Permeability reduction effect and correction for low-permeability tight sandstone reservoirs under bound water saturation

Tight sandstone reservoirs with low-permeability under formation conditions are highly prone to bound water saturation, and the permeability of dried samples obtained by standard methods can not accurately reflect the reservoir properties. Rock samples from different reservoir types in the Western Sichuan Depression were collected for water-bearing formation permeability experiments, including the Jurassic Penglaizhen Formation (JP) and Jurassic Shaximiao Formation (JS) of the Xinchang Gas Field, and JS of the Zhongjiang Gas Field. The microscopic characteristics of reservoirs were systematically characterized using thin-section observation, X-ray diffraction (XRD), cathodoluminescence, scanning electron microscopy (SEM), and energy spectrum analysis. The influencing factors of the reduction in effective permeability under bound water saturation were analyzed, and a permeability correction method was established. This study indicated that the total amount of illite-smectite (I/S) mixed-layer minerals and the relative content of smectite in I/S mixed-layer minerals were the main factors influencing the permeability reduction under bound water saturation. Compared to the Xinchang JP gas reservoir, the diagenetic environment for the Zhongjiang JS and Xinchang JS gas reservoirs was more favorable for the development of chlorite and illite, and the smectite content in I/S mixed-layer minerals was lower. Among them, the Zhongjiang JS gas reservoir was the most favorable and exhibited the smallest permeability reduction and the least difference across reservoirs with different physical properties under bound water saturation. Given similar clay mineral development characteristics, particle size, pore size, and carbonate cement were other important factors that affected permeability reduction in water-bearing formations. The Xinchang JP gas reservoir had the smallest clastic particles and the largest difference in reservoir physical properties, and the permeability reduction times and differences across reservoirs with different physical properties were the largest under bound water saturation. The Xinchang JS gas reservoir had smaller clastic particles and pore sizes compared to the Zhongjiang JS gas reservoir, and its carbonate cements were unevenly distributed, resulting in poor pore connectivity. Overall, the Xinchang JS gas reservoir was more prone to bound water saturation and permeability reduction. A permeability correction method for tight sandstone reservoirs was established. This method is universally applicable and can be widely promoted, providing more accurate physical parameters for reserve estimation and productivity prediction.