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The impact of faults on the occurrence of coal bed methane in Renlou coal mine, Huaibei coalfield, China
Journal of Natural Gas Science and Engineering 17 (2014) 151e158
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Journal of Natural Gas Science and Engineering journal homepage: www.elsevier.com/locate/jngse
The impact of faults on the occurrence of coal bed methane in Renlou coal mine, Huaibei coalfield, China Pinkun Guo a, b, Yuanping Cheng a, b, *, Kan Jin a, b, Yiping Liu c a
School of Safety Engineering, China University of Mining & Technology, Xuzhou 221116, China National Engineering Research Center for Coal & Gas Control, China University of Mining & Technology, Xuzhou 221116, China c Renlou Coal Mine, Anhui Hengyuan Coal-Electricity Group Co., Ltd., Suzhou 234000, China b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 25 July 2013 Accepted 13 December 2013 Available online 22 February 2014
The fault has an important impact on coal bed methane (CBM) occurrence. The complexity of the fault exacerbates the variability of CBM occurrence, which increases the difficulty of mine gas prevention and thus threats to mining safety seriously. The coal-bearing strata in the Renlou coal mine located in the Linhuan mining area of the Huaibei coalfield have undergone three transformations caused by major tectonic movements since its formation. In addition, the large-angle tectonic stress superposition leads to structure characteristic and mechanical properties of faults transform in the area or just make it become multiplicity. The normal faults F3 and F7 and reverse fault F5 in southern Renlou coal mine are open structures. The coal seam between faults F7 and F5 is in the emission range of the two faults (partially in the superimposed range) in which a significant amount of gas escapes. The formation between faults F5 and F7 is horst, and it turns from NS to EW. The stress in the formation is large, which results in the presence of numerous fractures and cleats. Therefore, there is difference of methane occurrence between the two regions. Between faults F7 and F5, the gas pressure gradient of coal seam No. 72 is 0.00356 MPa/m with a maximum value of 0.39 MPa (633.5 m). However, the gas pressure gradient of coal seam No. 72 between faults F3 and F2 is 0.00726 MPa/m and the maximum measured gas pressure is 1.7 MPa (692 m). The relative methane emission (RME) of working face 7257 located between faults F7 and F5 was 2.01 m3/t on average with a maximum value of 4.96 m3/t. The average RME of working face __7211 located between faults F3 and F2 reached 12 m3/t in the area of 0e600 m away from F3 fault and increased to 30 m3/t in the region of >600 m away from fault F3. Ó 2013 Elsevier B.V. All rights reserved.
Keywords: Fault CBM occurrence Gas pressure gradient Relative methane emission
1. Introduction Coal bed methane (CBM) is one of the coalification products during the coal-forming process as high as 458 m3/t for anthracite (Dai et al., 2001; Han et al., 2013; Yu, 1992). Most of the methane produced in the coal-forming process has been escaped in the long geological ages (Yu, 1992). The methane present in the current coal bed is only a small fraction of the produced methane. The CBM content depends on the condition of methane migration to the surface and storage capacity of coal. Factors affecting CBM content are primarily the coal metamorphic grade, burial depth, outcrops, geological structure, etc (Cheng et al., 2010; Wu et al., 2012, 2010;
* Corresponding author. National Engineering Research Center for Coal & Gas Control, China University of Mining & Technology, Xuzhou 221116, China. Tel.: þ86 516 83995759; fax: þ86 516 83995097. E-mail addresses: [email protected] (P. Guo), [email protected] (Y. Cheng). http://dx.doi.org/10.1016/j.jngse.2013.12.003 1875-5100/Ó 2013 Elsevier B.V. All rights reserved.
Yu, 1992; Zhao et al., 2012). And the fault has an important impact on the CBM occurrence especially. Methane is a major threat to coal mine safety. The coal mine gas control methods are based on the CBM occurrence seriously. As a result, the complexity of a fault exacerbates the variability of CBM occurrence, which increases the difficulty of mine gas prevention by making it difficult to implement coal mine gas control measures properly. Yu (1992) summarized the impact of faults on the CBM occurrence based on the opening or closure of the fault and the permeability of the fault wall in contact with the coal seam. The CBM content of a coal seam near an open fault exhibits a low value, which becomes even lower when the rock permeability of the fault wall incontact with the coal seam is high. A closed fault leads to high CBM content when the rock permeability of the fault wall in contact with the coal seam is low. Creedy (1988) investigated that a higher CBM content in the West Pennine coalfields compared with those in the East Pennine coalfields is most likely partially due to the higher frequency of large faults in the west, undoubtedly
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Nomenclature P H CBM RME
the gas pressure [MPa] the buried depth [m] coal bed methane relative methane emission [m3/t]
providing gas migration channels over geological time, effectively allowing cross-measures movement of gas. Su et al. (2003) found that there is an under-pressure area in the AnjianshaneWupu area, Eastern Ordos Basin in China. This lower pressure is due to the influence of the F1, F2, and F3 fault zones which the CBM escaped along. Wei and Han (2003) investigated that the CBM content in the west is lower than that in the middle even although the west is deeper than the middle in the Daning, Fanzhuang and Panzhuang mining areas of the southern Qinshui coalfield, China. The lower CBM content is the result of the CBM having escaped along the Shitou fault and its associated faults in the west of this area. In the Deerlick Creek field of the southern Appalachian Black Warrior foreland basin in USA, the half-graben region between two normal faults has a low level of CBM production because of the presence of well-developed open fractures along which methane has escaped (Groshong Jr. et al., 2009). In the Huaibei coalfield in China, the CBM occurrence is affected by faults (Jiang et al., 2009). For example, the CBM content is typically low in the Linhuan mining area of the Huaibei coalfield due to well-developed tensile and brittle normal faults that destroys the CBM storage conditions seriously (Jiang et al., 2010). However, the coal-bearing strata have undergone many transformations caused by major tectonic movements since its formation (Chu et al., 2008; Han, 1990; Song and Duan, 1999; Wan, 1993; Zhang and Guo, 2009). Tectonic stress fields of different nature have changed the characteristics of the fault (Jiang et al., 2005; Wang et al., 2011), thereby leading to different storage conditions in different geologic periods. As a result, the impact of faults on the CBM occurrence becomes more complex. Therefore, we have to investigate not only the current properties of the fault but also the formative period and the subsequently changing properties when analyzing the impact of faults on the CBM occurrence. In this article, we present the investigation of the changing properties of faults since their formation and the gas pressure of coal seam No. 72 combined with the relative methane emission (RME) of working faces in Renlou coal mine to determine the impact of faults on the occurrence of CBM. 2. Geological backgrounds The Renlou coal mine is located in the Linhuan mining area of the Huaibei coalfield in eastern China, as shown in Fig. 1. 2.1. Regional tectonics Geographically, the Huaibei Coalfield is located in the central and southern area of the XueSu arcuate thrust nappe structure, east of the Henan-Huai depression at the southeastern margin of North China ancient plate (Jiang et al., 2010; Zheng et al., 2008). The coalfield lies adjacent to He-Huai subsidence area on the west, to the Tan-Lu fault zone on the east, to the nearly EW-trending FengPei on the north and to Bengbu uplifts on the south as shown in Fig. 1 (Jiang et al., 2010; Wang et al., 1992). The regional basic geological structure of Huaibei Coalfield is determined by the plate
margin active zone at the south and east sides. The folds and faults trending near NS controlled by the Tanlu fault superimpose and cut the early period structure trending EW. And it forms many near netlike fracture-block style uplift-depression tectonic systems. Low-order structures trending NW and NE, primarily trending NE, are located in blocks. With the formation and development of Xue Su arcuate thrust nappe structure, a series of the thrust imbricate fan faults thrusting from SEE to NWW accompanied by a set of supine, skew and closed linear folds are formed, which is made more complicated by the later effect of rifting, gravity sliding and compression. The XueSu arcuate thrust nappe structure is located between Feihuanghe fault in the north and North Suzhou fault in the south and is divided into three parts from N to S (i.e., north, middle and south parts), that is, NE-trending fold belt in the north, arcuate fold-fault belt in the middle and NW-trending fold belt in the south, due to Bulaohe and North Suzhou faults trending near EW. Each part of them exhibits unique structural features (Wang et al., 1998). As a result of the tectonic influence during different geologic periods and the various directions with different types and strength, there are many near netlike fracture-block structures, namely, EW-trending faults and NNE-trending faults crisscross in Huaibei Coalfield. The Huaibei Coalfield is divided into two parts (i.e. north and south parts) by the North Suzhou fault. The Linhuan mining area is located in the south part, between Feng-Guo fault in the west and Nanping syncline in the east. There are well developed normal faults in the Linhuan mining area, which indicates that this area has exposed to an extension tectonic stress field. The depositions of coal seams in Huaibei coalfield has been strongly reformed by tectonic activities during different geological periods and various directions with different types and strength, which significantly impacted the generation, migration, enrichment and preservation of the CBM in these coal seams (Wu et al., 2011). Thus, the CBM occurrence largely varies from one mining area to the other, resulting in distinct differences of the CBM occurrence in different mining areas. The tectonic deformation after coal deposition may be a key factor that determines the physical properties of the coal reservoirs and the enrichment of the CBM in the Huaibei coalfield. 2.2. Mine structure The Renlou coal mine is located in the eastern Linhuan mining area (Fig. 1). The mine lies adjacent to the Sutuan coal mine bounded by the Jiegou fault on the north and the Xutuan coal mine bounded by the fault F8 on the south. The Renlou coal mine is located in the southeast and south limbs of the Tongting anticline. Therefore, the mine appears as a monoclonal structure sloping to the east in the area north of fault F3. However, the strike direction of formation in the area west of fault F3 gradually changes to NWW (Fig. 2). There are 35 faults of throw larger than 5 m in the Renlou coal mine and they are developed directionally and regionally, as shown in Table 1. In the Renlou coal mine, the reverse faults trend both near EW (i.e. fault F5) and NNE (i.e. fault F21). The normal faults distributed in the NW and NNW directions (i.e. faults F3 and F4) are tensile-shear, but the ones for which the distribution lines horizontally exhibit shapes of “S” and arcs with the orientations along NE and NNE (i.e. faults F13, F1, F2 and F11) are compressive-shear after tensile. There are also tensile after compressive faults trending NNW (i.e. fault F16) and parallel faults with compressive and tensile coexistence (i.e. faults FX2 and FX3). The characteristics of faults indicate that the coal-bearing strata have undergone transformation caused by major tectonic
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Fig. 1. Regional tectonic sketch of the HuaiBei coalfield (Jiang et al., 2010; Wang et al., 1992). The red rectangle represents the location of the Renlou coal mine. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
movements three times since their formation. Additionally, the large-angle tectonic stress superposition leads to structure characteristic and mechanical properties of faults transform in the area or just make it become multiplicity. 2.3. Coal-bearing strata The Carboniferous and Permian strata are the coal-bearing strata in the Renlou coal mine. The Carboniferous contains seven
coal seams with an average total thickness of 3.36 m; however, these coal seams are unminable. The Carboniferous contains 10 coal seams with an average total thickness of 14.88 m (Fig. 3). The Nos. 31, 51, 72, and 82 coal seams are the primary minable coal seams, of which the average total thickness is 7.52 m. The Nos. 52 and 73 coal seams are minable, of which the average total thickness is 3.00 m. The other coal seams are unminable. For this article, the main object of the study is focused on the coal seam No. 72.
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Fig. 2. Tectonic sketch, the location of gas pressure and working faces of the Renlou coal mine.
3. CBM occurrences In this article, we make the statistics of the gas pressure of coal seam No. 72 and the relative methane emission of working faces 7257 and __7211 to study the CBM occurrence in two gas geological units (i.e., the region between faults F5 and F7 and the region between faults F3 and F2). 3.1. Gas pressure The gas pressure of coal seam No. 72 was measured by the direct measurement method during mining. The method involves the process of exposing coal seams by drilling, the installation of measurement instrumentation and the borehole sealing, and measurement of the gas equilibrium pressure at the place of exposure using the natural infiltration principle of gas. The measured gas pressures were collected presented in Table 2 and Fig. 4. There are many factors such as boreholes sealing quality that affect the measured gas pressure which could lead to a deviation from actual values. The sealing length of the boreholes is longer than 10 m in practice. Therefore, the measured data do not possess basic conditions for regression methods. Based on a statistical analysis of actual measured results of coal-seam gas pressure in a Table 1 Summary statistics of the faults of throw larger than 5 m in the Renlou coal mine. Nature
Normal faults
Reverse faults
Total
Orientation
NNE NE NEE NNW NW EW Sub-total NNE SN NWW Sub-total
Fault throws (m)
Total
5 < H 30 m
30 < H 100 m
Fx1, FD52, FD48, FD47 F13, F1, H23
F11 F2-1, F14
F19, FX2, F16, F15, F160
F4 F7, F7-1
F3
6
3
F8 1 7
F5 1 4
F22, FX4, Fx6, FX7, FX8, FX8-1 18 F21, FD50 FX3 F5-1, F18, F17 6 24
H > 100 m
F2 JieGou
5 6 1 6 3 6 27 2 1 5 8 35
same geological section, the safety line method was developed by Wang et al. (2012). When predicting the gas pressure, two symbol measured points are selected to make a line and the other measured points should be below the line. The variation of the gas pressure of coal seam No. 72 with buried depth was analyzed by the safety line method (Wang et al., 2012). In the region between faults F7 and F5, the gas pressures Nos. 3 and 4 in Table 2 were selected as the symbol points to calculate the gas pressure gradient which was determined to be 0.00356 MPa/m. The gas pressure of coal seam No. 72 in the region between F5 and F7 faults can be calculated using Eq. (1)
P ¼ 0:00356 H 1:861
(1)
where P is the gas pressure, MPa; H is the buried depth, m. In the region between faults F3 and F2, the gas pressures Nos. 12 and 17 in Table 2 were chosen as the symbol points to calculate the gas pressure gradient the value of which was 0.00726 MPa/m. The gas pressure of coal seam No. 72 in the region between F3 and F2 faults can be calculated using Eq. (2)
P ¼ 0:00726 H 3:324:
(2)
Obviously, there is a considerable difference between the CBM occurrences in the two regions. Between faults F7 and F5, the gas pressure gradient of coal seam No. 72 is 0.00356 MPa/m, and the maximum measured gas pressure reaches 0.39 MPa (633.5 m). However, the gas pressure gradient of coal seam No. 72 between faults F3 and F2 is 0.00726 MPa/m and the maximum measured gas pressure is 1.7 MPa (692 m).
3.2. Relative methane emission The relative methane emission (RME) is the amount of methane emission during the production of a ton coal per day on average, in m3/t (Cheng et al., 2010; Yu, 1992). The RME is primarily affected by the CBM content and other factors, such as the mining sequence and methods. The variation of the RME, to a large extent, could indicate the variation of the CBM content. Therefore, we investigate the RME of two working faces (working faces 7257 and __7211) to obtain the impact of faults on the CBM occurrence in the Renlou coal mine. The locations of the two working faces are shown in Fig. 2.
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Fig. 3. Stratigraphic columns of the coal-bearing strata in the Renlou mining field.
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Table 2 Gas pressure statistics of the coal seam No. 72. The gas pressures Nos. 1 to 4 of coal seam No. 72 were located between faults F7 and F5; and the Nos. 5 to 17 were located between faults F3 and F2. Number
Location
Sealing length (m)
Buried depth (m)
Gas pressure (MPa)
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17
Outburst zoning roadway in Fifth district Outburst zoning roadway in Fifth district Outburst zoning roadway in Fifth district 7257 Return airway Fourth station of __1 district __1 district bottom station H8 point of in __2 district 50 m before H9 point of return-air dip in __2 district 720 m return airway of Sixth district 7218 Sump in First district 5 m after Y9 point of 7245 crosscut 13 m before three-way door of South roadway in First district L2 point nearby sump of bottom station in __2 district Middle station in __1 district West ingate auxiliary shaft West ingate auxiliary shaft 720 m south roadway
50 37 67 33 18 18 19 19 19 14.6 14.6 14.6 20 12 16 14.2 60
633.4 617.6 633.4 591.2 681 708 597 616 678 571.3 522.2 513 608.4 594.3 772.5 765.2 692
0.14 0.12 0.39 0.24 0.90 0.97 0.70 0.74 0.89 0.32 0.36 0.4 1 0.45 0.615 0.617 1.7
Ventilation and drainage are the main technical measures to control the CBM releasing from the coal threat to the safety in underground coal mining (Cheng et al., 2010; Yu, 1992). The RME consists of two parts: one is the amount of methane exhausting through ventilation; the other is the drainage amount. The RME variations of two working faces are analyzed statistically as shown in Fig. 5 and Fig. 6. The RME of working face __7211 remains stable, with an average of 12 m3/t, in the area of 0e600 m away from fault F3. However, the RME increases to 30m3/t on average in the region of >600 m away from fault F3, which indicates a higher methane content region. The RME of working face 7257 is 2.01 m3/t on average, with a maximum value of 4.96 m3/t, which is much smaller than the one of working face __7211. Hence the CBM of coal seam No. 72 between faults F7 and F5 is less than the one between faults F3 and F2, which is consistent with the pressure occurrence in the two regions. 4. Discussions 4.1. Tectonic evolution and permeability The coal-bearing strata have undergone transformations caused by major tectonic movements three times since its formation (Chu et al., 2008; Han, 1990; Song and Duan, 1999; Wan, 1993; Zhang and
Fig. 4. Relationship between the gas pressure of the coal seam No.72 and the buried depth in the Renlou coal mine. The lines in this figure are the gas pressure predicted by the safety line method.
Remark
Symbol point Symbol point
Symbol point
Symbol point
Guo, 2009). Additionally, the large-angle tectonic stress superposition leads to structure characteristic and mechanical properties of faults in the area transform or become multiplicity. The early tectonic movement is the middle and late Indosinian tectonic movement (Chu et al., 2008; Han, 1990; Song and Duan, 1999; Wan, 1993; Zhang and Guo, 2009). The principal compressive stress direction of early tectonic movement was near NS and the direction of principal tensile stress was near EW. As a result, the reverse faults F5 and F8 trending near EW was formed, which was compressive. The near NS-trending tensile normal fault F16, the NEtrending compressive-shear faults F2, F14, F2-1, and F31, and the NWtrending tensile-shear faults F3 and F4 were also formed. The middle tectonic movement was developed in the early and middle period of Yanshan movement (Han, 1990; Song and Duan, 1999; Wan, 1993). And the principal compressive stress direction of this movement was near EW and the direction of principal tensile stress was near NS. The horizontal distribution and mechanical properties of the structures formed by the early tectonic movement were changed during this period. For example, the NWW-trending faults (i.e. faults F7 and F5) changed from compressive to tensileshear, and the fault F16 changed from tensile to compressiveshear. The distribution lines of the faults formed by the early tectonic movement horizontally exhibited shapes of ‘S’ or arcs due to the directions of the two tectonic movements that turned 100 counterclockwise (Song and Duan, 1999). The late tectonic movement was developed during Himalayan period (corresponding to NeogeneeEarly Pleistocene, Wan, 2011 and Wu et al., 2011) which changed the properties of faults formed earlier (Han, 1990; Wan, 1993, 2011). The near EW-trending large faults were compressed and sheared left-laterally and the NNE-trending faults were compressed and sheared right-laterally. The faults are mostly distributed in the NE, NW and near EW directions, with characteristics of long term development and changeable properties. The faults primarily appear as normal faults since the Himalayan period (Jiang et al., 2010; Ju and Wang, 2002). The large angle superposition of three times tectonic stress changes structures and mechanical properties of faults in the area. The NNW-SN-NE striking faults are compressive, shear or compressive following tensile and they are impermeable, gas barrier structures that provide good sealing conditions for CBM. The NEE-EW-NWW-NW striking faults, especially normal faults with high dip angles, are tensile-shear, tensile or tensile following compressive and they are open structures that enable gas to escape easily. The normal faults F3 and F7 and the reverse fault F5 in southern Renlou coal mine are open structure.
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Fig. 7. Schematic cross section of the horsts and grabens formed by faults in Renlou coal mine along line AeA0 in Fig. 1.
Fig. 5. RME variation of working face __7211 with distance from fault F3.
4.2. Coal bed methane occurrence The RME of working face __7211 increased from 12 m3/t to 30 m3/t on average in 600 m away from F3 fault. Therefore, we could obtain that the influence distance of fault F3 is 600 m. We map the fault emission regions based on a 600 m distance of fault emission, as shown in Fig. 2. The coal seam is in ranges of fault emission basically and partly in the overlay area. The emission of faults F5 and F7 interacting each other makes the CBM content or pressure less than the ones between faults F3 and F2. Furthermore, the horst between faults F7 and F5 with numerous open fractures also enhances the methane emission (Groshong Jr. et al., 2009), as shown in Fig. 7. So, there is difference of CBM occurrence in the two regions. The RME of working face 7257 located in the region between faults F7 and F5 is lower than that of the working face __7211 located in the region between faults F3 and F2. The measured gas pressure between faults F7 and F5 is lower than that between faults F3 and F2 at the same depth. The gas pressure gradient in the region between faults F7 and F5 is less than that in the other region. 5. Conclusions 1) The coal-bearing strata have undergone transformation caused by major tectonic movements three times since its formation. In
addition, the large-angle tectonic stress superposition leads to structure characteristic and mechanical properties of faults transform in the area or just make it become multiplicity. The NNW-SN-NE striking faults are compressive, shear or compressive following tensile and they are impermeable, gas barrier structures that provide good sealing conditions for CBM. The NEE-EW-NWW-NW striking faults, especially normal faults with high dip angles, are tensile-shear, tensile or tensile following compressive and they are open structures that enable gas to escape easily. 2) The normal faults F3 and F7 and reverse fault F5 in the southern Renlou coal mine are open structure. The coal seam between faults F7 and F5 is in the emission range of the two faults (partially in the superimposed range), in which a significant amount of gas escapes. 3) The formation between faults F5 and F7 is horst and turns from NS to EW. The stress in the formation is large, resulting in the presence of numerous fractures and cleats. Therefore, the CBM emission in the region is larger than that between faults F3 and F2. 4) Between faults F7 and F5, the gas pressure gradient of coal seam No. 72 is 0.00356 MPa/m and the maximum measured gas pressure reaches 0.39 MPa (633.5 m). However, the gas pressure gradient of coal seam No. 72 is 0.00726 MPa/m and the maximum measured gas pressure coal is 1.7 MPa (692 m) between faults F3 and F2. The average RME of working face __7211 located in the region between faults F3 and F2 is 12 m3/t in the area of 0e600 m away from F3 fault and increases to 30 m3/t in the region of >600 m away from fault F3. The RME of working face 7257 located in the region between faults F7 and F5 is 2.01 m3/t on average, with a maximum value of 4.96 m3/t, which is much smaller than that of working face __7211. Acknowledgment The authors are grateful for the support of the National Foundation of China (No. 51074160, No. 41202118, No. 51204173, No. 51004106, No. 51374204, No. 51304188), the National Basic Research Program of China (973 Program, No. 2011CB201204) and A Project Funded by the Priority Academic Program Development of Jiangsu Higher Education Institutions. References
Fig. 6. RME variation of working face 7257 during mining in the region between faults F7 and F5.
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Journal of Natural Gas Science and Engineering journal homepage: www.elsevier.com/locate/jngse
The impact of faults on the occurrence of coal bed methane in Renlou coal mine, Huaibei coalfield, China Pinkun Guo a, b, Yuanping Cheng a, b, *, Kan Jin a, b, Yiping Liu c a
School of Safety Engineering, China University of Mining & Technology, Xuzhou 221116, China National Engineering Research Center for Coal & Gas Control, China University of Mining & Technology, Xuzhou 221116, China c Renlou Coal Mine, Anhui Hengyuan Coal-Electricity Group Co., Ltd., Suzhou 234000, China b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 25 July 2013 Accepted 13 December 2013 Available online 22 February 2014
The fault has an important impact on coal bed methane (CBM) occurrence. The complexity of the fault exacerbates the variability of CBM occurrence, which increases the difficulty of mine gas prevention and thus threats to mining safety seriously. The coal-bearing strata in the Renlou coal mine located in the Linhuan mining area of the Huaibei coalfield have undergone three transformations caused by major tectonic movements since its formation. In addition, the large-angle tectonic stress superposition leads to structure characteristic and mechanical properties of faults transform in the area or just make it become multiplicity. The normal faults F3 and F7 and reverse fault F5 in southern Renlou coal mine are open structures. The coal seam between faults F7 and F5 is in the emission range of the two faults (partially in the superimposed range) in which a significant amount of gas escapes. The formation between faults F5 and F7 is horst, and it turns from NS to EW. The stress in the formation is large, which results in the presence of numerous fractures and cleats. Therefore, there is difference of methane occurrence between the two regions. Between faults F7 and F5, the gas pressure gradient of coal seam No. 72 is 0.00356 MPa/m with a maximum value of 0.39 MPa (633.5 m). However, the gas pressure gradient of coal seam No. 72 between faults F3 and F2 is 0.00726 MPa/m and the maximum measured gas pressure is 1.7 MPa (692 m). The relative methane emission (RME) of working face 7257 located between faults F7 and F5 was 2.01 m3/t on average with a maximum value of 4.96 m3/t. The average RME of working face __7211 located between faults F3 and F2 reached 12 m3/t in the area of 0e600 m away from F3 fault and increased to 30 m3/t in the region of >600 m away from fault F3. Ó 2013 Elsevier B.V. All rights reserved.
Keywords: Fault CBM occurrence Gas pressure gradient Relative methane emission
1. Introduction Coal bed methane (CBM) is one of the coalification products during the coal-forming process as high as 458 m3/t for anthracite (Dai et al., 2001; Han et al., 2013; Yu, 1992). Most of the methane produced in the coal-forming process has been escaped in the long geological ages (Yu, 1992). The methane present in the current coal bed is only a small fraction of the produced methane. The CBM content depends on the condition of methane migration to the surface and storage capacity of coal. Factors affecting CBM content are primarily the coal metamorphic grade, burial depth, outcrops, geological structure, etc (Cheng et al., 2010; Wu et al., 2012, 2010;
* Corresponding author. National Engineering Research Center for Coal & Gas Control, China University of Mining & Technology, Xuzhou 221116, China. Tel.: þ86 516 83995759; fax: þ86 516 83995097. E-mail addresses: [email protected] (P. Guo), [email protected] (Y. Cheng). http://dx.doi.org/10.1016/j.jngse.2013.12.003 1875-5100/Ó 2013 Elsevier B.V. All rights reserved.
Yu, 1992; Zhao et al., 2012). And the fault has an important impact on the CBM occurrence especially. Methane is a major threat to coal mine safety. The coal mine gas control methods are based on the CBM occurrence seriously. As a result, the complexity of a fault exacerbates the variability of CBM occurrence, which increases the difficulty of mine gas prevention by making it difficult to implement coal mine gas control measures properly. Yu (1992) summarized the impact of faults on the CBM occurrence based on the opening or closure of the fault and the permeability of the fault wall in contact with the coal seam. The CBM content of a coal seam near an open fault exhibits a low value, which becomes even lower when the rock permeability of the fault wall incontact with the coal seam is high. A closed fault leads to high CBM content when the rock permeability of the fault wall in contact with the coal seam is low. Creedy (1988) investigated that a higher CBM content in the West Pennine coalfields compared with those in the East Pennine coalfields is most likely partially due to the higher frequency of large faults in the west, undoubtedly
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Nomenclature P H CBM RME
the gas pressure [MPa] the buried depth [m] coal bed methane relative methane emission [m3/t]
providing gas migration channels over geological time, effectively allowing cross-measures movement of gas. Su et al. (2003) found that there is an under-pressure area in the AnjianshaneWupu area, Eastern Ordos Basin in China. This lower pressure is due to the influence of the F1, F2, and F3 fault zones which the CBM escaped along. Wei and Han (2003) investigated that the CBM content in the west is lower than that in the middle even although the west is deeper than the middle in the Daning, Fanzhuang and Panzhuang mining areas of the southern Qinshui coalfield, China. The lower CBM content is the result of the CBM having escaped along the Shitou fault and its associated faults in the west of this area. In the Deerlick Creek field of the southern Appalachian Black Warrior foreland basin in USA, the half-graben region between two normal faults has a low level of CBM production because of the presence of well-developed open fractures along which methane has escaped (Groshong Jr. et al., 2009). In the Huaibei coalfield in China, the CBM occurrence is affected by faults (Jiang et al., 2009). For example, the CBM content is typically low in the Linhuan mining area of the Huaibei coalfield due to well-developed tensile and brittle normal faults that destroys the CBM storage conditions seriously (Jiang et al., 2010). However, the coal-bearing strata have undergone many transformations caused by major tectonic movements since its formation (Chu et al., 2008; Han, 1990; Song and Duan, 1999; Wan, 1993; Zhang and Guo, 2009). Tectonic stress fields of different nature have changed the characteristics of the fault (Jiang et al., 2005; Wang et al., 2011), thereby leading to different storage conditions in different geologic periods. As a result, the impact of faults on the CBM occurrence becomes more complex. Therefore, we have to investigate not only the current properties of the fault but also the formative period and the subsequently changing properties when analyzing the impact of faults on the CBM occurrence. In this article, we present the investigation of the changing properties of faults since their formation and the gas pressure of coal seam No. 72 combined with the relative methane emission (RME) of working faces in Renlou coal mine to determine the impact of faults on the occurrence of CBM. 2. Geological backgrounds The Renlou coal mine is located in the Linhuan mining area of the Huaibei coalfield in eastern China, as shown in Fig. 1. 2.1. Regional tectonics Geographically, the Huaibei Coalfield is located in the central and southern area of the XueSu arcuate thrust nappe structure, east of the Henan-Huai depression at the southeastern margin of North China ancient plate (Jiang et al., 2010; Zheng et al., 2008). The coalfield lies adjacent to He-Huai subsidence area on the west, to the Tan-Lu fault zone on the east, to the nearly EW-trending FengPei on the north and to Bengbu uplifts on the south as shown in Fig. 1 (Jiang et al., 2010; Wang et al., 1992). The regional basic geological structure of Huaibei Coalfield is determined by the plate
margin active zone at the south and east sides. The folds and faults trending near NS controlled by the Tanlu fault superimpose and cut the early period structure trending EW. And it forms many near netlike fracture-block style uplift-depression tectonic systems. Low-order structures trending NW and NE, primarily trending NE, are located in blocks. With the formation and development of Xue Su arcuate thrust nappe structure, a series of the thrust imbricate fan faults thrusting from SEE to NWW accompanied by a set of supine, skew and closed linear folds are formed, which is made more complicated by the later effect of rifting, gravity sliding and compression. The XueSu arcuate thrust nappe structure is located between Feihuanghe fault in the north and North Suzhou fault in the south and is divided into three parts from N to S (i.e., north, middle and south parts), that is, NE-trending fold belt in the north, arcuate fold-fault belt in the middle and NW-trending fold belt in the south, due to Bulaohe and North Suzhou faults trending near EW. Each part of them exhibits unique structural features (Wang et al., 1998). As a result of the tectonic influence during different geologic periods and the various directions with different types and strength, there are many near netlike fracture-block structures, namely, EW-trending faults and NNE-trending faults crisscross in Huaibei Coalfield. The Huaibei Coalfield is divided into two parts (i.e. north and south parts) by the North Suzhou fault. The Linhuan mining area is located in the south part, between Feng-Guo fault in the west and Nanping syncline in the east. There are well developed normal faults in the Linhuan mining area, which indicates that this area has exposed to an extension tectonic stress field. The depositions of coal seams in Huaibei coalfield has been strongly reformed by tectonic activities during different geological periods and various directions with different types and strength, which significantly impacted the generation, migration, enrichment and preservation of the CBM in these coal seams (Wu et al., 2011). Thus, the CBM occurrence largely varies from one mining area to the other, resulting in distinct differences of the CBM occurrence in different mining areas. The tectonic deformation after coal deposition may be a key factor that determines the physical properties of the coal reservoirs and the enrichment of the CBM in the Huaibei coalfield. 2.2. Mine structure The Renlou coal mine is located in the eastern Linhuan mining area (Fig. 1). The mine lies adjacent to the Sutuan coal mine bounded by the Jiegou fault on the north and the Xutuan coal mine bounded by the fault F8 on the south. The Renlou coal mine is located in the southeast and south limbs of the Tongting anticline. Therefore, the mine appears as a monoclonal structure sloping to the east in the area north of fault F3. However, the strike direction of formation in the area west of fault F3 gradually changes to NWW (Fig. 2). There are 35 faults of throw larger than 5 m in the Renlou coal mine and they are developed directionally and regionally, as shown in Table 1. In the Renlou coal mine, the reverse faults trend both near EW (i.e. fault F5) and NNE (i.e. fault F21). The normal faults distributed in the NW and NNW directions (i.e. faults F3 and F4) are tensile-shear, but the ones for which the distribution lines horizontally exhibit shapes of “S” and arcs with the orientations along NE and NNE (i.e. faults F13, F1, F2 and F11) are compressive-shear after tensile. There are also tensile after compressive faults trending NNW (i.e. fault F16) and parallel faults with compressive and tensile coexistence (i.e. faults FX2 and FX3). The characteristics of faults indicate that the coal-bearing strata have undergone transformation caused by major tectonic
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Fig. 1. Regional tectonic sketch of the HuaiBei coalfield (Jiang et al., 2010; Wang et al., 1992). The red rectangle represents the location of the Renlou coal mine. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
movements three times since their formation. Additionally, the large-angle tectonic stress superposition leads to structure characteristic and mechanical properties of faults transform in the area or just make it become multiplicity. 2.3. Coal-bearing strata The Carboniferous and Permian strata are the coal-bearing strata in the Renlou coal mine. The Carboniferous contains seven
coal seams with an average total thickness of 3.36 m; however, these coal seams are unminable. The Carboniferous contains 10 coal seams with an average total thickness of 14.88 m (Fig. 3). The Nos. 31, 51, 72, and 82 coal seams are the primary minable coal seams, of which the average total thickness is 7.52 m. The Nos. 52 and 73 coal seams are minable, of which the average total thickness is 3.00 m. The other coal seams are unminable. For this article, the main object of the study is focused on the coal seam No. 72.
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Fig. 2. Tectonic sketch, the location of gas pressure and working faces of the Renlou coal mine.
3. CBM occurrences In this article, we make the statistics of the gas pressure of coal seam No. 72 and the relative methane emission of working faces 7257 and __7211 to study the CBM occurrence in two gas geological units (i.e., the region between faults F5 and F7 and the region between faults F3 and F2). 3.1. Gas pressure The gas pressure of coal seam No. 72 was measured by the direct measurement method during mining. The method involves the process of exposing coal seams by drilling, the installation of measurement instrumentation and the borehole sealing, and measurement of the gas equilibrium pressure at the place of exposure using the natural infiltration principle of gas. The measured gas pressures were collected presented in Table 2 and Fig. 4. There are many factors such as boreholes sealing quality that affect the measured gas pressure which could lead to a deviation from actual values. The sealing length of the boreholes is longer than 10 m in practice. Therefore, the measured data do not possess basic conditions for regression methods. Based on a statistical analysis of actual measured results of coal-seam gas pressure in a Table 1 Summary statistics of the faults of throw larger than 5 m in the Renlou coal mine. Nature
Normal faults
Reverse faults
Total
Orientation
NNE NE NEE NNW NW EW Sub-total NNE SN NWW Sub-total
Fault throws (m)
Total
5 < H 30 m
30 < H 100 m
Fx1, FD52, FD48, FD47 F13, F1, H23
F11 F2-1, F14
F19, FX2, F16, F15, F160
F4 F7, F7-1
F3
6
3
F8 1 7
F5 1 4
F22, FX4, Fx6, FX7, FX8, FX8-1 18 F21, FD50 FX3 F5-1, F18, F17 6 24
H > 100 m
F2 JieGou
5 6 1 6 3 6 27 2 1 5 8 35
same geological section, the safety line method was developed by Wang et al. (2012). When predicting the gas pressure, two symbol measured points are selected to make a line and the other measured points should be below the line. The variation of the gas pressure of coal seam No. 72 with buried depth was analyzed by the safety line method (Wang et al., 2012). In the region between faults F7 and F5, the gas pressures Nos. 3 and 4 in Table 2 were selected as the symbol points to calculate the gas pressure gradient which was determined to be 0.00356 MPa/m. The gas pressure of coal seam No. 72 in the region between F5 and F7 faults can be calculated using Eq. (1)
P ¼ 0:00356 H 1:861
(1)
where P is the gas pressure, MPa; H is the buried depth, m. In the region between faults F3 and F2, the gas pressures Nos. 12 and 17 in Table 2 were chosen as the symbol points to calculate the gas pressure gradient the value of which was 0.00726 MPa/m. The gas pressure of coal seam No. 72 in the region between F3 and F2 faults can be calculated using Eq. (2)
P ¼ 0:00726 H 3:324:
(2)
Obviously, there is a considerable difference between the CBM occurrences in the two regions. Between faults F7 and F5, the gas pressure gradient of coal seam No. 72 is 0.00356 MPa/m, and the maximum measured gas pressure reaches 0.39 MPa (633.5 m). However, the gas pressure gradient of coal seam No. 72 between faults F3 and F2 is 0.00726 MPa/m and the maximum measured gas pressure is 1.7 MPa (692 m).
3.2. Relative methane emission The relative methane emission (RME) is the amount of methane emission during the production of a ton coal per day on average, in m3/t (Cheng et al., 2010; Yu, 1992). The RME is primarily affected by the CBM content and other factors, such as the mining sequence and methods. The variation of the RME, to a large extent, could indicate the variation of the CBM content. Therefore, we investigate the RME of two working faces (working faces 7257 and __7211) to obtain the impact of faults on the CBM occurrence in the Renlou coal mine. The locations of the two working faces are shown in Fig. 2.
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Fig. 3. Stratigraphic columns of the coal-bearing strata in the Renlou mining field.
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Table 2 Gas pressure statistics of the coal seam No. 72. The gas pressures Nos. 1 to 4 of coal seam No. 72 were located between faults F7 and F5; and the Nos. 5 to 17 were located between faults F3 and F2. Number
Location
Sealing length (m)
Buried depth (m)
Gas pressure (MPa)
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17
Outburst zoning roadway in Fifth district Outburst zoning roadway in Fifth district Outburst zoning roadway in Fifth district 7257 Return airway Fourth station of __1 district __1 district bottom station H8 point of in __2 district 50 m before H9 point of return-air dip in __2 district 720 m return airway of Sixth district 7218 Sump in First district 5 m after Y9 point of 7245 crosscut 13 m before three-way door of South roadway in First district L2 point nearby sump of bottom station in __2 district Middle station in __1 district West ingate auxiliary shaft West ingate auxiliary shaft 720 m south roadway
50 37 67 33 18 18 19 19 19 14.6 14.6 14.6 20 12 16 14.2 60
633.4 617.6 633.4 591.2 681 708 597 616 678 571.3 522.2 513 608.4 594.3 772.5 765.2 692
0.14 0.12 0.39 0.24 0.90 0.97 0.70 0.74 0.89 0.32 0.36 0.4 1 0.45 0.615 0.617 1.7
Ventilation and drainage are the main technical measures to control the CBM releasing from the coal threat to the safety in underground coal mining (Cheng et al., 2010; Yu, 1992). The RME consists of two parts: one is the amount of methane exhausting through ventilation; the other is the drainage amount. The RME variations of two working faces are analyzed statistically as shown in Fig. 5 and Fig. 6. The RME of working face __7211 remains stable, with an average of 12 m3/t, in the area of 0e600 m away from fault F3. However, the RME increases to 30m3/t on average in the region of >600 m away from fault F3, which indicates a higher methane content region. The RME of working face 7257 is 2.01 m3/t on average, with a maximum value of 4.96 m3/t, which is much smaller than the one of working face __7211. Hence the CBM of coal seam No. 72 between faults F7 and F5 is less than the one between faults F3 and F2, which is consistent with the pressure occurrence in the two regions. 4. Discussions 4.1. Tectonic evolution and permeability The coal-bearing strata have undergone transformations caused by major tectonic movements three times since its formation (Chu et al., 2008; Han, 1990; Song and Duan, 1999; Wan, 1993; Zhang and
Fig. 4. Relationship between the gas pressure of the coal seam No.72 and the buried depth in the Renlou coal mine. The lines in this figure are the gas pressure predicted by the safety line method.
Remark
Symbol point Symbol point
Symbol point
Symbol point
Guo, 2009). Additionally, the large-angle tectonic stress superposition leads to structure characteristic and mechanical properties of faults in the area transform or become multiplicity. The early tectonic movement is the middle and late Indosinian tectonic movement (Chu et al., 2008; Han, 1990; Song and Duan, 1999; Wan, 1993; Zhang and Guo, 2009). The principal compressive stress direction of early tectonic movement was near NS and the direction of principal tensile stress was near EW. As a result, the reverse faults F5 and F8 trending near EW was formed, which was compressive. The near NS-trending tensile normal fault F16, the NEtrending compressive-shear faults F2, F14, F2-1, and F31, and the NWtrending tensile-shear faults F3 and F4 were also formed. The middle tectonic movement was developed in the early and middle period of Yanshan movement (Han, 1990; Song and Duan, 1999; Wan, 1993). And the principal compressive stress direction of this movement was near EW and the direction of principal tensile stress was near NS. The horizontal distribution and mechanical properties of the structures formed by the early tectonic movement were changed during this period. For example, the NWW-trending faults (i.e. faults F7 and F5) changed from compressive to tensileshear, and the fault F16 changed from tensile to compressiveshear. The distribution lines of the faults formed by the early tectonic movement horizontally exhibited shapes of ‘S’ or arcs due to the directions of the two tectonic movements that turned 100 counterclockwise (Song and Duan, 1999). The late tectonic movement was developed during Himalayan period (corresponding to NeogeneeEarly Pleistocene, Wan, 2011 and Wu et al., 2011) which changed the properties of faults formed earlier (Han, 1990; Wan, 1993, 2011). The near EW-trending large faults were compressed and sheared left-laterally and the NNE-trending faults were compressed and sheared right-laterally. The faults are mostly distributed in the NE, NW and near EW directions, with characteristics of long term development and changeable properties. The faults primarily appear as normal faults since the Himalayan period (Jiang et al., 2010; Ju and Wang, 2002). The large angle superposition of three times tectonic stress changes structures and mechanical properties of faults in the area. The NNW-SN-NE striking faults are compressive, shear or compressive following tensile and they are impermeable, gas barrier structures that provide good sealing conditions for CBM. The NEE-EW-NWW-NW striking faults, especially normal faults with high dip angles, are tensile-shear, tensile or tensile following compressive and they are open structures that enable gas to escape easily. The normal faults F3 and F7 and the reverse fault F5 in southern Renlou coal mine are open structure.
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Fig. 7. Schematic cross section of the horsts and grabens formed by faults in Renlou coal mine along line AeA0 in Fig. 1.
Fig. 5. RME variation of working face __7211 with distance from fault F3.
4.2. Coal bed methane occurrence The RME of working face __7211 increased from 12 m3/t to 30 m3/t on average in 600 m away from F3 fault. Therefore, we could obtain that the influence distance of fault F3 is 600 m. We map the fault emission regions based on a 600 m distance of fault emission, as shown in Fig. 2. The coal seam is in ranges of fault emission basically and partly in the overlay area. The emission of faults F5 and F7 interacting each other makes the CBM content or pressure less than the ones between faults F3 and F2. Furthermore, the horst between faults F7 and F5 with numerous open fractures also enhances the methane emission (Groshong Jr. et al., 2009), as shown in Fig. 7. So, there is difference of CBM occurrence in the two regions. The RME of working face 7257 located in the region between faults F7 and F5 is lower than that of the working face __7211 located in the region between faults F3 and F2. The measured gas pressure between faults F7 and F5 is lower than that between faults F3 and F2 at the same depth. The gas pressure gradient in the region between faults F7 and F5 is less than that in the other region. 5. Conclusions 1) The coal-bearing strata have undergone transformation caused by major tectonic movements three times since its formation. In
addition, the large-angle tectonic stress superposition leads to structure characteristic and mechanical properties of faults transform in the area or just make it become multiplicity. The NNW-SN-NE striking faults are compressive, shear or compressive following tensile and they are impermeable, gas barrier structures that provide good sealing conditions for CBM. The NEE-EW-NWW-NW striking faults, especially normal faults with high dip angles, are tensile-shear, tensile or tensile following compressive and they are open structures that enable gas to escape easily. 2) The normal faults F3 and F7 and reverse fault F5 in the southern Renlou coal mine are open structure. The coal seam between faults F7 and F5 is in the emission range of the two faults (partially in the superimposed range), in which a significant amount of gas escapes. 3) The formation between faults F5 and F7 is horst and turns from NS to EW. The stress in the formation is large, resulting in the presence of numerous fractures and cleats. Therefore, the CBM emission in the region is larger than that between faults F3 and F2. 4) Between faults F7 and F5, the gas pressure gradient of coal seam No. 72 is 0.00356 MPa/m and the maximum measured gas pressure reaches 0.39 MPa (633.5 m). However, the gas pressure gradient of coal seam No. 72 is 0.00726 MPa/m and the maximum measured gas pressure coal is 1.7 MPa (692 m) between faults F3 and F2. The average RME of working face __7211 located in the region between faults F3 and F2 is 12 m3/t in the area of 0e600 m away from F3 fault and increases to 30 m3/t in the region of >600 m away from fault F3. The RME of working face 7257 located in the region between faults F7 and F5 is 2.01 m3/t on average, with a maximum value of 4.96 m3/t, which is much smaller than that of working face __7211. Acknowledgment The authors are grateful for the support of the National Foundation of China (No. 51074160, No. 41202118, No. 51204173, No. 51004106, No. 51374204, No. 51304188), the National Basic Research Program of China (973 Program, No. 2011CB201204) and A Project Funded by the Priority Academic Program Development of Jiangsu Higher Education Institutions. References
Fig. 6. RME variation of working face 7257 during mining in the region between faults F7 and F5.
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