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Tectonically deformed coal types and pore structures in Puhe and Shanchahe coal mines in western Guizhou
Mining Science and Technology (China) 21 (2011) 353–357
Contents lists available at ScienceDirect
Mining Science and Technology (China) journal homepage: www.elsevier.com/locate/mstc
Tectonically deformed coal types and pore structures in Puhe and Shanchahe coal mines in western Guizhou Li Ming a,b,c,⇑, Jiang Bo a,b, Lin Shoufa a,c, Wang Jilin a,b, Ji Mingjun d, Qu Zhenghui a,b a
School of Resource and Earth Science, China University of Mining & Technology, Xuzhou 221008, China Key Laboratory of Coalbed Methane Resources and Reservoir-Forming of Ministry of Education, Xuzhou 221008, China c Department of Earth and Environmental Sciences, University of Waterloo, Waterloo, Canada N2L 3G1 d Jiangsu Hogan Group Co., Ltd., Xuzhou 221137, China b
a r t i c l e
i n f o
Article history: Received 28 September 2010 Received in revised form 12 November 2010 Accepted 2 December 2010 Available online 11 June 2011 Keywords: Puhe and Shanchahe coal mines Tectonically deformed coal Pore structure Coalbed methane
a b s t r a c t To evaluate the effect of tectonic deformation on coal reservoir properties, we provide an analysis of the types of tectonically deformed coal, macro- and microscopic deformation and discuss pore structural characteristics and connectivity based on samples from the Puhe and Shanchahe coal mines. Our research shows that the tectonically deformed coal mostly includes cataclastic structural coal, mortar structural coal and schistose structural coal of a brittle deformation series. The major pore structures of different types of tectonically deformed coal are transitional pores and micropores. The pore volumes of macropores and visible fracture pores produced by structural deformations vary over a large range and increase with the intensity of tectonic deformation. Mesopores as connecting passages develop well in schistose structural coal. According to the shapes of intrusive mercury curves, tectonically deformed coal can be divided into parallel, open and occluded types. The parallel type has poor connectivity and is relatively closed; the open type reflects uniformly developed open pores with good connectivity while the occluded type is good for coalbed methane enrichment, but has poor connectivity between pores. Ó 2011 Published by Elsevier B.V. on behalf of China University of Mining & Technology.
1. Introduction Coal is a special kind of rock that is uniquely sensitive to stress and strain. Tectonically deformed coal with various structural characteristics and types form under different stress–strain conditions and tectonic stresses [1,2]. The distribution of tectonically deformed coal in a coal seam is relatively limited, but it is widely developed in China’s coal fields and is the main factor affecting the distribution and enrichment of coalbed methane (CBM) [3]. The development of tectonically deformed coal enhances the heterogeneity and porosity of coal seams and also reduces their permeability and mechanical strength. The feasibility of CBM exploitation and the possibility of coal and gas outbursts are significantly affected by late structural transformations and pore-fracture structural characteristics of coal seams [4–7]. This provides a basis for further discussion of deformation characteristics and pore structures of tectonically deformed coal, necessary for further CBM exploitation, safe mining and production. The Puhe and Shanchahe coal mines are located in the west of Guizhou province, near Puding and Anshun cities (Fig. 1). They belong to the Zhina coal field, which is located in the southwest of the
⇑ Corresponding author. Tel.: +86 13151981375. E-mail address: [email protected] (M. Li).
Zunyi fault arch [8]. Structural traces in this zone trend mainly in a northeasterly direction. The folds are mainly gentle folds. The faults are well developed and move mainly in a normal sense. The geological structure in the Puhe and Shanchahe coal mines is relatively simple and exhibits several normal faults with a near north–south trend. The Longtan formation of the upper Permian is a major coal-bearing section in the study area, between 300 and 490 m thick, containing 10–22 coal seams. Coal seams Nos. 22, 23 and 34 are the main mineable coal seams in this coal field with anthracite rank. The semi-bright and semi-dull coals are of the dominant lithotypes, with secondary amounts of dull and bright coal.
2. Materials and methods In the Puhe and Shanchahe coal mines, the coal seams are buried to a shallow depth, their geological structure is simple and the type of tectonically deformed coal is not complex [9]. Based on geological surveys of the coal mine within a limited underground mining space, different types of typical tectonic coal samples were collected, as shown in Table 1. Systematic observations and descriptions of the mesoscopic deformation features of tectonically deformed coal, including bedding, joint, fracture and degree of development, were made on the
1674-5264/$ - see front matter Ó 2011 Published by Elsevier B.V. on behalf of China University of Mining & Technology. doi:10.1016/j.mstc.2011.05.002
Pore volume (cm3/g)
M. Li et al. / Mining Science and Technology (China) 21 (2011) 353–357
Me iz i g
ng Bu la
ua n
a nt ic l in
e
Jic Pune coal mine
Shanchahe coal mine
syn c
line
ha ng po
sy nc l
in e
354
N
0
Normal fault
igu Ca
Puding 5 km
Reverse fault
a
yn ns
ne c li
Syncline
Shanchahe coal mine
S1
Primary-cataclastic structural coal
S2
Cataclastic–mortar structural coal
S3
Schistose structural coal Cataclastic–mortar structural coal Cataclastic structural coal Primary-cataclastic structural coal
S4 S5
S6 Puhe coal mine
Tectonic coal type
P1
Schistose structural coal
P2
Mortar structural coal
S3
0.01
S4 S5 S6
0 V2
V3
V4
V5
P1 P2
Fig. 2. Contribution of each pore to total volume. V1 > 100,000 nm; V2 = 100,000– 1000 nm; V3 = 1000–100 nm; V4 = 100–10 nm; V5 < 10 nm; Vt as total pore volume.
reservoir. Transitional pores and micropores mainly consist of primary and metamorphic pores. Stage pore volumes and their contribution to the total volumes of coal samples are shown in Fig. 2 and Table 2.
City
Table 1 Statistics of tectonically deformed coal samples collected in Puhe and Shanchahe coal mines. Samples
S2
Pore size stage
Fig. 1. Structural outline of Puhe and Shanchahe coal mines.
Digging
S1
V1
Anshun
Anticline
0.02
Sampling location
Coal seam 34
#
Ventilation roadway dipentry of three mining area Haulage roadway dipentry of three mining area Hanging wall of fault
34#
Footwall of fault
34#
Near the outcrop of coal seam
22#
Near the outcrop of coal seam
22#
Ventilation roadway raise of 1410 mining face Ventilation roadway dipentry of 1410 mining face
23#
34#
3. Results and discussion 3.1. Types and characteristics of tectonically deformed coal 3.1.1. Primary-cataclastic structural coal (S1 and S6) Structural deformation is mainly characterized by brittle fractures, with the primary structures of coal, as a rule, well preserved. Fractures are sparsely distributed with small fracture apertures. Tension joints have rough joint planes and extend unsteadily along its trend. Shear joint planes are smooth and straight, shown as two joint sets in Fig. 3a. Tectonic stress is focussed and many second order fractures form at the transition parts of the joints. 3.1.2. Cataclastic structural coal (S5) The primary structure of coal remains relatively unchanged; only small scale fractures, distributed sparsely, are formed in brittle fracture deformation. The joint plane is rough and extends in an arcuate shape. Coal deforms along the joints but without apparent displacement and the fractures have a preferred orientation (Fig. 3b). There is, locally, a slight ductile deformation.
23#
collected samples in the laboratory. Microscopic deformation was observed and measured using a polarizing microscope under reflected light. The types and deformation characteristics of tectonically deformed coal were then analyzed and studied. In addition, the pore volume and pore size distribution were tested by Micromerities 9310 type mercury porosimetry with pore diameter tests ranging from 7 nm to 0.23 mm. Based on the standard set by Hotot and considering that the minimum distinguishable width by the naked eye is 0.1 mm, the micro-scale structures of pores can be classified as visible fracture pores (pore diameter P100,000 nm), macropores (1000 to <100,000 nm), mesopores (100 to <1000 nm), transitional pores (10 to <100 nm) and micropores (<10 nm), with pore diameters 100,000, 1000, 100 and 10 nm as boundaries [10–12]. Visible fracture pores, with pore diameters between 0.1 and 0.23 mm, consist largely of visible joints and fractures formed by structural deformation and coalification. The degree of development of visible joints and fractures in different samples is reflected in the visible fracture pore volumes. Macropores and mesopores are mainly microfractures, exogenous holes or mineral pores [13,14]. Visible fracture pores and macropores, with pore diameters larger than 1000 nm, are the main flow channels of CBM in coal reservoirs, suggesting that their content and connectivity determine the breathability and permeability of a coal
3.1.3. Mortar structural coal (S2, S4 and P2) Dense joints and fractures develop well in the coal mass and large fractures are often accompanied by secondary associated fractures forming fracture-concentrated belts. Many secondary associated fractures form at the intersections of two or more sets of fractures. Larger scale fracture planes that show brittle fracture features are smooth and stable, but secondary fractures exhibit arcuate shape with poorly preferred orientations on horizontal surfaces. The coal mass is segregated into different sized pieces by fracture sets and appears severely damaged showing a mortar texture (Fig. 3c). 3.1.4. Schistose structure coal (S3 and P1) Due to the effect of tectonic strain, mainly shearing, and the coal mass is sliced into thin sections. The dominant dense fracture set is well developed at an angle of about 45° from the coal bedding (Fig. 3d). There are many obvious sliding signs on smooth joint surfaces, which shows that schistose structural coal is formed by shear failure along a group of dense array joints. The structural deformation of the Puhe and Shanchahe coal mines is not very intense and mainly exhibits brittle fracturing. Due to the different intensity and property of stress, to which coal samples were subjected, there are specific differences in the density of fractures, the number of joint sets and the geometric shape of joints. The development of brittle fracturing increases coal mass breathability and improves primary structure coal seam permeability, which enhances migration and dissipation of CBM.
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M. Li et al. / Mining Science and Technology (China) 21 (2011) 353–357 Table 2 Stage pore volumes of coal samples. Sample
Pore volume (cm3/g) V1
V2
V3
V4
V5
Vt
V1/Vt
V2/Vt
V3/Vt
V4/Vt
V5/Vt
S1 S2 S3 S4 S5 S6 P1 P2
0.0017 0.0005 0.0032 0.0008 0.0008 0.0039 0.0019 0.0008
0.0015 0.0056 0.0089 0.0094 0.0119 0.0040 0.0143 0.0076
0.0015 0.0046 0.0026 0.0039 0.0021 0.0024 0.0164 0.0059
0.0144 0.0118 0.0124 0.0122 0.0182 0.0152 0.0180 0.0123
0.0081 0.0071 0.0074 0.0063 0.0093 0.0092 0.0088 0.0107
0.0272 0.0296 0.0345 0.0326 0.0423 0.0347 0.0594 0.0373
6.25 1.69 9.28 2.45 1.89 11.24 3.20 2.14
5.52 18.92 25.79 28.84 28.13 11.53 24.07 20.38
5.52 15.54 7.54 11.96 4.96 6.92 27.61 15.82
52.93 39.86 35.94 37.42 43.03 43.80 30.30 32.97
29.78 23.99 21.45 19.33 21.99 26.51 14.82 28.69
Pore volume ratio (%)
(a)
(b)
(c)
Porosity (%)
3.84 4.72 5.22 5.06 5.67 4.59 8.17 4.89
(d)
0.003 0.002 0.001 0 1
10
100
1000 100001000001000000
0.004
Stage pore volume (cm3/g)
0.004
Stage pore volume (cm3/g)
Stage pore volume (cm3/g)
Fig. 3. Micro-structural features of tectonically deformed coal (magnification: 4 10).
0.003 0.002 0.001 0 1
10
100
1000 100001000001000000
Pore size (nm)
Pore size (nm)
(a)
(b)
0.004 0.003 0.002 0.001 0 1
10
100
1000 10000 1000001000000 Pore size (nm)
(c)
Fig. 4. Relation between typical pore size and stage pore volumes. (a) Primary-cataclastic structural coal (S1), (b) schistose structural coal (P1), and (c) mortar structural coal (P2).
3.2. Pore structural characteristics of tectonically deformed coal Primary-cataclastic structural coal (S1 and S6) was deformed weakly by structural stress. Its pore volume is concentrated primarily between transitional pores and micropores, on average accounting for 42.95% and 29.24%, respectively of the total pore volume (Figs. 2 and 4a). The dominant transitional pores and micropores are mainly primary and metamorphic pores, implying that the primary structure of coal remained mostly unchanged with less structural deformation. Visible fracture pores and macropores account for 17.15%, while mesopores account for 10.67%. The latter mainly consists of micro-fractures that connect seepage fractures to occurrence pores of CBM. The pore volume of cataclastic structural coal (S5) consists largely of macropores and transitional pores. This is because the relative strengthening of tectonic stress action leads to the enhancement of joints, fractures and micro-fractures, which increase total pore volume. The effect of structural deformation on micropores is small and micropore volume remained relativity unchanged, which causes the proportion of micropores to total volume to decrease relatively; transitional pores and micropores as occurrence space of CBM account for 65.02% of total pore volume. Through further deformation, visible fracture pore and macropore volumes account for 30.02% of total pore volume, an increase of 2– 3 times compared to primary-cataclastic structural coal. Mortar structural coal (S2, S4 and P2) deformed more intensely than cataclastic structural coal from structural strain. Both porosity and total pore volume increased. As with primarycataclastic structural coal, pore volumes remained concentrated
in transitional pores and micropores, on average accounting for 36.75% and 24.00% of total pore volume (Figs. 2 and 4c). This is followed by macropores (22.72%) and mesopores (14.44%), while the lowest fraction consisted of visible fracture pores, accounting for only 2.09%. Compared with cataclastic structural coal, the proportion of macropores is nearly three times greater, suggesting that dense microfractures, formed through structural deformation, develop well. There is a close relationship between the increase in the proportion of mesopores and the development of crushed granule pores, i.e., the space between crushed granules formed by structural deformation in coal. The visible fracture pore volume of mortar structural coal is only about one-quarter that of cataclastic structural coal. This data correlates well with the phenomenon observed microscopically showing that fractures are sparse and wide in cataclastic structural coal; in contrast, fractures are narrow, dense and dispersed in mortar structural coal. The pore volume of visible fracture pores and macropores, on average adding up to 0.0082 cm3/g, is 1.5 times the pore volume found in primary-cataclastic structural coal. Although the proportion of mesopores increased and that of visible fracture pores and macropores decreased, their pore volumes all increased to varying degrees. Thus, intense structural deformation not only causes development of joints, fractures and micro-fractures, but is also responsible for the considerable development of tectonic micro-pores, such as frictional and crushed granular pores. On the other hand, structural deformation and reformation of primary pores causes the decrease of pore space and poor connectivity, which restricts dissipation of CBM.
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M. Li et al. / Mining Science and Technology (China) 21 (2011) 353–357
mercury curve, testing samples can be divided into parallel, open and occlusion types.
Schistose structural coal (S3 and P1), a structural deformation product resulting from the action of structural shear strain on coal seams, has its pore volume concentrated in transitional pores and macropores, on average accounting for 33.12% and 24.93%, respectively, of total pore volume (Figs. 2 and 4b). This is followed by the volumes of micropores, mesopores and visible fracture pores. Pores develop well in schistose structural coal, where porosity is about 1.4 times that of cataclastic structural or mortar structural coal. It is largely represented by significant increases in volumes of visible fracture pores and macropores, i.e., 0.0026 and 0.0116 cm3/g, respectively. The development of visible fracture pores and macropores consists mainly of joints and micro-fractures, enhancing the release and seepage of CBM. Its mesopore volume (0.0095 cm3/g) is about twice that of cataclastic structural or mortar structural coal. As determined from microscopic observations and scanning electron microscopy, the mesopores consist largely of interfragment pores, crushed granular pores and micro-fracture pores formed by structural deformation. Therefore, the schistose structural coal with good connectivity and high porosity is a reservoir favorable for the development of CBM. In conclusion, the contributions of various stage pore volumes to total volume are in the following descending order: transitional pores, micropores, macropores, mesopores and visible fracture pores. The largest proportion of pore volume of tectonically deformed coal, developed in the study area, is transitional pores, consists mainly of primary and metamorphic pores with small pore sizes. Micropores, the second highest in proportion, also mainly consists of primary and metamorphic pores. This micropore stage volume, with pore sizes between 7 and 10 nm only, accounts for 20%–30% of total pore volume. Due to their small pore sizes and poor connectivity, micropores contribute only negligibly to coal permeability from transitional pores and micropores, although they provide most of the space for CBM in its adsorption state. Pore volumes of macropores range widely and increase with the enhancement of structural deformation. Mesopores, which mainly act as channels linking micropores and microfractures, in general do not develop well, accounting for only a small proportion of total pore volume. But mesopores develop well in schistose structural coal, with its excellent functions promoting migration and seepage of CBM. Although visible fracture pores form the smallest percentage of total pore volume, possibly because of the narrow test range of pore diameters, they develop well in intensely deformed regions. Visible fracture pores’ developments are of practical importance in CBM seepage.
(1) Parallel type: the mercury injection and ejection curves are almost parallel and the difference in value of the two curves at the same pressure is low (Fig. 5a). Primary-cataclastic structural coal (S1 and S6) and cataclastic structural coal (S5) belong to this type, with well developed micropores and transitional pores and less well-developed macropores (Fig. 4a). (2) Open type: there is an acute angle between the mercury injection and ejection curves. The difference in value, between these two curves at the same pressure, increases with a decrease in pressure (Fig. 5b). Cataclastic–mortar structural coal (S2 and S4) and schistose structural coal (S3 and P1) belong to this type and are well developed at each stage of pore volume (Fig. 4b). As the stage pore volumes develop, both acute angle and permeability increase. (3) Occlusion type: the mercury injection and ejection curves nearly intersect locally as an ‘‘occlusive throat’’. The difference in value between the two curves at the same pressure first decreases locally and then increases with a decrease in pressure (Fig. 5c). Mortar structural coal (P2) belongs to this type. Changes as a function of pressure shown in the two curves at the same pressure reflect the connectivity of the coal mass. Given the ‘‘occlusive throat’’ phenomenon in the occlusion type intrusive mercury curve, the distribution curve of stage pore volume is always shaped like the letter ‘‘V’’ or ‘‘U’’, where the pore volume is smaller at the stage of the ‘‘occlusive throat’’ pore size than at both sides. This phenomenon is largely concentrated at the stage of transitional pores with pore sizes between 10 and 100 nm. Thus it can be seen that the not-well-developed transitional pores at the stage of partial pore sizes and overall well-developed transitional pores and micropores are the leading causes of occlusion type intrusive mercury curves. The shape of the intrusive mercury curve is controlled by such factors as pore size structure, pore-fracture properties and combination mode. According to its morphological characteristics, pore shape and connectivity of tectonically deformed coal can be evaluated in a preliminary fashion. Open type intrusive mercury curves represent open pores with uniform development and good connectivity. Parallel types with two parallel and nearly coincident mercury curves suggest bad connectivity and poor permeability. The occlusion type is mainly caused by notwell-developed transitional pores at the partial pore size stage. Overall well-developed transitional and micropores indicate the occurrence and enrichment of CBM, but bad connectivity and low permeability restrict migration and seepage of CBM. It is hard to exploit CBM in areas of this type of tectonically deformed coal.
3.3. Pore shape and connectivity of tectonically deformed coal
Mercury injection curve Mercury ejection curve
0.04 0.03 0.02 0.01 0 0.001 0.01
0.1
1
10
Pressure (MPa )
(a)
100
1000
0.07 0.06
Cumulative pore volum e (cm 3/g)
0.07 0.06 0.05
Cumulative pore 3 volum e (cm /g)
C umulative por e volume (cm3/g)
The effective pore volume in coal can be calculated directly by an analysis of intrusive mercury curves, from which the degree of development, type and connectivity of pores can be deduced by the combination of the shape of its mercury injection and ejection curves [15–17]. According to the shapes of the intrusive
0.05 0.04 0.03 0.02 0.01 0 0.001 0.01
0.1
1
10
Pressure (MPa )
(b)
100
1000
0.07 0.06 0.05 0.04 0.03 0.02 0.01 0 0.001 0.01
0.1
1
10
Pressure (MPa )
(c)
Fig. 5. Three kinds of intrusive mercury curves. (a) Parallel type (S1), (b) open type (P1), and (c) occlusion type (P2).
100 1000
M. Li et al. / Mining Science and Technology (China) 21 (2011) 353–357
4. Conclusions
(1) The types of tectonically deformed coal in the Puhe and Shanchahe coal mines are cataclastic structural coal, mortar structural coal and schistose structural coal, belonging to a brittle deformation series, where cataclastic structural coal is best developed. Its mesoscopic and microscopic deformation characteristics mainly involve the development of brittle fractures, but there are specific differences in the various types of tectonically deformed coal. (2) The largest proportion of pore volume contained in the different types of tectonically deformed coal is that of transitional pore, followed by micropores. Pore volumes of macropores are wide ranging and increase with the enhancement of structural deformation. Mesopores generally do not develop well, but they do so in schistose structural coal. Visible fracture pores develop well in intensely deformed regions and are of practical importance in the seepage of CBM. (3) According to the shapes of intrusive mercury curves, tectonically deformed coal types can be divided into parallel, open and occlusion types. The parallel type has poor connectivity and permeability; the open type reflects uniformly developed open pores with good connectivity and the occlusion type is good for coalbed methane enrichment, but has poor pore connectivity.
Acknowledgments This work is supported by the National Natural Science Foundation of China (No. 40672101), the Key National Natural Science Foundation of China (No. 40730422), the National Science and Technology Key Special Project from the Ministry of Technology of China (No. 2008ZX05034) and the China Scholarship Council (CSC). We thank Dr. Nathan Cleven and Dr. Fengjuan Lan for improving the manuscript.
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Contents lists available at ScienceDirect
Mining Science and Technology (China) journal homepage: www.elsevier.com/locate/mstc
Tectonically deformed coal types and pore structures in Puhe and Shanchahe coal mines in western Guizhou Li Ming a,b,c,⇑, Jiang Bo a,b, Lin Shoufa a,c, Wang Jilin a,b, Ji Mingjun d, Qu Zhenghui a,b a
School of Resource and Earth Science, China University of Mining & Technology, Xuzhou 221008, China Key Laboratory of Coalbed Methane Resources and Reservoir-Forming of Ministry of Education, Xuzhou 221008, China c Department of Earth and Environmental Sciences, University of Waterloo, Waterloo, Canada N2L 3G1 d Jiangsu Hogan Group Co., Ltd., Xuzhou 221137, China b
a r t i c l e
i n f o
Article history: Received 28 September 2010 Received in revised form 12 November 2010 Accepted 2 December 2010 Available online 11 June 2011 Keywords: Puhe and Shanchahe coal mines Tectonically deformed coal Pore structure Coalbed methane
a b s t r a c t To evaluate the effect of tectonic deformation on coal reservoir properties, we provide an analysis of the types of tectonically deformed coal, macro- and microscopic deformation and discuss pore structural characteristics and connectivity based on samples from the Puhe and Shanchahe coal mines. Our research shows that the tectonically deformed coal mostly includes cataclastic structural coal, mortar structural coal and schistose structural coal of a brittle deformation series. The major pore structures of different types of tectonically deformed coal are transitional pores and micropores. The pore volumes of macropores and visible fracture pores produced by structural deformations vary over a large range and increase with the intensity of tectonic deformation. Mesopores as connecting passages develop well in schistose structural coal. According to the shapes of intrusive mercury curves, tectonically deformed coal can be divided into parallel, open and occluded types. The parallel type has poor connectivity and is relatively closed; the open type reflects uniformly developed open pores with good connectivity while the occluded type is good for coalbed methane enrichment, but has poor connectivity between pores. Ó 2011 Published by Elsevier B.V. on behalf of China University of Mining & Technology.
1. Introduction Coal is a special kind of rock that is uniquely sensitive to stress and strain. Tectonically deformed coal with various structural characteristics and types form under different stress–strain conditions and tectonic stresses [1,2]. The distribution of tectonically deformed coal in a coal seam is relatively limited, but it is widely developed in China’s coal fields and is the main factor affecting the distribution and enrichment of coalbed methane (CBM) [3]. The development of tectonically deformed coal enhances the heterogeneity and porosity of coal seams and also reduces their permeability and mechanical strength. The feasibility of CBM exploitation and the possibility of coal and gas outbursts are significantly affected by late structural transformations and pore-fracture structural characteristics of coal seams [4–7]. This provides a basis for further discussion of deformation characteristics and pore structures of tectonically deformed coal, necessary for further CBM exploitation, safe mining and production. The Puhe and Shanchahe coal mines are located in the west of Guizhou province, near Puding and Anshun cities (Fig. 1). They belong to the Zhina coal field, which is located in the southwest of the
⇑ Corresponding author. Tel.: +86 13151981375. E-mail address: [email protected] (M. Li).
Zunyi fault arch [8]. Structural traces in this zone trend mainly in a northeasterly direction. The folds are mainly gentle folds. The faults are well developed and move mainly in a normal sense. The geological structure in the Puhe and Shanchahe coal mines is relatively simple and exhibits several normal faults with a near north–south trend. The Longtan formation of the upper Permian is a major coal-bearing section in the study area, between 300 and 490 m thick, containing 10–22 coal seams. Coal seams Nos. 22, 23 and 34 are the main mineable coal seams in this coal field with anthracite rank. The semi-bright and semi-dull coals are of the dominant lithotypes, with secondary amounts of dull and bright coal.
2. Materials and methods In the Puhe and Shanchahe coal mines, the coal seams are buried to a shallow depth, their geological structure is simple and the type of tectonically deformed coal is not complex [9]. Based on geological surveys of the coal mine within a limited underground mining space, different types of typical tectonic coal samples were collected, as shown in Table 1. Systematic observations and descriptions of the mesoscopic deformation features of tectonically deformed coal, including bedding, joint, fracture and degree of development, were made on the
1674-5264/$ - see front matter Ó 2011 Published by Elsevier B.V. on behalf of China University of Mining & Technology. doi:10.1016/j.mstc.2011.05.002
Pore volume (cm3/g)
M. Li et al. / Mining Science and Technology (China) 21 (2011) 353–357
Me iz i g
ng Bu la
ua n
a nt ic l in
e
Jic Pune coal mine
Shanchahe coal mine
syn c
line
ha ng po
sy nc l
in e
354
N
0
Normal fault
igu Ca
Puding 5 km
Reverse fault
a
yn ns
ne c li
Syncline
Shanchahe coal mine
S1
Primary-cataclastic structural coal
S2
Cataclastic–mortar structural coal
S3
Schistose structural coal Cataclastic–mortar structural coal Cataclastic structural coal Primary-cataclastic structural coal
S4 S5
S6 Puhe coal mine
Tectonic coal type
P1
Schistose structural coal
P2
Mortar structural coal
S3
0.01
S4 S5 S6
0 V2
V3
V4
V5
P1 P2
Fig. 2. Contribution of each pore to total volume. V1 > 100,000 nm; V2 = 100,000– 1000 nm; V3 = 1000–100 nm; V4 = 100–10 nm; V5 < 10 nm; Vt as total pore volume.
reservoir. Transitional pores and micropores mainly consist of primary and metamorphic pores. Stage pore volumes and their contribution to the total volumes of coal samples are shown in Fig. 2 and Table 2.
City
Table 1 Statistics of tectonically deformed coal samples collected in Puhe and Shanchahe coal mines. Samples
S2
Pore size stage
Fig. 1. Structural outline of Puhe and Shanchahe coal mines.
Digging
S1
V1
Anshun
Anticline
0.02
Sampling location
Coal seam 34
#
Ventilation roadway dipentry of three mining area Haulage roadway dipentry of three mining area Hanging wall of fault
34#
Footwall of fault
34#
Near the outcrop of coal seam
22#
Near the outcrop of coal seam
22#
Ventilation roadway raise of 1410 mining face Ventilation roadway dipentry of 1410 mining face
23#
34#
3. Results and discussion 3.1. Types and characteristics of tectonically deformed coal 3.1.1. Primary-cataclastic structural coal (S1 and S6) Structural deformation is mainly characterized by brittle fractures, with the primary structures of coal, as a rule, well preserved. Fractures are sparsely distributed with small fracture apertures. Tension joints have rough joint planes and extend unsteadily along its trend. Shear joint planes are smooth and straight, shown as two joint sets in Fig. 3a. Tectonic stress is focussed and many second order fractures form at the transition parts of the joints. 3.1.2. Cataclastic structural coal (S5) The primary structure of coal remains relatively unchanged; only small scale fractures, distributed sparsely, are formed in brittle fracture deformation. The joint plane is rough and extends in an arcuate shape. Coal deforms along the joints but without apparent displacement and the fractures have a preferred orientation (Fig. 3b). There is, locally, a slight ductile deformation.
23#
collected samples in the laboratory. Microscopic deformation was observed and measured using a polarizing microscope under reflected light. The types and deformation characteristics of tectonically deformed coal were then analyzed and studied. In addition, the pore volume and pore size distribution were tested by Micromerities 9310 type mercury porosimetry with pore diameter tests ranging from 7 nm to 0.23 mm. Based on the standard set by Hotot and considering that the minimum distinguishable width by the naked eye is 0.1 mm, the micro-scale structures of pores can be classified as visible fracture pores (pore diameter P100,000 nm), macropores (1000 to <100,000 nm), mesopores (100 to <1000 nm), transitional pores (10 to <100 nm) and micropores (<10 nm), with pore diameters 100,000, 1000, 100 and 10 nm as boundaries [10–12]. Visible fracture pores, with pore diameters between 0.1 and 0.23 mm, consist largely of visible joints and fractures formed by structural deformation and coalification. The degree of development of visible joints and fractures in different samples is reflected in the visible fracture pore volumes. Macropores and mesopores are mainly microfractures, exogenous holes or mineral pores [13,14]. Visible fracture pores and macropores, with pore diameters larger than 1000 nm, are the main flow channels of CBM in coal reservoirs, suggesting that their content and connectivity determine the breathability and permeability of a coal
3.1.3. Mortar structural coal (S2, S4 and P2) Dense joints and fractures develop well in the coal mass and large fractures are often accompanied by secondary associated fractures forming fracture-concentrated belts. Many secondary associated fractures form at the intersections of two or more sets of fractures. Larger scale fracture planes that show brittle fracture features are smooth and stable, but secondary fractures exhibit arcuate shape with poorly preferred orientations on horizontal surfaces. The coal mass is segregated into different sized pieces by fracture sets and appears severely damaged showing a mortar texture (Fig. 3c). 3.1.4. Schistose structure coal (S3 and P1) Due to the effect of tectonic strain, mainly shearing, and the coal mass is sliced into thin sections. The dominant dense fracture set is well developed at an angle of about 45° from the coal bedding (Fig. 3d). There are many obvious sliding signs on smooth joint surfaces, which shows that schistose structural coal is formed by shear failure along a group of dense array joints. The structural deformation of the Puhe and Shanchahe coal mines is not very intense and mainly exhibits brittle fracturing. Due to the different intensity and property of stress, to which coal samples were subjected, there are specific differences in the density of fractures, the number of joint sets and the geometric shape of joints. The development of brittle fracturing increases coal mass breathability and improves primary structure coal seam permeability, which enhances migration and dissipation of CBM.
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M. Li et al. / Mining Science and Technology (China) 21 (2011) 353–357 Table 2 Stage pore volumes of coal samples. Sample
Pore volume (cm3/g) V1
V2
V3
V4
V5
Vt
V1/Vt
V2/Vt
V3/Vt
V4/Vt
V5/Vt
S1 S2 S3 S4 S5 S6 P1 P2
0.0017 0.0005 0.0032 0.0008 0.0008 0.0039 0.0019 0.0008
0.0015 0.0056 0.0089 0.0094 0.0119 0.0040 0.0143 0.0076
0.0015 0.0046 0.0026 0.0039 0.0021 0.0024 0.0164 0.0059
0.0144 0.0118 0.0124 0.0122 0.0182 0.0152 0.0180 0.0123
0.0081 0.0071 0.0074 0.0063 0.0093 0.0092 0.0088 0.0107
0.0272 0.0296 0.0345 0.0326 0.0423 0.0347 0.0594 0.0373
6.25 1.69 9.28 2.45 1.89 11.24 3.20 2.14
5.52 18.92 25.79 28.84 28.13 11.53 24.07 20.38
5.52 15.54 7.54 11.96 4.96 6.92 27.61 15.82
52.93 39.86 35.94 37.42 43.03 43.80 30.30 32.97
29.78 23.99 21.45 19.33 21.99 26.51 14.82 28.69
Pore volume ratio (%)
(a)
(b)
(c)
Porosity (%)
3.84 4.72 5.22 5.06 5.67 4.59 8.17 4.89
(d)
0.003 0.002 0.001 0 1
10
100
1000 100001000001000000
0.004
Stage pore volume (cm3/g)
0.004
Stage pore volume (cm3/g)
Stage pore volume (cm3/g)
Fig. 3. Micro-structural features of tectonically deformed coal (magnification: 4 10).
0.003 0.002 0.001 0 1
10
100
1000 100001000001000000
Pore size (nm)
Pore size (nm)
(a)
(b)
0.004 0.003 0.002 0.001 0 1
10
100
1000 10000 1000001000000 Pore size (nm)
(c)
Fig. 4. Relation between typical pore size and stage pore volumes. (a) Primary-cataclastic structural coal (S1), (b) schistose structural coal (P1), and (c) mortar structural coal (P2).
3.2. Pore structural characteristics of tectonically deformed coal Primary-cataclastic structural coal (S1 and S6) was deformed weakly by structural stress. Its pore volume is concentrated primarily between transitional pores and micropores, on average accounting for 42.95% and 29.24%, respectively of the total pore volume (Figs. 2 and 4a). The dominant transitional pores and micropores are mainly primary and metamorphic pores, implying that the primary structure of coal remained mostly unchanged with less structural deformation. Visible fracture pores and macropores account for 17.15%, while mesopores account for 10.67%. The latter mainly consists of micro-fractures that connect seepage fractures to occurrence pores of CBM. The pore volume of cataclastic structural coal (S5) consists largely of macropores and transitional pores. This is because the relative strengthening of tectonic stress action leads to the enhancement of joints, fractures and micro-fractures, which increase total pore volume. The effect of structural deformation on micropores is small and micropore volume remained relativity unchanged, which causes the proportion of micropores to total volume to decrease relatively; transitional pores and micropores as occurrence space of CBM account for 65.02% of total pore volume. Through further deformation, visible fracture pore and macropore volumes account for 30.02% of total pore volume, an increase of 2– 3 times compared to primary-cataclastic structural coal. Mortar structural coal (S2, S4 and P2) deformed more intensely than cataclastic structural coal from structural strain. Both porosity and total pore volume increased. As with primarycataclastic structural coal, pore volumes remained concentrated
in transitional pores and micropores, on average accounting for 36.75% and 24.00% of total pore volume (Figs. 2 and 4c). This is followed by macropores (22.72%) and mesopores (14.44%), while the lowest fraction consisted of visible fracture pores, accounting for only 2.09%. Compared with cataclastic structural coal, the proportion of macropores is nearly three times greater, suggesting that dense microfractures, formed through structural deformation, develop well. There is a close relationship between the increase in the proportion of mesopores and the development of crushed granule pores, i.e., the space between crushed granules formed by structural deformation in coal. The visible fracture pore volume of mortar structural coal is only about one-quarter that of cataclastic structural coal. This data correlates well with the phenomenon observed microscopically showing that fractures are sparse and wide in cataclastic structural coal; in contrast, fractures are narrow, dense and dispersed in mortar structural coal. The pore volume of visible fracture pores and macropores, on average adding up to 0.0082 cm3/g, is 1.5 times the pore volume found in primary-cataclastic structural coal. Although the proportion of mesopores increased and that of visible fracture pores and macropores decreased, their pore volumes all increased to varying degrees. Thus, intense structural deformation not only causes development of joints, fractures and micro-fractures, but is also responsible for the considerable development of tectonic micro-pores, such as frictional and crushed granular pores. On the other hand, structural deformation and reformation of primary pores causes the decrease of pore space and poor connectivity, which restricts dissipation of CBM.
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mercury curve, testing samples can be divided into parallel, open and occlusion types.
Schistose structural coal (S3 and P1), a structural deformation product resulting from the action of structural shear strain on coal seams, has its pore volume concentrated in transitional pores and macropores, on average accounting for 33.12% and 24.93%, respectively, of total pore volume (Figs. 2 and 4b). This is followed by the volumes of micropores, mesopores and visible fracture pores. Pores develop well in schistose structural coal, where porosity is about 1.4 times that of cataclastic structural or mortar structural coal. It is largely represented by significant increases in volumes of visible fracture pores and macropores, i.e., 0.0026 and 0.0116 cm3/g, respectively. The development of visible fracture pores and macropores consists mainly of joints and micro-fractures, enhancing the release and seepage of CBM. Its mesopore volume (0.0095 cm3/g) is about twice that of cataclastic structural or mortar structural coal. As determined from microscopic observations and scanning electron microscopy, the mesopores consist largely of interfragment pores, crushed granular pores and micro-fracture pores formed by structural deformation. Therefore, the schistose structural coal with good connectivity and high porosity is a reservoir favorable for the development of CBM. In conclusion, the contributions of various stage pore volumes to total volume are in the following descending order: transitional pores, micropores, macropores, mesopores and visible fracture pores. The largest proportion of pore volume of tectonically deformed coal, developed in the study area, is transitional pores, consists mainly of primary and metamorphic pores with small pore sizes. Micropores, the second highest in proportion, also mainly consists of primary and metamorphic pores. This micropore stage volume, with pore sizes between 7 and 10 nm only, accounts for 20%–30% of total pore volume. Due to their small pore sizes and poor connectivity, micropores contribute only negligibly to coal permeability from transitional pores and micropores, although they provide most of the space for CBM in its adsorption state. Pore volumes of macropores range widely and increase with the enhancement of structural deformation. Mesopores, which mainly act as channels linking micropores and microfractures, in general do not develop well, accounting for only a small proportion of total pore volume. But mesopores develop well in schistose structural coal, with its excellent functions promoting migration and seepage of CBM. Although visible fracture pores form the smallest percentage of total pore volume, possibly because of the narrow test range of pore diameters, they develop well in intensely deformed regions. Visible fracture pores’ developments are of practical importance in CBM seepage.
(1) Parallel type: the mercury injection and ejection curves are almost parallel and the difference in value of the two curves at the same pressure is low (Fig. 5a). Primary-cataclastic structural coal (S1 and S6) and cataclastic structural coal (S5) belong to this type, with well developed micropores and transitional pores and less well-developed macropores (Fig. 4a). (2) Open type: there is an acute angle between the mercury injection and ejection curves. The difference in value, between these two curves at the same pressure, increases with a decrease in pressure (Fig. 5b). Cataclastic–mortar structural coal (S2 and S4) and schistose structural coal (S3 and P1) belong to this type and are well developed at each stage of pore volume (Fig. 4b). As the stage pore volumes develop, both acute angle and permeability increase. (3) Occlusion type: the mercury injection and ejection curves nearly intersect locally as an ‘‘occlusive throat’’. The difference in value between the two curves at the same pressure first decreases locally and then increases with a decrease in pressure (Fig. 5c). Mortar structural coal (P2) belongs to this type. Changes as a function of pressure shown in the two curves at the same pressure reflect the connectivity of the coal mass. Given the ‘‘occlusive throat’’ phenomenon in the occlusion type intrusive mercury curve, the distribution curve of stage pore volume is always shaped like the letter ‘‘V’’ or ‘‘U’’, where the pore volume is smaller at the stage of the ‘‘occlusive throat’’ pore size than at both sides. This phenomenon is largely concentrated at the stage of transitional pores with pore sizes between 10 and 100 nm. Thus it can be seen that the not-well-developed transitional pores at the stage of partial pore sizes and overall well-developed transitional pores and micropores are the leading causes of occlusion type intrusive mercury curves. The shape of the intrusive mercury curve is controlled by such factors as pore size structure, pore-fracture properties and combination mode. According to its morphological characteristics, pore shape and connectivity of tectonically deformed coal can be evaluated in a preliminary fashion. Open type intrusive mercury curves represent open pores with uniform development and good connectivity. Parallel types with two parallel and nearly coincident mercury curves suggest bad connectivity and poor permeability. The occlusion type is mainly caused by notwell-developed transitional pores at the partial pore size stage. Overall well-developed transitional and micropores indicate the occurrence and enrichment of CBM, but bad connectivity and low permeability restrict migration and seepage of CBM. It is hard to exploit CBM in areas of this type of tectonically deformed coal.
3.3. Pore shape and connectivity of tectonically deformed coal
Mercury injection curve Mercury ejection curve
0.04 0.03 0.02 0.01 0 0.001 0.01
0.1
1
10
Pressure (MPa )
(a)
100
1000
0.07 0.06
Cumulative pore volum e (cm 3/g)
0.07 0.06 0.05
Cumulative pore 3 volum e (cm /g)
C umulative por e volume (cm3/g)
The effective pore volume in coal can be calculated directly by an analysis of intrusive mercury curves, from which the degree of development, type and connectivity of pores can be deduced by the combination of the shape of its mercury injection and ejection curves [15–17]. According to the shapes of the intrusive
0.05 0.04 0.03 0.02 0.01 0 0.001 0.01
0.1
1
10
Pressure (MPa )
(b)
100
1000
0.07 0.06 0.05 0.04 0.03 0.02 0.01 0 0.001 0.01
0.1
1
10
Pressure (MPa )
(c)
Fig. 5. Three kinds of intrusive mercury curves. (a) Parallel type (S1), (b) open type (P1), and (c) occlusion type (P2).
100 1000
M. Li et al. / Mining Science and Technology (China) 21 (2011) 353–357
4. Conclusions
(1) The types of tectonically deformed coal in the Puhe and Shanchahe coal mines are cataclastic structural coal, mortar structural coal and schistose structural coal, belonging to a brittle deformation series, where cataclastic structural coal is best developed. Its mesoscopic and microscopic deformation characteristics mainly involve the development of brittle fractures, but there are specific differences in the various types of tectonically deformed coal. (2) The largest proportion of pore volume contained in the different types of tectonically deformed coal is that of transitional pore, followed by micropores. Pore volumes of macropores are wide ranging and increase with the enhancement of structural deformation. Mesopores generally do not develop well, but they do so in schistose structural coal. Visible fracture pores develop well in intensely deformed regions and are of practical importance in the seepage of CBM. (3) According to the shapes of intrusive mercury curves, tectonically deformed coal types can be divided into parallel, open and occlusion types. The parallel type has poor connectivity and permeability; the open type reflects uniformly developed open pores with good connectivity and the occlusion type is good for coalbed methane enrichment, but has poor pore connectivity.
Acknowledgments This work is supported by the National Natural Science Foundation of China (No. 40672101), the Key National Natural Science Foundation of China (No. 40730422), the National Science and Technology Key Special Project from the Ministry of Technology of China (No. 2008ZX05034) and the China Scholarship Council (CSC). We thank Dr. Nathan Cleven and Dr. Fengjuan Lan for improving the manuscript.
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