Study on the gas-water seepage law and influencing factors of drainage radius in tight sandstone gas reservoirs: A case study of central Sulige area

Studying the drainage radius in tight sandstone gas reservoirs is of great significance for understanding the inter-well reserve mobilization, determining optimal well spacing, and implementing well pattern infilling adjustments. Low porosity and low permeability characteristics of tight sandstone gas reservoirs result in seepage mechanisms that differ from those of conventional gas reservoirs, with the threshold pressure gradient and stress-sensitive permeability being key influencing factors. Based on actual core samples from three types of reservoirs in the central area of Sulige Gas Field, experiments on the threshold pressure gradient and stress sensitivity were conducted under varying physical properties and water saturation conditions. Building on this, a pseudo-steady gas-water two-phase flow model incorporating both effects was established. Developing a corresponding iterative computation program, the impacts of the threshold pressure gradient and stress sensitivity on the drainage radius and formation pressure distribution were analyzed, ultimately leading to the development of a chart for calculating the elastic drainage radius. The findings indicate that cores with worse pore-throat structures and higher water saturation exhibit a greater threshold pressure gradient, with differences becoming more pronounced under higher water saturation conditions. The gas-water relative permeability curves for the three reservoir types in the region of interest show significant variations, shifting toward higher water saturation and lower relative permeability as the pore-throat structure and physical properties deteriorate. The model's calculations demonstrate over 97% consistency with field production data, validating its applicability for estimating the elastic drainage range of vertical wells in the central Sulige Gas Field. The drainage radius is inversely proportional to both the threshold pressure gradient and stress sensitivity, with the latter having a more pronounced effect. Noteworthy, the variation in drainage radius differs considerably among different reservoir types, underscoring the necessity of reservoir classification to improve the accuracy of drainage range calculations. The findings contribute to clarifying the patterns of inter-well reserve mobilization in Sulige Gas Field and provide theoretical guidance for well pattern infilling adjustments during the late development stage.