Geochemical Characteristics and Generation Process of Mixed

Jun 25, 2014 - Technology, Xuzhou 221008, China. ABSTRACT: The exploration and development of coalbed methane (CBM) is often associated with the ...
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Geochemical Characteristics and Generation Process of Mixed Biogenic and Thermogenic Coalbed Methane in Luling Coalfield, China Yuan Bao,*,†,‡,§ Chongtao Wei,‡,⊥ Chaoyong Wang,‡,⊥ Guochang Wang,†,⊥ and Qingguang Li†,⊥ †

Key Laboratory of Computational Geodynamics of Chinese Academy Sciences, College of Earth Science, University of Chinese Academy of Sciences, Beijing 100049, China ‡ Key Laboratory of CBM Resource and Reservoir Formation Process, Ministry of Education, China University of Mining and Technology, Xuzhou 221008, China ABSTRACT: The exploration and development of coalbed methane (CBM) is often associated with the thermogenic gas. Because the mixed CBM derived from thermogenic and secondary biogenic gases was discovered in many coal-bearing basins of the world, secondary biogenic CBM which makes a significant contribution to the gas content is becoming a hot topic. In the present study, the origin of the gas in the Luling coalfield of China was first identified through molecular and stable isotope testing. Then, based on the basin evolution history, the generation process of thermogenic CBM and mixed CBM from biogenic and thermogenic gases was analyzed using two calculations. The results show that the carbon isotopic ratios of methane and carbon dioxide in Luling coalfield range from −67.6‰ to −50.5‰ and from −12.6‰ to −8.7‰, respectively, and the hydrogen isotopic ratios of methane range from −228‰ to −206‰. The isotope data indicate that the CBM in the Luling coalfield consists of both biogenic and thermogenic gases. Moreover, the generation process of mixed CBM can be divided into three stages: primary biogenic gas, thermogenic gas, and secondary biogenic gas. Cap outburst dissipation was determined to be the main migration mechanisms of hybrid CBM in Luling coalfiled, whereas diffusion was along with the whole process of CBM generation. Various factors, including maturity, temperature, and the time required for allochthonous methanogenic bacteria to move through the coal bed, were discussed in affecting the generation of secondary biogenic gas. These factors control the quantity, rate, and start and end of secondary biogenic CBM generation, respectively.

1. INTRODUCTION One of the current challenges facing China and many other countries is the anticipated shift in resource use from coal to natural gas. China has a large proven coal and coalbed methane (CBM) resources of approximately 14856.9 × 108 t and 1023.08 × 108 m3 (according to Ministry of Land and Resources of the People’s Republic of China and China United Coalbed Methane Corporation, Ltd., 2013), respectively. The CBM resources of Anhui Province account for 2.7% of those of the entire country. China is poised to take advantage of these resources as the global demand for energy increases. On the basis of its genetic mechanism, CBM can occur as biogenic gas, thermogenic gas,1 or a mixture of both types.2−4 And biogenic gas is further divided into primary and secondary biogenic gases.5 Since the early discovery of secondary biogenic CBM resources by Scott et al. at the San Juan Basin,5 CBM derived from a mixture of secondary biogenic and thermogenic gases has been reported in many basins, such as the Upper Silesian and Lublin Basins in Poland,6,7 Sydney and Bowen Basins in Austrialia,8 and the Xinji area in China.9 Researchers have taken great efforts in studying the origin of biogenic gas,10 two metabolic pathways (acetate fermentation and CO2 reduction),11 generation mechanisms,12 biogenic shale gas generation,13 and thermogenic CBM accumulation.14,15 For example, by testing the isotopic data of δ13CCH4 (−62.6‰), δDCH4 (−199.9‰), δ13CCO2 (from −25.4‰ to 3.42‰), and δDH2O (from −27.3‰ to −18.0‰), Warwick et al.10 thought that the © 2014 American Chemical Society

gas origin of Wilcox Group coal gases was generated in saline formation water by bacterial reduction of CO2. Huang et al.12 expressed that the bioavailability of coal limited the production of biogenic methane and that it can be enhanced by the pretreatment of coal with permanganate. However, there is a lack of data on the generation process associated with the mixture of bio- and thermogenic gases, particularly with respect to mixed gases in nonmarine environments. In this study, we used a laboratory-based approach and integrated previous results16−24 to determine the relation between the cumulative yield of biogenic gas and Ro,max. The gas generation models of the biogenic CBM were then modified and improved. The sampling sites in Luling coalfield of Anhui Province, China, where the most successfully areas in mining CBM, were chosen based on the basin evolution history and methane 13C depletion (from −67.6‰ to −50.5‰).25,26 In addition, the gas origin and generation process of CBM were analyzed by geochemical testing, basin analysis, and numerical simulation. The relative contents of diverse genetic CBM sources currently hosted in coal beds were also quantitatively studied. The factors controlling secondary biogenic gas generation were also examined. Despite being preliminary, the results reported in this manuscript may play a very important role in enriching the CBM accumulation theory of Received: March 17, 2014 Revised: June 24, 2014 Published: June 25, 2014 4392

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Figure 1. Structure map of Luling coalfield at Sudong syncline in Huaibei coalfield, China.

progressively. The roof lithology of coal seam No. 8 is mainly mudstone and siltstone (Figure 2). The thickness of corresponding deposited formations (Neocene and Quaternary) is from 222 to 458 m, and there is no large fault in well areas. Therefore, these maybe provide a good sealing condition for reducing the dissipation of hybrid CBM. The main coal-bearing strata in the Luling coalfield (from bottom to top) are the Shanxi formation, Xiashihezi formation,

low- and middle-rank coal and guiding CBM exploration and development at the Huaibei and Huainan coalfields of China.

2. GEOLOGICAL SETTING The Suzhou mining area is located southwest of the North China Plate and north of the Huaibei coalfield27 and is bounded by the Subei Fault in the north. From east to west, the Suzhou mining area can be divided into the Sudong syncline, Sunan syncline, and Sudong anticline. The Sudong syncline is located along the hanging wall of a thrust-nappe structure, and the Luling coalfield is located in the southwest section of the Sudong syncline (Figure 1). Previous studies28 have shown that the Luling coalfield subsided continuously from the late Carboniferous to the middle Triassic. Affected by the collision of the Southern China Plate and North China Plate from the late Triassic, a SN-trending tectonic compression intensified the regional stratum uplift trend along the EW direction. The stratum in the Luling coalfield was exposed to long-term weathering and erosion.28 Ju and Wang28 concluded that in the Jurassic, the formation subsided and was deposited in local lowlying areas. At the end of the late Jurassic, thrust-nappe tectonics and the Xisipo overthrust were formed in the Suzhou mining area under the impacts of the Pacific Plate. Furthermore, the coal bed burial depth of the upper well of the Xisipo overthrust became smaller. In addition, magmatic rock intruding into the coal beds along the fault at this time heated this area to a notable extent;28 consequently, thermogenic CBM could be quickly generated by magmatic thermal metamorphism. In the Paleogene, faults showing tensile extension provided a channel through which methanogenic bacteria could flow into coal beds with meteoric water, which may have provided an opportunity for the generation of secondary biogenic gas.26 In the Miocene, the crust began to slowly subside, and the burial depth of coal beds increased

Figure 2. Permian stratigraphic column of Luling coalfield (arrow indicates gas sampling zone). 4393

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Table 1. Molecular and Stable Isotopic Compositions of CBM in Luling Coalfield molecular composition (mol %) location

CH4

C2H6

CO2

N2

H2

dryness (C1/C1+)

δ CCH4

δ13CCO2

δDCH4

well L1

78.20 63.00 98.90 98.62 98.95 98.74 97.36 98.46 98.57 98.74

0.02 0.01 0.01 0.02 0.02 0.03 0.02 0.02 0.03 0.04

8.03 6.60 1.03 1.14 0.55 0.70 2.36 0.73 1.1 0.88

13.59 30.33 0.06 0.19 0.47 0.52 0.25 0.78 0.28 0.33

0.16 0.06 0.00 0.02

0.9997 0.9998 0.9999 0.9998 0.9998 0.9997 0.9998 0.9998 0.9997 0.9996

−52.2 −51.3 −50.7 −50.5 −64.2 −67.6 −65.7 −66.2 −65.9 −66.8

−8.7 −9.9 −12.6 −11.5

−227 −228 −226 −225 −206 −224 −216 −221 −218 −223

well L2 Lulinga

a

Stable isotopes (‰) 13

Data from Tong et al. (2013).26

isotope ratio mass spectrometry (Delta plus XL and Delta V) at Wuxi Institute of Petroleum Geology, Sinopec Petroleum Exploration, and Development Research Institute. The values of carbon and hydrogen isotopic compositions are represented in the δ-notation as V-PDB and V-SMOW standards, respectively. The analytical precisions of the carbon and hydrogen isotopic compositions were estimated to be ±0.3‰ and ±3‰, respectively. 3.2.2. Simulation of Biogenic Gas Generation. Simulations of biogenic gas generation were completed at the Biological Engineering Experiment Center of Chemical Engineering and Technology School, China University of Mining and Technology. All of the experiments were conducted within an YQX-II anaerobic glovebox in accordance with the Hungate anaerobic technique.30 First, following the enrichment, expansion, and second expansion of the original methanogenic bacteria culture, a sufficient amount of methanogenic bacterial culture was obtained. Then, one of the original peat samples and six of the pyrolysis samples (solid material) were added to 500 mL of the culture medium. The selection of culture medium was based on the results reported by Wang et al.31 The sample weight was 20 g, and all of the samples were cultured for 45 days or until the output gases reach a balanced state, at which point the output gases were collected immediately using the drainage gas gathering method.18 The method used to measure the output gases’ molecular and isotopic compositions was the same as the analysis of the pyrolysis gases. 3.2.3. Simulation of the Basin Evolution History. A basin evolution history model was built using the 1D package of the Petromod software program. First, the basic geological information, including the stratum deposition age, present thickness, erosion age, eroded thickness, and lithology, was inputted. The boundary conditions, such as the water depth, SWIT (sedimentary water interface temperature), and HF (heat flow), were then set. As a result, by iteratively adjusting the HF values to match the known paleogradient values and the maximum vitrinite reflectance, the basin evolution histories (such as tectonic burial history, thermal history, and maturity history of organic matter) of the Luling coalfield could be obtained. 3.2.4. Numerical Simulation of the CBM Generation Evolutionary History. The numerical simulation of the evolutionary history of CBM generation consists of two calculation processes. The first calculation involves simulating the evolution of thermogenic gas generation using the CBMHS1.0 software program,32 and the second calculation

and Shangshihezi formation of the Permian. Coal seams No. 8 to 10 are the main mining coal seams in the Luling coalfield (Figure 2). In this study, coal seam No. 8 was the target formation for the numerical simulation of mixed bio- and thermogenic CBM. Its thickness ranges from 0.14 to 16.62 m with an average of 10.99 m, and its Ro,max value ranges from 0.86% to 0.89%. The depth of coal samples is from 742.7 to 808.9 m. The dip angle ranges from 10 to 25 degrees. The lateral continuity is well but end at the southwestward, Xishipo overthrust, the main fracture in this region. The depositional environment of Permain coalseam belongs to the marine and continental transitional facies. The roof and floor lithology of coal seam No. 8 is mudstone and siltstone. Hydrogen and oxygen isotopic compositions26 indicated that the water within the coal seam No. 8 was from meteoric water and the recharge areas with meteoric water throughout the regional coal seam.

3. MATERIALS AND METHODOLOGY 3.1. Samples. Gas samples were collected from fractured CBM wells L1 and L2, which are located in the west flank and axis of the Sudong syncline, respectively (Figure 1). The gases are coal bed gases pumped from the No. 8 coal seam (Figure 2). The gas samples from wells L1 and L2 were collected in August 2012 using the saturated salt water displacement method29 and were tested immediately after collection. In order to simulate the generation process of CBM in coal seam No. 8 and build the relation between the secondary biogenic gas cumulative yield and maturity, an experiment was conducted to obtain a series of different maturity samples by heating immature peat to various temperatures. The peat sample was collected in the same depositional environment. Fortunately, a dark-brown woody swamp peat was collected from Heilongjiang Province in Northeast China in May 2012 at depths of 0.2−0.4 m. The geochemical parameters of the samples are the same as those discussed by Bao et al.25 By performing pyrolysis experiments on the original peat samples at different temperatures (150, 250, 300, 350, 375, and 400 °C), a series of solid pyrolysates with different maturities (Ro,max values are 0.34%, 0.54%, 0.81%, 1.29%, 1.57%, and 1.81% measured by microscope photometer) was obtained. The peat and solid pyrolysates were used as substrates for microbial degradation. 3.2. Experimental Approaches. 3.2.1. Gas Molecular and Isotopic Composition. The gas content and the carbon and hydrogen isotopic compositions were analyzed using gas chromatography (Varian CP-3800) and thermal conversion4394

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involves simulating the generation process of hybrid CBM derived from bio- and thermogenic gases based on a modified gas generation model. The biogenic gas generation model was incorporated into the hybrid modified gas model, and the former was constructed based on the relation between cumulative gas yield and maturity.

4. RESULTS AND ANALYSIS 4.1. Geochemical Characteristics and CBM Origin. The data presented in Table 1 are the molecular and isotopic compositions of CBM in the Xiashihezi formation of the Luling coalfield. Methane is the main gas component of CBM in the Luling coalfield, whereas ethane and nonhydrocarbon gas were observed in trace amounts (Table 1). The mole percentages of methane and ethane range from 63.00% to 98.95% and from 0.01% to 0.04%, respectively. Carbon dioxide and nitrogen are the main components of nonhydrocarbon gas, and a trace amount of hydrogen was detected. The gas dryness (C1/C1+) is from 0.9996 to 0.9999. The carbon isotopic ratios of methane and carbon dioxide range from −67.6‰ to −50.5‰ and from −12.6‰ to −8.7‰, respectively, and the hydrogen isotopic ratio of methane ranges from −228‰ to −206‰. Smith and Pallasser7 showed that δD values for methane derived from CO2 reduction is −227‰ and from aceticlastic reactions is −333‰ (V-SMOW). Therefore, the dominant pathway for secondary biogenic gas generation in Luling coalfield is CO2 reduction. The genetic character of CBM can be determined using the relations between δ13CCH4 and δDCH433−36 and between δ13CCH4 and δ13CCO2.37 By plotting the δ13CCH4, δDCH4, and δ13CCO2 values obtained for the Luling coalfield, as shown in Figure 3 and Figure 4, it can be determined that the CBM in the Xiashihezi formation in Luling coalfield is of mixed origin. 4.2. Basin Evolution History. The burial, thermal, and maturity histories of coal seam No. 8 at wells L1 and L2 of Luling coalfield, which are shown in Figure 5, were modeled using the 1D package of the Petromod software program. The method and operation process is shown in section 3.2.3. By adjusting the HF values to meet the paleogeothermal gradient

Figure 4. Genetic characterization of CBM from Luling coalfield using δ13CCH4 and δ13CCO2. The general genetic field boundaries shown in this figure are derived from Whiticar (1999).37

(e.g., from Carboniferous to Jurassic, the geothermal gradient was 2.5 °C/100m; from later Jurassic to early Cretaceous, it was from 5 to 7 °C/100m; from later Cretaceous to Neogene, it was from 1.69 to 1.94 °C/100m; at Quaternary, it was from 1.7 to 2.6 °C/100m17) and maximum vitrinite reflectance (Ro,max value ranges from 0.86% to 0.89%). After calculation, the simulated results of burial, geothermal, and maturity evolution history would be output (Figure 5). Table 2 presents the data obtained from the numerical simulation of the evolutionary CBM generation. 4.3. Model Modification. Primary biogenic gas is generated during the early stages of coalification (i.e., during peat formation) by acetic acid fermentation or CO2 reduction. When coal beds are buried to a certain depth and enter the gas window or are affected by plutonic metamorphism, thermogenic gas can be generated.38 If methanogenic bacteria are introduced into coal beds after the burial, coalification, and subsequent uplift and erosion of basin margins, the methanogenic bacteria metabolize the wet gas components and other organic matter at relatively low temperatures (generally below 56 °C).5 Thus, secondary biogenic gas can be generated. Based on the mass balance principle, models of evolutionary CBM generation were built by Wei et al.14,15 in 2007 and 2010. These models include generation, dissipation, preservation, and basin parameters. In the present study, a gas generation model for biogenic gas was incorporated into the aforementioned models because Wei et al.14,15 model considers only the generation process of thermogenic gas without accounting for biogenic gas generation, dissipation, and conservation. 4.3.1. Primary Biogenic Gas Model. Based on the few published data regarding the cumulative yield of primary biogenic gas16,17 and the data obtained from the biogenic gas simulations performed in this study (Table 3), the primary biogenic gas generation can be mathematically modeled as follows: ⎧ Q̅ p ,max ⎪ ⎪ × t (t ≤ t i ) Q P = ⎨ ti ⎪ ⎪0 (t > t i ) ⎩

Figure 3. Genetic characterization of CBM from Luling coalfield using δ13CCH4 and δDCH4. The gas genetic fields shown in this figure are derived from Whiticar et al. (1986).36 Luling data from Tong et al. (2013).26 4395

(1)

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Figure 5. Burial, geothermal, and maturity evolution curves of coal seam No. 8 at well L1 in Luling coalfield.

Table 2. Basin Evolution History Data for Coal Seam No. 8 at Wells L1 and L2 in Luling Coalfield burial depth (m)

temperature (°C)

system period

geological age (Ma)

evolutionary period (Ma)

L1

L2

L1

L2

L1

L2

late Permian middle Triassic late Triassic middle Jurassic late Jurassic early Cretaceous late Cretaceous Oligocene present

248.0 230.0 213.0 170.0 144.0 96.0 65.0 23.5 0.0

39.0 57.0 74.0 117.0 143.0 191.0 222.0 263.5 287.0

2011.50 2828.00 1889.40 2121.10 2291.30 1535.90 950.30 510.00 742.70

2082.30 2886.00 1965.50 2164.20 2350.90 1579.70 982.80 567.80 808.90

73.34 89.88 72.15 89.52 126.36 96.39 62.42 38.74 38.04

72.76 91.64 73.37 92.02 128.29 97.24 65.17 40.39 40.02

0.43 0.56 0.56 0.60 0.81 0.86 0.86 0.86 0.86

0.45 0.57 0.58 0.61 0.84 0.89 0.89 0.89 0.89

where Qp is a function describing the cumulative generation of primary biogenic gas; t is any time, or more specifically, any geological time; ti is the end time of primary biogenic gas generation according to the geological time at which Ro,max = 0.30%;5 and Q̅ p,max is the average cumulative yield of primary biogenic gas, which is equal to 12.18 m3/t. Equation 1 means before Ro,max maturated to 0.30%, the generation of primary biogenic gas is linear. After that, the primary biogenic gas will stop generation. The data shown in Table 3 are insufficient to establish the relation between the cumulative yield of primary biogenic gas and Ro,max. To model different types of kerogen and samples with different evolutionary histories, average values were used to simplify the complex relation. With an increase in the amount of available data, the relation should be more accurately defined in future studies. 4.3.2. Secondary Biogenic Gas Model. It is well known that methanogenic bacteria are introduced into coal beds with meteoric water and that this introduction leads to the generation of methane and carbon dioxide. However, there is no set start and end time for secondary biogenic gas generation in practice; the onset of secondary biogenic gas generation is primarily controlled by the coal bed temperature during evolutionary processes.5 During tectonic uplift, the temperature of a coal bed decreases; when the temperature reaches 56 °C, secondary biogenic gases are generated.5 To calculate the end

maturity (%)

time of secondary biogenic gas generation, many experiments on the microbial metabolization of coal-forming organic matter at different evolutionary stages have been performed in closed laboratory environments.18,21,22 The results show that the cumulative yield of gases generated by microbes metabolizing organic matter stabilizes between 40 and 400 d. However, under actual geological conditions, the generation of secondary biogenic gases through microbe-induced degradation is controlled and affected by many geological factors, such as temperature, formation uplift or sedimentation, coal rank, and water quality.39 Thus, secondary biogenic gas generation is initiated when the burial depth of coal beds becomes shallow, the temperature of the coal beds decreases to 56 °C, and there are live microbes and is terminated when the burial depth of coal beds becomes rapidly deeper and the temperature of the coal beds increases to a temperature higher than 56 °C. In summary, at any geological time t, the generation of secondary biogenic gas can be modeled as follows: ⎧ Q s ,max ⎪ (t j ≤ t ≤ t k ) ⎪ Q s = ⎨ 1 + exp[a − b(t − t j)] ⎪ ⎪0 t > tk , or t < t j ⎩

(2)

where Qs is the cumulative function of secondary biogenic gas generation; Qs,max is a function of Ro,max and tj; tj is the start time of secondary biogenic gas generation; tk is the end time of 4396

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Table 3. Experimental Data Regarding Biogenic Cumulative Gas Yield and Maturity of Samples type of biogenic gases primary biogenic gases

secondary biogenic gases

Ro,max (%)

sample number/description

gas cumulative yield (m3/t)

Qinglangang of Hainan No. 6 well of Sezhong ZK004 well of Yunnan ZK004 well of Yunnan Woody peat of Heilongjiang Average value Sangshantang of Yunnan

0.17 0.25 0.25 0.26 0.16 0.34

24.0 17.9 4.6 9.6 4.80 12.18 2.64

Kebao of Yunnan Hongni of Yunnan Dongsheng of Nei Monggol Yima of Henan E2 of Liulin E1 of Liulin coal sample of Fort Yukon area coal sample of Powder River Basin coal sample TX of Uvalde region of Texas Basin coal sample B93 of Bashaw area coal sample of Surat Basin

0.32 0.31 0.35 0.56 1.24 1.29 lignite A subbituminous coal subbituminous coal

2.54 2.12 7.10 6.82 6.08 7.34 0.13−0.28 0.14−0.38 1.75

subbituminous coal subbituminous coal to bituminous coal 0.34 0.54 0.81 1.29

1.50 6.50

Pyrolysis Pyrolysis Pyrolysis Pyrolysis

sample sample sample sample

of of of of

150 250 300 350

°C °C °C °C

secondary biogenic gas generation; a is a temporal factor capturing the flow of microbes through coal beds; and b is a factor related to temperature. Equation 2 means the quantity of secondary biogenic gas generation depends on the factors of maturity, temperature, and time. 4.4. Simulation of Evolutionary CBM Generation. Coal seam No. 8 of the Xiashihezi formation in the middle Permian strata of the Luling coalfield was deposited approximately 287 Ma. A computer simulation was used to output 5166 data points on the geological age, burial depth, vitrinite maximum reflectance, cumulative yield of generated gas, coal seam gas content, reservoir pressure, cumulative diffusion loss intensity, and cumulative cap outburst diffusion intensity at 1 Ma intervals. The evolutionary CBM generation was simulated in two steps: first, the generation of thermogenic gas was simulated using a previous model and second, the generation of biogenic and thermogenic gas was simulated using the modified gas generation models developed in this study. Figure 6a−h illustrates the evolution of coal seam No. 8 at wells L1 and L2 in terms of the cumulative yield of generated gas, coal seam gas content, reservoir pressure, cumulative diffusion loss intensity, and cumulative cap outburst diffusion intensity over time. By comparing the results of the two simulations, the generation process of hybrid CBM can be divided into three stages. The first stage is the generation process of primary biogenic gas, which corresponds to the evolutionary period from 0 to 32 Ma. In this stage, the generation process of hybrid CBM is characterized by the rapid generation of primary biogenic gas within a relatively short period followed by a gradual increase in yield (Figure 6a, b). At 1 Ma, the first cap outburst dissipation event occurs due to a shallow burial depth and weak rock strength. The corresponding loss in intensity ranges from 81.49 to 91.84 m3/m2 according to the simulation

4.21 6.50 7.74 3.82

data source Chen et al., 199116

Bao, 201317 This study Wang, 201018

Chen et al., 199116 Su et al., 201219 Li et al., 199720 Harris et al., 200821 Jones et al., 200822 Penner et al., 201023 Papendick et al., 201124 this study

results. The gas content of coal beds begins to increase after an evolutionary period of 20 Ma (Figure 6c, d), and the increase in gas content results in a decrease in the cumulative diffusion loss intensity (Figure 6e, f). The second stage is the generation process of thermogenic gas, which corresponds to an evolutionary period of 32 to 231 Ma (Figure 6). At 32 Ma, the generation process of hybrid CBM was characterized by the rapid, early generation of thermogenic gas with an initial increase in burial depth. At 118 Ma, because the burial depth and temperature of coal beds increases again, thermogenic gas is rapidly generated again. Therefore, the gas content of coal beds is characterized by repeated increases and decreases corresponding to changes in the characteristics of the cumulative yield of generated gas (Figure 6a−d). At 144 Ma, the second cap outburst dissipation event occurs in the well area L2 because the burial depth of the No. 8 coal seam is too large (2350.9 m). An excessively large burial depth will cause an excessively high reservoir pressure and may exceed the critical point of cap outburst. The reservoir pressure before the second cap outburst occurs is 33.4 MPa. The cumulative cap outburst dissipation strength of well L2 is 113.21 m3/m2 at 144 Ma. Well L1 does not undergo the second cap outburst dissipation, which is the main reason why the gas content of coal seam No. 8 in well L1 does not rapidly decrease at 144 Ma (Figure 6c, d). The third stage is the generation process of secondary biogenic gas, which corresponds to the period from 231 to 287 Ma. At 231 Ma, coal beds are uplifted to approximately 1000 m near the surface (Figure 5). Methanogenic bacteria can be easily introduced into coal beds with meteoric water.26 When the coal bed temperature decreases to less than 56 °C and there is a sufficient amount of methanogenic bacteria, secondary biogenic gas generation is initiated. The yield is affected by temperature and the time required for the methanogenic bacteria to move 4397

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Figure 6. Evolutionary generation curves of coal seam No. 8 in Luling coalfield (the black curve describes the evolutionary process of thermogenic gas and the red curve describes the evolutionary process of hybrid CBM derived from biogenic and thermogenic gases.) (a) and (b) Cumulative yields of generated gas versus the evolutionary time in wells L1 and L2, respectively. (c) and (d) Gas content of coal seam versus the evolutionary time. (e) and (f) Cumulative diffusion dissipation strength versus the evolutionary time. (g) and (h) Cumulative cap outburst dissipation strength versus the evolutionary time.

Figure 7. Relation between cumulative yield of secondary biogenic gas as a function of Ro,max.

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Figure 8. Trend of the cumulative yield of secondary biogenic gas during evolution (a) and its incorporation into gas generation models (b and c).

where Qs,max represents the cumulative yield of secondary biogenic gas. 5.1.2. External Factors. The overall trend of microbial growth can be modeled by four stages: lag, exponential, stationary, and decline. Similarly, the secondary biogenic gas generation curve can also be described by a slow increase, quick increase, slow increase, and balance period (Figure 8a). The time required for allochthonous methanogenic bacteria to move through coal beds controls the onset of secondary biogenic gas generation and the length of the first slow increase stage (Figure 8b). The temperature controls the start and end times and the yield of secondary biogenic gas generation as well as the slope of the curve (Figure 8c). Equation 4 reflects the characteristics of the aforementioned factors that control secondary biogenic gas generation42

through coal beds. In this stage, the cumulative yield of gas generated and the gas content of coal beds are characterized by a quick increase within 1 Ma (Figure 6a−d). Diffusion is the main dissipation mechanism of hybrid CBM (Figure 6e, f). By comparing the results of the two simulation results, it can be concluded that the gas content of primary biogenic gas in coal seam No. 8 presently ranges from 0.40 to 0.97 m3/t and that of secondary biogenic gas ranges from 7.92 to 7.95 m3/t (Figure 6c, d).

5. DISCUSSION 5.1. Factors Affecting Secondary Biogenic Gas Generation. The main internal (Ro,max) and external (temperature and time required by allochthonous methanogenic bacteria to move through coal beds) factors affecting secondary biogenic gas generation are discussed in the following sections. 5.1.1. Internal Factors. Figure 7 shows the relation between the cumulative yield of secondary biogenic gas and maturity (Ro,max) according to the few data shown in Table 3. When Ro,max ranges from 0.30% to 1.50%, the cumulative yield of secondary biogenic gas tends to increase and then decrease (Figure 7). The maximum cumulative gas yield occurs when the value of Ro,max is equal to 0.87%. This peak is observed because a value of Ro,max equal to 0.80% corresponds to the maximum generation of thermogenic wet gas,5 that is, when the value of Ro,max exceeds 0.80%, there is abundant organic matter for methanogenic bacterial degradation. There are reports of the degradation of volatile organic compounds of bituminous coal and anthracite (Ro,max value greater than 1.50%) by microbes cultured with nutrient solution.40,41 However, the Ro,max value of Luling coalfield was observed to be less than 1.50%. Thus, the relation between the cumulative yield of secondary biogenic gas and maturity (Ro,max ranges from 0.30% to 1.50%) built in this study is suitable for the CBM of Luling coalfield. Based on very little published data16,18−24 and our experimental data (Table 3), the cumulative yield of secondary biogenic gas can be expressed by the following equation:

Qs =

R2 = 0.563)

1 + e a − bt

(4)

where Qs is the cumulative yield of secondary biogenic gas at any geological time t; Qs,max is a function of factor Ro,max and geological time t; a is a parameter that describes the time required for allochthonous methanogenic bacteria to move through coal beds; and b is a temperature-based factor. In addition, water quality is also a restraint on methanogenesis, which is characterized by negative δ13CDCI, and the combination of enriched δ13CDIC and enriched δDH2O is a robust indicator of large quantities biogenic gas has generated.39 However, for the generation amount of biogenic gas impacted by water quality, it is worthy of further attention. 5.2. Comparison of Quantitative and Simulated Results. Following the creation of a binary hybrid model of biogenic and thermogenic gas generation, the estimated proportion of each gas generated can be quantitatively identified using the δ13CCH4 value as the end member of different genetic types of methane.9,25 Bao et al.25 established the relation between the value of δ13CCH4 of thermogenic gas and maturity (Ro,max) using a pyrolysis experiment as follows:

Q s,max = −15.58(R o,max )2 + 27.19(R o,max ) − 3.63 (0.30% ⩽ R o,max ⩽ 1.50%

Q s,max

δ13CCH4 = −26.20log R o,max − 34.12 (R o,max < 1.30%)

(3) 4399

(5)

dx.doi.org/10.1021/ef500599s | Energy Fuels 2014, 28, 4392−4401

Energy & Fuels δ13CCH4 = 25.85log R o,max − 43.08

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major role in replenishing the gas content of coal seam presently.

(R o,max ≥ 1.30%)

■

(6)

The Ro,max values of coal seam No. 8 at wells L1 and L2 in Luling coalfield are 0.86% and 0.89%. In the absence of any secondary biogenic gas mixing into the coal beds of Luling coalfield, the δ13CCH4 value in Luling coalfield was calculated to range from −32.8‰ to −32.4‰ based on equation 5. However, the laboratory measured δ13CCH4 value ranged from −52.2‰ to −50.5‰. A comparison between the tested and calculated data shows that the measured δ13CCH4 value in Luling coalfield is much lower than the calculated value, suggesting that the coal bed gases of Luling coalfield are currently mixed with secondary biogenic gas. The distribution of δ13CCH4 values of biogenic CBM ranges from −90.0‰ to −55.0‰34 or from −80.0‰ to −52.0‰.43 Through statistics of 576 δ13CCH4 values of biogenic CBM in the word-wide, Tao et al.9 thought −70.0‰ is in the majority. Therefore, if −70.0‰ is assumed to be the value of δ13CCH4 for secondary biogenic gas and the value of δ13C CH4 for thermogenic gas is calculated based on equation 5, the estimated proportion of thermogenic gas in coal seam No. 8 of Luling coalfield is calculated to range from 48.54% to 52.15%, whereas that of secondary biogenic gas ranges from 47.85% to 51.46%. The groundwater and methanogenic pathways may also affect the value of methane isotopic compositions. However, these factors are beyond the topic of this research, they will be being considered in the next study. The actual gas content of coal seam No. 8 was measured to be 16.57 m3/t. The previous numerical simulation results reveal that the gas content of primary biogenic gas in coal seam No. 8 ranges from 0.40 to 0.97 m3/t, that of secondary biogenic gas ranges from 7.92 to 7.95 m3/t and that of thermogenic gas ranges 7.68 to 8.22 m3/t. If the gas contents are converted to percentages, the proportion of primary biogenic gas ranges from 2.41% to 5.85%, that of secondary biogenic gas is between 47.80% and 47.98%, and that of thermogenic gas ranges from 46.35% to 49.61%. The comparison of the measured and simulated results shows that thermogenic and biogenic gases are the main coal bed gases in coal seam No. 8 of Luling coalfield, the contents of which are comparable, which indirectly indicates that the simulation results are reliable to a certain degree.

AUTHOR INFORMATION

Corresponding Author

*Y. Bao. E-mail: [email protected]. Present Address §

College of Earth Science, University of Chinese Academy of Sciences, 19 A Yuquan Road, Beijing, China. Author Contributions ⊥

These authors contributed equally. The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.

Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS The authors gratefully acknowledge the financial support from the following institutes: China Postdoctoral Science Foundation (Grant No. 2014M550809), the National Science and Technology Major Project (Grant Nos. 2011ZX05034-005 and 2011ZX05060-005), and Key Program of the National Science Foundation of China (Grant Nos. 41272177 and 41372213). In addition, data related to gas isotopes are tested by us, and published data about gas cumulative yield are cited in references. We thank Scott A. Quillinan and three anonymous reviewers who provided contrasting viewpoints and many helpful suggestions for improving the manuscript.

■ ■

ABBREVIATIONS CBM = coalbed methane SWIT = sedimentary water interface temperature HF = heat flow d = days REFERENCES

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