For nearly 20 years, CO₂ has been injected into coal seams to enhance the recovery of methane in a process known as enhanced coal bed methane (ECBM). However, there is a huge complexity associated with this process, mainly due to the generating complex coal chemicophysical structure rearrangement. This review paper aims to comprehensively discuss two main influencing factors upon CO₂ injection in deep coal seams: (1) mobilization of hydrocarbon and (2) coal matrix swelling. CO₂ injection into deep coal seams may remove available polycyclic aromatic hydrocarbons (PAHs) from the coal matrix and mobilize them in the coal seam. The amount of hydrocarbon that is mobilized from the coal matrix by the injected CO₂ is dependent on coal rank, maceral content, type of available hydrocarbons in the coal mass (both dissolving and nondissolving types), and phase state of the injected CO₂ in the seam. Supercritical CO₂ has greater solvent ability and, therefore, has the ability to extract a greater percentage of hydrocarbon from the coal matrix. This mobilization of the organic constituents of the coal matrix by the injected CO₂ causes many environmental issues. For examples, PAHs that exist in highly volatile bituminous coal are harmful to biota and the environment, even at relatively low concentrations. On the other hand, adsorption of the injected CO₂ into the coal mass causes it to be swelled, leading to significant alternations in its internal coal mass structure, resulting in great modifications in its flow and strength properties. This CO₂-adsorption-induced coal matrix swelling process is reduced with increasing temperature, exhibits an inverted-U shaped variation with coal rank, and is largely dependent on the pressure and the physical state of the injected CO₂, where supercritical CO₂ creates a much greater swelling effect, compared to gas/liquid CO₂, because of its higher chemical potential. Potential coal seams for CO₂ sequestration process are available at extremely deep locations and there is a high possibility of phase change from gas/liquid to supercritical state and, thus, they likely have high swelling rates.

中文翻译：

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煤层深部CO ₂注入的影响研究进展

在将近20年的时间里，CO ₂已被注入煤层中，以提高煤层气的回收率，这一过程被称为增强煤层气（ECBM）。然而，该过程存在巨大的复杂性，这主要归因于产生复杂的煤化学物理结构重排。这篇综述旨在全面讨论深部煤层中注入CO _2的两个主要影响因素：（1）油气的运移和（2）煤基质膨胀。向深煤层中注入CO ₂可能会从煤基质中去除可用的多环芳烃（PAH），并在煤层中动员它们。通过注入的CO ₂从煤基质中运出的碳氢化合物的量取决于煤的等级，总含量，煤块中可用烃的类型（溶解和非溶解类型）以及煤层中注入的CO _2的相态。超临界CO ₂具有更大的溶剂能力，因此具有从煤基质中提取更大百分比的碳氢化合物的能力。注入的CO ₂对煤基质的有机成分的这种动员引起许多环境问题。例如，即使在相对较低的浓度下，高挥发性烟煤中存在的多环芳烃也对生物群和环境有害。另一方面，所注入的CO _2的吸附进入煤团会导致其膨胀，从而导致其内部煤团结构发生重大变化，从而导致其流动性和强度特性发生重大变化。这种CO ₂吸附引起的煤基质膨胀过程随温度升高而降低，表现出随煤级的倒U形变化，并且很大程度上取决于注入的CO ₂的压力和物理状态，在该压力和物理状态下会形成超临界CO _2。与气/液CO ₂相比，具有更大的溶胀效果，因为它具有更高的化学势。CO _2的潜在煤层 螯合过程可在极深的位置进行，并且很有可能从气/液相转变为超临界状态，因此它们的溶胀率很高。