Numerical simulation study on the coal-bed methane displacement effect of different gas injection components
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摘要: 注气促抽瓦斯的注气成分主要有N2，CO2和空气，但目前针对不同注入成分的置驱效果对比研究较少。针对该问题，建立了考虑裂隙气体渗流和基质孔隙气体扩散的注气成分置驱瓦斯数学模型，在对该模型验证的基础上，模拟注入气体置驱煤样瓦斯气体过程，并对比研究相同注气压力和煤体渗透率条件下不同注气成分（N2，CO2和空气）对瓦斯的置驱效果。结果表明：① 相同注气时间下，从注气端到排气端，注入气体的体积分数逐渐降低，在注气端附近注入气体的体积分数最高；瓦斯体积分数逐渐增加，在排气端附近瓦斯体积分数最高；随着注气时间增加，注入气体的体积分数增加区域逐渐向排气端移动直至覆盖整个煤样，瓦斯体积分数降低区域也逐渐向排气端移动直至覆盖整个煤样，表明煤样中的瓦斯逐渐被置换出来，进而被驱替出整个煤样。② 在相同注气时间内，从注气端到排气端，N2，CO2和空气3种注入气体体积分数和瓦斯体积分数具有相似的变化规律，即从注气端到排气端，注入气体体积分数逐渐降低、瓦斯体积分数逐渐升高，随着注气时间增加，注入气体的体积分数增高区域增加。相同注气时间、煤样相同位置处注入气体的体积分数和瓦斯体积分数互补，即相加为100%。③ 3种注入气体对瓦斯的置驱效果排序为CO2>空气>N2。④ 对排气端气体的体积分数分析可知，排气端气体的体积分数随时间变化可分为突破阶段、平衡进行阶段和置驱完成阶段。不同注入气体3个阶段持续时间不同，注入N2突破时间和置驱完成时间分别为30，90 min；注入CO2突破时间和置驱完成时间分别为20，80 min；注入空气突破时间和置驱完成时间分别为28，87 min。⑤ 在现场应用时，应根据具体煤层的吸附解吸能力、煤层自燃特性等选择合适的注入气体。Abstract: The main components of gas injection to promote methane extraction are N2, CO2, and air, but there is currently limited research on the comparison of displacement effects for different injection components. In order to solve the above problems, a mathematical model for gas injection displacement considering fracture gas seepage and matrix pore gas diffusion is established. Based on the validation of the model, the process of injecting gas into coal samples for methane displacement is simulated. The effects of different injection components (N2, CO2, and air) on methane displacement are compared and studied under the same injection pressure and coal permeability conditions. The results show the following points. ① Under the same injection time, the volume fraction of injected gas gradually decreases from the injection end to the exhaust end, with the highest injected gas volume fraction near the injection end. The methane volume fraction gradually increases, with the highest methane volume fraction near the exhaust end. As the injection time increases, the area with an increase in injected gas volume fraction gradually moves towards the exhaust end until it covers the entire coal sample. The area with a decrease in methane volume fraction also gradually moves towards the exhaust end until it covers the entire coal sample. It indicates that the methane in the coal sample is gradually displaced and driven out of the whole sample. ② Within the same injection time, from the injection end to the exhaust end, the volume fractions of N2, CO2, and air injected gases and methane have similar changes. That is, from the injection end to the exhaust end, the volume fraction of injected gas gradually decreases and the methane volume fraction gradually increases. With the increase of injection time, the area of increase in injected gas volume fraction increases. The volume fraction of injected gas and methane at the same injection time and at the same position of the coal sample complement each other, that is, they add up to 100%. ③ The ranking of the methane displacement effects of three types of injected gases is CO2>air> N2. ④ The analysis of the gas volume fraction at the exhaust end shows that the gas volume fraction at the exhaust end can be divided into breakthrough stage, equilibrium stage, and displacement completion stage over time. The duration of the three stages of injecting different gases varies, with N2 breakthrough time and displacement completion time of 30 and 90 minutes, respectively. The breakthrough time for CO2 injection and the completion time for displacement are 20 and 80 minutes, respectively. The breakthrough time for air injection and the completion time for displacement are 28 and 87 minutes, respectively. ⑤ When applied on site, appropriate injection gases should be selected based on the adsorption and desorption capacity of specific coal seams, as well as the spontaneous combustion features of coal seams.
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图 1 排气端CO2，CH4体积分数随时间变化规律
Figure 1. Volume fraction of CO2 and CH4 at the exhaust end changes with time
图 3 数值模拟结果与试验结果对比
Figure 3. Comparation between numerical simulation results and experimental results
图 4 注N2不同时间煤样内N2体积分数分布规律
Figure 4. Distribution pattern of N2 volume fraction in coal samples at different times of N2 injection
图 5 注N2不同时间煤样内瓦斯体积分数分布规律
Figure 5. Distribution pattern of methane volume fraction in coal samples at different times of N2 injection
图 6 注CO2不同时间煤样内CO2体积分数分布规律
Figure 6. Distribution pattern of CO2 volume fraction in coal samples at different times of CO2 injection
图 7 注CO2不同时间煤样内瓦斯体积分数分布规律
Figure 7. Distribution pattern of methane volume fraction in coal samples at different times of CO2 injection
图 8 注空气不同时间煤样内空气体积分数分布规律
Figure 8. Distribution pattern of air volume fraction in coal samples at different times of air injection
图 9 注空气不同时间煤样内瓦斯体积分数分布规律
Figure 9. Distribution pattern of methane volume fraction in coal samples at different times of air injection
图 10 注N2煤样内瓦斯体积分数分布曲线
Figure 10. Methane volume fraction distribution curve in coal samples injected with N2
图 11 注CO2煤样内瓦斯体积分数分布曲线
Figure 11. Methane volume fraction distribution curve in coal samples injected with CO2
图 12 注空气煤样内瓦斯体积分数分布曲线
Figure 12. Methane volume fraction distribution curve in coal samples injected with air
图 13 注气50 min时3种注入气体体积分数和瓦斯体积分数对比
Figure 13. Comparison of volume fraction of three gas injection and methane volume fraction during 50 min gas injection
图 14 排气端气体体积分数随时间变化规律
Figure 14. The variation law of gas volume fraction at the exhaust end with time
表 1 模拟参数
Table 1. Simulation parameters
参数 值 煤体孔隙率 0.075 煤体密度/（kg$\cdot $m−3） 1 400 CH4标况密度/（kg$\cdot $m−3） 0.668 CH4动力黏度/（Pa$\cdot $s） 1.03×10−5 CH4摩尔质量/（kg$\cdot $mol−1） 0.016 CO2标况密度/（kg$\cdot $m−3） 1.977 CO2动力黏度/（Pa$\cdot $s） 1.38×10−5 CO2摩尔质量/（kg$\cdot $mol−1） 0.044 气体常数/（J$\cdot $mol−1$\cdot $K−1） 8.314 煤体温度/K 293 
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表 2 N2和空气参数
Table 2. Parameters of N2 and air
参数 值 N2标况密度/（kg$\cdot $m−3） 1.165 N2动力黏度/（Pa$\cdot $s） 1.76×10−5 N2摩尔质量/（kg$\cdot $mol−1） 0.028 空气标况密度/（kg$\cdot $m−3） 1.293 空气动力黏度/（Pa$\cdot $s） 1.5×10−5 空气摩尔质量/（kg$\cdot $mol−1） 0.029
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