Research on the Variation Patterns of Reservoir Characteristics After Water Flooding in Tight Oil Reservoirs—By Taking the Chang6 of S37 in the Ansai Area as an Example

Abstract

To address unclear changes in reservoir characteristics before and after water flooding within the S37 tight oil reservoirs of Ansai Oilfield, integrated core analysis techniques including X-ray diffraction (XRD), scanning electron microscopy (SEM), cast thin-section examination, mercury injection capillary pressure (MICP) tests, relative permeability experiments, and real sandstone microscopic seepage modeling were employed. A comparative analysis assessed variations in reservoir physical properties, pore structure, clay mineral content, wettability, and relative permeability. This facilitated an in-depth investigation into the mechanisms and influencing factors driving these characteristic alterations. Results indicate that post-water-flooding, inter-granular pores and facial porosity increased variably, while the overall clay mineral content decreased significantly, particularly chlorite, calcite, and matrix materials dissolving substantially. Comparative analysis of rock samples before and after long-term water exposure revealed distinct behaviors based on initial quality: samples with initially good physical properties exhibited an enlarged median pore-throat radius, enhanced permeability, reduced heterogeneity, and a “polarization” trend characterized by increased proportions of both high and low permeability/porosity zones. Conversely, samples with initially poor properties showed a reduced median pore-throat radius, diminished permeability, and heightened heterogeneity. Water-flooding shifted relative permeability curves rightward overall, narrowed the oil-water two-phase co-seepage region, increased rock hydrophilicity, lowered residual oil saturation, led to earlier water breakthrough, and accelerated water cut rise. Furthermore, affected by reservoir high-permeability zones and long-term plugging of pore throats by injected water solids, absorbent layer thickness declined, the swept volume diminished, and the water-flood oil displacement efficiency deteriorated. These findings provide a crucial theoretical foundation for describing tight oil reservoirs, evaluating development performance, and optimizing field development strategies.