Abstract: The mixed injection of N₂ and CO₂ shows great potential for enhancing the productivity of low-yield wells and for CO₂ sequestration. This process involves complex interactions, including multicomponent gas competitive adsorption, multiphase porous flow, mass transfer, and heat exchange. However, the differential responses of coal reservoirs with different lithotypes to N₂-CO₂ mixed gas injection for enhanced coalbed methane (ECBM) recovery and their optimal injection ratios remain unclear. This study focuses on high-rank coals in southern Qinshui Basin. Using a multi-scale pore structure characterization approach, the pore and fracture characteristics of four coal samples were systematically analyzed, as well as their differences in N₂, CO₂, and CH₄ adsorption behaviors. The evolution patterns of gas-water and CO₂-CH₄ systems during the displacement process were investigated. Furthermore, thermo-hydro-mechanical (THM) coupled numerical simulations were conducted to reveal the differential response processes and underlying mechanisms of various coal reservoirs during N₂-CO₂ injection for ECBM recovery. The results show as follows . (1) The pore systems of all coal types are dominated by micropores, with the specific surface area of micropores accounting for up to 99%, indicating significant CO₂ sequestration potential. Mesopores and macropores are relatively well developed in semi-dull and dull coals. The overall adsorption capacity for multiple gases increases with increasing vitrinite content. Bright and semi-bright coals exhibit stronger adsorption capacities for N₂, CO₂, and CH₄, as well as more significant variances in Langmuir volume, implying more remarkable competitive adsorption effects. (2) During the displacement process, fracture water and seepage pore water are readily expelled, whereas adsorbed water requires higher pressure for adsorption-driven displacement. The volume of displaceable water increases nonlinearly with rising gas pressure and gradually approaches saturation. Due to its stronger adsorption affinity, CO₂ can significantly enhance CH₄ production efficiency and increase CO₂ sequestration capacity under high-pressure conditions. However, its displacement effectiveness is controlled by the adsorption properties and seepage conditions of the coal matrix, reflecting the strong coupling among gas displacement, water migration, and mechanical response during the ECBM process by CO₂ injection. (3) Numerical simulation results indicate that the mixed injection of N₂ and CO₂ promotes the migration of high-concentration CH₄ toward production wells under the effect of "far flooding and near displacement". Compared with pure CO₂ injection, the mixed injection increases CH₄ production by an average of 65.08%and enhances CO₂ sequestration capacity by 17.71%. Among different coal lithotypes, semi-dull and dull coals exhibit a more intense CH₄ concentration response and greater carbon storage capacity; however, their lower diffusion coefficients result in relatively reduced CH₄ production rates. (4) During the mixed gas injection process, the relatively rapid migration of N₂ tends to cause early breakthrough, thereby shortening the production cycle. Overall, as the proportion of CO₂ in the mixture increases, the N₂ breakthrough time exhibits a trend of first decreasing and then increasing, with the shortest production cycle occurring at a CO₂ fraction of 50%. For optimal CO₂ sequestration in reservoirs with different coal lithotypes, the recommended CO₂ concentration in gas mixture ranges from 55%to 90%, with the main differences reflected in the extent of CH₄ production enhancement. Comprehensive evaluation indicates that the optimal CO₂ injection ratios for different coal lithotypes are as follows: 55%-65%for bright and semi-bright coals, 60%-70%for semi-dull coal, and 65%-75%for dull coal, balancing the enhancement of CH₄ production and the maximization of CO₂ sequestration. These results provide a theoretical basis and technical support for the implementation of ECBM recovery by N₂-CO₂ mixed gas injection in reservoirs with different coal lithotypes.

Key words: mixed gas injection, CO₂ sequestration, competitive adsorption, numerical simulation, high-rank coal, Qinshui Basin

摘要： N₂-CO₂混合气注入在低产井增产改造和CO₂封存方面展现出很大潜力,该过程涉及多元气体竞争吸附、多相渗流和传质换热等复杂相互作用。目前,不同煤岩类型储层对N₂-CO₂混合气注入增强煤层气开采(ECBM)的响应差异及其最优注入组合尚不明确。以沁水盆地南部高阶煤为研究对象,基于多尺度孔隙结构联合表征的方法,系统分析了4类煤岩的孔隙/裂隙结构特征及其对N₂、CO₂和CH₄吸附性能的差异,探究了驱替过程中气-水、CO₂-CH₄的演变规律,并结合热-水-力耦合数值模拟揭示了不同煤岩类型储层在N₂-CO₂混合气注入ECBM工程中的差异响应过程及其机理。研究结果表明:①各类煤的孔隙均以微孔为主,微孔的孔隙比表面积占比高达99 %,具有较大的CO₂封存潜力。半暗煤和暗淡煤中的介孔和宏孔较为发育。多元气体的吸附能力整体上随镜质组含量增加而增强。光亮煤和半亮煤表现出更强的N₂、CO₂、CH₄气体吸附能力及较大的Langmuir体积差异,意味着其竞争吸附效应更为显著。②驱替过程中,裂隙水和渗流孔水易于被驱出,而吸附水则需较高压力通过吸附置换排出,可驱替水量随气体压力升高呈非线性增长并趋于饱和。CO₂凭借更强的吸附竞争力,在高压下可显著提升CH₄的产出效率并增加CO₂的封存能力,但其置换效果受控于煤岩吸附特性和渗流条件,反映了CO₂注入ECBM工程中气体置换、水分运移与力学响应之间的强耦合关系。③数值模拟结果显示,N₂-CO₂混合气注入通过"远驱近替"效应促进高浓度CH₄向生产井迁移。相对于纯CO₂注入,混合气注入可平均提升65.08%的CH₄产量和17.71%的CO₂封存量。在不同煤岩类型中,半暗煤和暗淡煤储层的CH₄浓度响应更剧烈、碳封存量更大,但因其扩散系数较低,CH₄产量相对较低。④在混合气注入过程中,N₂运移速度较快易导致早期突破,从而缩短生产周期。整体上,随着混合气中CO₂的比例增大,N₂突破时间呈先减后增的趋势,CO₂占比为50%时生产周期最短。不同煤岩类型储层对CO₂封存效果最优的混合气注入的CO₂配比为55%~90 %, 主要差异体现在CH₄增产效果上。综合评估显示,不同煤岩类型的最优混合气注入的CO₂配比分别为:在光亮煤和半亮煤中为55%~65%、在半暗煤中为60%~70%、在暗淡煤中为65%~75 %, 以兼顾CH₄增产与CO₂封存最大化。上述成果为不同煤岩类型储层开展N₂-CO₂混合气注入ECBM工程实践提供了理论支撑和技术依据。

关键词: 混合气注入, CO₂封存, 竞争吸附, 数值模拟, 高阶煤, 沁水盆地