Study on Microscopic Pore Structure and Gas-Driven Water Seepage Characteristics of Tight Sandstone Using Nuclear Magnetic Resonance

Tight sandstone gas resources hold significant potential, but their reservoir development is often hindered by low porosity, low permeability, and strong microscopic heterogeneity. The microscopic pore structure is a critical factor influencing gas-driven water seepage behavior; however, the quantitative relationship between pore structure and two-phase (gas-water) flow remains unclear. This study focuses on the Upper Shihezi Formation (He 8 Member) tight sandstone in the Yan'an Gas Field, Ordos Basin. Using nuclear magnetic resonance (NMR) technology, we systematically analyzed the pore structure characteristics of core samples and conducted gas-driven water seepage experiments to investigate the influence of different pore structures on gas displacement efficiency. The results indicate that the NMR T₂ spectra of the He 8 Member tight sandstone generally exhibit a bimodal distribution. Based on the skewness of the spectra, the pore structures can be classified into two types: bimodal fine-skewed and bimodal coarse-skewed. The former is dominated by small pores, with poor connectivity and low movable fluid saturation, leading to severely limited gas displacement efficiency. In contrast, the latter shows a significant increase in medium-sized pores, a coarser pore-throat structure, and superior seepage capacity. Furthermore, compared to the centrifugal method, the gas-driven water experiments demonstrated enhanced fluid mobility signals in larger pores on the T₂ spectrum, confirming the additional mobilization effect of gas fingering and bypassing in medium-to-large pore systems during actual gas reservoir development. This study provides key insights into microscopic seepage mechanisms, offering important theoretical support for the evaluation and development optimization of tight sandstone gas reservoirs.