Fluid flow mechanisms in tight gas reservoirs: A case study of Xujiahe formation in the central Sichuan basin

Tight sandstone gas reservoirs exhibit low gas abundance and extensive gas-bearing lithology, presenting promising prospects for future exploration and development. However, their exploitation in the Sichuan Basin is severely constrained by low recovery efficiency, high water saturation, and pronounced retrograde condensation phenomena. In this study, we aim to analyze the complex fluid flow behavior within these reservoirs to overcome these challenges. To achieve this, a comprehensive experimental program is utilized, encompassing nuclear magnetic resonance, micro-computed tomography, core flooding, and retrograde condensation tests. The analysis reveals that tight sandstone reservoirs feature low porosity, low permeability, and poor pore connectivity, with pore structures predominantly composed of nanometer-to 10-μm-size micropores. Pore size distributions in both Type I and Type II reservoirs exhibit multimodal characteristics, rendering them highly sensitive to stress changes and condensate-induced formation damage. Permeability demonstrates significant hysteresis effects during cyclic changes in confining pressure. Precisely, after two complete cycles of increasing and decreasing confining pressure, average permeability damage rates reach 52.08% and 46.1%, respectively. This indicates damage levels ranging from moderate-weak to moderate-strong. In contrast, variations in pore pressure exhibit minimal impact on permeability. Under irreducible water saturation conditions, the average gas phase damage rate is 64%. Furthermore, initiating fluid flow within the reservoir requires a specific threshold pressure. The average damage rate caused by retrograde condensation is 24.7%. This experimental investigation elucidates the reservoir's pore-throat characteristics, the dynamics governing gas-water and gas-oil flow, and the damage patterns associated with retrograde condensation. These insights provide critical foundational data for optimizing development strategies and designing effective stimulation measures to achieve enhanced recovery.