Microscope dynamic characterization of oil charging in tight sandstone using a physical simulation experiment

Abstract

Physical processes of oil charging in tight sandstone were simulated and analyzed using oil displacement water in sandstone micro-visualization models. Microscope dynamic characterizations of oil charging were obtained, and factors controlling the oil charging process in tight sandstones were elucidated in combination with thin section analysis, scanning electron microscopy (SEM) and rate-controlled porosimetry (RCP). Results indicate that three types of residual water exist in tight sandstone reservoirs: water-in-oil droplets; corner residual water; and connect-pore residual water. The charging rate of oil varied in the direction of migration, and uniform oil-water interfaces were not recorded during oil charging. The dynamic process of oil charging was classified as: (i) fingerlike and (ii) netlike. During fingerlike oil charging processes, the frontal movement formed obvious dominant paths, and the residual water area was large. During netlike oil charging processes, oil migrated through multiple channels forming a network of paths, having high terminal oil saturation. Due to the narrow pore-throat, oil charging characteristics of tight sandstones are not sensitive to variations of pore-throat radii; charging characteristics depend on the effective storage space and connectivity of the pore-throat. Connectivity of the dissolution pore-throat was found to be good, and the effective storage space of dissolution combined pores was large. Our results, therefore, indicate that oil charging of tight sandstone is controlled by pore-throat structures formed by dissolution.