Mechanism of mechanical strength degradation and microstructure evolution of anthracite induced by supercritical carbon dioxide
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摘要:
为揭示深部煤层注入CO2过程中，超临界CO2对无烟煤力学强度与微观结构的影响规律，以无烟煤为研究对象，对2种恒定温度（40、60 ℃）条件下超临界CO2对煤体劣化特性进行探究，利用自主研制的超临界CO2浸泡设备搭配煤体单轴加载装置对其力学强度进行初步测定，借助CT扫描系统表征孔裂隙等结构，通过分析不同浸泡时间（0、1、3、5、7 d）对孔裂隙的物化效应，揭示超临界CO2浸泡后无烟煤宏观强度损失与微观结构演变的内在联系。结果表明：超临界CO2对无烟煤宏观强度的劣化具有一定的时间效应，伴随着浸泡时间的增加，劣化效应逐渐减弱，逐渐趋于某一定值，其劣化主要时期为浸泡0～5 d，同时破坏模式改变，破坏颗粒的平均尺寸逐渐增大。同比恒温60 ℃，恒温40 ℃状态下的超临界CO2对煤体劣化作用较为明显；借助CT 扫描系统发现，经超临界CO2浸泡后白色矿物质消失，“溶蚀孔洞”逐渐扩大，新生孔裂纹不断发育，裂隙开度增加。煤样内部孔裂隙在0～5 d迅速发育成较为连续的孔隙团，此后内部孔裂隙缓慢发育，逐渐趋于稳定；超临界CO2侵入煤体内部，其通过萃取煤基质中的有机物与溶蚀碳酸盐矿物组分，形成“溶蚀孔洞”，破坏晶体结构，导致内部孔隙团逐渐发育。伴随着比表面积的增大，煤体吸附能力增加，其“溶胀效应”进一步增加孔裂隙发育，最终导致宏观力学强度的改变；由宏观强度损失数学模型分析得到，煤体经过超临界CO2浸泡后，强度包络线向右偏移，摩尔应力圆向左偏移，内摩擦角与黏聚力均变小，最终导致煤体宏观强度的损失。
Abstract:In order to reveal the effect of supercritical carbon dioxide on the mechanical strength and microstructure of anthracite in the process of CO2 injection into deep coal seams, takes anthracite as the research object, the degradation characteristics of coal by supercritical carbon dioxide under two constant temperatures (40 ℃ and 60 ℃) were investigated. The self-developed supercritical carbon dioxide immersion equipment combined with coal uniaxial loading device was used to preliminarily determine its mechanical strength. The structures such as pores and cracks were characterized by CT scanning system, and the physicochemical effects of different soaking days (0, 1, 3, 5, 7 d) on pores and cracks were analyzed. The intrinsic relationship between macroscopic strength loss and microstructure evolution of anthracite after supercritical carbon dioxide immersion was revealed. The results show that supercritical carbon dioxide has a certain time effect on the deterioration of the macroscopic strength of anthracite. With the increase of soaking time, the deterioration effect gradually weakens and gradually reaches a certain value. The main period of the deterioration is within 0−5 days of soaking, and the average size of the damaged particles gradually increases with the change of failure mode. Compared with the constant temperature of 60 ℃, the supercritical carbon dioxide under constant temperature of 40 ℃ has a more obvious degradation effect on coal. With the help of CT scanning system, it was found that after the supercritical carbon dioxide immersion, the white minerals disappeared, the “solution holes” gradually expanded, the cracks in the new holes continued to develop, and the crack opening increased. The internal pore and fissure of the coal sample developed rapidly into a relatively continuous pore group within 0−5 days, and then the internal pore and fissure developed slowly and gradually became stable. The supercritical carbon dioxide intrudes into the coal, and by extracting organic matter in the coal matrix and dissolution of carbonate mineral components, it forms “dissolution pores”, destroys the crystal structure, and leads to the gradual development of internal pore groups. With the increase of specific surface area, the adsorption capacity of coal increases, and the“swelling effect”further increases the development of pore and fracture, and finally leads to the change of macroscopic mechanical strength. According to the analysis of the macroscopic strength loss mathematical model, after the coal is soaked in supercritical CO2, the strength envelope shifts to the right, the molar stress circle shifts to the left, and the internal friction Angle and cohesion become smaller, resulting in the macroscopic strength loss of the coal.
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图 2 ScCO2浸泡条件力学测试与微观结构测试系统
Figure 2. ScCO2 soaking condition mechanical test and microstructure test system
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图 3 各试验条件下代表性煤样应力－应变曲线
Figure 3. Stress-strain curves of representative coal samples under various test conditions
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图 6 煤样典型宏观破坏及过渡示意
Figure 6. Typical macroscopic failure and transition diagram of coal sample
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图 7 煤样宏观破坏碎屑分布统计
Figure 7. Distribution Statistical distribution of macroscopic failure fragments in coal samples
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图 9 ScCO2浸泡后煤样三维孔隙渲染图
Figure 9. 3D pore rendering of coal sample after ScCO2 immersion
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图 11 ScCO2对煤体强度劣化示意
注：C0与C分别为考虑ScCO2吸附前、后的煤样黏聚力。$ r_0^{\mathrm{e}} $与$ r_{\mathrm{p}}^{\mathrm{e}} $分别代表煤样内部有、无ScCO2的摩尔应力圆半径。
Figure 11. Schematic diagram of strength deterioration of coal by ScCO2
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图 12 基于工程尺度CO2地质封存构想示意
Figure 12. Schematic diagram of CO2 geological storage based on engineering scale
表 1 浸泡实验方案
Table 1 Immersion experiment scheme
组号 温度/℃ 浸泡时间/d A 20（室温） 0 B，C，D，E 40 1，3，5，7 F，G，H，I 60 1，3，5，7 
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表 2 煤样试件在不同实验条件下的峰值强度及劣化程度
Table 2 Peak strength and deterioration degree of coal samples under different experimental conditions
浸泡时间/d 恒定温度40 ℃ 恒定温度60 ℃ 抗压强度/MPa 总劣化度/% 阶段劣化度/% 抗压强度/MPa 总劣化度/% 阶段劣化度/% 0 15.321 0 0 15.321 0 0 1 9.391 38.71 38.71 10.214 33.33 33.33 3 6.467 57.79 19.08 8.164 46.71 13.38 5 5.154 66.36 8.59 7.268 52.56 5.85 7 4.247 72.28 5.92 6.543 57.29 4.73
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表 3 煤样试件在不同实验条件下的破坏特征
Table 3 Failure characteristics of coal samples under different experimental conditions
组号 恒定温度/℃ 浸泡时间/d 破坏特征 失稳类型 A 20 0 拉剪破坏 突发失稳 B 40 1 拉剪破坏 突发失稳 C 40 3 拉剪破坏 突发失稳 D 40 5 剪切破坏 准突发失稳 E 40 7 剪切破坏 渐进失稳 F 60 1 拉剪破坏 突发失稳 G 60 3 拉剪破坏 突发失稳 H 60 5 拉剪破坏 渐进失稳 I 60 7 剪切破坏 渐进失稳
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表 4 不同ScCO2浸泡时间CT扫描图像切片
Table 4 CT scan image sections for different supercritical CO2 soaking days
CT扫描 浸泡时间/d 0 1 3 5 7 第1层 
第2层 
第3层 
第4层 
第5层 
第6层 
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表 5 不同ScCO2浸泡时间试件内部孔裂隙分布
Table 5 Distribution of internal pore cracks in specimens with different supercritical CO2 soaking days
样品 恒定温度/℃ 浸泡时间/d 孔裂隙数量/条 孔隙率/% 样品1 20(室温) 0 10019 3.275 40 1 23048 8.972 40 3 35328 10.882 40 5 39751 12.633 40 7 42623 13.784 样品2 20(室温) 0 10325 3.395 60 1 21642 8.021 60 3 30912 9.181 60 5 36142 11.883 60 7 38956 12.386
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