Analysis Method of Full-Scale Pore Distribution Based on MICP, CT Scanning, NMR, and Cast Thin Section Imaging-A Case Study of Paleogene Sandstone in Xihu Sag, East China Sea Basin

Using different experimental methods, the pore radius ranges vary greatly, and most scholars use a single experiment to study pore structure, which is rarely consistent with reality. Moreover, the numerical models used in different experiments vary and cannot be directly compared. This article uniformly revised all experimental data into a cylinder model. Quantitative analysis of the full-scale pore distribution is established by mercury withdrawal-CT data, and semi-quantitative distribution is obtained by mercury-NMR-cast thin section imaging. In this paper, we introduce the tortuosity index (tau) to convert the CT ball-and-stick model into a cylinder model, and the pore shape factor (eta) of the cast is used to convert the plane model into the cylinder model; the mercury withdrawal data is applied to void the influence of narrow throat cavities, and the NMR pore radius distribution is obtained using the mercury-T-2 calibration method. Studies have shown that the thickness of bound water is 0.35 similar to 0.4 mu m, so the pores with different radius ranges were controlled by different mechanisms in the NMR tests, with pores < 0.35 similar to 0.4 mu m completely controlled by surface relaxation, including strong bound water and weak bound water; pores in the 0.4 similar to 4 mu m reange were controlled by surface relaxation; and pores > 10 mu m were completely controlled by free relaxation. The surface relaxivity rate of fine sandstone was 18 similar to 20 mu m/s. The tortuosity index tau was generally 1 similar to 7; the larger the value, the more irregular the pores. The pore shape factor eta was generally 0.2 similar to 0.5; the smaller the value, the more irregular the pores. Mercury withdrawal-CT scan data can quantitatively determine the pore radius distribution curve. The coefficient of the logarithm is positive considering porosity, and the constant is negative considering porosity. Permeability controls the maximum pore radius, with a max pore radius > 100 mu m and a permeability > 1 mD. Mercury withdrawal-NMR-cast thin section imaging data can semi-quantitatively establish a pore radius distribution histogram. The histogram represents quasi-normal, stepped, and unimodal data. When 60 mu m is the inflection point, if a large proportion of pores measure > 60 mu m, good reservoir quality is indicated. If a large proportion of pores measures < 60 mu m, the permeability is generally <0.5 mD.