Article pubs.acs.org/EF
Partial Coal Pyrolysis and Its Implication To Enhance Coalbed Methane Recovery: A Simulation Study Yidong Cai,*,† Dameng Liu,† and Zhejun Pan‡ †
Coal Reservoir Laboratory of National Engineering Research Center of CBM Development & Utilization, China University of Geosciences, Beijing 100083, China ‡ CSIRO Energy Flagship, Private Bag 10, Clayton South, Victoria 3169, Australia ABSTRACT: A simulation study of partial coal pyrolysis to improve the petrophysics of coal seams and ultimately extract higher methane yields with accompanying pyrolysis gases was conducted, which was used to investigate the feasibility of partial coal pyrolysis for enhancing coalbed methane (CBM) recovery. Enhancing CBM production is an important subject for current CBM development, especially for low permeability coals. CBM exists mainly in an adsorption state in multiple micropores, which increases the complexity of CBM production. Pore volume and porosity in the low rank coal increased with increasing temperature. The permeability of the low rank coal increased exponentially with increasing temperature (300−400 °C) due to the generated pore−fractures. The excessively high temperature of pyrolysis could result in the coals with the highest pore volume possessing the lowest methane adsorption capacity due to the extent of graphitization. Partial coal pyrolysis in a subsurface can increase the gas content, and improve the seepage ability of a CBM reservoir. CBM gas-in-place can be vastly increased (almost seven times the original CBM gas-in-place) by thermal treatment. The results indicated that the peak of daily gas production greatly increased and the gas yield peak arrived in advance for different rank coals with thermal treatment. By means of F.A.S.T. numerical software, gases in place and gas production for coal reservoirs after thermal treatment were acquired, which demonstrated that injected heat could promote CBM desorption, increase the coal permeability, and improve CBM production. Therefore, this technique may have significant implications for enhancing CBM recovery. Previous field trials in China and Poland suggested that alternating injection of air and steam can heat the coal and the surrounding strata and can produce high-quality gases by recuperating the heat from the surrounding strata.3−5 Coal geomechanics and gas flow can be linked by two important petrophysical parameters, namely, the permeability and porosity. These two parameters combined with gas content are critical to CBM production. Therefore, heating a CBM reservoir can effectively improve the permeability, pore pressure, and gas production during low-temperature heating processes (less than 130 °C).6 In this paper, the gas production for the CBM reservoir treated with high-temperature heating (higher than 100 °C until 600 °C) will be simulated using a F.A.S.T. CBM simulator. A two-way sequential coupling of experimental modeling is used to investigate the variation of reservoir parameters.1 Then, a gas production numerical simulator was used to model the gas production history of the heat-treated coal reservoirs. To accurately and continuously achieve the suitable maximum temperature for enhancing CBM recovery, numerical simulation was adopted to analyze gas and water production of different rank coals at elevated temperatures (20 °C−600 °C).
1. INTRODUCTION Previous massive laboratory experiments on three coal rank samples monitored changes in mass loss, product composition, pore structure, porosity, methane adsorption, and permeability accompanying coal pyrolysis were conducted, which investigate the feasibility of enhanced coalbed methane (ECBM) recovery with partial coal pyrolysis.1 The work only discussed the concept from a systemic experimental perspective to determine if it was worth pursuing. Although multiple equations for a coal reservoir that was proceeded by heat treatment were developed to design and study the variations in petrophysical parameters of the heattreated coal reservoirs,2 the assessment of ECBM recovery through heat treatment was never modeled in any degree of detail. Integrated analysis of those petrophysical parameters showed that a number of potential issues could arise during its application when constructing the prediction for CBM production. This paper focuses on numerical modeling of reservoir parameter variations and gas production of coals by heat injection. A conceptual model for gas production in heat-treated coal seams was proposed by previous research based on extensive experimental measurements of multiple petrophysical parameters, including pore structure, porosity, methane adsorption, and permeability, which found that coals treated by heat injection play a significant role in contributing to petrophysical parameters.1 Methane adsorption ability is related not only to the pore structure but also to the extent of graphitization. Furthermore, gas produced from coals during heat injection may also provide considerable overall gas content for ECBM recovery. © 2017 American Chemical Society
2. GEOLOGICAL BACKGROUND AND MODEL SETUP 2.1. Field Description and Reservoir Properties. Three Chinese CBM production areas (Figure 1), namely, the Baode Received: January 20, 2017 Revised: April 19, 2017 Published: April 20, 2017 4895
DOI: 10.1021/acs.energyfuels.7b00219 Energy Fuels 2017, 31, 4895−4903
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3, 8 + 9, and 11 coal seams, which belong to the Shanxi and Taiyuan groups, respectively. 2.1.1. Field Description. The Baode block with welldeveloped coal seams is located in the northeast of the Ordos basin, which covers an area of 495 km2. The Baode block is situated in a west-inclined monoclinic structure with a dip angle between 5° and 10° and is becoming the most quickly developing district of the CBM industry in the low- to medium-rank coals in China. The No. 8 + 9 coal seams are the targeted seams (Figure 2). Among the existing production wells, the highest CBM production is approximately 16,000 m3/d per well. Because of this significant gas potential, the Baode block is considered favorable for CBM exploration and development.7 The Hancheng block, with an area of 1690 km2, is situated in the Weibei uplifted belt in the Ordos basin, which is a northwestinclined monoclinic structure.8 The coal-bearing strata are the Upper Carboniferous Taiyuan group and the Lower Permian Shanxi group; the main mineable coal seams are Nos. 2, 3, 5, and 11. The No. 3 coal seam has a thickness of 5 m and contains 1−2 mudstone bands. The thickness of the No. 11 coal seam ranges from 2 to 8 m, and this coal seam has a stable seam structure. The No. 3 coal seam is the target seam in this study. The Zhengzhuang block has an area of 771 km2 and is located in the Jingcheng coal district of the Southern Qinshui basin dual syncline, which is horseshoe shaped with a dip of 3−13° and an orientation of 5°NW.9 A few faults and folds with axial striking of NNE-SSW and near N−S are common (Figure 1). The main coal seams in this area are Nos. 3 and 15; the total thickness of the coal seams ranges from 7 to 16 m. In this study, the target seam is the No. 3 coal seam. 2.1.2. Reservoir Parameters. For the Baode block, coal samples were delivered to the laboratory for experimental tests, such as the vitrinite reflectance test, proximate analysis, and fractures determination. The maximum vitrinite reflectance (Ro, m) of the coals ranges from 0.6% to 0.97%, with an average of 0.8%. The thickness of Nos. 8 + 9 is on the order of 5−14.2 m, with an average of 10.2 m. The maceral composition reveals that
Figure 1. Research areas with differently ranked coals from the Zhengzhuang, Hancheng, and Baode areas, North China.
area of the northeastern Ordos Basin (low-medium rank coal), the Hancheng area of the eastern Ordos Basin (medium rank coal), and the Zhengzhuang area of the southern Qinshui Basin (high rank coal), were selected to simulate the effect of partial coal pyrolysis on ECBM recovery. The stratigraphy structure can be divided into two systems (Carboniferous and Permian) and two groups (Taiyuan and Shanxi). The main coal seams are Nos.
Figure 2. Stratigraphic section of coal-bearing strata in the Zhengzhuang, Hancheng, and Baode areas, North China. 4896
DOI: 10.1021/acs.energyfuels.7b00219 Energy Fuels 2017, 31, 4895−4903
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Figure 3. Simulation design: (a) simulation grids; (b) conceptual model for gas movement in coal reservoir; (c) thermal gas recovery from coal reservoir ((b) and (c) Reproduced with permission from ref 6. Copyright 2011 Society of Petroleum Engineers.
Table 1. Static Parameters for CBM Wells from Different Rank Coals Determined values Parameter Type
Static parameters
Nos. 8 + 9 (BD)
No. 3 (HC)
No. 3 (ZZ)
Parameter specification
Reservoir parameters
Coal thickness (m) Reservoir pressure (MPa) Porosity (%) Permeability (×10−3 μm2) Coal density (g/cm3) Langmuir Vol. (m3/t)/Pressure (MPa) Well temperature (°C) Wellbore radius (cm) Burial depth (m)
10.2 4.2 6.8 5.2 1.49 10.1/1.9 20 3.8 700−715
5.3 3.35 6.5−6 1.96 1.45 24.5/2.4 20 3.8 369−449
6.11 6.34 5.4 0.5 1.46 42.09/2.69 30 3.8 705−809
Well logging Injection/fall off well test
Well data
Sample test Experimental test Field data
Reservoir permeability is commonly less than 0.5 mD but locally reaches 1 mD. Coal permeability, depending on the cleat system in the coal structure, determines the ability to conduct fluids. 2.2. Model Setup and Modeling Parameters. A 400 m × 400 m (length × width) model (Figure 3) domain was constructed to simulate CBM production with and without heat treatment, which also shows the petrophysical parameters’ variation. Differences between reality and simulation cannot be avoided; the simulation can be more intuitive to show the influences of different factors. However, necessary assumptions are adopted, which are as follows: (1) the coal reservoir is isotropic; (2) the thermophysical and dielectric parameters are constant;14 (3) the temperature homogeneously diffuses in the coal reservoir; (4) pyrolysis gas can be effectively preserved in the coal reservoir. The thickness of the coal to be accessed by the CBM exploitation was assumed to be 10 m for the Baode block, 5 m for the Hancheng block, and 6 m for the Zhengzhuang block. All coal seam reservoir parameters and the range of values used in the simulations and explained in the paragraphs above are given in Table 1.
the coals have high vitrinite content (more than 70%). The gas content varies from 2 to 12 m3/t. The reservoir permeability is relatively high, in the range of 2.4 to 8 mD. For the Hancheng block, the main coal lithology is semibright and semidull coal, which accounts for ∼40% and 30%, respectively. The maceral composition is primarily vitrinite (60%∼85%). The fractures in the coals are well developed. The gas content is 7.4−16.97 m3/t. The porosity is on the order of 3.99%−10.43%, with an average of 6%. The permeability varies from 0.023 to 16.2 mD, which generally decreases with depth, as increasing stress causes cleats to close.9−11 For the Zhengzhuang block, the thickness of the No. 3 coal seam is between 2.3 and 7.37 m, with an average of 6.11 m. The maximum vitrinite reflectance (Ro, m %) is between 1.95 and 3.49%. The Langmuir volume and pressure are in the range of 28.1 to 57.1 m3/t and 1.91 to 3.47 MPa, respectively. The gas content is high, between 10 and 37 m3/t.12,13 The porosity is between 1.5% and 9.3% and is usually less than 5%. Maceral composition is variable, containing 18.5%−97.4% vitrinite, 2.4%−81.4% inertinite, and a small amount of other minerals. 4897
DOI: 10.1021/acs.energyfuels.7b00219 Energy Fuels 2017, 31, 4895−4903
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Figure 4. Gas and water production curves of CBM wells from different coal districts, North China.
d, with an average of 5.03 m3/d. The long-term stable drainage decompression steadily improves the CBM production (Figure 4). The Hancheng block is the first commercial CBM production block in medium-rank coals. In this study, well HC-1 was selected to exhibit gas and water production characteristics. For well HC1, after a 4-month period of unstable water production (normally well HC-1 > well BD-1. This is mainly because the improved reservoir permeability in high-rank coal is relatively weak, whereas a large amount of pyrolysis gas can be created from the organics. Therefore, from the perspective of CBM development, thermal treatment of a high-rank coal reservoir may be more valuable.
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4. CONCLUSIONS The simulation is parametrized based on experimental measurements using differently ranked coal samples with a unique partial coal pyrolysis and coupled with an ordinary CBM simulator (F.A.S.T. CBM) to investigate the effects of thermal treatment of a coal reservoir on gas and water production and on thermalECBM recovery. Although many assumptions were used, the simulation results indicate that CBM gas-in-space can be vastly increased due to the increased temperature for thermal treatment. Additionally, the consistent trends for CBM wells from differently ranked coals are as follows: the gas peak of daily production greatly increased and gas yield peak arrived in advance. However, the stable production period of CBM wells from a thermally treated coal reservoir is well ZZ-1 > well HC-1 > well BD-1, which is mainly because the improved reservoir permeability in high rank coal is relatively weak, whereas a large amount of pyrolysis gas can be created from the organics. Therefore, from the perspective of CBM development, thermal treatment for a high-rank coal reservoir may be more valuable. The current work only considered a homogeneous reservoir under the hypothetical conditions. Future work will be focused on the actual anisotropic coal under actual reservoir conditions. 4902
DOI: 10.1021/acs.energyfuels.7b00219 Energy Fuels 2017, 31, 4895−4903
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Energy & Fuels (25) Yang, X. L. Study on mechanism of injection heat increasing production in coal-bed gas of low permeability coal seam; Doctoral dissertation of Liaoning Technical University, 2009.
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DOI: 10.1021/acs.energyfuels.7b00219 Energy Fuels 2017, 31, 4895−4903
