多元杂质对CO2咸水层封存对流混合过程的影响机制研究

The geological storage of CO2 in deep saline aquifers is one of the important options for significantly reducing net carbon emissions. The dissolution of CO2 in the formation brine would result in a local density increase of the aqueous phase at the two-phase interface, which leads to convective mixing. The mixing greatly accelerates the dissolution rate of CO2 and thus plays an important role in the security and permanence of CO2 storage. Many practical projects usually inject CO2 with multiple impurities such as N2 and O2 into deep saline aquifers to reduce the costs. These impurities would hinder or promote the occurrence and development of the convective mixing process. The pattern and mechanism of the convective mixing process of impure CO2 storage is supposed to be different with that of the pure CO2 case. This study intends to employ experimental research, numerical simulation and theoretical analysis to investigate the effects of typical impurities such as N2, O2 on the convective mixing process. Firstly, we will carry out laboratory experiments of convective mixing at constant temperature and atmospheric pressure. In the mean time, we will collect relevant physical data (such as solubility) under the conditions of high temperature and high pressure in the literature. Both the experiments and physical data will be compared with the results of our existing multiphase multicomponent numerical program. Necessary modifications to the numerical program will be made until an appropriate numerical program is accomplished. Secondly, this study will perform three-dimensional high resolution numerical simulation to investigate the physical association mechanism between the number, type as well as concentration of impurities and the onset of convection, dissolution rate of CO2, fingering distribution, etc. Finally, the corresponding mathematical representation model of the convective mixing of CO2 containing multiple impurities in deep saline aquifers will be established. These models can complement the theoretical model of the existing pure CO2 convective mixing mechanism. Moreover, this study can provide practical reference for the type and content of impurities for impure CO2 storage in deep saline aquifers and improve the economy and security of CO2 geological storage projects.

咸水层封存是CO2大规模减排的重要方法，CO2溶解于咸水引发的对流混合过程能够提高CO2的溶解速率，对封存安全和长期稳定起重要作用。为了降低成本，实际工程项目注入的CO2通常含有N2、O2等多种杂质组分，这些杂质可能阻碍或促进对流混合的发生和发展，其混合模式及机制与现有的纯CO2模式不同。本项目拟采用实验研究、数值模拟和理论分析相结合的方法，开展N2、O2等典型杂质对CO2对流混合过程的影响研究：（1）进行对流混合室内实验，并参考文献相关物性数据（如溶解度），验证和完善申请人已有的多相多组分数值程序；（2）进行三维高精度数值模拟，研究和揭示对流混合过程的发生时间、CO2溶解速率、指进分布等与杂质数量、类型及浓度的物理关联机制，并建立数学表征模型。本项目建立的对流混合机制模型，是现有纯CO2理论模型的有效补充；同时为界定CO2气流中的允许杂质类型及浓度，提高封存项目的经济性和安全性提供参考。

本项目利用室内实验、数值模拟和理论分析相结合的方法，研究和分析了各种典型杂质（N2、O2、Ar、CH4、H2S、SO2等）对对流混合过程的各特征参数的影响。首先利用Hele-Shaw和砂槽实验，并参考文献相关物性数据（如气体溶解密度和偏摩尔体积），验证和完善了申请人已有的多相多组分数值程序；然后开展三维高精度数值模拟，研究和揭示了对流混合过程的发生时间、CO2溶解速率、指进分布等与杂质数量、类型及浓度的物理关联机制，建立了非纯净CO2地质封存中对流开始时间-瑞利数、对流衰减时间-瑞利数、舍伍德数-瑞利数等的数学表征模型；初步证实了N2可在一定程度上替代其他杂质（如O2、Ar）在对流混合过程研究中的影响；同时证实了预溶解H2S杂质比共注入杂质（N2、CH4、H2S）对分层咸水层中CO2迁移和溶解具有更强的影响。本项目的实施圆满完成了预定的研究目标，建立了非纯净CO2地质封存中的对流混合机制模型，是现有纯CO2理论模型的有效补充，对可为界定CO2气流中的允许杂质类型及浓度，提高封存项目的经济性和安全性提供理论参考。