Comprehensive Multi-Method Characterization of Pore Structure and Its Impact on Fluid Mobility of Tight Sandstones—A Case Study of the Shaximiao Formation in the Central Sichuan Basin

The pore structure and fluid mobility of gas reservoirs in tight sandstones significantly influence the assessment of reservoir quality and the potential for gas reservoir development. This study focuses on analyzing tight sandstone samples extracted from the Shaximiao Formation in the Central Sichuan Basin. Experimental techniques such as scanning electron microscopy (SEM), high-pressure mercury injection (HPMI), nuclear magnetic resonance (NMR), and micrometer CT (micro-CT) scanning were employed to investigate the pore structure characteristics of tight sandstone reservoirs and their impact on fluid mobility. The findings indicate that the tight sandstones in the study region exhibit three distinct types of pore throat structures, which correspond to varying characteristics in the distribution of mobile fluids. The characteristics of a Class I pore throat structure are a large pore throat, predominantly ranging from 0.1 to 10 μm, exhibiting robust connectivity, and a straightforward pore throat structure. It possesses an average fractal dimension of 2.5347, along with the highest levels of movable fluid saturation and movable fluid porosity, averaging 69.48% and 7.00%, respectively. The pore throat of Class II pore throat structure is concentrated between 0.01 and 0.1 μm, exhibiting weak connectivity and a complicated pore throat structure. The average fractal dimension is 2.6604, while the averages for movable fluid saturation and movable fluid porosity are 46.60% and 3.94%, respectively. The pore throat of Class III pore throat structure is small, exhibiting the lowest connectivity. The coordination number and pore-throat ratio are 1.60 and 4.82, respectively, while the fractal dimension is 2.7388. The fluid mobility is suboptimal, and the mobile fluid saturation is at 19.88%. Nanopores and micropores constituted 30.77% and 30.55%, respectively, serving as the primary pore throats within the tight sandstone reservoir. The microscopic pore structure of tight sandstone is the main factor influencing fluid mobility. Higher-quality pore structures in tight sandstone correlate with enhanced fluid movement. In addition, the physical properties of tight sandstone reservoirs and clay mineral types also have some influence on fluid mobility. There is a direct correlation between the quality of physical characteristics in tight sandstone reservoirs and the amount of fluid that can be easily moved inside them. A rise in chlorite content promotes an increase in moveable fluid content, whereas the opposite is observed for the illite and smectite interlayer (I/S).