Microscale Changes in Residual Oil under Low Salinity Water Flooding in Offshore Sandstone Reservoirs Using X-ray Technology

Abstract

Low salinity water flooding is an effective enhanced oil recovery (EOR) method for offshore sandstone reservoirs, but conventional core flooding struggles to capture the microscopic distribution of residual oil. This study focuses on a sandstone reservoir from the Bohai Oilfield, utilizing X-ray computed tomography (X-CT) to perform in situ scans on core samples under 10 conditions. A 3D model of pore structures and fluid distribution was constructed to classify and quantify residual oil. Changes in oil and water saturation, interfacial tension and pH values were tracked by segmenting pore spaces across varying pore volume (PV) of injected fluid. The results demonstrate that low salinity water flooding significantly enhances crude oil recovery. Oil saturation decreases markedly with increasing PV, although recovery slows beyond 70 PV, ultimately reaching an efficiency of 66.05% at 1000 PV. Furthermore, a detailed analysis of residual oil distribution revealed the proportions of various oil clusters and their positioning within pore spaces. Pores with radii between 10–25 μm, particularly those containing network-type residual oil, were found to be the most mobilizable. This study provides crucial insights into the behavior of residual oil during low salinity water flooding and offers guidance for the optimization of flooding strategies in high water-cut oilfields.