Characterization of Micro-nano Pore Structure of Tight Sandstone Based on Nuclear Magnetic Resonance Experiments (NMR)

The samples of Chang 8 member of Yanchang Formation in Jiyuan area were tested by nuclear magnetic resonance (NMR) with saturated water and different centrifugal states, and the core thin section, scanning electron microscope (SEM), X-ray diffraction and physical properties were tested to qualitatively and quantitatively characterize the pore type, pore size distribution (PSD), pore connectivity and movable fluid distribution (MFD) characteristics of sandstone reservoirs. The results show that: (1) The pore types of Chang 8 member are mainly (residual) intergranular pores, dissolution pores, intergranular pores and a few micro-fractures; (2) Based on the NMR T2 spectrum morphology, the pore structure can be divided into three types (I, II and III), and the T2 spectrum curve shows “trimodal and quadrimodal”, The reservoir property and percolation capacity deteriorate gradually, and the heterogeneity of pore structure increases gradually; (3) The types (I, II) reservoirs mainly develop macro-pores and micro-fractures, and the PSD is between 1.0–1000.0 ms (15 nm–5.0 μm). The physical properties and connectivity are the well, and the obvious “trimodal and quadrimodal” morphology, and the fractal dimension Di is 2.54–2.76, Types III reservoirs have small pores, the pore size is mainly between 0.1–10 ms (1–50 nm), the connectivity is poor, and most of them are “trimodal” shape. The pores are narrow and the internal structure is complex, and the fractal dimension D is 2.71–2.81; (4) There are obvious differences in the movable fluid saturation between different types of reservoirs. The movable fluid saturation of Types I and II high-quality reservoirs is 50.2% to 73.2%, which is mostly distributed in macropores and microfractures. Therefore, the qualitative and quantitative characterization of microscopic pore structure and MFD in tight sandstone reservoir was realized based on NMR experiments, which has guiding significance for fluid displacement in late production of tight sandstone.