CO2在多孔介质中的扩散机理及其与前缘推进速度的量化关系研究

Carbon dioxide utilization and geological sequestration have become a topic of considerable interest worldwide as a substantial increase in use of fossil fuels have released a correspondingly large amount of CO2 into the atmosphere. As a key evaluation parameter, diffusion front determination and prediction, is a crucial step; and also is one of the most challenging areas of work in CO2 sequestration. Many recent studies have focused on the dispersion induced by pressure difference and gravity effects; however, much less work has been conducted on the diffusion induced by concentration differences in reservoir conditions. In particular, there is a deficit of study of the relationship between diffusion coefficient and the molecule moving front. This investigator has built a new multipoint measurement model in one dimension that can be combined with other measurement methods such as Raman spectroscopy, tracking of Dynamic pH value, and NMR in order to investigate the diffusion mechanism of CO2-water-oil compound systems in a reservoir condition. This investigator has also examined the quantitative relations between diffusion coefficient and the moving front of compound systems, both in bulk phase and porous medium. Given this analysis the investigator proposes a method to predict the CO2 moving front, which will ultimately expose the key diffusion mechanisms by a model that quantitative describes the relationship between the diffusion coefficient and its associated moving front. The findings therein are also expected to contribute to studies of the diffusion characteristics of CO2 flooding and geological sequestration and to the prediction of displacement front and displacement effect evaluation.

CO2在地层中扩散前缘的预测是"CO2资源化利用及地质埋存"这一国际热点问题中亟待解决的难点之一。前人对扩散的研究主要集中在压力差及重力场下物质的流动及对流传递，而浓度差导致其在地层中的扩散却较少涉及。且对于浓度梯度下的扩散研究主要基于简单体系，而对CO2在复杂条件下的扩散系数及与其在多孔介质中的前缘推进速度关系的研究尚属空白。针对以上问题，本课题通过建立一维多点测定实验模型，利用拉曼光谱、动态pH值测定、核磁共振等方法，开展油藏条件下CO2在油、水构成的复杂体系中的扩散机理、CO2扩散系数与其在体相及多孔介质中前缘运移速度之间的关系、实际油藏条件下CO2扩散前缘的预测方法等方面的研究。通过本课题的研究，揭示CO2在复杂油藏条件下的扩散机理及相关规律，建立描述CO2在油藏中扩散系数与前缘推进速度的量化模型，为CO2驱油和地质埋存过程中扩散行为的描述、驱替前缘及驱油效果的预测奠定基础。

将CO2注入地下油层进行提高原油采收率、地下盐水层埋存及海洋埋存等是实现CO2埋存及资源化利用的主要途径，但注入到油层、海水中的总量及其在空间中的分布情况取决于其扩散行为，因此扩散系数的测定及扩散行为的描述是该问题的关键。在实际应用中如CO2吞吐提高采收率过程中，扩散系数还没有被引入到组分模型中，目前仅用溶解平衡常数描述其在原油中的溶解量，因此难以给出注入量和作用半径之间的关系。课题通过搭建实验装置研究了CO2在不同介质体相及孔隙介质中的扩散规律，并通过模型研究其扩散系数与扩散前缘量化关系。通过近4年的研究，搭建了两套实验装置用于测量CO2在体相及孔隙中的扩散行为：（1）集成细管扩散实验装置；（2）带取样口高压填砂模型。结合拉曼光谱等浓度测量手段，研究了CO2在不同浓度盐水及不同粘度烷烃中的扩散行为。结果表明，CO2在体相中的扩散系数及其在空间中的分布强烈的受介质性质及初始压力的影响，随着初始压力的增加，其扩散系数呈增加的趋势，随着介质粘度的增加，其扩散系数及其在空间分布均降低；在相同的介质及初始压力环境下，CO2在孔隙中的扩散系数比其在孔隙中高1-2个数量级。随着孔隙渗透率的降低，扩散系数呈降低的趋势，其与渗透率的对应关系还有待于进一步的深入研究。