Coalbed methane resources and reservoir characteristics of NO. II 1 coal seam in the Jiaozuo coalfield, China

Transcription

1 ENERGY EXPLORATION & EXPLOITATION Volume 27 Number pp Coalbed methane resources and reservoir characteristics of NO. II 1 coal seam in the Jiaozuo coalfield, China Xiaodong Zhang 1*, Yanhao Liu 1, Geoff Wang 2 and Hao Liu 1 1 College of Resources & Environment, Henan Polytechnic University, Jiaozuo , China 2 School of Chemical Engineering, the University of Queensland, Brisbane, 4072, Australia * Author for corresponding. z_wenfeng@163.com (Received 2 September 2009; accepted 26 October 2009) Abstract Jiaozuo coalfield is located in the northwest of Henan province, China, and close to the Southern Qinshui coal basin, the most successfully commercial CBM resource developed area in China. The No. II 1 coal seam is the main economic coal seam in Jiaozuo coalfield and its average thickness exceeds 5.36m. The maximum reflectance of vitrinite (R O,max ) of the No. II 1 coal across the Jiaozuo is between 3.16% and 4.78%. The coalbody structure of the No. II 1 coal seam changes greatly in different part and can be generally divided into 1~3 sub-layers. The micropores in the No.II 1 coal seam is the major pores, secondly are transitional pores, and then less macropores and mesopores. The No. II 1 coal seam has stronger adsorption, and the reservoir natural permeability has an evident heterogeneity vary from to 83.71mD. High permeability region is often near fault structure or the boundary of fault block. The CBM genetic type is homologous thermal cracking gas of humic coal with high matunity. Gas content with the burial depth of 163~1070m varies very greatly from 4.65 to 45.75m 3 /t, with an average value of 18.3m 3 /t, and gradually increases from northeast to southwest. According to the latest evaluation for CBM resource in Jiaozuo coalfield, the existing total in-place CBM resources in the No. II 1 coal seam with the depth of shallower than 2000 m are close to m 3, most of them mainly distribute in the depth of 1000 ~ 1500 m. The existing total inplace CBM resources is dominated by the inferred CBM resource reserves (more than 70%), which distribute the undrilled places with few coal geological knowledge and deeper than 1000m. The resource concentration of the No. II 1 coal seam in Jiaozuo coalfield is in the range of ( ) 10 8 m 3 /km 2, with an average value of m 3 /km 2. Based on the CBM resource investigation and reservoir evaluation, the most prospective target zones for CBM production in Jiaozuo coalfield include Guhanshan coal mine, Jiulishan coal mine and the west part of Qiangnan coal district. Keywords: Coalbed methane, Jiaozuo coalfield, reservoir characteristics, CBM resource assessment

2 308 Coalbed methane resources and reservoir characteristics of NO. II 1 coal seam in the Jiaozuo coalfield, China 1. INTRODUCTION Coal is a source, reservoir and trap for significant quantities of methane and minor amounts of other gases (Bustin and Clarkson, 1998). Coalbed methane(cbm) mainly occurs in the coal seam and its surrounding rock, and its make-up is primarily methane, with minor amounts of heavier hydrocarbons, carbon dioxide, nitrogen, oxygen, hydrogen, and helium. Because of its explosive and outburst hazard during the mining, the gas was recognized as harmful gases and directly emitted into atmosphere, contributing to the greenhouse effect. It has generally been overlooked as an economic resource mainly used as unconventional natural gas by the oil and gas industry since the 1970s, when CBM got its start in Pennsylvania with the drilling of degasification holes prior to mining (Antonette and Markowski, 1998). Commercial utilization of these resources is mostly limited to basins where enough knowledge and experience as well as adequate reservoir properties exist (Karacan and Okandan, 2001). China has abundant coal and CBM resources. According to the newest oil and gas resource assessment by Ministry of Land and Resources in 2008, the CBM resource is more than m 3 (shallower than 1500m) and m 3 (shallower than 2000m) in China (Li et al, 2009). That is to say, more CBM resource exists in the depth between m in China, which makes the CBM development more different. With the development of geological research and exploration technology, more CBM resources have achieved commercial development in different coalfields, where the coal ranks are even different. for example, Jincheng Coalfield, located in Southeast of Qinshui basin, Shanxi Province, China, the coal classification being anthracite coal, having the largest drainage well groups in China and more than m 3 /d CBM production, has been the first basin to be developed commercially in China. Jiaozuo coalfield is situated in the northwest of Henan province, China, and adjoins to Jincheng Coalfield separated by Taihang Mountain. The coalfield is 60-km long (E- W) and 15-km wide (N-S), with a total area of 970km 2. It is one of most important high-quality anthracite producing mines in China. Many estimations about the CBM resources have been done since 1970s in Jiaozuo coalfield, and the potential gas resources have been estimated from m 3 (shallower than 2000 m, deeper than the weathered zone, and with a gas content >4m 3 /t) to more than m 3 (shallower than 2000 m) (Ye et al., 1998; Zhang et al., 2002). The latest estimation by Department of Land and Resources of Henan Province in 2007 shows the total CBM resource of No. II 1 coal seam (shallower than 2000 m, deeper than the weathered zone, and with a gas content >8m 3 /t) was more than m 3 and the resources abundance near to m 3 /km 2 in Jiaozuo coalfield, indicating a very favorable promise for CBM exploration and development. In addition, Jiaozuo coalfield is one of most serious areas of coal and gas outburst accidents in China. A total of 276 coal and gas outburst accidents occurred in 11 pairs Out of the 13 pairs of mines from 1955 to 2000 (Zhang et al., 2008). Moreover, there is an increasing tendency of coal and gas outburst with the increasing of the mining depth. In Jiaozuo coalfield, there are near 1000 coal exploration bores and approximately 42 CBM wells identified as being drilling prior to the end of 2008 and many investigations have been published on the geology of CBM reservoir in Jiaozuo coalfield (Qin et al., 1990; Zhang et al., 1998; Meng and Peng, 1998; Song and Meng,

3 ENERGY EXPLORATION & EXPLOITATION Volume 27 Number ; Yan, 2003; Yuan, 2005; Zhang, 2007; Gao and Wang, 2008), so many geological data is available for evaluating the characteristic of coal reservoir and with the progress of the old coal mine depth and increase of new coal mines, more and more data will be available. In the paper, coal geology and CBM reservoir properties of the No. II 1 coal seam in the study area were comprehensively studied. The CBM resources in-place were calculated and prospective areas of CBM resources recovery were plotted out. This paper aims to be a guide to the further exploration and development of CBM in Jiaozuo coalfield. 2. GEOLOGICAL SETTING 2.1. Regional tectonics and formation Jiaozuo coalfield lies in the south of late Paleozoic North China coal-accumulating Basin. According to unit of tectonics dividing, it s also at the south margin of Taihang fault-uplift zone of Taihang tectonic region in North China plate. There are widely various structural features formed since Yanshanial Movement within the study area. Relatively, fault structures, including regional and small faults, developed very much in the field, but folds develop very weak and only several widely slow folds spread on the eastern part and southern part of the coalfield. Fault structures are mainly highangle normal faults in the study area. The whole coalfield is divided into two tectonic zones by Phoenix fault, which is near EW strike. The south zone is latitudinal tectonic belt, where the main structure features are near EW fractures and the near EW-axis folds. The north part is New Nathaysian tectonic belt where mainly structural feature is near NE fracture, followed by NW fracture and rarely NNE fractures (Fig. 1). Figure 1. Location map of Jiaozuo coalfield in China and the structure contour map of the floor of the No. II 1 coal seam.

4 310 Coalbed methane resources and reservoir characteristics of NO. II 1 coal seam in the Jiaozuo coalfield, China The strata, in order of geologic time, are Archean, Jixian System of Mesoproterozoic, Cambrian and Ordovician System of Lower Paleozoic, Carboniferous and Permian System of Upper Paleozoic, Triassic System of Mesozoic, Neogene and Quaternary System of Cenozoic (Dai et al., 2002, 2003, 2006). Generally, the strata is a monoclinic structure which strikes NE-NNE, dips to SE and dip angle changes from 5 to 20 controlled by region structure Coal-bearing formation and coal distribution The North China Craton basin was eroded from Silurian to Mississippian time but subsided and received sediment from Pennsylvanian to Triassic time. The coal-bearing formation includes Pennsylvanian Benxi and Taiyuan Formation, the Permian Shanxi, Xiashihezi and Shangshihezi, of which the Taiyuan and Shanxi formation are averagely 170m thick and main coal-bearing units (Dai et al., 2006; Sun et al., 2002; Sun, 2003; Sun and Horsfield, 2005). There are 15 coal-bearing formations in the study area, and average total coal thickness is about 16m. Two coal seams, including No. II 1 coal seam of the Shanxi unit, No. I 2 seam of the Taiyuan unit, are minable within Jiaozuo coalfield, of which the No. II 1 coal seam develops the most widely and stably with a thickness from 0 m to m and an average thickness of 5.36 m and is the main economic coal seam. The No. I 2 seam is partially minable with thickness is from 0 m to 5.14 m and with average thickness of 1.34 m, which is minable in south and west part of the coalfield. Generally, the depth of the No. I 2 seam is about 100 m deeper than that of the No. II 1 coal seam (Fig. 2). Many researches in China reveal that there are great differences in coal body structure under different stress and structure setting and put forward many classification of coalbody structure (Yuan, 1985; Jiang et al., 2004; Ju et al., 2005). According to coal morphological appearance and coalbody destruction degree, Yuan et al. divided coal into four primary constructional coal (also named undeformed coal), cataclastic coal, granular coal and mylonitic coal, of which the former two are often called hard coal and the latter soft coal (Yuan, 1985) and the classification is generally recognized in China. By the coal profile observation under all mines within Jiaozuo coalfield, coalbed structures show regular change. In the middle part of the coalfield, the coal seam shows different coalbody structure with the Jiulishan fault and Beibei fault as the border. Coalbed structure can be classified into three sub-layers in southwest part of Jiulishan fault: The top is mylonitic coal sub-layer with nearly 0.3 m~1.0 m thick, the middle is a massive undeformed coal sub-layer with 3.0 m~4.5 m thick and the bottom is mylonitic coal or granular coal sublayer with 0.25~1.5 m thick. Between the northeast part of Jiulishan fault and the east part of Beibei fault, mylonitic coal is main coal body structure, followed by granular coal and then small lump hard coal. In the west part of the coalfield, including the part between in the northeast of Jiulishan fault and the west part of Beibei fault, the coalbed structure can be generally divided into two layers: The top coal seam widely develops a mylonitic coal seam belt with 0.5 m to 2 m thickness and the low part does undeformed coal seam belt with about 3~4m thick.

5 ENERGY EXPLORATION & EXPLOITATION Volume 27 Number In the east part of this coalfield, the coalbody only show slightly deformed and there are dominated by undeformed coal, and then rare mylonitic coal or granular coal. Figure 2. Stragraphic column of the Permo- Carboniferous coal-bearing sequence in Jiaozuo coalfield.

6 312 Coalbed methane resources and reservoir characteristics of NO. II 1 coal seam in the Jiaozuo coalfield, China 3. EXPERIMENTAL METHOD 3.1. Coal sample In this study, a total of 47 fresh samples of the No. II 1 coal seam were investigated from 10 underground mines in Jiaozuo coalfields, of which 17 samples can be chosen as measured ones in this study (see Table 1. for sampling locations). Except Fangzhuang Coalmine, Baizhuang Coalmine and Xiaoma Coalmine, two coal samples including one undeformed coal and one mylonitic coal could be chosen each coal mine in the other coalmines. Most of the selected undeformed coals were big blocks with a weight of about 8 kg each, some of which can be applied to the permeability testing. All the samples were collected following the Chinese Standard Method GB/T , carefully packed and immediately carried to the laboratory for testing. For the samples used for permeability testing, the original occurrences were recorded and tagged. Table 1. The position of coal samples and their petrographic constituent, proximate and ultimate analyses, and coalbody structure types in Jiaozuo coalfield. No Coalmine R max V I MM C daf H daf O daf M ad A d V daf ARD BDS G-1 Guhanshan M G-2 Guhanshan U J-1 Jiulishan U J-2 Jiulishan M Y-1 Yanma M Y-2 Yanma U Z-1 Zhongma M Z-2 Zhongma U H-1 Hanwang M H-2 Hanwang U F-1 Fengying M F-2 Fengying U FZ-1 Fangzhuang U Bai Baizhuang U ZC-1 Zhucun M ZC-2 Zhucun U X-1 Xiaoma U Abbreviations: V, I, MM are the percentage content of vitrinite, inertinite and mineral matter (vol.%), respectively; Cdaf, Hdaf, Odaf are the percentage content of Carbon, Hydrogen and Oxygen (%, dry ash-free basis), respectively; Mad is the moisture content (vol. %, air dried basis), Ad is ash yield (vol. %, air dried base), Vdaf(%) is the volatile matter content (vol. %, dry ash-free basis); ARD is apparent of coal samples (10 3 kg/m 3 ); BDS denotes the coalbody structure type, U is undeformed coal and M is mylonitic coal.

7 ENERGY EXPLORATION & EXPLOITATION Volume 27 Number Experiments Conventional test In this study, ultimate analysis (by GB/T ) was applied to measure the contents of main elements, including carbon, hydrogen, oxygen, and proximate analysis (by GB/T ) includes four tests for: ash, volatile matter, fixed carbon and moisture of the 17 samples (Table 1). Vitrinite reflectance and coal maceral composition of these samples were performed under a Leitz Laborlxe 12 POL microscope. The two testing was done by GB/T and GB/T (Table 1), respectively Pore structure test Mercury injection experiment (by Sy/T ) was done to test the pore structure parameters, including pore volume, pore specific surface area and pore size of the samples. The experiment device model is No 9310 manufactured by Micromeritics Instrument Corp, in the USA. Its working pressure range is ~ MPa, resolution 0.1 mm 3, Dilatometer volume of powder cm 3, and lower pore-radius limit of 7.2nm. In the study, all the samples must be dried at 106 C for 24h before the testing and the usage is about 3g per testing. Volume intrusion/extrusion curves were automatically obtained, and the data, including the change of pore radius and pressure and the amount of mercury, were collected by computer. But the parameters, for example pore volume, pore specific surface area and pore radius distribute, need to be future analyzed based on data or curves above Permeability test Gas phase absolute permeability is also named Kleinberg permeability and approximately equal to the Helium gas-phase permeability (Fu and Qin, 2003). Absolute permeability of coal samples (by SY/ T ) was tested under the Whole Core flow System, manufactured by Terra Tek Inc., USA. The device includes pressure system, temperature system, control system and the core holder, etc. The experimental system also has a complete set of rock sample preparation and routine core analysis equipment. The maximum of surrounding pressure and fluid pressure of the instrument is respectively 70MPa and the max is 65MPa. In the study, 5 undeformed blocky coal samples with very few cracks were selected to measure the single-phase permeability of helium and water and the gas-water relative permeability. Before the testing, all the samples were drilled paralleling to the bedding direction into cylindrical shape with a diameter of 25 mm and a height of 50 mm. The gas used in the testing was Helium and each experiment was performed in room temperature and 6 different pressure spots. All the measurements were under simulated in-situ stress conditions using a tri-axial cell with isotropic ambient pressure of 2.5 MPa and 3.5 MPa. During the testing, the gas permeability in coal was calculated in each pressure under no change of effective stress and temperature according to the following formula: K g 2pq 0 gµ gl 10 = A p 2 p 2 ( 1 0 ) 2 (1)

8 314 Coalbed methane resources and reservoir characteristics of NO. II 1 coal seam in the Jiaozuo coalfield, China Where, Kg is the gas permeability under each pressure spot, 10-3 m 2 ; p0 is standard atmospheric pressure (ATM), MPa; qg is the gas flux, cm 3 /s; µ g is the gas viscosity, mpa s; L is the coal sample s length, cm; A is the coal sample s cross section, cm 2 ; p1 is the entrance pressure, MPa. Further, the gas absolute permeability can be calculated by the following formula: K g b = K0 + 1 p m (2) Where, Kg is the same as formula (1); K 0 is the absolute permeability, 10-3 m 2 ; pm is the average pressure; b is a constant relevant to the gas property and pore structure Adsorption test Adsorption testing is performed under the Isothermal Adsorption/Desorption System Model IS-100 made by Terra Tek Company in USA. The upper temperature limit of the instrument is 50 C and the upper pressure limit is 30 MPa. In this study, all the samples were made into equilibrium moisture sample by the ASTM D The absorbate is methane with a purity of 99%, and the maximum initial pressure is 15MPa. The experiment temperatures are 30 C for all samples, 40 C and 50 C for 4 samples of these samples. Qian et al. (1996) described experimental equipment and methods in detail. The principles of the instrument testing are the volume method and the Langmuir equation [Langmuir] used to draw isothermal adsorption curves and calculate related parameters. The gas sorption isotherms in coal were modeled using the Langmuir isotherm: Or, P V 1 P = + bv V VP L V = P + P Where P is the equilibrium gas or vapor pressure, MPa; V is the volume of gas adsorbed, commonly reported at standard temperature and pressure (STP), per unit mass of coal, m 3 /t or cm 3 /g; VL is the Langmuir monolayer volume, m 3 /t or cm 3 /g; b is an empirical constant, MPa-1; PL is the Langmuir pressure, equal to 1/b, MPa. 4. RESULTS AND DISCUSSIONS 4.1. Geological characteristics of CBM reservoirs Coal petrography In Jiaozuo coalfield, macroscopic lithotypes of the No. II 1 coals are dominated by bright coals and semi-bright coals, but the No. I 2 coal is dominated by semi-bright and semi-dark coals. The main body of the coal seam is a uniform sequence of clarain and L L L

9 ENERGY EXPLORATION & EXPLOITATION Volume 27 Number durain, and thin layers of vitrain and fusion are restricted to these intervals showing lenticular shape. The No. II 1 coal composition is dominated by more than 86% organic macerals components, in which vitrinite reachesr 38~76% and the rest is inertinite (Table 1) Coal rank and coal quality The maximum reflectance of vitrinite (R max ) of the No. II 1 coal across the Jiaozuo is between 3.16% and 4.78% (Fig. 3) (Qin et al., 1990). The distribution of coal rank is controlled by the burial history and the Superposition of palaeoheat flow imported by the fracture and shows obvious coal-class sub-zone (Qin et al., 1990; Su et al., 2005). The coal metamorphism gradually increases trend from the east to the west, the northeast to the southwest, and the same coal rank occurred as northeast-strike. Coal quality data across Jiaozuo coalfield are listed in Table 1. The coal quality of the No. II 1 coal shows distribution Law in plane scale to a certain extent as follows: the volatile matter content and H-element content have an increase tendency but the C-element content presents downtrend from the west to the east. The distribution tendency of coal quality is in keeping with that of coal rank. On the whole, the No. II 1 coal belongs to anthracite and No. 3 shows the characteristics of lower ash-content, especially lower sulfur-content, lower phosphide-content, and mid-to-high caloric value (Hao et al., 2005). Figure 3. R max distribution map of the No. II 1 coal in Jiaozuo coalfield (modified from Qin et al., 1990) Physical properties of CBM reservoirs Pore and crack Coal is a complex polymeric material with a complicated porous structure that is difficult to classify (Clarksona and Bustinb, 1999). Previous studies (C1ose, 1993; Gamson et al., 1998) show coal is a double porous medium which includes matrix pores and cracks. The size, shape, porosity, permeability and connectivity of the pores and cracks determine the reservoir, migration and output of CBM, thereby constraining the flow of CBM and its production.

10 316 Coalbed methane resources and reservoir characteristics of NO. II 1 coal seam in the Jiaozuo coalfield, China Crack In study area, the cracks in the coal seam develop heterogeneously. The observation under the mines shows that various creaks with different sizes develop in coal, including the large cracks through the whole coal seam and even into the roof and the floor or through the tonsteins but suspending in the roof and the floor, the small creaks distributing in different coal delaminations and suspending in the tonsteins and the micro creaks only developing in vitrain strips. These cracks mainly are oriented in NNW, NE, NNW, NNE or near SN directions (Fig. 4), and most of them are highobliquity cracks with inclination of over 50. There are two sets of cracks oriented in different directions in most of coal mines, of which the larger often cut the smaller, and the same set of creaks show en echelon style with 0.2~7 cm interval. But there are three sets of creaks in local area, such as Guhanshan mine field. The observation to hand specimen by the naked eye and to polished specimen by microscope reveals that cleats of the No. II 1 coal are a little weak and only develop in vitrain strips, some of which are filled or half-filled by kaolinite and calcite. Further, the secondary interstices of different section even in the same mine have different distribution, showing the heterogeneity in creak development and permeability of the coal seam. The micro-cracks are mainly NE, near EW and NEE strike, roughly the same to regional fractures and macro-creaks (Fig. 4). a b Figure 4. The rose diagram of microfracture strikes (a) and trends (b) in the No.II 1 coal seam in Jiaozuo coalfield Pore Pore structure includes the pore size, size distribution and geometry/morphology of the interconnecting pore network (Liu et al., 2009). Previous studies put forward many classification methods according to the pore size and the action in gas storage and the pore into many types (XoπoT, 1966; Sang et al., 2005). XoπoT divided the pore into four types: micropore (pore diameter: <10 nm), transitional pore (pore diameter: 10~100 nm), mesopore (pore diameter: 100~1000 nm) and macropore (pore diameter: >1000 nm). The decimal classification has been the most widely used. In order to avoid the influence on macropores statistics by the gap between the particles during the

11 ENERGY EXPLORATION & EXPLOITATION Volume 27 Number testing process, the pores, with a diameter of more than nm, were excluded from macropores statistics in the study. Previous results showed that the porosity of the No. II 1 coals was moderate in Jiaozuo coalfield. The porosity values generally range from 7% to 12% and show an increase trend from the east to the west, the shallow to the deep (Meng and Peng, 1998). The data shows micropores and transitional pores contribute to the specific surface area of the whole coal sample much larger than mesopores and macropores (Table 2). In the measure scope of mercury injection experiment, the specific surface area is dominated by 58.6% micropores with an average of cm 2 /g, followed by 40.4% transitional pores with an average value of cm 2 /g, and then 0.9% mesopores and 0.1%macropores. The contribution to the pore volume follows the sequence as transitional pores (average percentage: 49.3%) > micropores (average percentage: 32.2%) > mesopores (average percentage: 10.8%) > macropores (average percentage: 0.7%). Thus, the small diameter pores are much richer than large diameter pores in the No. II 1 coal. In the study area, the whole pore volume and the whole specific surface area of the mylonitic coal samples range from 273 to cm 3 /g, 6.6~8.7m 2 /g, more than those of the undeformed coal samples. The two parameters of different pore sizes in mylonitic coal samples have a similar contrast between two kinds of coal samples (Fig. 5). Table 2. The pore-structure parameters of the No. II 1 coal in Jiaozuo coalfield. Items Macropore Mesopore transitional pore Micropores Total cm 2 /g 0 ~183 0 ~ ~ ~ ~ specific average surface % 0 ~ ~ ~ ~ 69.7 area average cm 3 /g 0 ~100 0 ~ ~ ~ ~ 597 average pore volume % 0 ~ ~ ~ ~ 46.6 average Additional information for pore structures can be obtained from their mercury intrusion/extrusion curves. This method has been widely used as a petrophysical tool for evaluating traditional petroleum reservoir such as sandstones and carbonates, and it is also useful for coals (Qin, 1994; Chen and Tang, 2001; Zhang, 2005). According to the hysteresis loops formed by the difference between the intrusion curves and the extrusion curves, the basic shape and the connectivity of pores in coal can be evaluated (Qin, 1994). But in view of the pores complexity and all kinds of pores coexist in the same pore diameter, the hysteresis loops only reflects the main pore types and the whole properties of all the pores (Zhang, 2005). The evident hysteresis loops of all the coal samples in Jiaozuo coalfield show that there are many open pores. For the large pore diameter range, the half- closed pores in mylonitic coals is less than those in

12 318 Coalbed methane resources and reservoir characteristics of NO. II 1 coal seam in the Jiaozuo coalfield, China undeformed coals, but the open pores in mylonitic coals are more than those in undeformed coals (Fig. 6). However, for the small pore diameter range, the comparison between the contents of open pores and half-closed pores in two kinds of coalbody structure coals are On the contrary. Precious study show that micropores in coal provide the gas with adsorption space, transitional pores with the area of capillary condensation and diffusion, and macropores and mesopores with the area of seepage and laminar (Sang, 2005). Thus, the No. II 1 coals have stronger adsorption and diffusion capacity, but the capacity of seepage and laminar flow are weaker. The gas storage capacity in mylonitic coals is more than that in undeformed coals, but the gas migration in mylonitic coals is more difficult than that in undeformed coals. a b Figure 5. The distribution curve of the pore size of mylonitic coal (a: G-1) and undeformed coal (b: G-2). Compositional data for these coals are summariazed in Table 1. Figure 6. The mercury intrusion/extrusion curves of mylonitic coal (G-1) and undeformed coal (G-2). Compositional data for these coals are summariazed in Table 1.

13 ENERGY EXPLORATION & EXPLOITATION Volume 27 Number Permeability Permeability is a critical factor controlling CBM production and an important parameter for evaluating the economic production of natural gas from coal seams (Clarkson and Bustin, 1997; Thomas et al., 2007; Andrew et al., 2006). The permeability of coals can be calculated by many methods, such as coal core samples testing in laboratory, conversion of the geophysical logging curve, conversion of insitu permeability coefficient, well-test in CBM well and numerical simulation of CBM reservoir. Because of the difference in testing principles, methods and coal samples or test position, the measure results have great differences, which have their own advantages and disadvantages. Results from the 6 groups of well-tests, which distribute in Guhanshan coal mine and Zhongma coal mine in the center and Encun coal mine in the south, show that the reservoir natural permeability vary greatly from to md. Besides, the permeability value of CBM well could be increased 5~10 times by using hydraulic fracture (Table 3). Moreover, the permeability coefficient, tested under the coalmines in the center and the south of the study area, shows the permeability value also vary greatly from 0.004~3.6 md. Table 3. The well-test permeability of the CBM wells in Jiaozuo coalfield. Coal mine Guhanshan Z hongma Encun Well code T1 T2 T3 T4 ZM1 CQ6 natural permeability(md) ~ ~ ~ Permeability after hydrofrac treatment(md) ~ In this study, 5 blocks of coal samples were chosen to test the permeability. In the experiment, the gas-water relative permeability curve in 5 coal samples could be obtained because the permeability value of other coal samples is so low that the testing result could not be obtained in the experiment (Table 4; Fig. 7). The testing results showed the single-phase permeability of all coal samples was low: the gas-phase permeability vary greatly from 0.02 to 0.945mD under isotropic ambient pressure of 2.5 MPa and from 0.013~0.552mD under isotropic ambient pressure of 3.5 Mpa. The water-phase permeability was lower and the water-phase permeability values of 2 coal samples with the lowest gas-phase permeability value were too low to be measured. The water-helium relative permeability values obtained from the water-helium relative permeability curve is only 2.4 md, far lower than the areas with better conditions in CBM development, for example, San Juan Basin and Black Warrior in United States and Warrior Basin in Australia.

14 320 Coalbed methane resources and reservoir characteristics of NO. II 1 coal seam in the Jiaozuo coalfield, China Figure 7. The relation of helium-water relative permeability and gas saturation in Jiaozuo field. Table 4. The permeability testing results of the coal samples in the laboratory. No coalmine burial depth(m) ambient pressure(mpa) K(mD) Kw(mD) G-2 Guhanshan FZ-1 Fangzhuang FZ-2 Fangzhuang Bai Baizhuang ZC-2 Zhucun Abbreviations: K, Kw are respectively the single-phase permeability tested by helium and water in the Lab Based on the analysis of several kinds of the permeability testing results and the observation on cores and coal mines, the permeability is mainly controlled by several factors: coalbed depth, regional structure, cleat development, coalbody structure. Generally, there is higher permeability near fault structure or the boundary of fault block in the study area. For example, the four CBM wells in Guhanshan coalmine are located the downthrown Block of normal faults. Thus the well-test permeability are higher than that in the other coalmines, where are in a state of tensile stress and the factures generally develop because of the tensile stress of the boundary faults. However, the well-test permeability values vary greatly from 1.58 to 83.71mD because

15 ENERGY EXPLORATION & EXPLOITATION Volume 27 Number of the difference in the fracture development level. As the higher well-test permeability value in the four CBM wells, The T3 and T4 wells are closer to Tuanxiang fault zone, so their well-test permeability value are higher than those of the other two CBM wells. Moreover, the permeability of syncline axle is very low because of the action of extrusion stress. For example, the well CQ6 is located near the Shaft of Qiangnan Syncline, and its well-test permeability is only 0.001~0.08mD, the minimum permeability value in 6 groups of well-test. According to the permeability-test results in laboratory, the helium permeability under isotropic ambient pressure of 2.5 MPa are all higher than that under isotropic ambient pressure of 2.5 MPa, and the helium permeability of coal sample from the depth of 160 m is much higher than that of coal samples from the depth of 520 m. Thus, we can draw a conclusion that the permeability is inverse relationship with the burial depth. Precious study showed that the permeability of undeformed coal seam is better than tectonic coal seam (Su and Sheng, 1999; Zhong et al., 2004; Peter et al., 1998). Therefore, the reservoir permeability of the No. II 1 coal seam in the east coalfield is better than that in the middle and the west, and can be inferred according to the coalbody structure distribution. In a word, the reservoir permeability in Jiaozuo coalfield is generally low and has an evident heterogeneity. There are higher-permeability region for CBM exploration, for example, near fault structure or the boundary of fault block. Moreover, the reservoir permeability may be so significant improved after hydraulic fracture. All of these indicate that high permeability districts for CBM exploration and development may be workable in the study area Coal adsorption In the study area, the No. II 1 coals have high adsorption capacity. The testing results of isotherm adsorption experiment in previous study and this study show Langmuir volume parameter (VL) of equilibrium-moisture-bearing coal samples of the No. II 1 range from 22.68~46.67 cm 3 /g, with an average value of cm 3 /g, under the temperature of 30 C and Langmuir pressure parameter (PL) from 1.17 to 4.96 MPa, with an average value of 3.18 MPa. Gas sorption by coal is closely related to its physical and chemical properties (Peter et al., 1998; Sun et al., 2009). Generally, the adsorption capacity of coal is influenced by many factors, including coal rank (coal metamorphism), maceral composition, macro- and micro-lithotype, pore, moisture, temperature, pressure, and so on. For the study area, the parameter VL of mylonitic coals varies from to cm 3 /g, with an average value of 38.33cm 3 /g. It is a little higher than that of undeformed coals, with an average value of cm 3 /g. The parameter PL varies from 2.37 to 3.98MPa, with an average value of 3.63cm 3 /g, and is far higher than that of undeformed coals, with an average of 3.04 MPa. Thus, the conclusion can be draw that mylonitic coal has stronger adsorption capacity than undeformed coals (Fig. 8). The main reason of the difference is that the mylonitic coal has larger pore content, especially macropore content than undeformed coals.

16 322 Coalbed methane resources and reservoir characteristics of NO. II 1 coal seam in the Jiaozuo coalfield, China T=30 C T=40 C T=50 C Figure 8. High-pressure methane adsorption isotherms for the mylonitic coal (G-1) and undeformed coal (G-2) coal. Samples with different coalbody structure in different temperature. Compositional data for these coal are summariazed in Table 1. The symbol and refers to the mylonitic coal and the undeformed coal respectively. Temperature is another factor on the change of coal adsorption. In general, as the temperature increases, the adsorption capacity of coal dropped. This experiment also proved this conclusion (Fig. 8). Further study shows that, as temperature increase, the declination of adsorption volume of coals under higher pressure section is faster than that under lower pressure section. For example, under the pressure range of 0~4 MPa, the decay rate for the coal sample from the Guhanshan coal mine is 0.18 m 3 /(t C) from 30 to 40 C, about 1.8 times of the decay rate of 0.10 m 3 /(t C) from 40 to 50 C. But under the higher pressure range of 4~20 MPa, the decay rate is about 0.28 m 3 /(t C) from 30 to 40 C, about 1.4 times of the decay rate of 0.20 m 3 /(t C) from 40 to 50 C. Moreover, as temperature increases, the difference in adsorption capacity between mylonitic coal and undeformed coal decreases Gas content and composition Gas content determination techniques generally fall into two categories: (1) direct methods which actually measure the volume of gas released from a coal sample sealed into a desorption canister and (2) indirect methods based on empirical correlations, or laboratory derived sorption isotherm gas storage capacity data. (William and Steven, 1998) In the study area, more than 1000 coal bores had been drilled in nearly 100 years, providing with many data, including the burial depth, coal thickness, the roof and the floor, gas content, gas composition, and so on. The gas content in the No. II 1 coal seam with the burial depth of 163~1070 m varies very greatly from 4.65 to m 3 /t, with an average of 18.3m 3 /t. Generally, gas content gradually increases from northeast to southwest (Fig. 9 and Table 5), and the highest gas content region distributes in the south-central coalfield. The gas content in the deep fault blocks is higher than that in the shallow. In the same fault block, the gas content increases as the burial depth of coal seam increases, but not in an unlimited increase, such as Encun mine field, the highest gas-content region is not synclinal axis but on the north side of synclinal.

17 ENERGY EXPLORATION & EXPLOITATION Volume 27 Number Therefore, the gas content must tend to a saturation value when coal seam reaches a certain depth in the same fault block with little tectonic transformation. In the study area, gas composition in the No. II 1 coal seam varies very greatly. Under the CH 4 weathering zone, in general, the gas composition is mainly CH 4, followed by N2 and CO 2, and then less than 1% heavy hydrocarbon, and the CH 4 percentage in the whole gas composition increases from the shallow to the deep. The δ 13 value of methane is -3.41%, the δ 13 value of C 2 H 6 is %, the δ 13 value of CO 2 is %, and the C 1 /C 1-5 value is These data show that the genetic type of CBM in the study area is homologous thermal cracking gas of humic coal with high matunity. The factors controlling on gas content mainly include the structure, coalbed depth, effective burial depth, the roof and the floor, and so on. More analysis can be seen in the paper by Zhang (2007). Table 5. In-place CBM resource and CBM abundance in Jiaozuo coalfield. Qiannan-W, -E and N are respectively the west part, the east part and the north part of the Qiangnan coal mine. Coalmine Area (Km 2 ) Coal thickness (m) Coal reserve (10 6 t) Gas content (m 3 /t) CBM resource (10 8 m 3 ) CBM abundance (10 8 m 3 /km 2 ) Encun Zhongma Guhanshan Qiangnan-W Qiangnan-E Qiangnan-N Jiulishan Jiaonan Yanma Xinhe Fengying Fangzhuang Xiuwu Zhaojing Dazhaoying Zhaogu Fangzhuang Wuliyuan Fengcheng Kuaicunying Chizhuang Bobi Total

18 324 Coalbed methane resources and reservoir characteristics of NO. II 1 coal seam in the Jiaozuo coalfield, China Figure 9. Gas-content contours (in m 3 /tone) of the No. II 1 coal seam in Jiaozuo coalfield CBM RESOURCE AND PRODUCIBILITY POTENTIAL CBM resource The geologic reserves of CBM resources are calculated based on the usual volumetric method (Charles et al., 1998; Drobniak et al., 2004; Langenberg et al., 2006). This method can briefly be as follows: Gi = A i H i D Ci (1) Where Ai is surface area of the block (km 2 ); H represents average net coal thickness of the block (m); D is average coal density (t/m 3 ); C is the gas content from estimation or direct test of the block (m 3 /t); and Gi is the GIP by volumetric resource estimation methodology of the block (m 3 ). Firstly, according to region structure, coal thickness and whether they are mined, 4 large blocks were divided into. On this division basis, 24 sub-blocks and 79 smallest calculated blocks divided into. In each smallest calculation blocks, the gas resource can be obtained by the volumetric equation (1). Then the geologic reserves in the study area can be estimated by the equation (2) G = Gi (2)

19 ENERGY EXPLORATION & EXPLOITATION Volume 27 Number Where G is the geologic reserves of the No. II 1 coal seam (m 3 ); n is the calculation blocks, equal to 79 here. Since the collected data are from the shallow basin, with less than 1000 m deep, and the drilled area mainly during the coal geological exploration, we could use different methods to estimate for the deep and undrilled areas: 1) for the shallow coal seam with less than 1000 m deep, where there is undrilled but many data can be available nearby, the gas content gradient method can be used; 2) for the undrilled area, where few data can be valid or the deep with more than 1000 m, the adsorption isotherm method can be used (Saulsberry et al., 1996; Yao et al., 2009). In the calculation on gas resource, the coal density was chosen according to the average density of the every block or nearby blocks, and the values range from 1.45 to 1.49t/m 3. And the coal thickness in undrilled area can be calculated by interpolation method in coal thick isoline chart. The existing total in-place CBM resources in the No. II 1 coal seam presented in Jiaozuo coalfield are estimated to be m 3 (Table 5), most of them mainly distribute in the depth of 1000 ~ 1500 m, followed by the depth of 1500 ~ 2000 m and 500~100m, and then the depth of shallower than 500m (Table 6). The existing total CBM resources in-place are dominated by more than 70% the inferred CBM resource reserves, which distribute the undrilled places with few coal geological knowledge and deeper than 1000m. There are six coal districts with the total in-place CBM resources of more than m 3, respectively. The six coal districts are Fengcheng coal district, Xiuwu coal district, Encun coal mine, Zhaojing coal district, Qiangnan coal mine and Wuliyuan coal district (Table 5). Gas resource concentrations (gas-in-place in m 3 /km 2 ), an important index for dividing the target CBM area, are given in Table 5 and Figure10. The resource concentration of the No. II 1 coal seam in Jiaozuo coalfield is in the range of ( ) 10 8 m 3 /km 2, with an average of m 3 /km 2. The areas with high CBM resource concentration (generally higher than m 3 /km 2 ) in Jiaozuo coalfield include (1) Xiuwu coal district, (2) Qiannan coal mine and (3) Encun coal mine. Table 6. CBM resource in different depth and different level reserves. Depth(m) <500m 500 ~ -1000m 1000 ~ 1500m 1500 ~ 2000m CBM resource(10 8 m 3 ) CBM resource producibility potential CBM resource producibility potential relate to economic and geological factors. The geological factors, including geological properties and reservoir physical properties, play important action on the CBM resources recoverability. Based on the fuzzy mathematics and comprehensive analysis of main geological parameters, the CBM resources assessment indexes in Jiaozuo coalfield have been put forward (Table 7).

20 326 Coalbed methane resources and reservoir characteristics of NO. II 1 coal seam in the Jiaozuo coalfield, China Figure 10. Isopach map (in m 3 /km 2 ) of CBM resource abundance in Jiaozuo coalfield. Table 7. Comprehensive assessment criteria of for favorable CBM resource exploration areas selected in Jiaozuo coalfield. type Resource properties Workable properties Parameter The classified criteria II III IV Parameter wight Coal thickness(m) > 6 6 ~ 5 5 ~ 3 < Gas content(m 3 /t) > ~ ~ 10 < CBM abundance ( m /km ) >2 2 ~ ~0.5 <0.5 + Area(km 2 ) >40 40 ~ ~ 10 < 5 + Burial depth(m) 300 ~ ~ ~ 1500 > Pressure gradient (kpa/m) > ~ 6 6 ~4 < 4 + Pc/P(%) > ~ ~ 0.2 < natural permeability >10 10 ~1 1 ~ 0.1 < (md) Gas saturation(%) > ~ ~ 40 < Coalbody structure Undeformed Cataclastic Granular Mylonitic Tectonic development level Hydrogeology condition Simple. Undeveloped structure Simple. Easier to reduce pressure by drainage Relative simple. Few faults and larger folds Relative simple, Larger displacement, Stable groundwater source Relative complex. Developed large folds Relative complex. Water-bearing distribution controlled by faults Very complex. Discontinuous coal seam Very complex. Greatly change in Water-bearing Explanation: Pc is the critical desorption pressure, and P is the reservoir pressure. The number of the symbol + expresses the importance of the parameter, more numbers of symbol +, more important

21 ENERGY EXPLORATION & EXPLOITATION Volume 27 Number In the assessment system, in the light of concrete conditions, the four parameters including coal seam thickness, gas content, primary permeability and coalbody structure are the most important in all parameters, and influence on the CBM resource estimation. In other words, if coal seam thickness is less than 3m, or gas content of the coal seam lower than 10m 3 /t, or natural permeability of the coal seam lower than 0.1mD, or the whole coal seam belongs to mylonitic coal, the coal seam can be classified to non-extraction coal seam. Further, according to the hierarchical priority principle of the other parameters, the CBM resources recoverability in Jiaozuo coalfield has been estimated and favorable zones for CBM exploration and development have been indicated. According to the assessment results, four types of CBM exploration and development prospects in Jiaozuo coalfield are identified: Type I is chosen as the most favorable zone; and it is the highest priority to mine the CBM resources areas in nowadays mining and technical conditions. Type II is a favorable zone; and its mining priority is lower than type I and can be used as back-up of the minable area in the near future. Type III is a relatively favorable zone; it is the non-minable area in the short run. Type IV is an unfavorable zone. It is non-minable area. To a certain extent, there is a great potential for CBM production when it is with high gas content, high CBM resource concentration in Jiaozuo coalfield. The most favorable zones in Jiaozuo coalfield are Guhanshan coal mine, Jiulishan coal mine and the west part of Qiangnan coal district, in which there are better permeability, mainly undeformed coal, higher gas-content and gas saturation, relatively simple tectonic and hydrogeology condition, and especially more information available in Guhanshan and Jiulishan coalmines. 5 CONCLUSIONS (1) The No. II 1 coal seam, with an average thickness of 5.36 m, is the main economic coal seam of CBM in Jiaozuo coalfield. Macroscopic lithotypes of the No. II 1 coals are dominated by bright coals and semi-bright coals, and its organic macerals components are dominated by vitrinit. The coalbody structure of the No. II 1 coal seam is very different in different part and can be generally divided into 1~3 sub-layers. (2) The cracks in the study coal seam develop heterogeneously and mainly are oriented in NNW, NE, NNW, NNE or near SN directions, and most of them are high-obliquity cracks with inclination of over 50. The micropores in the No. II 1 coal seam are the major pores, secondly are transitional pores, and then less macropores and mesopores. The No. II 1 coal seam has stronger adsorption with an average maximum adsorption capacity of 38.07cm 3 /g, and monlyitic coals have stronger adsorption than undeformed coals, and the difference in adsorption capacity between mylonitic coal and undeformed coal decreases as the temperature increases. The permeability of the No. II 1 coal seam is generally lower and has an evident heterogeneity. But there are higher-permeability region for CBM exploration, for example, near fault structure or the boundary of fault block. Moreover, the reservoir permeability may be so significant improved after hydraulic fracture. All of these indicate