Investigation on coal seam distribution and gas occurrence law in ...

Original Article

Investigation on coal seam distribution and gas occurrence law in Guizhou, China

Energy Exploration & Exploitation 0(0) 1–25 ! The Author(s) 2018 DOI: 10.1177/0144598718758068 journals.sagepub.com/home/eea

Qingsong Li1,2,3, Xin He2,4, Jiahao Wu5 and Shu Ma1,2,3

Abstract In order to enhance the management level of coal mine safety production and promote the “safe, accurate and efficient” preventive treatments for gas in Guizhou of China, the occurrence and other prominent features of coal and gas are investigated. The characteristics and regularities of coal mine accidents in Guizhou during 2001–2015 are summarized to analyze the commonness of gas accidents in general and determine the characteristics of gas preventive treatment. Geological data, gas basic parameters, and physical properties of coal of 386 mines and 761 sets of coal seams in Guizhou are also statistically analyzed. Based on step control theory of gas occurrence structure and the regionally tectonic regularity of coal-bearing stratum distribution, the deformations of coal measures in Guizhou mine area are mainly caused by great variation of stratigraphic occurrence, complicated geological structure, and high crustal stress. The regional occurrence of coal seam is obvious with the highest content of Tongzi–Zunyi–Liuzhi–Xingyi line, which gradually reduces to the both east and west sides. Influence factors and weights of gas occurrence are expounded from geological and coal factor by mathematical statistics, and the main influence factors of gas occurrence are the sedimentary environment, syncline structure, and metamorphic grade in proper sequence. Combined with the risk prediction of coal and gas outburst area, the prediction of gas pressure by gas content is not suitable under the special occurrence conditions. The initial velocity of gas emission, the solidity coefficient, and the damage type in more than 77%

1

School of Safety Engineering, China University of Mining and Technology, Xuzhou, China Mine Safety Science Research Institute of Guizhou Province, Guiyang, China 3 Engineering Research Center of Prevention and Control for Coal and Gas Outburst of Guizhou, Guiyang, China 4 Mining College, Guizhou University, Guiyang, China 5 Key Laboratory of CBM Resources and Dynamic Accumulation Process, Ministry of Education, School of Resources and Earth Science, China University of Mining and Technology, Xuzhou, China 2

Corresponding author: Jiahao Wu, School of Resources and Earth Science, China University of Mining and Technology, No. 1, Daxue Road, Xuzhou, Jiangsu 221116, China. Email: [email protected] Creative Commons CC BY: This article is distributed under the terms of the Creative Commons Attribution 4.0 License (http://www.creativecommons.org/licenses/by/4.0/) which permits any use, reproduction and distribution of the work without further permission provided the original work is attributed as specified on the SAGE and Open Access pages (https://us.sagepub.com/en-us/nam/open-access-at-sage).

2

Energy Exploration & Exploitation 0(0)

of minable seams all exceed the critical value. This work provides guidance in improvement of the governance situation for gas control in Guizhou. The index prediction system which is suitable for mining conditions of special coal mines in Guizhou should be established. Keywords Coal and gas outburst, coal-bearing stratum, gas occurrence, geological structure, metamorphic degree

Introduction In recent decades, the distribution characteristics of coal seam methane were investigated by many researchers. Their works showed that distribution of gas was controlled by geological factors with regularity of uneven distribution, which was related to tectonics and structure, stratigraphy and sedimentology, coal rank, permeability, gas content, and hydrodynamics (Flores, 1998; Kaiser et al., 1994; Pashin, 1998; Scott, 2002). Creedy (1988) put forward that geological structure dominated the occurrence and distribution of gas in coal measures strata, and it suggested strengthening the research on evolution of geological structure and the gas geology law. Shepherd et al. (1981) studied the relation between geological structure with gas outburst and he considered that the occurrence state of coal seam gas and the geological condition affecting gas disaster were the results of the evolution of all previous tectonic movements in coal-bearing strata. Bibler et al. (1998) studied the phenomenon of global gas emission and pointed out that the tectonic movement of coal mine not only affected the formation condition of seam gas, but also affected the storage condition of gas. Frodsham and Gayer (1999) suggested that the influence of geological structure on coal seam was that the destroyed structure of coal seam was widely developed of tectonic coal formed under the tectonic extrusion and shearing action, which provided a carrier for gas enrichment. Li and Ogawa (2001) proved that the tectonic coal was the rich collective of gas, and they also pointed out that tectonic coal was widely developed near the geological structure of the coalfield by simulation experiments. Guizhou is located in the Yangtze landmass, which is the vulcanorium of Qinghai–Tibet Plateau to Sichuan basin and Yunnan–Guizhou Plateau and significantly influenced by Indosinian, Yanshan, and Himalayan movements with the typical features of Karst topography and geomorphology. Its complex geological condition and high original crustal stress cause that the occurrence of coal seam is mainly based on the thin coal seam groups with near distance. Most of the coal seams currently mined in Guizhou have high gas content. Meanwhile, they are soft and prone to outbursts, which significantly influence mining safety (Lu et al., 2011). Coal mining in Guizhou is mainly based on medium and small mines for a long time and it is restricted by geological and economic conditions. Gas control is extremely difficult with frequent coal and gas outburst accidents. Statistics of 4571 coal mine accidents and deaths in Guizhou during 2001–2015 is shown in Figure 1. There is still a giant gap compared to the national average though the gas accident has been effectively controlled in general and the numbers of accidents and deaths have declined significantly. Especially the number of

Li et al.

3

Figure 1. Statistics of coal mine accidents in Guizhou during 2001–2015.

deaths in gas accidents accounted for a large part of coal accidents, and gas accident mortality per million tons of coal production remained high. In order to further reveal the gas occurrence law in mine area and promote the efficient governance for coal mine gas in Guizhou, 761 sets of coal and gas parameters of 386 mines and 285 coal-bearing geological structures in nine major mining areas were analyzed by the progressive control theory of gas occurrence structure in this work. Based on the investigation reports of the 66 coal and gas outburst accidents in the nearly 10 years and the characteristics of coal seam distribution and gas occurrence in Guizhou coal mine areas, regional geological characteristics of gas are comprehensively analyzed from multiangle by mathematical statistics.

Occurrence characteristics of coal seams in Guizhou Classification of coal-bearing stratum and mining area Guizhou is situated in the composite part of the south of the third fold subsidence zone and the Nanling zonal tectonic belt of the new Cathaysian. Wumeng Mountain, Dalou Mountain, Miaoling Mountain, and Wuling Mountain constitute the terrain skeleton of the whole province which belongs to the joined zone of Western Tethys tectonic domain and the eastern Pacific tectonic domain. Folds and fractures are relatively developed, and it forms the terrain of high north, low south, and gently bias from northwest to southeast. The main coal mines in Guizhou are located in the passive marginal fold zone in south of the upper Yangtze landmass according to the geotectonic division. As shown in Figure 2, it covers six IV class tectonic units of broad and gentle fold fault area in Tongren and fold fault area in north-south of Fenggang. The coal-bearing stratums in Guizhou are mainly the upper Permian series, the geological control type of coal seam belonging to regional tectonic compression depression control type, which is synergistically affected by numerous factors of sedimentary environment, geological structure, variation of coal thickness, properties of roof and floor, etc. The occurrence condition of coal seam is complex with the typical features of the Karst topography and the Jura-type folds. By the end of 2016, there are 1189 booked mines of Guizhou with the design production scale of 320 million tons each year. The production capacity of the 679 production mines is

4


Figure 2. Sketch of tectonic unit zoning of Guizhou. Note: I class structure—Yangtze Landmass (V); II class structure—Upper Yangtze Landmass (V2); III class structure—Passive marginal fold zone in the south of the Yangtze Landmass (V2-7).

170 million tons each year. They are divided into nine mining areas according to the sedimentary characteristics of Late Permian epoch, coal-bearing difference, and administrative division. The distribution of the mining area is shown in Figure 3 and the characteristics of coal seam of each mining area are shown in Table 1.

The general law of coal-bearing stratum Based on the analysis of typical coal mines in Guizhou and regional geological characteristics, tectonic evolution history and characteristics of coal accumulation are studied in this work. And the regularity study is conducted from three aspects of the overall geological law, the main coal seam distribution in each mining area and the general characteristics of coal, and the details are stated below. Geological law. (1) Intense plate movement and complex geological structure As influenced by tectonic structures and plate motion (Figure 2), the geological structure in Guizhou coal mine area has developed, and there are as many as 285 tectonic units within

Li et al.

5

Figure 3. Sketch map of coal mine area division in Guizhou.

Table 1. Characteristics table of coal seams of coal mining area in Guizhou. Mining area

Coal number (layer)

Coal thickness (m)

Layer spacing (m)

Dip angle ( )

Panjiang Watertown Zhina North Guizhou Northwest Guizhou Puxing Lindong HongMao

3–27 13–80 2–16 6–20 1–5 1–6 7–9 2–3

0.8 0.9 1.4 1.5 1.7 1.2 1.6 1.4

6.0–27.2 1.5–35.0 6.0–30.0 2.0–35.0 4.0–29.0 3.0–40.0 8.0–25.0 4.0–20.0

3–8 10–33 8–43 10–41 3–18 14–25 14–30 9–23

the district according to incomplete statistics, where 79 main coal-bearing tectonic belts are concentrated in the central-western region. The folded structures in Guizhou are various and the syncline structure accounts for more than 54%, which regionally controls the deposition of coal-bearing stratum. Under the comprehensive influence of mutual combination of structural belts and cutting modification, the geological structure of each mining area is complicated and coal measures are disordered. The fractures and faults have developed, the proportion of small faults (1–5 m) has exceeded 90%, and the density of local area is as high as 188 per square kilometer, which ranks first in the country. Faults not only affect the

6


integrity of the coal seam and the sealing condition of gas, but also affect the coal structure, the microscopic characteristics of coal petrography, and the permeability of coal with different extents. Particularly in Zhina and northern Guizhou, the coal measures are strongly deformed in characterization of compressional fold belt in eastern Yunnan–western Guizhou, which is a typical representative of complex geological conditions of high gas. The coal mines with complex geological structure account for 100%, which is 2.78 times of national average proportion (36%), ranking first in the country. (2) Numerous massifs and generally higher crustal stress The majority of coal mines in Guizhou is located in the eastern Qinghai–Tibetan Plateau and the tectonic transition zone to the Sichuan basin and the Yunnan–Guizhou Plateau, which is one of the most violent areas of new tectonic movements and earthquakes in China. The crustal stress decreases from two plateaus to the Sichuan basin with obvious effects. As the impact of the Himalayan tectonic movement, the crustal rock mass structure deforms and the Cenozoic era of the India Ocean plate continues to wedge northward, which makes the crustal stress in coal mine area generally higher. As shown in Figure 4, the whole province is divided into three classes of stress zones according to the size of stress distribution based on statistically geological prospecting data.

Figure 4. Stress zoning map of Guizhou.

Li et al.

7

Figure 4 shows that the regional average crustal stress first decreases and then rises in general from the north to south, and it steadily increases from east to west. The crustal stress is less than 15 MPa from the east of the low stress zones of Yuping–Zhenyuan–Kaili–SaDu, the crustal stress is more than 25 MPa in the high stress zones from the west of Ziyun– Shuicheng–Weining and the north of Yanhe–Wuchuan–Daozhen. The central region is the transition stress zone with imbalanced stress distribution. The crustal stress changes greatly in the local range as simultaneously affected by syncline structure. High in situ stress will appear with the disaster forms of mine pressure, outburst, rock burst, etc. in coal mining, which has brought immeasurable losses to the safe production of coal mines. The induced effect of crustal on outburst and accidents of roof and floor are most significant according to statistics. In terms of gas accidents, the minimum initial depth in Guizhou is 96 m, which is much lower than that of 200–300 m of most coal mines in China. With the increase of mining depth, more and more attention should be paid to crustal stress in the outburst inducements. Seam distribution law. (1) Concentrated occurrence and obvious regionalism of coal The distribution of coal resources in Guizhou is relatively concentrated from the regional point of view. Figure 5 manifests that the occurrence abundance gradually increases

Figure 5. Sketch map of coal resources distribution in Guizhou.

8


Table 2. Stratigraphic characteristics of coal seam occurrence in Guizhou. Erathem

System

Series

Sketch

Cenozoic Erathem

Tertiary system

The upper third series The lower third series

Mesozoic Erathem

Jurassic system Triassic Permian

Middle and lower series The upper series The upper series The lower series

Sandwiched lignite: 0–10 layer Lignite line with partially clamped lenticular shape Even clamped coal line Middle and upper clamped coal line Main coal-bearing seam Bottom clamped coal line

Paleozoic Erathem

expanding from the boundary of Yanhe–Shiqian–Weng’an–Duyun–Libo to west with the east of no coal area. The coal reserves shows a decreasing tend to the west of Hezhang– Weining. More than 90% of the reserves are in the mining areas of Liupanshui, northern Guizhou and northwest Guizhou, Zhina, which are at the western Zhengan–Suiyang– Zunyi–Guiyang–Anshun–Xingyi, and the abundance occurrence of coal seam presents the typical regional characteristics. (2) Concentrated coal occurrence position The main coal-bearing stratum in Guizhou is located in Longtan Formation (P2l) and Changxing Formation (P2c) of the upper Permian system of Palaeozoic Erathem. The coal resources account for 98% of the total amount of the whole province, and the occurrence position and characteristics of the coal seam are shown in Table 2. Tables 1 and 2 manifest that the coal seam spacing in Guizhou is smaller, which brings great difficulties to the actual production. Not only the development and deployment time is extended and the economic cost investment is increased, but also leads to disasters caused by crossing the coal seam by mistake. Besides, the partial area even appears the coal seam crossing due to small seam spacing, which makes extraction efficiency poor and thus causes lower accuracy of gas control. (3) Mainly based on coal seam mining of close distance At present, the coal seam mined in Guizhou coal mine area is generally relatively intensive and the layer spacing is more concentrated at 1.5–38 m with great change, some areas can even reach hundreds of meters. The majority of coal-rich areas in the west of Guizhou is coal seam mining with close range. One or even a few coal seam groups can be developed in the vertical direction in the same area, and the proportion of coal seams in coal-bearing areas is as high as 85%. The single coal seam is mined only in the coal-bearing areas of Guiyang, Tongren, and southeast Guizhou, which corresponds to the occurrence horizon of the above coal seam. The average workable coal seam in the whole province is 20 layers, which is far more than the typical coal-producing areas in China, such as Sichuan (nine layers), Yunnan (14 layers), Huainan (15 layers), and it ranks first in the country. Furthermore, coal-mining depth is shallow in Guizhou with the average mining depth of only 248 m, it is generally smaller compared to the domestic coal mine. With the improvement of the economic status of coal industry in Guizhou, the production capacity is also increasing with mining gradually toward deep, and the development and mining of deep well will require higher level of gas control technology in the future.

Li et al.

9

Figure 6. Vitrinite reflectance of coal in upper Permian series of Guizhou.

(4) The coal species are complete range of coal with generally high degree of metamorphism, and most are anthracite The degree of coal metamorphism varies greatly from low metamorphic lignite to high metamorphic anthracite in Guizhou. The large degree of coal rank variation has different requirements for coal mining technology and gas control difficulty. From the view of volatile, the volatile of the central Guizhou of Bijie–Jinsha–Zhijin–Anshun–Guanling–Jinsha– Xingyi is less than 10% and belongs to the anthracite regions. It gradually increases to the east and west sides and is up to 40% and forms the transition from meager coal to gas coal, only a small amount of lignite exists in some areas. From the vitrinite reflectance (Ro) in Figure 6, it can be seen that the vitrinite reflectance in east and west Guizhou and the north of Shuicheng–Liuzhi Sinan–Wengan–Majiang is 0.75–1.30; it is 2.20–4.08 in northern Guizhou, central Guizhou, southwestern Guizhou, and southern region of Panjiang. It gradually increases with metamorphic degree, and the Ro is more than 4 in the southern region of Panjiang which is the region with the highest degree of evolution. The distribution of coal species reflected by it is almost the same as that of volatiles. According to Figure 5, the distribution of anthracite is relatively concentrated and the occurrence area is also the coal-rich area above, which is mainly concentrated in the coal

10


mine area in Zhina and north Guizhou. The anthracite reserves account for 75% of the total coal resources in Guizhou based on statistics, high metamorphic coal will produce a large number of coal seam gases, and the prospects for exploitation and utilization of coal seam gas are relatively promising. (5) Thin coal seam is in the majority The reserves of coal resources in Guizhou are large, but most of occurrence is distributed regionally with schistose form and stratiform. The reserves of thin coal seam below 1.3 m account for 32.7% of the proved ones, which is in the forefront in the country. It can be seen from Table 1 that the thickness of workable coal seam in the whole province is between 0.6 and 3.7 m with an average of 1.2 m. It is one of the most urgent and important development directions of coal mining for Guizhou that is raising the mechanization degree of thin seam mining and developing to automation, intelligence, and unmanned mining. (6) Significant effect of folds and faults, large dip angle of coal seam Affected by geologically tectonic movement, the coal mining area in Guizhou is uplifted by plate compression and the fold is widely distributed with large dip angle of coal seam (4– 85 ). The dig angle varies greatly near the geologically tectonic belt, local areas are even overturned, which easily leads to potential safety hazard of coal mining and gas control. Characteristic and law of coal mass. (1) Low permeability and poor breathability of coal seam Generally the stratification of coal measures strata of Guizhou is disordered due to significant sedimentation during the coal-forming stage together with intense plate tectonic movement in the later stage. The plugging of pores and fissures is serious under the effect of fault cutting failure, which leads to low permeability of coal seam. Table 3 shows that coal seam permeability of Guizhou is poor as affected by geological structure, and most of coal seam gas reservoirs belong to low permeability reservoirs with the lowest one of 8.7 106 mD. They are less than other domestic serious mining area by 2–6 orders of magnitude in general. The low permeability of coal seam makes most coal seam gas difficult to be extracted. It is difficult to extract coal seam under the condition of no unloading. Moreover, the rate of extraction is still low in the anthracite mining area under the circumstances of good pressure relief. It not only increases the time cost and economic cost of gas control, but also the extraction efficiency is not ideal, which causes Table 3. Comparison list of coal seam permeability in various regions. Region

Mining depth (m)

Permeability of coal seam (mD)

Guiyang Chongqing Yunnan Huainan Sichuan

80–400 300–1000 400–600 300–1000 200–500

8.7 106 8.6 105 2.2 104 1.1 103 2.3 101

Li et al.

11

tense replacement of mine excavation and even induces coal and gas outburst (Zhou et al., 2014). (2) Soft coal quality The main coal seams in various mining areas of Guizhou are generally soft, which is mainly characterized by small firmness coefficient and high failure type as shown in Figures 7 and 8. The figure reflects that the firmness coefficient of the coal seam in the mining area is relatively large with an average of 0.54, and the firmness coefficient of the remaining coal seams is less than 0.5. The firmness coefficient of the workable coal seams is generally low in the whole province, only 5% of the samples of coal seams have the firmness coefficient of more than 1 according to statistics. It can be seen that the main failure type of coal in Guizhou is type III from Figure 8. The proportion of destruction type III and above is as high as 78%, which demonstrates that nearly 80% of the coal seams index exceed the critical value during the outburst prediction.

Figure 7. Firmness coefficient of various mining areas in Guizhou.

Figure 8. Scale map of failure types in Guizhou.

12


It also confirms that the occurrence condition of coal seam in Guizhou is seriously damaged by geological action, resulting in low permeability which makes it difficult to predict and control gas accidents.

Law of gas occurrence of Guizhou Guizhou is located in the high gas zone of upper Yangtze-Guizhou Yunnan with the upper Permian coal mine gas reserves of 3.60 trillion m3. Its reserves of resources rank second in the country, which belongs to the super-huge type of coal seam gas reserves. The average resource abundance of the province is 1.2 108 m3/km2. As an efficient new energy, the gas is rich in reserves and has great potential for exploitation. It is divided into three high gas belts and one gas belt according to the distribution of coal mine gas in Guizhou, which contains Liupanshui—high gas zone, North Zhina Liuzhi Guiyang—high gas zone, North Guizhou—high gas zone, and East Guizhou—gas zone (Jia et al., 2014). Coalbed methane resources are mainly concentrated in five mining areas of Liuzhi Panjiang, Shuicheng, Zhina, north Guizhou. The resources amount accounts for 93% of the province’s total resources with an average resource abundance of 230 million m3/km2, which is far higher than two major coal seam gas industry base in China at present (both Qinshui Basin and Ordos Basin are 150 million m3/km2). It is beneficial for exploitation and utilization of coalbed methane resources to master the law of the gas occurrence in preventing gas accidents and realizing the safety production of coal mine at the same time. Law of gas occurrence in coal mine area of Guizhou is intensively studied by the theory of regional geological evolution and the control theory of tectonic hierarchy in this paper. Statistics is conducted according to the gas pressure and the extreme value of the content in the mining area by counting the geological data of Guizhou coal mine area and the measured data of gas in each production mine, as shown in Table 4.

Table 4. Gas characteristic values of each mining area. Gas pressure (MPa) Mining area

Interval

Mean

Gas pressure gradient (MPa/100 m)

Liuzhi Panjiang Shuicheng Zhina North Guizhou Northwest Guizhou Puxing Lindong Hongmao Total

0.10–4.49 0.12–6.30 0.09–3.35 0.12–3.35 0.08–4.12 0.10–1.50 0.10–0.85 0.08–0.48 0.12–0.42 0.08–6.30

0.60 0.84 0.96 1.08 0.60 0.46 0.38 0.22 0.21 0.72

0.27 0.33 0.29 0.28 0.26 0.27 0.26 0.20 0.21 0.26

Gas content (m3/t) Interval

Mean

Gas content gradient (m3/t 100 m)

0.37–29.92 1.22–17.86 2.43–22.43 4.50–26.39 3.45–20.60 1.46–22.19 4.25–6.58 3.00–6.08 – 0.37–29.92

11.45 9.23 11.83 12.03 12.20 10.04 5.30 4.90 – 11.23

3.25 3.54 2.89 6.09 3.14 2.40 1.72 1.36 – 3.05

Note: Hongmao mining area is not considered as an important object of analysis due to lack of gas content data.

Li et al.

13

Influence factors of gas occurrence Gas is the product of geological processes and the present occurrence state of coal seam gas is the result of the evolution of coal-bearing strata through complicated geological history. Geological processes comprehensively control the formation, migration, and preservation of gas (Bustin and Clarkson,1998; Fisne and Esen, 2014; Moore, 2012). The main influence factors of gas occurrence in Guizhou can be divided into two aspects of geological factors and physical characteristics of coal mass. The geological factors determine the accumulation law and regional storage law of coal mine gas, and the physical characteristics of coal mass affect the efficiency of gas emission (Cheng et al., 2011; Karacan et al., 2011; Lunarzewski, 1998). Geological factors. (1) Geological structure The structure of coal mines in Guizhou can be broadly divided into the following three categories. • Closed-type structure: compressive or torsional structures make it easy to form the soft tectonic coal enclosed area with a great potential of gas, which is the main place of gas occurrence. • Open-type structure: it is the structure formed under the action of tensile stress, which is conducive for the emission of gas. • Semiopen and semiclosed structure: it is the structure formed by the simultaneous action of compression and tensile stress with a certain gas potential and water transmissibility. The geological structure has significant influence on gas occurrence, and the control action has the progressive property (Li, 2001). The structure controls the evolution of the coal-bearing basin, and thus it affects the generation and distribution of gas (Cai et al., 2011; Hou et al., 2012; Su et al., 2005). Coal accumulation basin of Guizhou has formed many anticline, syncline, and fault structure after the Indosinian and Yanshan movements. The coal measures are all preserved in the syncline tectonic units controlled by fracture structures, and stress distribution of syncline structure and extrusion is mainly barrier-type fold. The eroded areas of anticline and syncline abdomen are small, which are beneficial to preserve the gas in coal-bearing stratum and coal seams. At the same time, different tectonic forms play a pivotal role in the accumulation and storage of gas in later stage. The coal seam is destroyed, the strength of the coal decreases, and the gas in the coal seam relatively increases near the small fault and the cusp. The faults in Guizhou coal mine area are widely distributed, and most of them are the closed faults. Its piezotropy, compression torsion, nonhydraulic faults, and the breathability of stratum contacted with the coal seam are low. As a result, the emission of coal seam gas can be effectively prevented, and the gas storage condition is relatively good in general. (2) Crustal stress Crustal stress is an important factor affecting gas occurrence and the migration of coal seam gas is controlled by crustal stress (Xu et al., 2006; Yang et al., 2011). In the stress

14


Table 5. List of nationwide typical coal mining areas. Coal mining area

In situ stress (MPa)

Buried depth (m)

Stress gradient (kPa/m)

Liupanshui, Huainan Hegang En Hong Weibei Jincheng Anyang Tiefa Kailuan

15.5–28.3 9.5–22.3 12.2–19.5 13.4–18.2 8.5–16.9 3.1–13.6 8.7–11.7 8.3–10.2 7.6–9.4

560–1245 506–974 650–955 510–568 630–720 225–697 570–670 590–730 720–983

21.0–33.5 15.6–23.5 14.0–25.7 26.7–33.1 13.1–28.0 9.7–21.6 15.2–17.6 8.6–12.2 10.6–11.2

concentration area of coal seam, a great deal of new fissures and cracks are produced due to the extrusion and shearing of high stress. Meanwhile, more cracks are closed, which causes the gas accumulation. Figure 2 and Table 5 show that the crustal stress is lateral, vertical, and other forms. The overall stress is larger than other typical coal mines in China, and high crustal stress makes it difficult to conduct the desorption inside coal seam gas and diffuse to the earth’s surface. From Figures 3 and 4, it can be seen that the coal mine areas in Guizhou are located in the region where the in situ stress is relatively high. The effect of crustal stress on gas plugging remains remarkable even if the mining depth is shallow. (3) Buried depth (overlying pedestal rock thickness) The buried depth is the main factor affecting the gas content. The gas is easy to lose in the coal seam exposed to the surface, and the gas content in coal seam is low. The distance of gas migration to the surface increases with the increase of overlying pedestal rock thickness, and gas sequestration is preferable. Generally, the gas content increases as the increase of overlying pedestal rock thickness below the gas-weathering belt. With the increase of the buried depth, the increased overall distribution law of gas content in Guizhou coal mine area is Q ¼ 0.0162H þ 5.0155, and its correlation coefficient of regression equation is R2¼0.7288. As Table 4 shows, gas content increases with the buried depth, especially in Zhina mining area. With the extension of mining to depth, the crustal stress is increasing continuously and gas sealing ring forms in both wings of syncline structure. The thickening of overlying strata makes the gas difficult to lose to the surface, and the mutual interaction plays a positive role in the gas enrichment. However, the sealing effect is relative. The lithology of overlying wall rock, the breathability of the coal mass, and the underpart aquifer are also the critical factors. (4) Lithology of roof and floor The late Permian in Guizhou coal mine area is mainly the delta deposition of interaction between land and sea. The roof and floor of the coal seam are mainly mudstone and silty mudstone with poor breathability and stronger capping capacity for gas. The influence of lithology of roof and floor of coal seam on gas occurrence is mainly as follows: • Overlying strata in Panjiang mining area is mainly the clay minerals, tight mudstone, and sandy mudstone. The development of cracks and micro gap is poor and the breathability

Li et al.

15

Table 6. Relation between hydrogeology and gas distribution in each mining area. Mining area

Water storage capacity

Water and coal seam spacing

Influence form

Liuzhi Panjiang Shuicheng Zhina North Guizhou Northwest Guizhou Puxing Lindong

Abundant Abundant Abundant Abundant Abundant Abundant Abundant Abundant

Moderate Moderate Moderate near near far Moderate Moderate

Sealing or plugging Sealing or plugging Sealing or plugging Migration or loss Sealing or plugging Migration or loss Sealing or plugging Sealing or plugging

water water water water water water water water

is extremely poor as affected by swelling deformation and timely closure of argillaceous rock mass. The gas loss is still difficult even if the faults or folds are widely distributed, and gas content of coal seam is very large. • The overlying strata in the mining area of northern Guizhou is mainly the contact-type cementation siltstone, stratiform fine sandstone, and sandy mudstone with poor permeability. • The overlying strata are mostly sandstone, conglomerate, and limestone in Lin Dong coal mine. The fracture development is fine, and its permeability has significant effect on dispersion action of gas. Therefore, the gas content is low. (5) Hydrogeology The numerous coal-bearing basins in the west of Guizhou form the regional water separation boundary by the upper Permian series and Feixianguan formation of lower three Triassic series and then forms an independent hydrogeological unit. As overlying a large number of mudstone and sandy mudstone, the underlying thick basaltic tuffs all have good effect of water separation and the water closed ring of gas forms in geological structural units, which is conductive for gas occurrence. The spacing relation among the water storage capacity, aquifer water-bearing stratum, and coal-bearing stratum in main mining area of Guizhou is shown in Table 6. As shown in Table 4, the gas content in mining area with high water storage capacity is generally higher. The mining areas of Liuzhi, Panjiang, Shuicheng, and northern Guizhou have favorable water seal trap condition, and main coal-bearing stratum has poor waterbearing property. There is no direct relation between coal seam and surface water and stratum with higher aquifer, which is beneficial to form the water seal trap for gas and it is an important area of gas enrichment. The gas content in the weak watery mining area is generally low with an average gas content of 4.8 m3/t. The weak watery property of hydrogeological conditions reduces the effect of hydraulic power on gas plugging or sealing in the case of similar conditions. It can be seen from this that the influence of hydrogeology of Guizhou on gas occurrence is mostly the plugging or sealing. In particular, the underlying coal seam is affected by long-term hydraulic migration due to the distance between the coal seam and aquifer is short near Zunyi in the northern of Guizhou. Thus, the gas content in the underpart seam is lower than that of the upper side because of gas loss. Overlying and underlying aquifers in the coal measure strata of Zhina mining area are all separated by sandstone and basalt, and the aquifer is basically not in

16


contact with the coal-bearing strata. Thus, it has little effect on the gas occurrence in the coal seam. Physical characteristics of coal mass. The characteristics of coal petrography and coal quality can not only reflect the degree of resistance to the stress failure for coal mass, but also reflect the degree of gas enrichment in coal seam (Beamish and Crosdale, 1998). (1) Metamorphic degree The influence of metamorphic degree on the gas content of coal seam is mainly reflected by the control for gas content and the adsorption capacity of coal. First of all, as the coal gas adsorption, the higher the degree of metamorphism is, the higher gas amount is for coal seam in the same storage conditions. Second, the adsorption capacity of coal to methane increases gradually (except ultrahigh metamorphic anthracite) with the increase of metamorphic degree. According to the statistics of gas content in Liupanshui coal mine with the same buried depth (300–500 m), it can be seen that the degree of metamorphism of coal is positively related to gas content. The gas content in medium and low metamorphic coal stage steadily increases with the metamorphic degree, and high metamorphic coal has higher gas content than medium and low metamorphic coal. However, the relation between gas content and metamorphic degree is not obvious in high metamorphic coal. The metamorphic degree of coal in Guizhou is mainly controlled by plutonic metamorphism, that is the metamorphic degree of coal increases with the burial depth. And the total variation trend of coal volatility decreases downward in the vertical direction. On the other hand, the effect of magmatic thermal metamorphism in coal-rich area is significant as influenced by intense magmatic activity during the Yanshan period. The high metamorphic degree of coal in Guizhou therefore is the result of the combined effects of multiple metamorphic types. (2) Coal thickness Coal seam is a kind of highly dense rock formation with low permeability. The upper and lower layers have strong capping effects on the part of the middle layer. The greater the thickness of coal seam is, the larger the gas generation amount and the longer the diffusion path of the middle layer gas to the top and bottom plate will be. The diffusion resistance will also be greater, which is more conductive for the preservation of the gas. Thick coal belts are gas-enriched zones in general. Especially the areas with large variation of coal thickness are more likely to lead to uneven distribution of gas. For example, the trend of gas content increases with the increase of coal seam thickness in the first mine of Heitang and Baishui exploration areas of Zhina coalfield. And the thickness of coal seam and gas content are also positively related in most of the exploration areas or mine field in Liupanshui. (3) Dip angle of coal seam The gas content of coal seam has a certain correlation with the dip angle of coal seam when the buried depth and wall rock property are same. It is easier for gas to flow in horizontal direction than that in vertical direction. The smaller the dip angle of coal seam

Li et al.

17

is, the greater the gas content will be. The dip angle of coal seam of Guizhou varies greatly and the influence on gas emission is not obvious. The sealing effect is favorable on local areas.

Gas occurrence characteristics It can be drawn from the above analysis that most of the coal mining area in Guizhou is located in the transitional sedimentary area of land and sea. The gas occurrence is greatly affected by sedimentary environment. Various faults and their combined forms are developed in the syncline. The folds, faults, and secondary structure are numerous, which makes the gas form local agglomeration. The compression torsion and crumple cause messy occurrence of coal seam and structural coal morphology forms, and the transverse movement of gas is difficult (Cao et al., 2001). The breathability coefficient of overlying strata and coal seam is generally poor, leading to difficult loss for gas to the surface. The effect of hydrogeology on loss of gas is not strong since the aquifers are often far from coal seams and hydrosphere plugging with closed structure forms in some areas. The coal species of Guizhou are all in readiness and mainly based on anthracite. The metamorphic degree of coal is high in each mining area, especially in the mining areas in Zhina and northern Guizhou. Anthracite in the coal mining accounts for more than 90% with large gas generation quantity. On the other hand, the mining coal seam in Guizhou is mainly based on thin coal seam and the thickness of coal seam varies greatly. The structure of coal is disordered with extremely uneven gas distribution, and the risk of coal and gas outburst is higher. The dip angle of coal seam has certain influence on gas emission. But it has not found stronger regularity characteristics in the whole province, so it is regarded as a secondary factor. Therefore, the importance degree of factors that determine the gas enrichment degree in coal mine area of Guizhou is as follows. Overall factors: Geological factors > Physical characteristics of coal mass; Geological factors: Depositional environment > Synclinal structure > Overlying bedrock > Hydrogeology; Physical characteristics of coal mass: Metamorphic grade > Coal thickness > Dip angle. Different geological structure and their parts have diverse effects on the gas storage, migration. Especially, the faults of coal mine area in Guizhou are dense and the faults have significant effect on the accumulation and dissipation of coal seam gas. But it cannot be generalized as the density degree, type, property, and size of faults are different in each mining area. The local characteristics of lithology of roof and floor, coal thickness, and dip angle of coal seam are obvious. However, the general law is not strong. Thus, it is better to determine the gas geology law of the mined coal seam by analyzing in the light of circumstances in actual production.

Distribution law of gas pressure and content Gas pressure is the basic parameter that determines the coal seam gas-bearing capacity and the dynamic characteristics of gas (Aguado and Nicieza, 2007; Chen, 2011; Xue et al., 2011). At present, the current codes and regulations in China take the gas pressure as an important index of coal and gas outburst identification and regional prediction for coal seam. Usually the relationship between gas content and gas pressure is calculated since the time for gas

18


pressure measurement is long. The gas pressure of 0.74 MPa corresponds to the gas content of 8 m3/t according to the Langmuir monomolecular layer adsorption model. Based on the long-term and large number statistical results of the nationwide outburst measurements, the critical value is not exact due to the particularity of coal seam gas occurrence in Guizhou. (1) Law of gas pressure As shown in Table 4, the average gas pressure gradient of 100 m is Panjiang > Shuicheng > Zhina > Liuzhi ¼ northwestern Guizhou > northern Guizhou ¼ Puxing > Hongmao > Lindong. Figure 9 shows that maximum mean of gas pressure in Guizhou is Zhina area with the value of 1.08 MPa. And the maximum gas pressure in Guizhou appears in Tucheng Mine No. 13 coal seam of Panjiang mine area with the value of 6.30 MPa. The average gas pressure in the province is 0.72 MPa, where the regional prediction value of coal and gas outburst in Panjiang, Shuicheng, Zhina are all more than 0.74 MPa. The workable coal seam gas pressure of 35.24% of the whole province’s coal mines exceeds 0.74 MPa which is the critical value in China’s regulations. The isoline map of gas pressure in Guizhou is drawn by collecting 715 sets of measured value of coal seam gas pressure based on the gas geological map of Guizhou and the set program diagram of coal mine rights in Guizhou. It can be acquired from Figure 10 that the gas pressure in Guizhou is most affected by syncline structure, and the isolines are mostly around the tectonic belt and are affected by faults at the same time. Gas pressure varies greatly in some areas, and the influence of buried depth on gas pressure is also obvious according to the analysis on buried depth. Most of the recoverable coal seams are located under the mountain due to numerous mountains in Guizhou. The less exposed coal seams cause that gas pressure increases indistinctively with the buried depth, which is also related to the larger crustal stress and gas gradient in Guizhou. The distribution of gas pressure isoline in the whole province is uniform in general in addition to Lindong and Hongmao mining areas. As mentioned above, the central part is not only the coal-rich area but it is also the anthracite mine area on behalf of Zhina and northern Guizhou mine areas. The gas pressure in central region is not high, but it is small with uniform distribution compared to Liuzhi, Panjiang, Shuicheng mining areas. On the other hand, the lateral variation is not obvious.

Figure 9. The statistical chart of mean gas pressure in each mining area.

Li et al.

19

(2) Gas content law As shown in Table 4, the average gas content gradient of 100 m is Zhina > Panjiang > Liuzhi > northern Guizhou > Shuicheng > northwestern Guizhou > Puxing > Lindong. It can be seen from Figure 11 that the coal mine area with the largest gas content in Guizhou is northern Guizhou mining area with the value of 12.20 m3/t; the maximum value appears in No. 18 and No. 19 coal seams of Tushantian coal mine in Liuzhi with the value of 29.92 m3/t. The average gas content of the whole province is 11.23 m3/t. The regional prediction value of coal and gas outburst in the mining areas of Liuzhi, Panjiang, Shuicheng, Zhina, north Guizhou, northwest Guizhou is all more than 8 m3/t which is recommended by the “Provisions for the prevention and control of coal and gas outbursts.” The gas content in the minable seam of 75.90% of the whole province’s coal mines exceeds this critical value. This is also consistent with the law of large anthracite production of Guizhou, that is the gas pressure is relatively moderate and the gas content is generally higher. As Figure 12 shows, the isoline map of gas content in Guizhou is drawn by collecting 307 sets of measured values of coal seam gas content based on the research work of gas geology in Guizhou.

Figure 10. Isoline map of gas pressure in Guizhou.

20


Figure 11. Statistical chart of gas content in each mining areas.

Figure 12. Isoline map of gas content in Guizhou.

It can be acquired from Figure 12 that the gas content in Guizhou is relatively high, and the main external factors of the poor permeability of axial structure of syncline and overlying strata are all favorable for gas sealing. Though the mining depth is relatively shallow, the gas content is still over 8 m3/t at this stage. This is extremely unfavorable for the safe mining of coal mines. As shown in Figure 6, the gas content in the middle anthraciteproducing area is the highest in the whole province and then gradually reduces to the east

Li et al.

21

Figure 13. Corresponding relation diagram of measured gas pressure and gas content of Guizhou.

Figure 14. Langmuir regression of gas pressure and gas content in Guizhou.

and west. The gas content of anthracite area varies greatly where exactly is the geological tectonic belt. That is, the degree of gas dispersion is larger than the small surrounding areas. This also demonstrates that the anthracite with high degree of metamorphism has the highest impact on gas content in the case of little difference in external environment (Cao et al., 2000). (3) Relationship between gas pressures with content The gas content in the anthracite-producing area in Guizhou is far above the national average with the gas pressure within normal range which brings challenges to coal mining of Guizhou and proposes higher requirements for forecasting and warning of coal and gas outburst. The collected measured gas pressure and gases content as well as their reciprocal values of typical coal mines in Guizhou are plotted and the corresponding relationships are shown in Figures 13 and 14. As Figure 13 shows, the corresponding gas content is between 9.4 and 17.9 m3/t when the gas pressure of Guizhou is 0.74 MPa. The content of combustible gas adsorbed of gas and gas pressure obeys the Langmuir equation, and the free gas content of coal is linear with gas pressure. According to the calculated regression analysis, the gas pressure of 0.74 MPa corresponds to gas content of 9.6 m3/t. On the whole, the gas pressure and gas content of coal seam in Guizhou are basically consistent with the Langmuir adsorption model with the

22


correlation coefficient of 70%. But the gas content is generally higher than the national average level in case of same gas pressure. It mainly lies in most anthracite mining for Guizhou and the high degree of metamorphism makes the measured gas pressure value higher than the corresponding value measured by gas pressure. The mechanism research on coal and gas outburst should be strengthened in future work, especially the research on the theoretical basis of W–P correlation. The reasonable index critical value that can satisfy the regional prediction of coal mines in Guizhou will be found out. The forecasting and warning system for coal and gas outburst in line with Guizhou characteristics will be established so as to guide the control and prevention work for coal mine gas of Guizhou.

Characteristics and law of coal and gas outburst in Guizhou The gas geological law is the key to predict the regional risk of coal and gas outburst as well as the gas emission amount (Noack, 1998). According to the relevant regulations and specifications of “Provisions for the prevention and control of coal and gas outbursts,” the determinant index of prediction of regional risk of coal and gas outburst contains initial velocity of gas emission (Dp), types of coal failure, consistent coefficient (f), and gas pressure of coal seam (P). The initial velocity of gas emission and the gas pressure are the characterization values of outburst dynamic factors. The greater the value is, the greater the outburst risk will be. And failure types and consistent coefficient can reflect the physical properties of the coal mass, which is the characterization value of the resistance factor that outburst occurs. The greater the value is, then the smaller the outburst danger will be (Lama and Bodziony, 1998). The critical values and exceeding standard situations of regional prediction indexes for coal and gas outburst in each mining area of Guizhou are summarized in Table 7. As Table 7 shows, the four indexes of risk prediction in outburst area of Guizhou coal mine are seriously exceeding the standard, in which 35.24% of workable coal seam gas pressure exceeds 0.74 MPa and 75.90% of workable coal seam gas content is more than 8 m3/t. The variation law of gas pressure and content is special in the anthracite mining areas with high metamorphic degree of Zhina and northern Guizhou, etc., and there is a Table 7. The proportion of exceeding standard of outburst prediction index in each mining area. Index

Dp

f

Failure mode

P

Critical value

10 cm3/s

0.5

III, IV, V

0.74 MPa

Liuzhi Panjiang Watertown Zhina North Guizhou Northwest Guizhou Puxing Lindong Hongmao Total

90.54% 85.71% 87.96% 99.11% 98.77% 94.12% 100.00% 93.10% 66.67% 93.61%

75.34% 80.52% 86.32% 69.09% 75.63% 73.53% 71.11% 89.66% 83.33% 77.13%

87.10% 87.32% 85.06% 70.59% 74.07% 75.68% 68.18% 93.33% 91.67% 78.52%

32.89% 41.56% 47.93% 53.33% 30.91% 22.86% 13.04% 0.00% 0.00% 35.24%

Li et al.

23

great difference compared to other mining areas. According to the rules and regulations, the coal seam can be determined with outburst risk and the seam as outburst coal seam when all four indexes reach or exceed the critical conditions. When calculating based on gas pressure, 19.92% of the coal seams in the whole province have outburst risk. When calculating based on gas content, 43.03% of the coal seams in the whole province have outburst risk, and the difference almost doubles. This also illustrates that the occurrence of coal seam and gas in Guizhou is complex, which brings great difficulty to safe production. And the applicability of index value of current outburst regional prediction is not efficient due to the especially geological condition of Guizhou. The suitable and comprehensive index system for the characteristics of coal and gas occurrence of Guizhou can be established by strengthening the gas geological study and quantifying the weight of outburst indexes combined with dynamic deformation characteristics of gas geological map and composite index BP.

Conclusions and suggestions (1) The geological law of coal seam distribution of Guizhou is systematically studied and the general laws of coal-bearing stratum are divided into geological action law, coal seam distribution law, and coal mass characteristic law. General laws such as the occurrence of coal seam with obviously lateral region property, concentrated longitudinal positions, the thin seam mining of close range with large anthracite content, and dip angle are illustrated through the comparative analysis on main coal-bearing tectonic units. The low permeability and soft coal characteristics in Guizhou are explained by intraregional comparisons and analysis on characteristic index values. (2) The influence of geological evolution history in Guizhou on the gas generation, storage, transportation, and cover of coal measures strata is analyzed in this work. The main factors affecting gas occurrence in Guizhou are the geological and physical characteristics of coal mass. The causes and gas controlling law of gas accumulation such as geological structure, crustal stress, burial depth, and metamorphic degree are analyzed and their impact degrees are weighed. We also conclude that the main factors determining gas enrichment of coal mine area of Guizhou are the sedimentary environment and the development situation of syncline structure. The coal mass factors such as gas production during metamorphism and the influence of coal thickness on gas emission are the secondary factors. (3) Distribution laws of gas pressure and content in Guizhou are analyzed combining with the occurrence characteristics of coal seam. The gas pressure reckoned by gas content is not suitable as the prediction index when predicting the regional risk of coal and gas outburst in anthracite mine area. And the Langmuir adsorption model does not fit the adsorption characteristics of anthracite. (4) Comparing the critical value of risk prediction index with the linear regression of measured parameters values in Guizhou, more than 77% of the initial velocity of gas emission, the consistent coefficient, the failure modes, and the gas content are beyond critical value. More than 35% of gas pressure are beyond critical value. The eigenvalue law is remarkable. The crustal stress and gas content increase especially when increasing the mining depth, which brings adverse effect to coal mining. In order to improve prevention measures and avoid unnecessary losses of labor, materials, and finance, the investment of forecast is supposed to enhance in future work and the index prediction system which is suitable for mining conditions of special coal mines in Guizhou should be established.

24


Declaration of conflicting interests The author(s) declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

Funding The author(s) disclosed receipt of the following financial support for the research, authorship, and/or publication of this article: This work is supported by the Guizhou Science and Technology Found ([2016]1083).

Reference Aguado BD and Nicieza CG (2007) Control and prevention of gas outbursts in coal mines, Riosa– Olloniego coalfield, Spain. International Journal of Coal Geology 69(4): 253–266. Beamish BB and Crosdale PJ (1998) Instantaneous outbursts in underground coal mines: An overview and association with coal type. International Journal of Coal Geology 35(1): 27–55. Bibler CJ, Marshall JS and Pilcher RC (1998) Status of worldwide coal mine methane emissions and use. International Journal of Coal Geology 35(1): 283–310. Bustin RM and Clarkson CR (1998) Geological controls on coalbed methane reservoir capacity and gas content. International Journal of Coal Geology 38(1): 3–26. Cai Y, Liu D, Yao Y, et al. (2011) Geological controls on prediction of coalbed methane of No. 3 coal seam in Southern Qinshui Basin, North China. International Journal of Coal Geology 88(2): 101–112. Cao Y, He D and Glick DC (2001) Coal and gas outbursts in footwalls of reverse faults. International Journal of Coal Geology 48(1): 47–63. Cao Y, Mitchell GD, Davis A, et al. (2000) Deformation metamorphism of bituminous and anthracite coals from China. International Journal of Coal Geology 43(1): 227–242. Chen KP (2011) A new mechanistic model for prediction of instantaneous coal outbursts – Dedicated to the memory of Prof. Daniel D. Joseph. International Journal of Coal Geology 87(2): 72–79. Cheng Y, Wang L and Zhang X (2011) Environmental impact of coal mine methane emissions and responding strategies in China. International Journal of Greenhouse Gas Control 5(1): 157–166. Creedy DP (1988) Geological controls on the formation and distribution of gas in British coal measure strata. International Journal of Coal Geology 10(1): 1–31. Fisne A and Esen O (2014) Coal and gas outburst hazard in Zonguldak Coal Basin of Turkey, and association with geological parameters. Natural Hazards 74(3): 1363. Flores RM (1998) Coalbed methane: From hazard to resource. International Journal of Coal Geology 35(1): 3–26. Frodsham K and Gayer RA (1999) The impact of tectonic deformation upon coal seams in the South Wales coalfield, UK. International Journal of Coal Geology 38(3): 297–332. Hou Q, Li H and Fan J (2012) Structure and coalbed methane occurrence in tectonically deformed coals. Science China Earth Sciences 55(11): 1755–1763. Jia T, Wang Y, Yan J, et al. (2014) Control law and zonation division of coalmine gas occurrence structure in Guizhou. Earth Science Frontiers 21(6): 281–288. Kaiser WR, Hamilton DS, Scott AR, et al. (1994) Geological and hydrological controls on the producibility of coalbed methane. Journal of the Geological Society of London 151: 417–420. € Ruiz FA, Cote` M, et al. (2011) Coal mine methane: A review of capture and utilization Karacan CO, practices with benefits to mining safety and to greenhouse gas reduction. International Journal of Coal Geology 86(2): 121–156. Lama RD and Bodziony J (1998) Management of outburst in underground coal mines. International Journal of Coal Geology 35(1): 83–115.

Li et al.

25

Li H (2001) Major and minor structural features of a bedding shear zone along a coal seam and related gas outburst, Pingdingshan coalfield, northern China. International Journal of Coal Geology 47(2): 101–113. Li H and Ogawa Y (2001) Pore structure of sheared coals and related coalbed methane. Environmental Geology 40(11): 1455–1461. Lu T, Zhao Z and Hu H (2011) Improving the gate road development rate and reducing outburst occurrences using the waterjet technique in high gas content outburst-prone soft coal seam. International Journal of Rock Mechanics and Mining Sciences 48(8): 1271–1282. Lunarzewski LW (1998) Gas emission prediction and recovery in underground coal mines. International Journal of Coal Geology 35(1): 117–145. Moore TA (2012) Coalbed methane: A review. International Journal of Coal Geology 101(6): 36–81. Noack K (1998) Control of gas emissions in underground coal mines. International Journal of Coal Geology 35(1): 57–82. Pashin JC (1998) Stratigraphy and structure of coalbed methane reservoirs in the United States: An overview. International Journal of Coal Geology 35(1): 209–240. Scott AR (2002) Hydrogeologic factors affecting gas content distribution in coal beds. International Journal of Coal Geology 50: 363–387. Shepherd J, Rixon LK and Griffiths L (1981) Outbursts and geological structures in coal mines: A review. International Journal of Rock Mechanics and Mining Sciences & Geomechanics Abstracts 18(4): 267–283. Su X, Lin X, Liu S, et al. (2005) Geology of coalbed methane reservoirs in the Southeast Qinshui Basin of China. International Journal of Coal Geology 62(4): 197–210. Xu T, Tang C, Yang T, et al. (2006) Numerical investigation of coal and gas outbursts in underground collieries. International Journal of Rock Mechanics and Mining Sciences 43(6): 905–919. Xue S, Wang Y, Xie J, et al. (2011) A coupled approach to simulate initiation of outbursts of coal and gas-model development. International Journal of Coal Geology 86(2): 222–230. Yang T, Xu T, Liu H, et al. (2011) Stress–damage–flow coupling model and its application to pressure relief coal bed methane in deep coal seam. International Journal of Coal Geology 86(4): 357–366. Zhou H, Yang Q, Cheng Y, et al. (2014) Methane drainage and utilization in coal mines with strong coal and gas outburst dangers: A case study in Luling mine, China. Journal of Natural Gas Science and Engineering 20: 357–365.