Quantitative characterization of pore structure and NMR fractal based on nuclear magnetic resonance and fractal theory

Based on nuclear magnetic resonance (NMR) technology and fractal theory, this paper systematically studies the microscopic pore structure of tight sandstone reservoirs from Dahaize Coal Mine and its influence on fluid occurrence and migration. The T ₂ spectra of six groups of rock samples in saturated water and bound water were obtained by low field nuclear magnetic resonance experiment, and the complexity and heterogeneity of pore structure were quantitatively characterized by fractal dimension. The results show that the reservoir is characterized by low porosity and low permeability, but the microscopic pore structure is significantly different. Some samples are mainly medium-small pores, with good pore connectivity, high movable fluid saturation and good seepage capacity. Most of the samples are dominated by micropores, with dense structure and poor fluid mobility. The cut-off value of T ₂ (T _2C ) and effective porosity further reveal the difference of fluid mobility in reservoirs, and high values usually correspond to high-quality reservoirs. In addition, through fractal dimension analysis, the essential difference in structural complexity between bound fluid pores ( D _min average 2.144 ) and movable fluid pores ( D _max average 2.959 ) is clarified, indicating that the seepage pore system has higher heterogeneity and tortuosity. This study verifies the effectiveness of the combination of NMR and fractal theory in the quantitative characterization of tight sandstone pore structure, and provides a scientific basis for reservoir evaluation and CO ₂ geological storage potential prediction.