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Characteristics of coal fractures and the influence of coal facies on coalbed methane productivity in the South Yanchuan Block, China
Journal of Natural Gas Science and Engineering 22 (2015) 625e632
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Characteristics of coal fractures and the influence of coal facies on coalbed methane productivity in the South Yanchuan Block, China Teng Li a, b, Caifang Wu a, b, *, Qiang Liu a, b a b
School of Mineral Resources and Geosciences, China University of Mining & Technology, Xuzhou, Jiangsu Province, 221008, China Key Laboratory of Coalbed Methane Resource and Reservoir Formation Process, Ministry of Education, Xuzhou, Jiangsu Province, 221008, China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 13 November 2014 Received in revised form 6 January 2015 Accepted 7 January 2015 Available online 12 January 2015
Micro-fractures and macro-fractures are abundant in coal reservoirs, and the formation of such fractures depends on the coal-bearing environment. In addition to the development of fractures, the gas content and coalbed methane productivity are also controlled by the coal-bearing environment. Research on micro-fracture characteristics is conducted through microscopic observation, and research on macrofracture characteristics is conducted by investigating coal cores. The method of quantitative statistics is adopted for the measure of the submaceral, and the coal facies parameters are calculated. The coal structure is explained using logging data. Then, the coalbed methane productivity controlled by the coal facies is examined. The results show that Type D micro-fractures are well developed, followed by Type C micro-fractures. The micro-fractures are primarily tensional fractures, implying that the formation of micro-fractures is controlled by external stress. Micro-fractures pass through the macropores and effectively link with other micro-fractures, whereas macro-fractures are primarily developed in clarain bands and cataclastic texture coal. The gas content increases with the gelatification index (GI) and ratio of vitrinite to intertinite (V/I) in the coal reservoir, which are favourable coal facies geological conditions for coalbed methane production. However, whereas the production of the coalbed methane wells is in its initial stage, the methane produced is mostly free and strongly desorbed gas. The coal structure primarily contributes to the permeability of the coal reservoir. The development of Type II coal can increase the production of methane, whereas the development of Type III coal has the opposite effect. © 2015 Published by Elsevier B.V.
Keywords: Coalbed methane Macro-fractures Micro-fractures Coal facies Coalbed methane productivity
1. Introduction Coal consists of a matrix, pores and fractures. The pore-fracture system not only provides storage space but also serves as the migration pathway of coalbed methane (Laxminarayana and Crosdale, 1999). Research on pore-fracture systems has evolved from studies on dual porosity systems to studies on triple porosity systems (Fu and Qin, 2003). The types of fractures that are observed include macroscopic and microscopic fractures, and different observation methods lead to different fracture classifications. Outside China, systematic fractures in coal are referred to, in ancient mining terms, as cleats, including face cleats and butt cleats (Gamson et al., 1993; Laubach et al., 1998; Paul and Chatterjee,
* Corresponding author. Room 338, Key Laboratory of Coalbed Methane Resource and Reservoir Formation Process, Ministry of Education, South Jiefang Road, Xuzhou, Jiangsu Province, 221008, China. E-mail address: [email protected] (C. Wu). http://dx.doi.org/10.1016/j.jngse.2015.01.014 1875-5100/© 2015 Published by Elsevier B.V.
2011). Huo and Zhang (1998) proposed origin fractures and joint fractures, and Su et al. (2001) presented micro-fractures and macro-fractures. Su et al. (2001) concluded that special fractures depend on the fluid pressure in a coal reservoir. Stress leads to the formation of fractures. The origins of stress are varied, and internal stress and exterior stress are the primary types (Nickelsen and Hough, 1967; Ting, 1977; Karacan and Okandan, 2000). The intrinsic tensile force and matrix shrinkage are the main types of internal stress, and tectonic stress is the main type of exterior stress. The development of endogenetic fractures is related to the lithotype of coal, coal rank and coal thickness (Laubach et al., 1998). Exogenetic fractures are primarily formed by crustal stress. The classification method of macro-fractures and micro-fractures is adopted in this study. Fractures that can be observed with the naked eye are termed macro-fractures, and fractures observed through a microscope are termed micro-fractures. The reticular, isolated and random fractures occur in coal reservoirs (Su et al., 2001), and the sizes of such fractures are vary. The fractures formed by tectonic stress are large, whereas unstressed fractures
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appear small (Gamson et al., 1993; Laubach et al., 1998; Su et al., 2001). The fracture characteristics influence the interstitial flow and production of coalbed methane (Karacan and Okandan, 2000; Fu et al., 2001a, 2001b; Su et al., 2005; Acosta et al., 2007; Meng et al., 2014), fractures and pores contribute to the permeability of coal reservoirs (Dawson and Esterle, 2010; Qu et al., 2011), and fracture surface morphology influences the methane flow (Karacan and Okandan, 2000). The exogenetic fractures are the primary location for interstitial flow, and endogenetic fractures are the bridge between the desorption and interstitial flow of coalbed methane (Fu et al., 2004; Su et al., 2005; Liu et al., 2012). Previous studies have focused on the description of fractures, including the width, length, spacing, connectedness, aperture and degree of mineral fill, and quantitative methods for evaluating coal reservoirs have also been presented (Laubach et al., 1998; Su et al., 2001; Yao et al., 2007). However, these studies have not considered the coalbearing environments. In fact, the components and properties of peat would be much different in different coal-bearing environments (Wang and Chen, 1995), which would result in different mechanical properties of coal reservoirs. The characteristics of coal facies controlled by the coal-bearing environments have an important influence on the gas content of coal reservoirs as well as on coalbed methane productivity. In this study, coal samples were collected from the South Yanchuan Block, and the polished surfaces were examined under a fluorescence microscope and scanning electron microscope to investigate the characteristics of micro-fractures. The characteristics of macro-fractures were observed using coal cores. The content of the submaceral was quantitatively counted, and the parameters of coal facies were calculated, including the tissue preservation index (TPI), transportation index (TI), gelatification index (GI) and the ratio of vitrinite to intertinite (V/I). In addition, the influence of coal facies on coalbed methane productivity was investigated.
2. Geological setting The South Yanchuan Block is situated in the southeast of the Ordos Basin, at the junction of Shanxi Province and Shaanxi Province. The tectonic system is a simple, monoclinal structure with a northeast trend and western dip. The area can be divided into five third-class structural units; the Xi Baigou gentle slope belt, Bai E fault nose structure belt, Tan Ping gentle slope belt, Bai Shansi fault nose structure belt, and Wan Baoshan gentle slope belt are distributed from east to west (Fig. 1). The primary coal-bearing strata are the Upper Carboniferous - Lower Permian Taiyuan Formation (C2eP1t) and Lower Permian Shanxi Formation (P1s), and the No. 2 coal seam of the Shanxi Formation is the target stratum for the development of coalbed methane. The burial depth of the No. 2 coal seam ranges from 659.10 m to 1502.00 m; the coal thickness ranges from 2.70 m to 7.50 m, with an average thickness of 5.03 m; the gas content ranges from 6.10 m3/t to 20.40 m3/t, with an average gas content of 10.13 m3/t, and most of the reservoir gas content exceeds 8.00 m3/t. Currently, there are seventeen coalbed methane wells in the South Yanchuan Block, and production times have surpassed one year. According to existing data, there is no magmatic activity in the research area, and the well temperature gradient ranges from 1.12 to 1.49 C/100 m, indicating that the earth temperature is flat. The reservoir pressure gradient ranges from 3.14 to 7.53 kPa/100 m, indicating that the reservoir is under pressure. The coal samples collected for this study were derived from the coal cores of coalbed methane wells. Because they are limited to the drilling conditions under which they were collected, the samples are incomplete. 3. Methods The coal sample from the S5 well is characterised by a polished surface based on the method of preparing coal samples for coal
Fig. 1. Location of the coalbed methane block.
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petrographic analysis (GB/T 16773-2008, Chinese national standard). With a 50 fluorescence microscope, the 30 30 mm coal polished section was divided into nine different areas of 10 10 mm each; then, the quantities of different types of microfractures were counted. According to the length and width of the micro-fractures, the micro-fractures were divided into Type A, Type B, Type C and Type D fractures (Table 1). The interstitial flow of methane occurs in Type A fractures; the development of Type B fractures favours the diffusion and migration of methane; Type C fractures are the migration passageway for methane; and Type D fractures are the medium for linking the matrix pores and fractures. Moreover, the link between the pores and fractures and the shape of fractures were observed with the scanning electron microscope. For the macro-fractures, the fracture density was counted using the coal cores. The coal samples were collected at 30 cm equidistant spacings from the coal reservoir, and the characteristics of the coal facies could be accurately determined using a well-distributed sampling method. Then, the polished surfaces were examined. According to a maceral analysis of the polished surfaces of whole rocks (SY/T 64141999, Chinese petroleum and natural gas industry standard), with a grid distance of 1 mm 1 mm, the content of submarceral and inorganic minerals was determined. At least 800 available points are counted, and the parameters of the coal facies were calculated using Eqs. (1)e(4), including TPI, TI, GI and V/I (Marques, 2002). According to the clustering method, the apparent resistivity, interval transit time and natural gamma were determined to explain the coal structure (Fu et al., 2009).
627
Fig. 2. Fractures in the coal sample of the S5 well.
TPI ¼ ðtelovitrinite þ semifusinite þ fusiniteÞ =ðdetrovitrinite þ inertodetriniteÞ
(1)
GI ¼ ðtotal vitriniteÞ=ðtotal inertinite; excluding micriniteÞ (2) V=I ¼ ðtotal vitriniteÞ=ðtotal inertiniteÞ
(3)
TI ¼ ðgelovitrinite þ detrovitrinite þ mineral matterÞ =ðtelovitriniteÞ
(4)
fractures that develop in clarain bands are influenced by coalbearing environments. Oxidising reduction environments in the coal forming period determine the coal macerals, and the development of fractures is controlled by mechanical properties and the content of different coal macerals, whereas internal stress controls the formation of the fractures. Fractures that cross through the coal bands are controlled by external stress, whereas the fractures terminating in the coal bands indicate that the formation of fractures is synthetically controlled by internal and external stress (Fu et al., 2007).
4.1.2. Characteristics of micro-fractures In the South Yanchuan Block, Type D micro-fractures are primarily developed, followed by Type C micro-fractures, whereas Type A and B micro-fractures are undeveloped (Table 2). The development of Type C and D micro-fractures favours the
4. Results and discussion 4.1. Characteristics of fractures 4.1.1. Characteristics of macro-fractures The fracture density in the vertical stratification direction is greater than that in the horizontal stratification direction. In the horizontal stratification direction, the fracture density ranges from 2 pcs/25 cm2 to 19 pcs/25 cm2, with an average of 6 pcs/25 cm2, and the fracture density ranges from 3 pcs/25 cm2 to 25 pcs/25 cm2 in the vertical stratification direction, with an average fracture density of 10 pcs/25 cm2. The lengths of the macro-fractures are different. Some macro-fractures only develop in clarain bands, whereas others develop across several coal bands (Fig. 2). The macro-
Table 1 Types of micro-fracutres. Dimensions
Type A
Type B
Type C
Type D
Width Length
>5 mm >10 mm
>5 mm 1 mme10 mm
<5 mm 300 mme1 mm
<5 mm <300 mm
Table 2 Coal micro-fracture statistics in the South Yanchuan Block. Samples Type A pcs
Type B pcs
S5-2
0
4
DH2#-1 DH2#-2 DH2#-3 HS1 HS2 HS3 WLG2#1 WLG2#2 WLG2#3
0 0 0 0 0 0 0
Type C pcs
Type D pcs
Total pcs
Data sources
3
15
22
1 2 3 2 1 0 0
19 19 27 21 24 8 13
18 42 51 27 52 2 12
38 63 71 50 80 10 25
Actual measurement Chen et al. (2013).
0
0
14
13
27
0
1
11
7
19
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communication between micro-fractures and pores in the coal matrix. The deficiency of Type A and B micro-fractures makes it difficult for coalbed methane to migrate and seep. Scanning electron microscopy images show that the exogenetic fractures are more developed; the exogenetic fractures appear wavy and serrated, whereas the endogenetic fractures are undeveloped (Fig. 3e). The secondary scale fractures always develop around the large-scale fractures (Fig. 3a, c), and the fractures connect to each other (Fig. 3a). In addition to direct communication, fractures can be connected through the macropores (Fig. 3b, d). Clay minerals are distributed on the surface of the coal matrix, at the edges of fractures and around pores (Fig. 3b, e, f); the minerals are banded and bulky and are at a disadvantage in the development of coalbed methane. 4.2. Control of coal-bearing environments and external stress on fractures 4.2.1. Control of coal-bearing environments on fractures The development and accumulation of fractures in the clarain bands indicate that the coal macerals are selective, a feature that is greatly related to the coal-bearing environments. The tissue preservation index (TPI), transportation index (TI), gelatification index (GI) and the ratio of vitrinite to intertinite (V/I) can be used to
reflect the characteristics of coal-bearing environments (Marques, 2002). The relationship between coal facies parameters and the horizontal stratification direction fracture density is distinct; there is a positive correlation between the GI and fracture density and between V/I and the fracture density, whereas there is a negative correlation between the TPI and fracture density and between the TI and fracture density (Fig. 4). Fractures in the clarain bands have endogenetic fractures, which is strongly correlated with the high vitrinite content. During the spoilage process of coal, with an increase in the vitrinite's hydrocarbon-generating intensity, a high instantaneous gas rate typically appears. The homogeneous organic macerals, especially telocollinite, are an ideal location for the production of this high instantaneous gas rate (Wang et al., 1996). During the formation of coal, in environments characterised by high reduction and low flowing water, gelatinisation occurs more easily, and the vitrinite content is high in the coal macerals, which results in a high V/I in the coal reservoir. Vitrinite is brittle and endogenetic fractures are more easily generated with the same geological stress; therefore, the clarain bands can generate many more fractures. 4.2.2. Control of external stress on fractures Coal deformation during geologic processes results in the
Fig. 3. Scan of electron microscope photos of coal samples.
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Fig. 4. The relationship between coal facies parameters and coal fracture.
destruction of the primary structure of coal. Fractures controlled by external stress develop into macro-fractures. The weak deformation of coal can accelerate the transport of methane, whereas strong deformation may form tectonic coal, which would make desorption difficult. Because the coal samples examined in this study were collected from drilling holes, the distribution in the coal reservoir could not be determined. Thus, the coal structure was used to represent the characteristics of fractures. The coal structure in the research area was determined based on the combined results obtained from coal cores and logging data (Fu et al., 2009). We divided the coal structures observed into three types: primary texture (Type I), cataclastic texture (Type II) and mylonitic texture (Type III) (Table 3). Primary texture coal can maintain its primitive condition; cataclastic texture coal can burn fractures influenced by geological structure; and because mylonitic texture coal is pulverous, the development of fractures was not observed. The fracture density characteristics of primary texture coal and cataclastic texture coal
Table 3 Coal structure statistics from the well log interpretation method. Wells
S1 S2 S3 S4 S5 S6 S7 S8 S10 S12 S13 S11 S15 S16 S17 S18 S20
Type I coal
Type II coal
Type III coal
Thickness m
Ratio %
Thickness m
Ratio %
Thickness m
Ratio %
4.90 0.00 4.50 0.00 2.70 0.00 0.00 0.70 0.00 0.65 5.30 5.10 0.00 2.72 0.00 0.00 0.00
100.00 0.00 100.00 0.00 58.70 0.00 0.00 17.28 0.00 10.48 100.00 100.00 0.00 41.72 0.00 0.00 0.00
0.00 0.00 0.00 0.00 1.90 4.80 4.10 1.75 3.30 2.00 0.00 0.00 1.60 3.80 2.10 0.00 0.00
0.00 0.00 0.00 0.00 41.30 100.00 100.00 43.21 55.93 32.26 0.00 0.00 41.03 58.28 53.85 0.00 0.00
0.00 6.60 0.00 5.10 0.00 0.00 0.00 1.60 2.60 3.55 0.00 0.00 2.30 0.00 1.80 4.20 3.40
0.00 100.00 0.00 100.00 0.00 0.00 0.00 39.51 44.07 57.26 0.00 0.00 58.97 0.00 46.15 100.00 100.00
were compared. The fracture density in primary texture coal was less than 6 pcs/25 cm2. The fracture density in cataclastic texture coal could reach 19 pcs/25 cm2, whereas primary texture coal could reach twice that density in the same coalbed methane well (Fig. 5). This result illustrates that cataclastic texture coal contributes more towards the development of fractures. 4.3. Coalbed methane productivity influenced by coal facies and coal structure Coal facies control the reservoir properties, including gas content and permeability, which influence coalbed methane production. Coalbed methane wells are in the initial recovery stage in the South Yanchuan Block, where most of the coalbed methane wells' methane productivity is low. To ensure the validity of the data, the coalbed methane wells with a steady recovery history beyond 450 days were selected as the targets for this study. 4.3.1. Gas content influenced by coal facies 4.3.1.1. GI and TPI. The methane content increases with an increase in the GI and V/I (Fig. 6). The hydrocarbon generation rate of vitrinite is lower than that of exinite, but the adsorption capacity of methane is higher than that of exinite and that of inertinite (Fu et al., 2007). Furthermore, the high GI also indicates that the submacerals with enrichment micropores are more developed and are conducive to adsorption of methane. 4.3.1.2. TI. The coal reservoir with a high TI indicates that there are strong hydrodynamic conditions during the coal forming period. The coal environment, which exhibits strong oxidisability, leads to the high inertinite content, which has a negative influence on the development of the adsorption pore and the hydrocarbon generation rate, and the methane content is low (Fig. 6). 4.3.1.3. TPI. The effect of the TPI on the methane content influenced is complex; and can be divided into two different stages based on a threshold TPI value of 0.8 (Fig. 6). When the TPI is less than 0.8, the coal-bearing environments are reductive. Due to high gelatification, hydrocarbon generation has an advantage because of
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Fig. 5. The relationship between coal structure and fracture density.
Fig. 6. The relationship between coal facies parameters and methane content.
the high adsorption pore and vitrinite content. When the TPI is greater than 0.8, the coal-bearing environments are influenced by the ratio of coal forming plants and the rate of peat accumulation and oxidation reduction environments. There is no major control factor. 4.3.2. Coalbed methane productivity influenced by coal facies The coal facies parameters affect the reservoir gas content, which controls the productivity of the coalbed methane wells. The high GI and V/I should have provided favourable coal facies conditions for the high productivity of the coalbed methane. However, the actual situation showed the opposite trend (Fig. 7). The high V/I indicates that the vitrinite content is high, which favours the development of fractures, and the vitrodetrinite content in the research area is high, which signifies that the mechanical properties of the coal reservoir in the research area are weak. Under the
same tectonic stress, the coalbed could show many more fractures, which would result in the development of tectonic coal, and the coalbed methane production may be low. With respect to the relationship between coalbed methane production and TPI, the coalbed methane production and TI reflect a strong oxidising environment. The vitrinite content is low in the coal macerals, making it unfavourable for hydrocarbon generation (Fig. 7). The high TPI and TI also indicate that the oxidation macerals' content is high and that the desorption of methane is strong. Currently, the production period of coalbed methane wells in the research area is only one or two years. Coalbed methane is primarily produced as free gas or as weakly adsorbed or strongly desorbed methane within the coal matrix. Eq. (5) is used to calculate the drainage radius of the coalbed methane wells in the research area (Lv et al., 2012). The drainage radius maximum is only 77.68 m (Table 4), which is equal to the fracturing radius,
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631
Fig. 7. The relationship between coal facies parameters and coalbed methane productivity.
supporting the conjecture that the methane produced occurs in the form of free gas and strongly desorbed gas.
Qw ¼ pr 2 h4ð1 Swo Þ
(5)
where Qw is the cumulative water production, m3; r is the drainage radius of the equivalent circle model, m; h is the reservoir thickness, m; 4 is the porosity; and Swo is the irreducible water saturation, which ranges from 47.6% to 74.1%. 4.3.3. Coalbed methane productivity influenced by coal structure The coal structure contributes to the permeability of the coal reservoir, and the development of Type III coal depresses reservoir permeability (Fig. 8a). As shown in Fig. 8a, the reservoir coal structure of S2, S4 and S20 is a complete mylonitic texture, especially that of S20 well, and the permeability is low. Compared with the other wells, the primary texture coal and cataclastic texture coal are more developed and the reservoir permeability is higher. In the coalbed methane wells, with productivity rates beyond 1000 m3/d, the rates of Type I and Type II are generally high. Type I coal can maintain the primary structure of a coal reservoir, and the development of Type II coal can increase the productivity of a single well. The development of Type II coal can increase the development of the fractures and raise the conductivity of the reservoir. With a thick reservoir and high gas content, the coalbed methane productivity is high. In the coalbed methane wells, the productivity rate is under 500 m3/d, and Type III coal is more developed, which signifies that the mylonitic texture coal is more developed. With an increase in the structural complexity of coal, particularly an increase in the production rate of Type III coal, the productivity of the coalbed methane wells is reduced (Fig. 8b). In these low productivity coalbed methane wells, where coal dust is discharged, it can be conjectured that the coal dust is derived from Type III coal. With the discharge of coal dust during gas production, pores and fractures are easily blocked. Furthermore, clay minerals may cause the
pathway of fluid flow to be tortuous (Harpalani and McPherson, 1984; Harpalani and Schraufnagel, 1991; Roberts et al., 1994); and may reduce the percolation capacity, which hamper coalbed methane production. 5. Conclusion (1) In the South Yanchuan Block, Type D micro-fractures are the most developed, followed by Type C micro-fractures. These micro-fractures are well linked to pores; thus, undeveloped Type A and B fractures make the migration and interstitial flow of methane difficult. The exogenetic fractures are more developed, and the endogenetic fractures are rare, indicating that the formation of micro-fractures is primarily controlled by external stress. The development of macro-fractures is controlled by the coal-bearing environment and geological structure. Clarain bands and cataclastic texture coal favour the development of macro-fractures. (2) Gas content and permeability are the primary factors that affect coalbed methane productivity, which are controlled by coal facies. With an increase in the GI and V/I, the gas content increases. A high GI indicates that the submacerals with enrichment micropores are more developed. The methane content is low with a high TI, which is a disadvantage for the high productivity of methane. The relationship between the TPI and methane content is characterised by segmentation. TPI values less than 0.8, with the high gelatification of plants, facilitate hydrocarbon generation, whereas TPI values greater than 0.8, the relationship between the TPI and methane content is inconspicuous. The relationships between methane productivity and GI and between the methane productivity and V/I could not be systematically understood. Thus, the coalbed methane wells remain in production infancy, and the methane produced primarily occurs as free gas and strongly desorbed gas. The coal structure contributes to
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References
Table 4 Equivalent drainage radius of coalbed methane wells. Maximum Wells Cumulative water Porosity Thickness Minimum production m3 % m drainage radius drainage radius m m S3 S4 S5 S8 S10
142.90 658.02 1040.56 239.39 463.54
4.39 4.53 4.61 4.09 4.52
4.50 4.10 4.60 3.40 6.10
20.97 46.40 54.61 32.35 31.97
29.82 66.00 77.68 46.01 45.47
Fig. 8. The relationship between coal structure and coalbed methane productivity.
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Time delay effect due to pore pressure changes and existence of cleats on borehole stability in coal seam. Int. J. Coal Geol. 85 (2), 212e218. Roberts, S.B., Stanton, R.W., Folres, R.M., 1994. A debris flow deposit in alluvial, coalbearing faces, Bighorn basin, Wyoming, USA: evidence for catastrophic termination of a mire. Int. J. Coal Geol. 25 (3e4), 213e241. Su, X.B., Feng, Y.L., Chen, J.F., Pan, J.N., 2001. The characteristics and origins of cleat in coal from Western North China. Int. J. Coal Geol. 47 (1), 51e62. Su, X.B., Lin, X.Y., Liu, S.B., Zhao, M.J., Song, Y., 2005. Geology of coalbed methane reservoirs in the Southeast Qinshui Basin of China. Int. J. Coal Geol. 62 (4), 197e210. Ting, F.T.C., 1977. Origin and spacing of cleats in coal beds. J. Press. Vess. Technol. 99 (4), 624e626. Wang, S.W., Chen, Z.H., 1995. Advances in the research of pore-fracture system in coal reservoir. Geol. Sci. Technol. Inf. 14 (1), 53e59. Wang, S.W., Zhang, M., Zhuang, X.L., 1996. 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Characteristics of coal fractures and the influence of coal facies on coalbed methane productivity in the South Yanchuan Block, China Teng Li a, b, Caifang Wu a, b, *, Qiang Liu a, b a b
School of Mineral Resources and Geosciences, China University of Mining & Technology, Xuzhou, Jiangsu Province, 221008, China Key Laboratory of Coalbed Methane Resource and Reservoir Formation Process, Ministry of Education, Xuzhou, Jiangsu Province, 221008, China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 13 November 2014 Received in revised form 6 January 2015 Accepted 7 January 2015 Available online 12 January 2015
Micro-fractures and macro-fractures are abundant in coal reservoirs, and the formation of such fractures depends on the coal-bearing environment. In addition to the development of fractures, the gas content and coalbed methane productivity are also controlled by the coal-bearing environment. Research on micro-fracture characteristics is conducted through microscopic observation, and research on macrofracture characteristics is conducted by investigating coal cores. The method of quantitative statistics is adopted for the measure of the submaceral, and the coal facies parameters are calculated. The coal structure is explained using logging data. Then, the coalbed methane productivity controlled by the coal facies is examined. The results show that Type D micro-fractures are well developed, followed by Type C micro-fractures. The micro-fractures are primarily tensional fractures, implying that the formation of micro-fractures is controlled by external stress. Micro-fractures pass through the macropores and effectively link with other micro-fractures, whereas macro-fractures are primarily developed in clarain bands and cataclastic texture coal. The gas content increases with the gelatification index (GI) and ratio of vitrinite to intertinite (V/I) in the coal reservoir, which are favourable coal facies geological conditions for coalbed methane production. However, whereas the production of the coalbed methane wells is in its initial stage, the methane produced is mostly free and strongly desorbed gas. The coal structure primarily contributes to the permeability of the coal reservoir. The development of Type II coal can increase the production of methane, whereas the development of Type III coal has the opposite effect. © 2015 Published by Elsevier B.V.
Keywords: Coalbed methane Macro-fractures Micro-fractures Coal facies Coalbed methane productivity
1. Introduction Coal consists of a matrix, pores and fractures. The pore-fracture system not only provides storage space but also serves as the migration pathway of coalbed methane (Laxminarayana and Crosdale, 1999). Research on pore-fracture systems has evolved from studies on dual porosity systems to studies on triple porosity systems (Fu and Qin, 2003). The types of fractures that are observed include macroscopic and microscopic fractures, and different observation methods lead to different fracture classifications. Outside China, systematic fractures in coal are referred to, in ancient mining terms, as cleats, including face cleats and butt cleats (Gamson et al., 1993; Laubach et al., 1998; Paul and Chatterjee,
* Corresponding author. Room 338, Key Laboratory of Coalbed Methane Resource and Reservoir Formation Process, Ministry of Education, South Jiefang Road, Xuzhou, Jiangsu Province, 221008, China. E-mail address: [email protected] (C. Wu). http://dx.doi.org/10.1016/j.jngse.2015.01.014 1875-5100/© 2015 Published by Elsevier B.V.
2011). Huo and Zhang (1998) proposed origin fractures and joint fractures, and Su et al. (2001) presented micro-fractures and macro-fractures. Su et al. (2001) concluded that special fractures depend on the fluid pressure in a coal reservoir. Stress leads to the formation of fractures. The origins of stress are varied, and internal stress and exterior stress are the primary types (Nickelsen and Hough, 1967; Ting, 1977; Karacan and Okandan, 2000). The intrinsic tensile force and matrix shrinkage are the main types of internal stress, and tectonic stress is the main type of exterior stress. The development of endogenetic fractures is related to the lithotype of coal, coal rank and coal thickness (Laubach et al., 1998). Exogenetic fractures are primarily formed by crustal stress. The classification method of macro-fractures and micro-fractures is adopted in this study. Fractures that can be observed with the naked eye are termed macro-fractures, and fractures observed through a microscope are termed micro-fractures. The reticular, isolated and random fractures occur in coal reservoirs (Su et al., 2001), and the sizes of such fractures are vary. The fractures formed by tectonic stress are large, whereas unstressed fractures
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appear small (Gamson et al., 1993; Laubach et al., 1998; Su et al., 2001). The fracture characteristics influence the interstitial flow and production of coalbed methane (Karacan and Okandan, 2000; Fu et al., 2001a, 2001b; Su et al., 2005; Acosta et al., 2007; Meng et al., 2014), fractures and pores contribute to the permeability of coal reservoirs (Dawson and Esterle, 2010; Qu et al., 2011), and fracture surface morphology influences the methane flow (Karacan and Okandan, 2000). The exogenetic fractures are the primary location for interstitial flow, and endogenetic fractures are the bridge between the desorption and interstitial flow of coalbed methane (Fu et al., 2004; Su et al., 2005; Liu et al., 2012). Previous studies have focused on the description of fractures, including the width, length, spacing, connectedness, aperture and degree of mineral fill, and quantitative methods for evaluating coal reservoirs have also been presented (Laubach et al., 1998; Su et al., 2001; Yao et al., 2007). However, these studies have not considered the coalbearing environments. In fact, the components and properties of peat would be much different in different coal-bearing environments (Wang and Chen, 1995), which would result in different mechanical properties of coal reservoirs. The characteristics of coal facies controlled by the coal-bearing environments have an important influence on the gas content of coal reservoirs as well as on coalbed methane productivity. In this study, coal samples were collected from the South Yanchuan Block, and the polished surfaces were examined under a fluorescence microscope and scanning electron microscope to investigate the characteristics of micro-fractures. The characteristics of macro-fractures were observed using coal cores. The content of the submaceral was quantitatively counted, and the parameters of coal facies were calculated, including the tissue preservation index (TPI), transportation index (TI), gelatification index (GI) and the ratio of vitrinite to intertinite (V/I). In addition, the influence of coal facies on coalbed methane productivity was investigated.
2. Geological setting The South Yanchuan Block is situated in the southeast of the Ordos Basin, at the junction of Shanxi Province and Shaanxi Province. The tectonic system is a simple, monoclinal structure with a northeast trend and western dip. The area can be divided into five third-class structural units; the Xi Baigou gentle slope belt, Bai E fault nose structure belt, Tan Ping gentle slope belt, Bai Shansi fault nose structure belt, and Wan Baoshan gentle slope belt are distributed from east to west (Fig. 1). The primary coal-bearing strata are the Upper Carboniferous - Lower Permian Taiyuan Formation (C2eP1t) and Lower Permian Shanxi Formation (P1s), and the No. 2 coal seam of the Shanxi Formation is the target stratum for the development of coalbed methane. The burial depth of the No. 2 coal seam ranges from 659.10 m to 1502.00 m; the coal thickness ranges from 2.70 m to 7.50 m, with an average thickness of 5.03 m; the gas content ranges from 6.10 m3/t to 20.40 m3/t, with an average gas content of 10.13 m3/t, and most of the reservoir gas content exceeds 8.00 m3/t. Currently, there are seventeen coalbed methane wells in the South Yanchuan Block, and production times have surpassed one year. According to existing data, there is no magmatic activity in the research area, and the well temperature gradient ranges from 1.12 to 1.49 C/100 m, indicating that the earth temperature is flat. The reservoir pressure gradient ranges from 3.14 to 7.53 kPa/100 m, indicating that the reservoir is under pressure. The coal samples collected for this study were derived from the coal cores of coalbed methane wells. Because they are limited to the drilling conditions under which they were collected, the samples are incomplete. 3. Methods The coal sample from the S5 well is characterised by a polished surface based on the method of preparing coal samples for coal
Fig. 1. Location of the coalbed methane block.
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petrographic analysis (GB/T 16773-2008, Chinese national standard). With a 50 fluorescence microscope, the 30 30 mm coal polished section was divided into nine different areas of 10 10 mm each; then, the quantities of different types of microfractures were counted. According to the length and width of the micro-fractures, the micro-fractures were divided into Type A, Type B, Type C and Type D fractures (Table 1). The interstitial flow of methane occurs in Type A fractures; the development of Type B fractures favours the diffusion and migration of methane; Type C fractures are the migration passageway for methane; and Type D fractures are the medium for linking the matrix pores and fractures. Moreover, the link between the pores and fractures and the shape of fractures were observed with the scanning electron microscope. For the macro-fractures, the fracture density was counted using the coal cores. The coal samples were collected at 30 cm equidistant spacings from the coal reservoir, and the characteristics of the coal facies could be accurately determined using a well-distributed sampling method. Then, the polished surfaces were examined. According to a maceral analysis of the polished surfaces of whole rocks (SY/T 64141999, Chinese petroleum and natural gas industry standard), with a grid distance of 1 mm 1 mm, the content of submarceral and inorganic minerals was determined. At least 800 available points are counted, and the parameters of the coal facies were calculated using Eqs. (1)e(4), including TPI, TI, GI and V/I (Marques, 2002). According to the clustering method, the apparent resistivity, interval transit time and natural gamma were determined to explain the coal structure (Fu et al., 2009).
627
Fig. 2. Fractures in the coal sample of the S5 well.
TPI ¼ ðtelovitrinite þ semifusinite þ fusiniteÞ =ðdetrovitrinite þ inertodetriniteÞ
(1)
GI ¼ ðtotal vitriniteÞ=ðtotal inertinite; excluding micriniteÞ (2) V=I ¼ ðtotal vitriniteÞ=ðtotal inertiniteÞ
(3)
TI ¼ ðgelovitrinite þ detrovitrinite þ mineral matterÞ =ðtelovitriniteÞ
(4)
fractures that develop in clarain bands are influenced by coalbearing environments. Oxidising reduction environments in the coal forming period determine the coal macerals, and the development of fractures is controlled by mechanical properties and the content of different coal macerals, whereas internal stress controls the formation of the fractures. Fractures that cross through the coal bands are controlled by external stress, whereas the fractures terminating in the coal bands indicate that the formation of fractures is synthetically controlled by internal and external stress (Fu et al., 2007).
4.1.2. Characteristics of micro-fractures In the South Yanchuan Block, Type D micro-fractures are primarily developed, followed by Type C micro-fractures, whereas Type A and B micro-fractures are undeveloped (Table 2). The development of Type C and D micro-fractures favours the
4. Results and discussion 4.1. Characteristics of fractures 4.1.1. Characteristics of macro-fractures The fracture density in the vertical stratification direction is greater than that in the horizontal stratification direction. In the horizontal stratification direction, the fracture density ranges from 2 pcs/25 cm2 to 19 pcs/25 cm2, with an average of 6 pcs/25 cm2, and the fracture density ranges from 3 pcs/25 cm2 to 25 pcs/25 cm2 in the vertical stratification direction, with an average fracture density of 10 pcs/25 cm2. The lengths of the macro-fractures are different. Some macro-fractures only develop in clarain bands, whereas others develop across several coal bands (Fig. 2). The macro-
Table 1 Types of micro-fracutres. Dimensions
Type A
Type B
Type C
Type D
Width Length
>5 mm >10 mm
>5 mm 1 mme10 mm
<5 mm 300 mme1 mm
<5 mm <300 mm
Table 2 Coal micro-fracture statistics in the South Yanchuan Block. Samples Type A pcs
Type B pcs
S5-2
0
4
DH2#-1 DH2#-2 DH2#-3 HS1 HS2 HS3 WLG2#1 WLG2#2 WLG2#3
0 0 0 0 0 0 0
Type C pcs
Type D pcs
Total pcs
Data sources
3
15
22
1 2 3 2 1 0 0
19 19 27 21 24 8 13
18 42 51 27 52 2 12
38 63 71 50 80 10 25
Actual measurement Chen et al. (2013).
0
0
14
13
27
0
1
11
7
19
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communication between micro-fractures and pores in the coal matrix. The deficiency of Type A and B micro-fractures makes it difficult for coalbed methane to migrate and seep. Scanning electron microscopy images show that the exogenetic fractures are more developed; the exogenetic fractures appear wavy and serrated, whereas the endogenetic fractures are undeveloped (Fig. 3e). The secondary scale fractures always develop around the large-scale fractures (Fig. 3a, c), and the fractures connect to each other (Fig. 3a). In addition to direct communication, fractures can be connected through the macropores (Fig. 3b, d). Clay minerals are distributed on the surface of the coal matrix, at the edges of fractures and around pores (Fig. 3b, e, f); the minerals are banded and bulky and are at a disadvantage in the development of coalbed methane. 4.2. Control of coal-bearing environments and external stress on fractures 4.2.1. Control of coal-bearing environments on fractures The development and accumulation of fractures in the clarain bands indicate that the coal macerals are selective, a feature that is greatly related to the coal-bearing environments. The tissue preservation index (TPI), transportation index (TI), gelatification index (GI) and the ratio of vitrinite to intertinite (V/I) can be used to
reflect the characteristics of coal-bearing environments (Marques, 2002). The relationship between coal facies parameters and the horizontal stratification direction fracture density is distinct; there is a positive correlation between the GI and fracture density and between V/I and the fracture density, whereas there is a negative correlation between the TPI and fracture density and between the TI and fracture density (Fig. 4). Fractures in the clarain bands have endogenetic fractures, which is strongly correlated with the high vitrinite content. During the spoilage process of coal, with an increase in the vitrinite's hydrocarbon-generating intensity, a high instantaneous gas rate typically appears. The homogeneous organic macerals, especially telocollinite, are an ideal location for the production of this high instantaneous gas rate (Wang et al., 1996). During the formation of coal, in environments characterised by high reduction and low flowing water, gelatinisation occurs more easily, and the vitrinite content is high in the coal macerals, which results in a high V/I in the coal reservoir. Vitrinite is brittle and endogenetic fractures are more easily generated with the same geological stress; therefore, the clarain bands can generate many more fractures. 4.2.2. Control of external stress on fractures Coal deformation during geologic processes results in the
Fig. 3. Scan of electron microscope photos of coal samples.
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Fig. 4. The relationship between coal facies parameters and coal fracture.
destruction of the primary structure of coal. Fractures controlled by external stress develop into macro-fractures. The weak deformation of coal can accelerate the transport of methane, whereas strong deformation may form tectonic coal, which would make desorption difficult. Because the coal samples examined in this study were collected from drilling holes, the distribution in the coal reservoir could not be determined. Thus, the coal structure was used to represent the characteristics of fractures. The coal structure in the research area was determined based on the combined results obtained from coal cores and logging data (Fu et al., 2009). We divided the coal structures observed into three types: primary texture (Type I), cataclastic texture (Type II) and mylonitic texture (Type III) (Table 3). Primary texture coal can maintain its primitive condition; cataclastic texture coal can burn fractures influenced by geological structure; and because mylonitic texture coal is pulverous, the development of fractures was not observed. The fracture density characteristics of primary texture coal and cataclastic texture coal
Table 3 Coal structure statistics from the well log interpretation method. Wells
S1 S2 S3 S4 S5 S6 S7 S8 S10 S12 S13 S11 S15 S16 S17 S18 S20
Type I coal
Type II coal
Type III coal
Thickness m
Ratio %
Thickness m
Ratio %
Thickness m
Ratio %
4.90 0.00 4.50 0.00 2.70 0.00 0.00 0.70 0.00 0.65 5.30 5.10 0.00 2.72 0.00 0.00 0.00
100.00 0.00 100.00 0.00 58.70 0.00 0.00 17.28 0.00 10.48 100.00 100.00 0.00 41.72 0.00 0.00 0.00
0.00 0.00 0.00 0.00 1.90 4.80 4.10 1.75 3.30 2.00 0.00 0.00 1.60 3.80 2.10 0.00 0.00
0.00 0.00 0.00 0.00 41.30 100.00 100.00 43.21 55.93 32.26 0.00 0.00 41.03 58.28 53.85 0.00 0.00
0.00 6.60 0.00 5.10 0.00 0.00 0.00 1.60 2.60 3.55 0.00 0.00 2.30 0.00 1.80 4.20 3.40
0.00 100.00 0.00 100.00 0.00 0.00 0.00 39.51 44.07 57.26 0.00 0.00 58.97 0.00 46.15 100.00 100.00
were compared. The fracture density in primary texture coal was less than 6 pcs/25 cm2. The fracture density in cataclastic texture coal could reach 19 pcs/25 cm2, whereas primary texture coal could reach twice that density in the same coalbed methane well (Fig. 5). This result illustrates that cataclastic texture coal contributes more towards the development of fractures. 4.3. Coalbed methane productivity influenced by coal facies and coal structure Coal facies control the reservoir properties, including gas content and permeability, which influence coalbed methane production. Coalbed methane wells are in the initial recovery stage in the South Yanchuan Block, where most of the coalbed methane wells' methane productivity is low. To ensure the validity of the data, the coalbed methane wells with a steady recovery history beyond 450 days were selected as the targets for this study. 4.3.1. Gas content influenced by coal facies 4.3.1.1. GI and TPI. The methane content increases with an increase in the GI and V/I (Fig. 6). The hydrocarbon generation rate of vitrinite is lower than that of exinite, but the adsorption capacity of methane is higher than that of exinite and that of inertinite (Fu et al., 2007). Furthermore, the high GI also indicates that the submacerals with enrichment micropores are more developed and are conducive to adsorption of methane. 4.3.1.2. TI. The coal reservoir with a high TI indicates that there are strong hydrodynamic conditions during the coal forming period. The coal environment, which exhibits strong oxidisability, leads to the high inertinite content, which has a negative influence on the development of the adsorption pore and the hydrocarbon generation rate, and the methane content is low (Fig. 6). 4.3.1.3. TPI. The effect of the TPI on the methane content influenced is complex; and can be divided into two different stages based on a threshold TPI value of 0.8 (Fig. 6). When the TPI is less than 0.8, the coal-bearing environments are reductive. Due to high gelatification, hydrocarbon generation has an advantage because of
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Fig. 5. The relationship between coal structure and fracture density.
Fig. 6. The relationship between coal facies parameters and methane content.
the high adsorption pore and vitrinite content. When the TPI is greater than 0.8, the coal-bearing environments are influenced by the ratio of coal forming plants and the rate of peat accumulation and oxidation reduction environments. There is no major control factor. 4.3.2. Coalbed methane productivity influenced by coal facies The coal facies parameters affect the reservoir gas content, which controls the productivity of the coalbed methane wells. The high GI and V/I should have provided favourable coal facies conditions for the high productivity of the coalbed methane. However, the actual situation showed the opposite trend (Fig. 7). The high V/I indicates that the vitrinite content is high, which favours the development of fractures, and the vitrodetrinite content in the research area is high, which signifies that the mechanical properties of the coal reservoir in the research area are weak. Under the
same tectonic stress, the coalbed could show many more fractures, which would result in the development of tectonic coal, and the coalbed methane production may be low. With respect to the relationship between coalbed methane production and TPI, the coalbed methane production and TI reflect a strong oxidising environment. The vitrinite content is low in the coal macerals, making it unfavourable for hydrocarbon generation (Fig. 7). The high TPI and TI also indicate that the oxidation macerals' content is high and that the desorption of methane is strong. Currently, the production period of coalbed methane wells in the research area is only one or two years. Coalbed methane is primarily produced as free gas or as weakly adsorbed or strongly desorbed methane within the coal matrix. Eq. (5) is used to calculate the drainage radius of the coalbed methane wells in the research area (Lv et al., 2012). The drainage radius maximum is only 77.68 m (Table 4), which is equal to the fracturing radius,
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631
Fig. 7. The relationship between coal facies parameters and coalbed methane productivity.
supporting the conjecture that the methane produced occurs in the form of free gas and strongly desorbed gas.
Qw ¼ pr 2 h4ð1 Swo Þ
(5)
where Qw is the cumulative water production, m3; r is the drainage radius of the equivalent circle model, m; h is the reservoir thickness, m; 4 is the porosity; and Swo is the irreducible water saturation, which ranges from 47.6% to 74.1%. 4.3.3. Coalbed methane productivity influenced by coal structure The coal structure contributes to the permeability of the coal reservoir, and the development of Type III coal depresses reservoir permeability (Fig. 8a). As shown in Fig. 8a, the reservoir coal structure of S2, S4 and S20 is a complete mylonitic texture, especially that of S20 well, and the permeability is low. Compared with the other wells, the primary texture coal and cataclastic texture coal are more developed and the reservoir permeability is higher. In the coalbed methane wells, with productivity rates beyond 1000 m3/d, the rates of Type I and Type II are generally high. Type I coal can maintain the primary structure of a coal reservoir, and the development of Type II coal can increase the productivity of a single well. The development of Type II coal can increase the development of the fractures and raise the conductivity of the reservoir. With a thick reservoir and high gas content, the coalbed methane productivity is high. In the coalbed methane wells, the productivity rate is under 500 m3/d, and Type III coal is more developed, which signifies that the mylonitic texture coal is more developed. With an increase in the structural complexity of coal, particularly an increase in the production rate of Type III coal, the productivity of the coalbed methane wells is reduced (Fig. 8b). In these low productivity coalbed methane wells, where coal dust is discharged, it can be conjectured that the coal dust is derived from Type III coal. With the discharge of coal dust during gas production, pores and fractures are easily blocked. Furthermore, clay minerals may cause the
pathway of fluid flow to be tortuous (Harpalani and McPherson, 1984; Harpalani and Schraufnagel, 1991; Roberts et al., 1994); and may reduce the percolation capacity, which hamper coalbed methane production. 5. Conclusion (1) In the South Yanchuan Block, Type D micro-fractures are the most developed, followed by Type C micro-fractures. These micro-fractures are well linked to pores; thus, undeveloped Type A and B fractures make the migration and interstitial flow of methane difficult. The exogenetic fractures are more developed, and the endogenetic fractures are rare, indicating that the formation of micro-fractures is primarily controlled by external stress. The development of macro-fractures is controlled by the coal-bearing environment and geological structure. Clarain bands and cataclastic texture coal favour the development of macro-fractures. (2) Gas content and permeability are the primary factors that affect coalbed methane productivity, which are controlled by coal facies. With an increase in the GI and V/I, the gas content increases. A high GI indicates that the submacerals with enrichment micropores are more developed. The methane content is low with a high TI, which is a disadvantage for the high productivity of methane. The relationship between the TPI and methane content is characterised by segmentation. TPI values less than 0.8, with the high gelatification of plants, facilitate hydrocarbon generation, whereas TPI values greater than 0.8, the relationship between the TPI and methane content is inconspicuous. The relationships between methane productivity and GI and between the methane productivity and V/I could not be systematically understood. Thus, the coalbed methane wells remain in production infancy, and the methane produced primarily occurs as free gas and strongly desorbed gas. The coal structure contributes to
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References
Table 4 Equivalent drainage radius of coalbed methane wells. Maximum Wells Cumulative water Porosity Thickness Minimum production m3 % m drainage radius drainage radius m m S3 S4 S5 S8 S10
142.90 658.02 1040.56 239.39 463.54
4.39 4.53 4.61 4.09 4.52
4.50 4.10 4.60 3.40 6.10
20.97 46.40 54.61 32.35 31.97
29.82 66.00 77.68 46.01 45.47
Fig. 8. The relationship between coal structure and coalbed methane productivity.
the permeability of the coal reservoir, and with the development of mylonitic texture coal, the coalbed methane productivity is reduced. The coal dust originates from the mylonitic texture coal of the reservoir, and it may block the fractures in the coal reservoir, which offers no advantages in increasing the productivity of methane. Acknowledgements This work was supported by the National Major Special Project of Science and Technology of China (2011ZX05034), the Natural Science Foundation of China (41272178), and the Fundamental Research Funds for the Central Universities (China University of Mining and Technology) (2014ZDPY26).
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