碳封存砂岩储层的孔隙迂曲度分形特征研究

Research on Tortuosity Fractal Characteristics of Sandstone Pores Based on Nuclear Magnetic Resonance Imaging Technology

目前“双碳”减排的背景下,砂岩咸水层是最具潜力的二氧化碳封存介质,其孔隙结构特征决定了流体的运移及封存效率,对其进行研究具有重要的科学意义。分形维数常用于定量表征砂岩孔隙结构在三维空间中的分布规律,但以往对砂岩孔隙迂曲度分形特征进行研究时,多依赖于孔隙率、平均孔隙半径、平均迂曲度等特征值计算分形维数,并不能很好的反映岩石孔隙排列分布及孔隙连接的非均质性。文章将砂岩孔隙结构概化为具有迂曲度的毛细管,首先通过核磁共振成像(MRI)技术获取岩石片层图像,以盒维数法获得孔径分布分形维数;随后以孔径分布分形维数为目标值,结合不同迂曲度下的毛细管数量与孔径的分形标量关系,迭代计算迂曲度分形维数。与传统的分形维数计算方法相比,该研究确定的迂曲度分形维数更能体现岩石内部不同孔隙排列方式引起的迂曲度差异。

Under the context of “double carbon” emission reduction, saline aquifer of sandstone is the most potential medium for carbon dioxide storage. The study of pore-structure characteristics determines the fluid migration and storage efficiency, which has great scientific significance. Fractal dimension is usually used to quantitatively characterize the distribution of sandstone pore structures in three-dimensional space. However, when studying the fractal dimension of sandstone pores in the past, the fractal dimension was calculated based on the characteristic values such as porosity, average pore radius, and average tortuosity, which could not well reflect the heterogeneity and arrangement of pore structures. Therefore, in this paper, pore structures of sandstone are generalized into capillaries with tortuosity. First, the image of rock slices is obtained by means of nuclear magnetic resonance imaging(MRI) technology, and the fractal dimension of pore distribution obtained by the box-counting dimension method is used.The fractal dimension of the pore distribution is the target parameter, and the fractal dimension of the tortuosity is calculated iteratively by combining the fractal scalar relationship between the number of capillaries and the pore size under different tortuosities. Compared with the traditional fractal dimension calculation method, the fractal dimension of tortuosity determined in this paper is larger, and the difference between different rock samples is more obvious, which can better reflect the heterogeneous characteristics of pore distribution.