<p>我国煤层渗透率普遍较低，原始煤层瓦斯抽采困难，因此常用人工增透和瓦斯解吸技术提高抽采效率。CO₂驱替置换煤层CH₄具有渗流驱替及置换解吸的双重作用，是一项具备工程应用潜力的瓦斯强化抽采技术。然而，由于工程实践中未充分考虑目标驱替煤层本身变质程度和原始瓦斯压力等赋存条件，以及实施过程中的关键技术参数对驱替效果的影响，导致现场瓦斯抽采效果不甚理想。揭示CO₂驱替置换煤层CH₄的作用机制，确定影响瓦斯抽采效果的关键技术参数，并掌握关键技术参数对驱替置换效果的影响规律，对于CO₂驱替置换煤层CH₄技术的推广应用具有重要科学意义和应用价值。</p> <p>鉴于此，论文采用理论分析、物理相似模拟、数值模拟及工程试验相结合的研究方法，围绕CO₂与CH₄竞争吸附过程中的合理置换压力、CO₂驱替煤层CH₄过程中的合理驱替压力与驱替效果定量指标、煤层中CO₂渗透系数的变化规律与渗流影响半径等主要内容进行了系统地研究，确定煤层注CO₂驱替置换CH₄的关键技术参数，最终通过工程试验进行效果检验。取得的主要研究成果如下：</p> <p>（1）通过开展不同变质程度煤样CH₄、CO₂等温吸附实验，得出煤样对CH₄、CO₂等温吸附特性曲线，明确了不同煤样CH₄与CO₂吸附量差异；依据吸附势理论，构建了煤样对CH₄、CO₂吸附势与吸附空间的量化对应关系，确定了CO₂置换不同变质程度煤层CH₄的合理置换压力阈值：无烟煤（2.3~4.3MPa）、焦煤（0.5~2.5MPa）、弱粘煤（3.8~7.0MPa）、长焰煤（0.5~0.9MPa），在合理置换压力阈值内，CO₂置换煤层CH₄效果显著。</p> <p>（2）开展了不同驱替压力条件下煤层（弱粘煤）注CO₂驱替CH₄物理模拟实验，分析了CO₂驱替煤层CH₄过程中气体浓度变化规律，明确了驱替效果的定量指标；结合驱替压力与驱替效率及驱替置换比之间的关系，确定了CO₂驱替煤层CH₄过程中的合理驱替压力阈值（＞3.75MPa）；综合合理置换压力和合理驱替压力，确定了合理的注入压力阈值为3.8~7.0MPa。基于驱替置换比及钻孔周边的CO₂含量分布规律，提出了CO₂驱替煤层CH₄钻孔合理压注量的确定方法。</p> <p>（3）开展了CO₂在煤样中的渗流实验，掌握了煤层中CO₂的渗透系数对注入压力的响应规律，得到CO₂渗透系数随着注入压力的增加呈现先减小后增大的趋势，证明煤层中CO₂渗透系数变化过程中存在合理压力梯度；开展了CO₂在煤层中的渗流特性原位试验，得到CO₂渗透系数变化规律与渗流实验中的基本一致。</p> <p>（4）构建了CO₂驱替煤层CH₄渗流扩散数值模型，研究驱替过程中CO₂和CH₄浓度变化规律，验证了数值模型的可靠性；进行了煤层注CO₂驱替CH₄的数值模拟计算，明确了驱替压力与CO₂渗流影响半径的对应关系：驱替压力为5.0~8.0MPa，CO₂渗流影响半径为22.0~25.0m；此结论验证了通过吸附特性和驱替效果综合确定的注入压力阈值的合理性。</p> <p>（5）基于对CO₂驱替置换CH₄过程中置换压力、驱替压力和CO₂渗流特性的研究，综合确定了合理注入压力、钻孔合理压注量、CO₂渗流影响半径及钻孔封孔指标等关键技术参数，开展了高瓦斯煤层顺层钻孔CO₂驱替置换煤层CH₄工程试验，并通过瓦斯抽采效果考察了关键技术参数的合理性。</p> <p>（6）通过工程试验的效果考察，对关键技术参数进行优选，确定了适合黄陵双龙煤矿顺层钻孔注CO₂驱替置换煤层CH₄的关键技术参数：合理注入压力阈值为5.8~7.0MPa；压注孔与抽采孔布置间距为20~30m；钻孔合理压注量为8.0~10.4m³。CO₂驱替置换煤层CH₄技术的实施，为煤层瓦斯高效抽采提供了新的工艺方法。</p>

<p>The low permeability of coal seams in China leads to the difficulties in original coal seam gas extraction. Therefore, artificial permeability enhancement and gas desorption technology were commonly used to improve extraction efficiency. CO₂ displacement and replacement of CH₄ in coal seams has the dual effects of seepage displacement and replacement desorption. It is a gas enhanced extraction technology with engineering application potential. However, due to the lack of full consideration of the occurrence conditions such as the metamorphic degree of the target displacement coal seam and the original gas pressure in the engineering practice, as well as the influence of the key technical parameters in the implementation process on the displacement effect, the field gas extraction effect was not ideal. It is of great scientific significance and application value for the popularization and application of CO₂ displacement and replacement of coal seam CH₄ technology to reveal the mechanism of CO₂ displacement and replacement of coal seam CH₄, determine the key technical parameters affecting gas extraction effect, and master the influence law of key technical parameters on displacement and replacement effect.</p> <p>In view of this, this paper adopted the research methods of theoretical analysis, physical similarity simulation, numerical simulation and engineering test, and systematically studies the reasonable displacement pressure in the process of competitive adsorption of CO₂ and CH₄, the reasonable displacement pressure and displacement effect quantitative index in the process of CO₂ displacing coal seam CH₄, the variation law of CO₂ permeability coefficient in coal body and the influence radius of seepage. The key technical parameters of CH₄ displacement by CO₂ injection in coal seam were determined, and the effect was tested by engineering experiment. The main research results are as follows:</p> <p>(1) By conducting CH₄ and CO₂ isothermal adsorption experiments on coal samples with different metamorphic degrees, the adsorption characteristics curves o for CH₄ and CO₂ are obtained, and the difference in adsorption capacity between CH₄ and CO₂ for different coal samples was clarified. Based on the adsorption potential theory, a quantitative correspondence between the adsorption potential and adsorption space of coal samples for CH₄ and CO₂ was established. The reasonable threshold for the displacement pressure of CO₂ in different metamorphic coal seams was determined: anthracite (2.3-4.3MPa), coking coal (0.5-2.5MPa), weakly caking coal (3.8-7.0MPa), long flame coal (0.5-0.9MPa). Within the reasonable replacement pressure threshold, the effect of CO₂ replacing CH₄ in coal seams is remarkable.</p> <p>(2) The physical simulation experiment of CO₂ displacement CH₄ in coal seam (weak viscous coal) under different displacement pressure conditions was carried out, and the change law of gas concentration in the process of CO₂ displacement of CH₄in coal seam was analyzed, and the quantitative index of displacement effect was clarified. Combined with the relationship between displacement pressure, displacement efficiency and displacement ratio, the reasonable displacement pressure threshold (&gt;3.75MPa) in the process of CO₂displacement of coal seam CH₄was determined, and the reasonable injection pressure threshold was determined to be 3.8~7.0MPa by comprehensively treating the displacement pressure and reasonable displacement pressure. Based on the displacement ratio and the distribution of CO₂ content around the borehole, a method for determining the reasonable pressure injection amount of CH₄ drilling hole in CO₂ displacement coal seam was proposed.</p> <p>(3) The seepage experiment of CO₂ in coal samples was carried out, and the response law of the permeability coefficient of CO₂in coal to the injection pressure was mastered, and the permeability coefficient of CO₂showed a trend of decreasing first and then increasing with the increase of injection pressure, which proved that there was a reasonable pressure gradient during the change of CO₂ permeability coefficient in coal. The seepage characteristic of CO₂ in coal seam was tested in situ, and the change law of CO₂ permeability coefficient was basically consistent with that in the seepage experiment.</p> <p>(4) The numerical model of CO₂ displacing CH₄ seepage and diffusion was constructed to study the variation of CO₂ and CH₄ concentration during the displacement process, and the reliability of the numerical model was verified. We conducted numerical simulations of CO₂ injection in coal seams to displace CH₄ in coal seams, mastered the laws of the CO₂ seepage, and clarified the relationship between displacement pressure and the radius of influence of seepage flow: the displacement pressure is 5.0-8.0MPa, and the CO₂ seepage influence radius is 22.0-25.0m; this conclusion verifies the rationality of the injection pressure threshold determined by the combination of adsorption characteristics and displacement effect.</p> <p>(5) Based on the research on the displacement pressure, displacement pressure and CO₂ seepage characteristics during the process of CO₂ displacement and CH₄ displacement, the key technical parameters such as reasonable injection pressure, reasonable pressure injection amount of borehole, radius of influence of CO₂ seepage and borehole sealing index were comprehensively determined, and the CH₄ engineering test of CO₂displacement coal seam was carried out along the high gas coal seam, and the rationality of key technical parameters was investigated by gas extraction effect.</p> <p>(6) Through the evaluation of the effect of engineering test and the optimization of key technical parameters, the key technical parameters suitable for the CO₂ displacement and replacement technology of coal seam CH₄ in the Huangling Shuanglong Coal Mine were determined: the reasonable injection pressure threshold is 5.8-7.0MPa; The spacing between injection holes and extraction holes is 20-30m; The reasonable injection volume threshold is 8.0-10.4m³. The implementation of the technology of replacing coal seam CH₄ by CO₂ displacement provides a new process method for efficient extraction of coal seam gas.</p>