Multi-Scale Quantitative Characterization of Residual Oil in Different Remaining States: A Case Study of Polymer Surfactant Flooding in Xinjiang Conglomerate Reservoirs

Field pilot tests of polymer-surfactant flooding in Xinjiang’s conglomerate reservoirs have proven successful. However, the unique bimodal pore structure of these reservoirs complicates the existence state of residual oil and obscures its continuous mobilization, thereby hindering efficient development and the potential for enhanced oil recovery (EOR). To improve the simulation accuracy of pore structure and oil residence, this research integrates microfluidic technology, dual-resolution CT, and digital core techniques. This integration enables quantitative assessment of residual oil at the nanoscale. We analyzed the dynamics of residual oil during the flooding process and studied the coupling relationships between core, oil, and water across multiple dimensions. Simulation results indicate that after water-flooding, 80% of residual oil exists in globular or porous forms. High-molecular-weight chemical systems at low to medium injection rates primarily mobilize globular oil, while low-molecular-weight systems at high injection rates can partially mobilize porous oil. After binary flooding, 60–90% of residual oil is in porous or other forms. Globular oil is mobilized by expanding the sweeping area, forming flow channels, and continuous mobilization. In contrast, porous and other residual oils are mobilized through emulsification, stripping, and reduction of cluster size. The research demonstrates that different residual oil types have distinct mobilization pressures and mechanisms. To enhance oil recovery, it is essential to optimize flooding system formulations and injection strategies based on pore structure and residual oil characteristics, especially when the targeted conglomerate reservoirs are in a high water-cut stage.