The injection of CO into deep unminable coal seam is a most promising carbon storage method that can simultaneously achieve the dual objectives of mitigating CO emission and enhancing coalbed methane recovery. In this context, the wettability of the coal-water-CO system plays an important role in the CO injection rate, storage capacity, and sequestration security. However, the factors affecting wettability alternation in the coal-water-CO system are still unclear until now. In this study, the sessile drop method was used to measure the contact angle (CA) in the coal-water-CO system under different pressures (3 – 8 MPa) and temperatures of (40–70 °C) with four different rank coals. Results indicate that with increasing pressure, the CA shows a steady increase below 5 MPa followed by a rapid increase at 5 – 8 MPa. The CA decreases with increasing temperature and coal rank. The CA is negatively correlated with the mes- and macro-pores, but positively correlated with micropores. It was found that these relationships are attributed to the CO density and CO adsorption capacity of coal. As the CO density increases, the coal-CO interaction force intensifies, leading to a tendency towards CO wetting for the coal-water-CO system. The adsorption capacity illustrates the strength of the coal-CO interaction, where the affinity of polar adsorption sites for CO molecules exceeds that of nonpolar adsorption sites. Finally, we conducted a comprehensive assessment of the wettability of the coal-water-CO system as it varies with burial depth by developing an integrated model that considers four influential factors, including pressure, temperature, coal rank, and pore structure. The findings of this study can be utilized to determine the optimal CO injection pressure in CO-ECBM projects and enhance sequestration safety.

中文翻译：

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解码煤-水-CO2系统的润湿性以增强封存安全性

将CO注入深部不可开采煤层是一种最有前途的碳封存方法，可以同时实现减少CO排放和提高煤层气采收率的双重目标。在此背景下，煤-水-CO系统的润湿性对CO注入速度、封存能力和封存安全起着重要作用。然而，影响煤-水-CO系统润湿性变化的因素目前仍不清楚。在这项研究中，采用坐滴法测量了四种不同煤级煤在不同压力（3 – 8 MPa）和温度（40 – 70 °C）下煤-水-CO体系中的接触角（CA） 。结果表明，随着压力的增加，CA 在低于 5 MPa 时稳定增加，然后在 5 – 8 MPa 时快速增加。 CA随着温度和煤阶的升高而降低。 CA与中孔和大孔呈负相关，但与微孔呈正相关。研究发现，这些关系归因于煤的 CO 密度和 CO 吸附能力。随着CO密度的增加，煤-CO相互作用力增强，导致煤-水-CO系统有CO润湿的趋势。吸附容量说明了煤-CO 相互作用的强度，其中极性吸附位点对 CO 分子的亲和力超过非极性吸附位点。最后，我们通过开发考虑压力、温度、煤阶和孔隙结构等四个影响因素的综合模型，对煤-水-CO系统的润湿性随埋深的变化进行了综合评估。本研究的结果可用于确定 CO-ECBM 项目中的最佳 CO 注入压力并提高封存安全性。