STUDY ON GAS ADSORPTION AND TRANSPORT BEHAVIOR IN SHALE ORGANIC NANOPORE
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摘要: 页岩储层孔隙结构复杂, 气体赋存方式多样. 有机质孔隙形状对受限空间气体吸附和流动规律的影响尚不明确, 导致难以准确认识页岩气藏气体渗流机理. 为解决该问题, 本文首先采用巨正则蒙特卡洛方法模拟气体在不同形状有机质孔隙(圆形孔隙、狭长孔隙、三角形孔隙、方形孔隙)内吸附过程, 发现不同形状孔隙内吸附规律符合朗格缪尔单层吸附规律, 分析了绝对吸附量、过剩吸附浓量、气体吸附参数随孔隙尺寸、压力的变化, 研究了孔隙形状对气体吸附的影响. 在明确不同形状有机质孔隙内气体热力学吸附规律基础上, 建立不同形状有机质孔隙内吸附气表面扩散数学模型和考虑滑脱效应的自由气流动数学模型, 结合分子吸附模拟结果研究了不同孔隙形状、孔隙尺寸有机质孔隙内吸附气流动与自由气流动对气体渗透率的贡献. 结果表明, 狭长孔隙内最大吸附浓度和朗格缪尔压力最高, 吸附气表面扩散能力最弱. 孔隙半径5 nm以上时, 吸附气表面扩散对气体渗透率影响可忽略. 本文研究揭示了页岩气藏实际生产过程中有机质孔隙形状对页岩气吸附和流动能力的影响机制.Abstract: The characteristics of shale gas reservoir are expressed in terms of complex pore structure and multiple gas occurrence pattern. Adsorbed gas and bulk gas coexist in nano-scale organic pores. The influence of organic pore shape on confined gas adsorption and flow behavior is not clear, which consequently causes the inaccuracy to understand gas transport mechanisms in shale gas reservoir. To solve this problem, different shapes of organic pore (circular pore, slit pore, triangle pore, square pore) are constructed in this study and Grand canonical Monte Carlo method is first applied to simulate gas adsorption process in different shapes of organic pore across different pore pressure. We found that gas adsorption behavior in different shapes of pores conforms well with the Langmuir adsorption pattern. The absolute adsorption, excess adsorption and gas adsorption parameter change with pore size and pore pressure is analyzed and the influence of pore shape on gas adsorption is studied on this basis. The mathematical model of adsorbed gas surface diffusion and free gas slip flow in different shapes of organic pore is established based on the understanding of thermodynamic gas adsorption pattern in different shapes of organic pore. The respective contribution of adsorbed gas flow and free gas flow on total gas permeability is studied in organic pores with different pore shape and pore size combining molecular simulation results with established gas transport models. The results indicate that the slit pore exhibits the largest value of maximum gas concentration, Langmuir pressure and the weakest adsorbed gas surface diffusion capacity compared with the other three pore shapes. The adsorbed gas surface diffusion influence on gas permeability can be neglected at pore radius larger than 5 nm for different pore shapes. This study reveals the influencing mechanism of organic pore shape on gas adsorption and flow ability during practical shale gas reservoir production.
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Key words:
- shale gas reservoir /
- gas flow /
- organic pore adsorption /
- permeability /
- Grand canonical Monte Carlo
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图 1 不同有机质孔隙形状高分辨率页岩扫描电镜图像
Figure 1. High resolution shale SEM images (the different pore morphologies are marked on the SEM image)
图 2 不同有机质孔隙模型横向剖面形状(孔隙半径2 nm)
Figure 2. Visualization of constructed 3D carbon pores lateral cross sections (r = 2 nm)
图 3 有机质孔隙模型纵向剖面(圆形孔隙为例, 孔隙半径2 nm)
Figure 3. Visualization of constructed carbon pore longitudinal cross section, circle pore as an example, r = 2 nm
图 5 不同形状有机质孔隙内气体分子分布(气体分子: 红色, 孔隙半径2 nm)
Figure 5. Gas molecule distribution in different shapes of organic pore (gas molecule: red color, r = 2 nm)
图 6 实验室等温吸附曲线测试原理示意图
Figure 6. Illustration of laboratory measured isothermal gas adsorption curve
7 温度400 K下不同压力、孔隙半径(1 nm ~ 8 nm)下不同形状孔隙中绝对吸附量变化
7. Absolute adsorption isotherms at 400 K in different structures of carbon pores with pore radii ranging from 1 nm to 8 nm
图 7 温度400 K下不同压力、孔隙半径(1 nm ~ 8 nm)下不同形状孔隙中绝对吸附量变化 (续)
Figure 7. Absolute adsorption isotherms at 400 K in different structures of carbon pores with pore radii ranging from 1 nm to 8 nm (continued)
图 8 朗格缪尔吸附参数随孔隙半径的变化
Figure 8. Langmuir adsorption parameters variation with pore radius
图 9 典型气藏生产条件下不同孔隙形状气体绝对吸附量随孔隙尺寸变化
Figure 9. Absolute adsorption concentration change with pore size in different pore structures at typical reservoir conditions
图 10 孔隙半径1.5 nm、温度400 K、压力40 MPa下气体赋存状态. (a)(d)(g)(j)吸附气和自由气赋存状态, (b)(e)(h)(k)自由气赋存状态, (c)(f)(i)(l)吸附气赋存状态. 三角形孔隙以中心三角形孔隙分析
Figure 10. Methane molecule distribution in different pore structures with pore radius 1.5 nm at 400 K, 40 MPa. (a)(d)(g)(j) Adsorbed gas and bulk gas total distribution, (b)(e)(h)(k) bulk gas distribution, (c)(f)(i)(l) adsorbed gas distribution. Triangle pore is studied using central triangle
图 11 温度400 K基于吸附层和基于孔隙整体空间最大吸附浓度随孔隙半径的变化
Figure 11. Variation of maximum adsorption gas concentration based on total pore space and adsorption layer with pore radius at 400 K
图 12 温度400 K、孔隙半径1 nm~ 8 nm下不同压力、不同形状孔隙中过剩吸附浓度变化
Figure 12. Excess adsorption isotherms at 400 K in different structures of carbon pores with pore radii ranging from 1 nm to 8 nm
图 13 温度400 K下不同孔隙形状、孔隙半径下过剩吸附线
Figure 13. Excess adsorption isotherms at 400 K in different structures of carbon pores with different pore radii
图 17 不同孔隙尺寸下吸附气和自由气赋存状态变化及其对气体渗透率影响(圆形孔隙为例, 40 MPa, 400 K)
Figure 17. Adsorbed gas and bulk gas distributions at different pore radii and its influence on gas permeability (circle pore as an example, 40 MPa, 400 K)
图 18 温度400 K, 不同孔隙半径、孔隙压力下吸附气表面扩散对气体渗透率贡献
Figure 18. Contribution of surface diffusion on total gas permeability at different pore radii, pore pressure and 400 K
表 1 力场参数
Table 1. Force field potential parameters
Potential parameter εff/kB/K σff/nm CH4 148 0.373 C 28 0.34 
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