon dioxide, and hydrogen
The depth distribution of the methane content in the study area includes two zones (Table 2, Figure 2). The upper one, which was naturally degassed in the geological past, is located at a depth of about 600 m below ground level, where methane is absent or occurs in very small quantities (0.0 to 0.8 m3/t coal daf), and the lower methane zone extends from a depth of about 600 m to the bottom of the deposit, where the methane content increases consistently, reaching a maximum measured value of more than 14 m3/t coal daf at a depth of approximately 1160 m (−910 m above sea level). The depth range of the methane zone has not been identified thus far.
This distribution of the methane content in the Rydułtowy deposit corresponds to the so-called northern pattern of methane distribution in the USCB (see, e.g., [14,29]), which characterises the several hundred meter-thick degassed zone of natural desorption occurring in the upper part of the Carboniferous strata. The lack of secondary accumulation of the methane on the Carboniferous roof of the deposit proves that the Miocene and Triassic overburden is not tight enough here for migrating gases [15].
The configuration of the methane seam roof, with a methane content above 4.5 m3/coal daf, as shown in Figure 3, is varied.
The position of the top surface spans between −540 m and −820 m above sea level. The top is clearly lowered to the north, because in the surface boreholes Jejkowice IG-1, being 1700 m deep, and Jejkowice 6, being 1300 m deep, located in the northernmost part of the area, the determined methane content of the coal seams does not exceed 1 m3/t coal daf in the entire profile of these wells.
The balance resources of methane as an accompanying commodity in the Rydułtowy coal deposit are estimated at 508 million m3, of which 183 million m3 are developed reserves [1]. They are concentrated at a depth of 600–800 m below the ground level. The demethanation of the works started in 2002, and since 2013, it has exhibited a steady downward trend. In 2020, just over 6 million m3 of gas was captured. In the best year, 2008, it was more than 15 million m3. A total of around 130 million m3 of methane has been captured in the last 15 years. Since 2016, the captured methane has been used to power cogeneration units producing electricity and heat that are sold to external consumers. The unused or sold gas is released into the atmosphere. The efficiency of using the captured methane in the Rydułtowy mine, i.e., the ratio of the methane consumed to the methane captured, was almost 89% in 2017 and 75.58% in 2020 [25], which means that 11% and 24.42% of the captured gas, respectively, was emitted into the atmosphere.

4.2. Sorption Capacity of the Coal

The sorption capacity is the volume of gas that coal is able to adsorb at a given temperature and pressure. The temperature and pressure of the rock mass increase with depth. The average geothermal gradient for the USCB Paralic and Upper Silesian Sandstone series ranges from approximately 2.75 to 4.75 °C/100 m [30] and is one of the highest in Poland. In the studied deposit, it is 3.25–3.75 °C/100 m [30]. Temperature significantly reduces the sorption capacity of coal, [17,18], while the overlying rocks’ pressure positively affects the amount of adsorbed methane. Since both factors counteract the accumulation of methane, most of the methane accumulates in a strictly defined depth range, called the optimum methane zone, as a compromise between these two influences [17]. Above this zone, too low pressure prevents the accumulation of significant amounts of gas, while below it, too high temperature limits the sorption capacity of the coal. In the USCB, such an optimum methane zone occurs within the 800–1500 m depth range, with the possibility of fluctuations [17]. The maximum determined methane content in the Rydułtowy deposit, amounting to 14 m3/t coal daf at a depth of approximately 1100 m, is within that range. However, as mentioned, the depth range of the optimum zone in the studied deposit has not been recognised.
The authors studied the sorption capacity of coals taken from the Rydułtowy coal mine from a depth around 1000–1200 m below ground level. The coal samples were placed in the gas sorption analyser (in the CLP-B laboratory) and saturated with methane at the rock mass temperatures (36–40 °C) under a pressure not exceeding 20 MPa. The Rydułtowy coals’ Langmuir sorption isotherms were around 15–16 m3/t coal (Figure 4).
The maximum methane content recorded at a depth of 900 m below sea level (~1200 m below ground level) was 14–15 m3/t coal, which means that the coal seams are saturated with 95% methane (Figure 5). The methane content in the Rydułtowy area generally increases with depth; therefore, shallower deposited, less methane-rich coals are less saturated with gas, with a content ranging from 30 to 78% (depth of 500 to 900 m below sea level) (Figure 5).
Teste performed in the CLP-B laboratory confirmed the negative influence of increasing temperature on the sorption capacity of the coal. The coal sorption properties decreased with increasing temperature at a rate of about 0.05 m3 of gas per 1 °C. The effective diffusion coefficient, which describes the gas molecules’ movement between the coal grains, was also studied at the rock mass temperatures (36–40 °C). The results were generally similar, centring around D = 4 × 10−10 cm2/s. The methane diffusivity of the USCB coals tested by Wierzbicki increased with rising temperature and pressure [31]. This dependence was demonstrates for the Rydułtowy coal samples, where the diffusion coefficient increased by 0.2 × 10−10 cm2/s with every increase of 1 °C. Taking into account the measured content of the methane in the coal seams in the studied deposit (up to approximately 14 m3/t coal daf), we can observe the almost full saturation of the coal seams with methane, which can be seen in the greater methane emissions from the coal during coal works in the deepest parts of the coal deposit.

4.3. Geological and Mining Factors Influencing the Methane Occurrence

4.3.1. Geological Evolution of the Study Area

Coal-bearing formations of the USCB were deposited during the Late Carboniferous (Pennsylvanian) between 323 and 305 Ma. According to computer modelling of the coalification degree carried out using the PetroMod software, the maximum depth at which the Carboniferous formation is buried was estimated to be between 3.3 km in the north-eastern part of the USCB and 5.5 km in the western part at the end of the Carboniferous period [19]. The coal-bearing formations in the Rydułtowy deposit located in the westernmost part of the basin are buried at a depth of approximately 5 km. During the Late Carboniferous, those sediments were subjected to a maximum heating temperature exceeding 90 °C [19,20]. The heating process resulted in the coalification of the seams, resulting in the present coal rank, which was accompanied by the production of gases, including methane (see, e.g., [18,32,33]). The USCB coals represent gas-prone type-III kerogen. Therefore, with the present-day degree of coalification of above 0.8% Ro in the western part of the basin, huge amounts of gas have been produced from the coal. The hydrocarbon potential of the coal seams of the Paralic and the Upper Silesian Sandstone series in the western part of the USCB was estimated at 45–65 and 65–75 mg of methane per gram of TOC, respectively [19]. According to the research conducted thus far [16,19,20,34], the methane generation took place in the Late Carboniferous and was completed by the end of the Variscan orogeny, at the turn of the Carboniferous and Permian. Later thermal events taking place in the Mesozoic did not lead to resumed methane generation but only caused its re-mobilization [19,20]. Due to the limited sorption capacity of the coal and subsequent erosion processes, not all of the methane produced was accumulated in the coal seams.
As already mentioned, after the burial of the coal-bearing sediments at the maximum depth at the turn of the Carboniferous and Permian, the area of the basin was subjected to uplift movements, with the most intense occurring in the Permian, which contributed to the erosion of the coal-bearing formations. The erosion process continued throughout the Mesozoic and the Paleogene up to the Miocene, when marine clays were deposited. The thickness of the eroded sediments has been estimated by various methods. According to Botor’s research, it ranges from 2000 m in the east of the USCB to more than 4000 m in the west [20]. These values are similar to the estimates of Gerslova et al. in the Czech part of the basin, ranging from 2500–3400 m [35]. In light of the presented values, the level of erosion of the Carboniferous deposits in the Rydułtowy coal deposit is significant and may amount to approximately 4400 m. Changes in the static and hydrodynamic pressure regimes, resulting in erosion, enabled methane desorption from the coal seams and, consequently, their natural degassing [14,15,18]. The escape of methane from the coal seams may have also been favoured by endogenous fires caused by long-term exposure to the seams and their contact with oxygen [36]. The result of these processes is the approximately 600-thick zone of natural desorption of gases observed in the studied deposit.

4.3.2. Coal Rank and Maceral Composition

Using the data obtained from the geological documentation of the deposit, the seams of 600 (Poruba layers) and 700 (Jaklovec layers) groups were sampled. The caloric value of the examined seams is 13,832–32,609 kJ/kg, the volatile matter content is 26.3–37.4%, and the total sulphur content is 0.1–3.0%. As mentioned, the deposit contains both subbituminous (non-coking) and high- to medium-volatile (coking) coal. The coal rank increases with the depth of the seams. The boundary between these two ranks of coal runs at a depth of 400 m in the southern part of the deposit and 900 m in the northern part.
The dominant group of macerals in the coal is the vitrinite group (50–66%). The inertinite group’s content varies between 12–25% and the liptinite varies between 7–18%. The vitrinite reflectance is within 0.8–1.0% and increases with depth (seam groups 600 and 700, [37]).
The coal rank and maceral composition of coal are both important for the amount of methane generated from the coal substance and for the sorption properties of coal. The coalification process is continuous, and various products, including methane, are generated during this process. The higher the coal rank is, the more methane is produced. The second coalification jump, which corresponds to coking coals, plays a special role here, since this is when the coal loses approximately 10% of its volatiles, which can affect the amount of gas generated [18]. Due to several subsequent processes, not all the produced gas is retained in the coal. The greatest levels of methane in the study area are accumulated below 600–800 m, which is partially consistent with the boundary between the steam and coking coal at a depth of 400–900 m. The maximum methane content in the study area (more than 14 m3/t coal daf) was found at a depth of about 1100 m, which corresponds to the presence of coking coal. The hydrocarbon potential of the Rydułtowy field is discussed in Section 4.3.1.
The coal rank also affects the sorption capacity of the coal seams. The research conducted by Dutka on one of the coal deposits in the southern part of the USCB demonstrated that the coal rank has a dominant effect (about 89%) on the reduction of the coal sorption capacity [21].
The fact that the coal rank has such a negative impact on the sorption capacity of coal requires special attention, because other studies, [17,38] suggested a positive correlation between these values. Research carried out on Australian coals [39] showed that the trend of the changes in the sorption capacity of coal with increasing rank is a second-order polynomial and differs between the cases of bright and dull coal (Figure 6). At the lower coal rank, this difference is greater than it is at the higher rank. Moreover, up to the value of 1.65% Ro, we observed a negative influence of the increasing rank on the sorption capacity of the coal, especially in the case of bright coal. From this point, starting with a 1.65% Ro value, the trend becomes clearly positive (Figure 6).
The negative influence of the increase in the coal rank on the amount of sorbed gases can be explained by the presence of occluded oil in the coal micropores at a lower rank (from subbituminous to medium-volatile bituminous coal), which is produced at the oil window stage (0.7 to 1.3% Ro) and then cracked decomposed to obtain a higher coal rank (>1.65 Ro) [39,40]. Oil-depleted micropores with a larger specific surface area are able to adsorb more gas. Thus, in the case of higher-ranked coal, the sorption capacity increases with increasing rank [39]. The coal rank in the study area, ranging from 0.8–1.0% Ro, may therefore indicate a negative influence of the increasing coalification degree on the sorption capacity of the coal, as demonstrated by Dutka [21].
Due to the fact that the coal rank increases with depth, the effect of reducing the sorption capacity of the coal should be enhanced with increasing depth [41,42]. Tests of the sorption capacity of the coal in the southern and western part of the USCB, based on the analysis of Langmuir sorption isotherms, showed values of 9–15 m3/t coal daf at a pressure of 2–12 MPa and a temperature of 40 °C [28], which is consistent with the results of the sorption capacity in the study area (15–16 m3/coal daf, Figure 4).
From the point of view of the maceral composition of coal, macerals from the vitrinite and inertinite groups play an important role in the gas sorption process. As indicated by global research [43,44], vitrinite-rich bright coal generally has a higher sorption capacity than inertinite-rich dull coal at the same rank, while domestic studies [45] have demonstrated a relationship between the methane sorption capacity of USCB coals and the presence of cellular macerals of fusinite and semifusinite. The maceral composition of coal from the Rydułtowy deposit (the total share of inertinite and vitrinite coal exceeds 60–70%) seems to be favourable for the gas sorption process.

4.3.3. Methane Migration and Accumulation

Geological factors, such as the lithology of the Carboniferous strata and overburden, fault tectonics, and hydrogeological conditions, are responsible for gas migration and accumulation [16]. Sandstones are considered to be permeable, facilitating the migration of methane, in contrast to claystones and mudstones, which are impermeable and thus provide a sealing screen for migrating gases. The decreased share of sandstones, along with the increasing share of impermeable claystones and mudstones, in the Jaklovec Beds may be one reason for the increasing methane content in the coal seams with depth, which reaches the maximum value (more than 14 m3/t coal daf) at a depth of over 1100 m. In this case, claystones and mudstones may act as a sealing screen for migrating gases and their accumulation at depths greater than 600 m. As can be seen from Figure 7, the upper boundary of the methane coal seams (G > 4.5 m3/t coal daf) approximates the boundary of the Poruba and Jaklovec layers. According to the deep shaft borehole profiles, continuous sandstone-conglomerate sediments over 130 m thick were documented in the Poruba layers, which may have caused degassing in the deep coal seams and helped the methane to migrate from the deeper to the shallower parts of the profile or into the atmosphere. In the Jaklovec Beds, alternately deposited claystones, mudstones, and shales, being a few meters thick, dominate the lithological profile.
The coal-bearing formations are covered with Miocene clay sediments in the western and northern parts of the study area, while in the northern part Triassic formations ar also present. The thickness of the overburden is not large (Figure 7), slightly exceeding 400 m. However, in the central part of the deposit, the Carboniferous formations are covered with only a thin layer of Quaternary formations. This nature of the overburden means that the deposit is open to the migration of gases from the rock series into the atmosphere. The free migration of methane from the coal seams into the atmosphere through the discontinuous and permeable overburden is another cause of the approximately 600 m-thick degassed zone.
As mentioned, the migration of methane can take place through permeable sandstones, but also through faults. Discontinuous tectonic zones (Figure 3 and Figure 7) are considered to be pathways for gas migration, as rapid increases or decreases in methane content have been observed around them. Faults can also act as sealings for gases or divide deposits into blocks with different gas capacities [13,17,46].
Carboniferous sediments are cut by numerous faults, with displacements up to 200 m with a latitudinal (predominant) and meridional course and an inclination of the fault planes of 45–85°. Zones of tectonic disturbance are up to 300 m wide; therefore, they can act as migration pathways for methane. Most likely, the majority of methane migrated through the faults and accompanying fractures in the past, contributing to the present-day distribution of the methane content in the deposit. The 4D modelling of the methane potential in the USCB [47] revealed that the decisive influence on the level of hydrocarbons accumulated today is the time of tectonic activity in the study area, i.e., the length of the fault opening interval.
Gas migration is often accompanied by water migration due to the permeable nature of the migration pathways (sandstones and permeable faults). The infiltration of meteoric waters into the Carboniferous formations is possible only in places where there is no tight clay cover of the Miocene formations, i.e., in the central and eastern parts of the area. The free infiltration of meteoric water from the surface into the Carboniferous deposits may have contributed to the dissipation of migrating gas in the past and may continue to contribute today, thus contributing to the expansion of the degassed gas zone of natural desorption [18,19,20,21,22,23,24,25,26,27,28,29,30,31,32,33,34,35,36,37,38,39,40,41,42,43,44,45,46,47,48].

4.3.4. Mining

Many years of coal mining in the Rydułtowy deposit have resulted in the relaxation of the rock mass, which has led to a pressure drop and, consequently, the desorption of methane from the coal seams. This process was further intensified by the demethanation of the deposit, which started in 2002 and continues today. The result of this is the methane migration from the coal seams and goafs to the methane drainage station, forced by negative pressure. This causes changes in the methane content (its significant reduction); thus, a different distribution from the natural (virgin) distribution of the gas content in the seams is observed. This applies in particular to the overlying and underlying seams, as the reach of mining degassing can vary from 60 to 100 m below and 120–200 m above the exploited seam [22,49]. Tests and observations of the operational range of degassing in a shallow coal mine in Australia [50] showed that the upper limit of the degassing range can reach up to 500 m above the exploited longwall; however, this study concerned a shallow deposit with coal seams of a high permeability, ranging from several dozen up to over 100 mD. In the case of the USCB, the coal permeability is much lower, often below 1 mD. Hence, the extent of degassing is not as large in the studied deposit. Nevertheless, the noted significant differences in methane content between coal seams at particular levels (sometimes in close vicinity) (Figure 2) may be the consequence of overlaps between the effects of exploitation on the natural factors influencing the distribution of the methane content.

4.4. Methane Emissions and Hard Coal Output in the Rydułtowy Coal Mine

The hard coal output was studied in the years 1994–2020 (Figure 8), while the methane emissions were studied in 2000–2020 (Figure 9), because earlier coal had been extracted from the methane-free coal seams; therefore, no CH4 emission was recorded. At the beginning of these studies, coal was extracted from the Poruba and Jaklovec layers at a depth ranging from 400 to 700 m below sea level, including mostly degassed, methane-free coals (Figure 10).
Since 2000, coal has been extracted almost exclusively in the southern part of the deposit, where the methane concentration tends to increase (G > 4.5 m3/t coal daf), and the tops of the methane-bearing coal seams are located at shallower depths than in the central and northern parts of the deposit. Methane emissions increased rapidly from less than 3 million m3 in 2000 to almost 35 million m3 in 2003. At the same time, the coal extraction was at around 2.3 million t per year. Coal extraction from deeper, more methane-rich seams was the main reason for the increase in CH4 emissions. In subsequent years (2004–2007), coal extraction and preparatory works were taken up for a short time in the western (less methane-rich) part of the Rydułtowy 1 deposit; therefore, emissions from the surrounding strata were not as evident and relevant as those observed in seams with higher methane contents (the southernmost area). Several years of coal production in comparatively harmless gaseous conditions resulted in a decrease in CH4 emissions (from ~26 million m3 in 2004 to 11 million m3 in 2007); however, the coal production also decreased (from 2.0 million t to just 0.90 million t) (Figure 8). Methane that was released into goafs was systematically drained in the following years in order to provide safer conditions in adjacent coal works.
Since 2008, the average depth of the works has exceeded 800 m below sea level (over 1000 m below ground level), which has resulted in the mining process being performed at higher pressures and temperatures. The methane content at those depths constantly increases to over G > 8 m3/t coal daf, which is related to the fact that the operation is performed within the primary gas-bearing zone, which constitutes the highest methane hazard in Polish mining (Figure 2). Mining in more methane-rich seams resulted in increasing amounts of gas released every year. The amount of released methane rose gradually from ~14 million m3 in 2008 to 32 million m3 in 2013 (Figure 9). In the following years, the methane emissions stabilised at 20–25 million m3 annually due to stable coal production, at 1.5–2.0 million t/year (Figure 8).
If methane emissions increase, the most harmful gas must be discharged and vented out of the mine to keep working conditions safe and prevent an explosive atmosphere. In the Rydułtowy mine, almost 70% of the released CH4 was directed to the ventilation shafts and then into the atmosphere during the entire research period (Figure 9). The rest of the methane was drained during mining (from the coal seams and goafs) and processed (for heat and power production) or released into the atmosphere (see Section 4.1).
Mining coal exclusively in the Jaklovec layers resulted in constant methane emissions into the coal works, combined with greater amounts of extracted coal, reaching ~2 million t of coal, and 32 million m3 of CH4 was emitted in 2013 (Figure 8 and Figure 9). The deeper deposited and most productive 712 and 713 coal seams, which have been mined in the southernmost E1 area (thickness~2.50 m), are characterised by methane contents in the range from G < 1 m3/t coal daf to G > 14 m3/t coal daf. Most methane is released from the crushing of the coal during the mining activities (road heading or blasting). The moderate firmness of the coal (f = 0.81 on average), combined with a low intensity of desorption (dp = 0.70 kPa on average), results in the not-so-rapid methane emissions from the coal. Still, most methane is released at the beginning, when the coal face is exposed, which is in compliance with Langmuir’s rule [51].
The surrounding strata in the Jaklovec layers consists of permeable sandstones (approximately 29% of the profile), which facilitate the migration of gases, such as methane, to the exploited wall environment. On the other hand, impermeable claystones and mudstones provide a sealing screen for migrating gases. The direct sandstone–coal contact may be burdened by intense methane migration and release during mining due to strata relaxation [52,53]. The last seven years of the study period (2014–2020) were characterised by the relatively constant methane release into the coal workings, amounting to ~24 million m3 CH4/year on average, and stable coal production from the Jaklovec layers, being ~1.9 million t/year on average. In the southern part of the Rydułtowy 1 deposit (e.g., the E1 area), there were many smaller faults (<1 m throw). These discontinuities may act as migration pathways for methane, contributing to coal degassing in the immediate vicinity of faults, which may be profitable in current and future coal production. However, at deeper levels, seams that are higher in methane will be subject to operations in the future (>1000 m below sea level), which may result in greater methane emissions and migration into the wall environment (>24 million m3 CH4/year).
Methane emitted with every tonne of coal indicates the actual gaseous danger posed by mining (specific methane emissions). The extension of coal production to the methane-dangerous part of the deposit is associated with gas migration from the surrounding strata, goafs, and coal. The specific methane emissions are around 14 m3/t coal, with periodic drops and rises (Figure 9). In the following years, coal will be produced from the deeper, more methane-rich seams. Gas will migrate through faults and breaks from the surrounding unmined seams, goafs, and mined coal, which may result in greater amounts of released gas per tonne of coal. Coal extraction from the methane-rich coal seams is burdened by the emission of significant amounts of gas released directly into the atmosphere, which is the cause of the enhancement of the greenhouse effect. In the Polish economy, the majority of methane released into the atmosphere comes from fuel emissions (including methane) (47%), agriculture (30%), and waste management (23%) [54]. There is an urgent need to limit the CH4 emissions in the general industries and economy. It is worth highlighting that coal mines located in the Upper Silesian Coal Basin emit around 25–30% of methane in relation to the entire Polish emissions, but the emitted methane contributes to just 3% of the total greenhouse gases emitted in the country [55].

5. Conclusions

  • After 2000, an increase in methane emissions into the mine workings of the Rydułtowy coal mine was noted, ranging from a few million to over 30 million m3 annually.
  • During the period from 2000–2020, these emissions fluctuated and stabilized at 20–30 million m3 per year.
  • The variability in methane emissions results from the interaction between natural (geological) and mining factors.
  • The Rydułtowy coal deposit exhibits a vertical zonation of methane. Up to a depth of approximately 600 m, the rock mass is naturally degassed, whereas deeper, the methane content rapidly increases to ca 14 m3/t coal daf at a depth of approximately 1100 m.
  • The sorption capacity of the coal at a depth of ca 1000 m is 15–16 m3/t coal daf, which means that the coal at this depth is almost fully saturated with methane (95% saturation).
  • The sorption capacity of the coal decreases with increasing temperature and the coalification degree of the seams, i.e., with the depth, which, given the high gas content of the seams, contributes to the high methane emissions into the mine workings.
  • The firmness of the coal, methods of exploitation, and complicated fault tectonics are other factors influencing the emission of methane into the mine workings.
  • Only 30% of the emitted methane is captured by methane drainage stations and then used. Increasing the collection of the emitted gas could reduce the amount of methane released into the atmosphere, which has approximately 30% more radiation power than carbon dioxide.

Author Contributions

Conceptualization, S.K. and M.D.; data curation, S.K. and M.D.; formal analysis, S.K. and M.D.; funding acquisition, S.K.; investigation, S.K. and M.D.; methodology, S.K. and M.D.; project administration, S.K. and M.D.; resources, S.K. and M.D.; supervision, S.K. and M.D.; validation, S.K. and M.D.; visualization, S.K. and M.D.; writing—original draft preparation, S.K. and M.D.; writing—review & editing, S.K. All authors have read and agreed to the published version of the manuscript.

Funding

This study was undertaken in the framework of the activities of the University of Silesia in Katowice and was funded by the University of Silesia, Institute of Earth Sciences (WNP/INOZ/2020_ZB32).

Acknowledgments

The authors are grateful to the employees of the Rydułtowy mine and of the CLP-B Sp. z o.o. in Jastrzębie-Zdrój, as well as the National Geological Archive, for their cooperation and assistance during the collection of data and materials for this research.

Conflicts of Interest

The authors declare no conflict of interest.

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Figure 1. The Rydułtowy coal deposit against the background of the lithology and stratigraphy of the Upper Silesian Coal Basin.
Figure 1. The Rydułtowy coal deposit against the background of the lithology and stratigraphy of the Upper Silesian Coal Basin.
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Figure 2. Depth distribution of the methane content in the Rydułtowy mine (Rydułtowy mine archive materials).
Figure 2. Depth distribution of the methane content in the Rydułtowy mine (Rydułtowy mine archive materials).
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Figure 3. Top of the methane seams with G > 4.5 m3/Mg coal daf against the background of the tectonics of the research area (modified after Rydułtowy mine archive materials): 1—boundary of the Rydułtowy mine field, 2—fault with the displacement value, 3—estimation point of the top of the methane seams, 4—line of the top of the methane seams (m above sea level), 5—presumed line of the top of the methane seams, 6—surface bore hole.
Figure 3. Top of the methane seams with G > 4.5 m3/Mg coal daf against the background of the tectonics of the research area (modified after Rydułtowy mine archive materials): 1—boundary of the Rydułtowy mine field, 2—fault with the displacement value, 3—estimation point of the top of the methane seams, 4—line of the top of the methane seams (m above sea level), 5—presumed line of the top of the methane seams, 6—surface bore hole.
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Figure 4. Langmuir sorption capacity of the coal from the Rydułtowy mine, taken from the depth of 1000 m.
Figure 4. Langmuir sorption capacity of the coal from the Rydułtowy mine, taken from the depth of 1000 m.
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Figure 5. Saturation of the coal seams with methane, depending on depth, in the Rydułtowy deposit.
Figure 5. Saturation of the coal seams with methane, depending on depth, in the Rydułtowy deposit.
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Figure 6. Trends in the Langmuir volume in relation to the coal rank [39].
Figure 6. Trends in the Langmuir volume in relation to the coal rank [39].
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Figure 7. Cross-section across the Rydułtowy coal deposit (modified after Rydułtowy mine archive materials): 1—overburden, 2—Poruba layers, 3—Jaklovec layers, 4—fault, 5—coal seam, 6—top of the methane zone with G > 4.5 m3/t coal daf, 7—shaft.
Figure 7. Cross-section across the Rydułtowy coal deposit (modified after Rydułtowy mine archive materials): 1—overburden, 2—Poruba layers, 3—Jaklovec layers, 4—fault, 5—coal seam, 6—top of the methane zone with G > 4.5 m3/t coal daf, 7—shaft.
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Figure 8. Hard coal output and specific methane emissions from 1994–2020 in the Rydułtowy mine.
Figure 8. Hard coal output and specific methane emissions from 1994–2020 in the Rydułtowy mine.
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Figure 9. Total methane emissions and outgassing and ventilation air methane from 2000–2020 in the Rydułtowy mine.
Figure 9. Total methane emissions and outgassing and ventilation air methane from 2000–2020 in the Rydułtowy mine.
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Figure 10. Changes in the average and total depth of the coal output from 1988–2020.
Figure 10. Changes in the average and total depth of the coal output from 1988–2020.
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Table
Table 1. Summarised lithological description of Rydułtowy 1 coal deposit (Rydułtowy mine archive materials).
Table 1. Summarised lithological description of Rydułtowy 1 coal deposit (Rydułtowy mine archive materials).
StratigraphyLithostratigraphic UnitCoal Seam Name Description
Quaternary--Thickness 5–30 m.
Sandy gravel deposits with glacial and fluvioglacial clays.
Miocene--Thickness 0–400 m.
Lack of deposit in central, east, and southeast parts of the area.
Loams, silts, fine sands and gravels, shales, sandstones.
Triassic--Thickness 0.1–56 m.
Deposited as local patches.
Loams, sands, marl loams, siltstones, mudstones.
(Carboniferous)
Pennsylvanian		Namurian B-CUpper Silesian Sandstone SeriesSaddle Layers
(500)		Coarse-grained sandstones (thickness up to 25 m) with 0.7–5.6 m thick coal seams.
Namurian AParalic SeriesPoruba Layers (600)Total of 60 coal seams with coal inserts were recognised in the profile. Coal seams are generally thin and accompany surrounding rocks deposited as shales, sandy shales, and fine-grained sandstones.
Jaklovec Layers (700)Main production level.
Total of 30 coal seams with coal inserts were recognised in the profile (<0.1–4.0 m).
Siltstones, mudstones layered with sandstones.
Hrusov Layers (800)Siltstones, mudstones, sandstones.
Numerous coal seam inserts but non-documented.
Recognised by deep boreholes only.
Table
Table 2. Parameters of the methane content at individual depth intervals.
Table 2. Parameters of the methane content at individual depth intervals.
Interval
(m above Sea Level)		Methane Content (m3/t coal daf)
MinimumMaximumAverageStandard DeviationNumber of Data
0 to −2000.0040.040.010.0114
−200 to −4000.0011.420.090.23264
−400 to −6000.0016.270.611.07916
−600 to −8000.0019.212.672.161104
−800 to −10000.00314.763.832.51306
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Kędzior, S.; Dreger, M. Geological and Mining Factors Controlling the Current Methane Conditions in the Rydułtowy Coal Mine (Upper Silesian Coal Basin, Poland). Energies 2022, 15, 6364. https://doi.org/10.3390/en15176364

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Kędzior S, Dreger M. Geological and Mining Factors Controlling the Current Methane Conditions in the Rydułtowy Coal Mine (Upper Silesian Coal Basin, Poland). Energies. 2022; 15(17):6364. https://doi.org/10.3390/en15176364

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Kędzior, Sławomir, and Marcin Dreger. 2022. "Geological and Mining Factors Controlling the Current Methane Conditions in the Rydułtowy Coal Mine (Upper Silesian Coal Basin, Poland)" Energies 15, no. 17: 6364. https://doi.org/10.3390/en15176364

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