Applying Fractal Theory to Characterize the Pore Structure of Lacustrine Shale from the Zhanhua Depression in Bohai Bay Basin, Eastern China

摘要

The complexity of the shale pore structure, which can be assessed by the fractal dimension, will affect the percolation and reservoir capability of a shale; thus, the pore structure is important for shale reservoir evaluation. For the pore structure and fractal characteristics of lacustrine shale to be investigated, a combination of X-ray diffraction (XRD), total organic carbon (TOC), scanning electron microscopy (SEM), mercury-injection capillary pressure (MICP), nuclear magnetic resonance (NMR), and Nano-CT experiments were performed on shale samples from the lower submember of the third member of the Eocene Shahejie Formation (Es-3(L)) in the Zhanhua Depression, Bohai Bay Basin. On the basis of fractal theory, the NMR fractal dimensions of the analyzed shale samples were determined by the transversal relaxation time (T-2) spectrum from the NMR experiment. The relationships between the NMR fractal dimension and mineral content, TOC, and pore structure parameter were discussed. The results indicate that the pore structure of the lacustrine shale in the study area is complex and exhibits strong heterogeneity. The pore types mainly include intergranular pores, intragranular pores, and some dissolved pores and microfractures. Calcite and clay are the dominant minerals, ranging from 9 to 91% (average 52.23%) and from 1 to 48% (average 18.63%), respectively. The TOC contents are relatively high with values from 0.06 to 9.32%. The calculated NMR fractal dimension (D) values are between 2.2544 and 2.439, which exhibit positive correlations with TOC content, quartz content, and clay mineral content. In contrast, a negative relationship occurs between the NMR fractal dimension and calcite content, indicating that the development of large, dissolved pores in shale samples could reduce the heterogeneity of the pore size distribution. Negative correlations are observed between the NMR fractal dimension and the T-2 cutoff value, T-2 geometric mean, porosity, and average pore throat radius, whereas the NMR fractal dimension exhibits a positive correlation with the displacement pressure (P-d) and has no obvious relationship with permeability. The different relationships suggest that the NMR fractal dimension is closely related to the pore structure; namely, the smaller the NMR fractal dimension is, the better the pore structure is in the shale samples. Four typical samples were chosen to verify the relationship between the NMR fractal dimensions and the shale pore structure in the logging profile of Well L69. Excellent application results were obtained, suggesting that the NMR fractal dimension can be used to indicate the effectiveness of the reservoir in the study area.