Knowledge regarding microseismic characteristics associated with longwall coal mining is crucial in evaluating the potential for underground mining hazards. Although microseismicity is induced by ...mining activities, it still remains uncertain as to what extent mining activities influence the spatial, temporal, and magnitude characteristics of microseismicity. To establish a thorough understanding of the relationship between microseismic characteristics and mining activities, a 27-month long microseismic monitoring campaign was conducted around a highly stressed coal zone and eight producing longwall panels at Coal Mine Velenje in Slovenia. Each microseismic event was classified to be associated with the producing longwall panel that triggered it, and the microseismic response to multi-panel longwall top coal caving face advance was analysed. Monitoring data have shown that locations of microseismic events coincided with stress concentrated regions. It was established that both seismic count and energy-intensive regions associated with coal mining in different panels are spatially connected, but they do not fully overlap with mined-out or stress concentrated areas. In addition, microseismic event counts frequency was found to be well correlated with mining intensity, while seismic energy magnitude and spatial distribution are poorly correlated with the same. Therefore, microseismic characteristics could not be explained solely by the mining-induced stress transfer and mining intensity, but are believed to be dominated by pre-existing natural fractures throughout the coal seam. Analyses of these observations helped the development of a conceptual seismic-generation model, which provides new insights into the causes of microseismicity in coal mining.
Mining-induced microseismicity has been extensively used to evaluate the potential for rock bursts and coal and gas outbursts in underground coal mines. In a research project completed a few years ...ago, it was observed that characteristics of microseismicity around a working longwall panel were fairly consistent over the monitoring period until a lithological heterogeneity zone with a relatively high coal strength was reached. The current research presented in this paper aims at achieving a better understanding of the effect of lithological heterogeneity on microseismic activity in longwall coal mining. The heterogeneous zone inferred from the tomography measurements was first digitalised and implemented into a 3D geomechanical model. A microseismicity modelling approach which combines deterministic stress and failure analysis together with a stochastic fracture slip evaluation was used to simulate the evolution of microseismicity induced by the progressive face advance passing through the heterogeneous zone. The heterogeneity was taken into account by varying the material strength and the fracture attributes of the elements within the high strength zone. Results have shown that both the high rock strength of coal lithotype and low power law scaling exponent of fractures within this zone contribute to the reduction in fitted b values from frequency-magnitude distribution of microseismicity and the increase in fitted Gaussian distribution parameters to the logarithmic event energy. These deviations are believed to result from the combined effects of increased stress drops and slipped fracture sizes when the heterogeneous zone is approached.
•Mining-induced microseismicity influenced by lithological heterogeneity was modelled.•Heterogeneities in both material strength and fracture attributes were considered.•Both high coal strength and low fracture scaling exponent lead to elevated seismic energy level.•The recorded seismic anomaly results from increased stress drops and slipped fracture sizes.
Uncontrolled and excessive gas emissions pose a serious threat to safety in underground coal mining. In a recently completed research project, a suite of monitoring techniques were employed to assess ...the dynamic response of the coal seam being mined to longwall face advance at Coal Mine Velenje in Slovenia. Together with continuous monitoring of gas emissions, two seismic tomography measurement campaigns and a microseismic monitoring programme were implemented at one longwall top coal caving panel. Over 2000 microseismic events were recorded during a period of four months. Over the same period, there also was a recorded episode of relatively high gas emission in the same longwall district. In this paper, a detailed analysis of the processed microseismic data collected during the same monitoring period is presented. Specifically, the analysis includes the spatial distribution of the microseismic events with respect to the longwall face advance, the magnitude of the energy released per week and its temporal evolution. Examination of the spatial distribution of the recorded microseismic events has shown that most of the microseismic activity occurred ahead of the advancing face. Furthermore, the analysis of the gas emission and microseismic monitoring data has suggested that there is a direct correlation between microseismicity and gas emission rate, and that gas emission rate tends to reach a peak when seismic energy increases dramatically. It is believed that localised stress concentration over a relatively strong xylite-rich zone and its eventual failure, which was also identified by the seismic tomography measurements, may have triggered the heightened microseismic activity and the excessive gas emission episode experienced at the longwall panel monitored.
•Over 2000 micro-seismic events were recorded at a longwall top coal caving panel.•The spatial distribution of microseismic events was evaluated against face advance.•An episode of increased gas emission was correlated with mining induced microseismic activities.•A zone with heterogeneous coal structure was successfully detected by seismic monitoring.
The application of multi-level longwall top coal caving mining method in ultra-thick coal seams generally yields a much higher productivity and is more efficient in comparison to a mechanised ...single-slice longwall panel. However, the greater productivity achieved by this mining method may further exacerbate the gas emission problems often faced in longwall mining. In order to establish a thorough understanding of gas pressure regimes, and gas emission patterns around a producing multi-level longwall top coal caving face, a suite of in-situ measurements on seam gas pressure, gas composition, and ventilation environment was conducted at Coal Mine Velenje in Slovenia. This paper focuses on the analysis of these field observations which helped develop a conceptual gas emission model for multi-level longwall top coal caving mining of ultra-thick coal seams. It has been found that, at Coal Mine Velenje, the coal zone within 40m ahead of the face can significantly contribute to the overall district gas emission. In addition, floor coal and roof goaf may both play a major role towards the total gas emitted during mining.
•The in situ pressure regimes and gas composition at different mining levels of an ultra-thick coal seam were monitored.•Gas pressure and composition response to face advance of the coal seam around a LTCC extraction have been evaluated.•A conceptual gas emission model for a longwall top coal caving panel was developed.
The longwall top coal caving method, which enables the most productive exploitation of thick/ultra-thick coal seams, may result in a distinct geomechanical response of strata and associated gas ...emission patterns around longwall layouts. A two-way sequential coupling of a geomechanical and a reservoir simulator for the modelling of gas emissions around a longwall top coal caving (LTCC) panels was developed building on the understanding established from the analysis of in-situ gas pressure and concentration measurements carried out at Coal Mine Velenje in Slovenia. Model findings have shown that the modelling method implemented can reproduce the dynamic changes of stresses and gas pressure around a LTCC face and predict the total gas emissions and mixed gas concentrations accurately. It was found that, in LTCC panels, although the rate of gas emission from mined coal depends highly on the coal face advance, floor coal and roof goaf act as a constant and steady gas source accounting for a considerable part of the overall gas emission. Research has shown that, at first and/or second mining levels of multi-level LTCC mining, a notable stress relief and pore pressure drop induced by fracturing of the mined and roof coal can be experienced within 40m ahead of the face-line. In the floor coal, on the other hand, the pore pressure change was found to extend to 20m below the mining horizon. Model results have clearly shown the permeability enhancement and gas mobilisation zones around the LTCC panel, which can be the target zones for gas drainage boreholes.
•A two-way sequential coupling approach between a geomechanical and a reservoir simulator was developed.•The model was applied to simulate gas emissions around a longwall top coal caving panel.•Gas emission zones around LTCC panels were successfully identified.
Coal is one of the main fossil fuel resources and remains the most important contributor to global power generation but coal utilization has severe negative impacts, mostly due to the release of CO2 ...and toxic elements into the environment. This means that determining the quality of coal is important to address the environmental and health problems related to coal combustion. Coal samples from the Velenje coalmine, one of the largest actively mined coal basins in Central Europe, were divided into organic rich and inorganic rich fractions according to the percentage of carbon. Oxides of the major elements (SiO2, Al2O3, Fe2O3 MgO, CaO, Na2O, K2O, TiO2, P2O5, MnO, Cr2O3), toxicologically and environmentally relevant elements (As, Ba, Co, Cu, Hg, Mo, Ni, Pb, Th, U, V, Zn, Se) and other trace elements (Ce, Cs, Dy, Er, Eu, Gd, La, Nb, Nd, Pr, Rb, Sm, Sr, Tb, Y, Zr) were measured in sample digests using inductively coupled plasma mass spectrometry (ICP-MS). The concentrations of the majority of analyzed elements were either equal to or below the global average for coal. Exceptions were Sr (1090 ± 510 μg g−1, 9 times higher), Ba (301 ± 184 μg g−1, 2 times higher) and Pb (9.12 ± 17.0 μg g−1, 1.4 times higher) in inorganic rich coal and Mo (7.76 ± 4.76 μg g−1, 3.5 times higher) and U (5.24 ± 3.23 μg g−1, 1.8 times higher) in organic rich samples. Eighteen elements (Ag, Au, Be, Bi, Cd, Ga, Hf, Ho, Lu, Sb, Sc, Se, Sn, Ta, Ti, Tm, Yb, W) were below the limit of detection in >70% of the samples. Speciation analysis revealed the presence of several organoarsenic compounds in the organic rich samples, with the tetramethylarsonium ion (TETRA, 0.01–1.10 μg g−1) and trimethylarsine oxide (TMAO, 0.01–0.29 μg g−1) as the most abundant. A comparison with coal samples from the Senovo, Kanižarica, and Trbovlje coal mines in Slovenia and from the Sokolov Basin, (Czech Republic) revealed that Velenje contains much higher amount of organoarsenic compounds (34.8 ± 16.9%) in comparison to the others (4.45 ± 4.19%).
•We investigated element composition in inorganic and organic rich lignite.•Major, minor and trace elements were determined in Velenje lignite.•Values of measured elements are comparable to world coal, except U and Mo.•Unexpected organoarsenic compounds were found in Velenje lignite.•Organoarsenic compounds in Velenje lignite are related to bacterial activity.
Coal gas outbursts (especially CO2) present a high risk in mining of lignite in the Velenje Coal Mine, located in the Velenje Basin in northern Slovenia. A programme of monitoring geochemical ...parameters was set up to help understand the behaviour of the coalbed gas distribution in advance of the working face using mass spectrometric methods to study its molecular and isotopic compositions and origin. Coalbed gas samples from four different excavation fields (G2/C and K.-130/A from the north and south Preloge mining area and K.-5/A and K.-50/C from the Pesje mining area), which were operational between the years 2010 and 2011 were investigated. The major gas components are CO2 and methane. Temporal changes in the chemical and isotopic composition of free seam gases were observed within boreholes as a function of the advancement of the working face. The study also revealed that at a distance of around 120m from the working face, the influence of coal exploitation by the Velenje Longwall Mining Method causes coalbed gas to migrate. At a distance of 70m the lignite structure is crushed causing desorption of fixed CO2 from the coal. Differences in coalbed gas composition at the longwall panels which underlie the unmined area or under previously mined areas were found. A high CDMI {=CO2/(CO2+CH4)100 (%)} index with values up to 95.6% was typical for areas of pre-mined excavation fields (South Preloge K.-130/A and Pesje area K.-5/A), while in excavation fields with no previous mining activity (North Preloge G2/C and Pesje area K.-50/C) up to 61.9 vol % of CH4 was detected. The concentration measurements and isotopic studies revealed endogenic CO2 (including CO2 originating from dissolution of carbonates) with δ13CCO2 values ranging from −7.0‰ to 5.5‰, microbial methane and CO2 with values ranging from −70.4 to −50‰ and from −11.0 to −7.0‰, respectively. Higher δ13CCH4 values ranging from −50 to −19.8‰ could be attributed to so-called secondary processes influencing the δ13CCH4 value, such as migration due to lignite excavation (escape of isotopically lighter methane). In excavation fields (G2/C and K.-50/C) with no-premining activity higher δ13CCH4 values could also be explained by migration of methane from deeper strata. The δ13CCH4 value also depended on the depth of the excavation field; at shallower levels of the excavation field (K.-5/A) a lower δ13CCH4 value was traced indicating microbial gas, while at deeper levels higher δ13CCH4 values were found.
•Coalbed gas distribution and origin were investigated in Velenje Basin.•CO2 is a major gas component at investigated excavation fields.•Rate of advancement of the working face influences coalbed gas distribution.•Methane was formed through microbial processes, while CO2 is endogenic in origin.•CH4 is accumulated at no pre-mined areas, while CO2 accumulates in pre-mined areas.
Gas samples were collected from 25 m long horizontal boreholes drilled into the excavation field at 10° inclination to the longwall face in two mining areas, Pesje and Preloge, in the Velenje Coal ...Mine, Slovenia, from 2013 to 2018. The degassing mechanism of coalbed gas and its stable isotopic composition (δ
13
C
CO2
, δ
13
C
CH4
, and δ
2
H
CH4
) were investigated in boreholes in advance of eight working faces. The major coalbed gas constituents were CO
2
and methane. Gas concentrations and isotope values revealed that the methane is biogenic in origin with δ
13
C
CH4
values of −69.4 to −29.5 ‰, δ
2
H
CH4
values of −301 to −222 ‰, and a fractionation factor (α
CO2
-
CH4
) of 0.998-1.073, suggesting that methane derives from microbial acetate fermentation and CO
2
reduction. The carbon dioxide methane index values ranged from 50.0-98.3 vol.% and δ
13
C
CO2
values from −11.8 to −0.5 ‰, indicating that CO
2
is biogenic and endogenic in origin. The degassing mechanism results in isotope fractionation of methane and CO
2
for carbon isotopes up to 39.9 ‰ and up to 8.5 ‰, respectively, depending on the position of the excavation fields in space, e.g. under pre-mined coal area, fresh overburden.
The geochemical and isotopic composition of surface waters and groundwater in the Velenje Basin, Slovenia, was investigated seasonally to determine the relationship between major aquifers and surface ...waters, water–rock reactions, relative ages of groundwater, and biogeochemical processes. Groundwater in the Triassic aquifer is dominated by HCO
3
–
, Ca
2+
, Mg
2+
and δ
13
C
DIC
indicating degradation of soil organic matter and dissolution of carbonate minerals, similar to surface waters. In addition, groundwater in the Triassic aquifer has δ
18
O and δD values that plot near surface waters on the local and global meteoric water lines, and detectable tritium, likely reflecting recent (<50 years) recharge. In contrast, groundwater in the Pliocene aquifers is enriched in Mg
2+
, Na
+
, Ca
2+
, K
+
, and Si, and has high alkalinity and δ
13
C
DIC
values, with low SO
4
2–
and NO
3
–
concentrations. These waters have likely been influenced by sulfate reduction and microbial methanogenesis associated with coal seams and dissolution of feldspars and Mg-rich clay minerals. Pliocene aquifer waters are also depleted in
18
O and
2
H, and have
3
H concentrations near the detection limit, suggesting these waters are older, had a different recharge source, and have not mixed extensively with groundwater in the Triassic aquifer.
The geochemical and isotopic composition of surface waters and groundwater in the Velenje Basin, Slovenia, was investigated seasonally to determine the relationship between major aquifers and surface ...waters, water–rock reactions, relative ages of groundwater, and biogeochemical processes. Groundwater in the Triassic aquifer is dominated by HCO₃ –, Ca²⁺, Mg²⁺ and δ¹³CDIC indicating degradation of soil organic matter and dissolution of carbonate minerals, similar to surface waters. In addition, groundwater in the Triassic aquifer has δ¹⁸O and δD values that plot near surface waters on the local and global meteoric water lines, and detectable tritium, likely reflecting recent (<50 years) recharge. In contrast, groundwater in the Pliocene aquifers is enriched in Mg²⁺, Na⁺, Ca²⁺, K⁺, and Si, and has high alkalinity and δ¹³CDIC values, with low SO₄ ²– and NO₃ – concentrations. These waters have likely been influenced by sulfate reduction and microbial methanogenesis associated with coal seams and dissolution of feldspars and Mg-rich clay minerals. Pliocene aquifer waters are also depleted in ¹⁸O and ²H, and have ³H concentrations near the detection limit, suggesting these waters are older, had a different recharge source, and have not mixed extensively with groundwater in the Triassic aquifer.
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