Surfactant–Polymer\nViscosity Effects on Enhanced\nOil Recovery Across Pore Structures: Microfluidic Insights from Pore\nScale to Darcy Scale

Viscosity is a fundamental fluid\nproperty affecting the multiphase\nflow behavior and enhanced oil recovery (EOR) efficiency. A higher\ninvading fluid to defending fluid viscosity ratio is often associated\nwith a more effective invading efficiency in porous media. However,\na higher viscosity ratio of displacing fluid in chemical EOR does\nnot necessarily result in higher EOR efficiency due to pore-blocking\nand reservoir damage. Therefore, the optimal viscosity range varies\nwith the pore structure and permeability. Nevertheless, the relationship\nbetween the optimal viscosity range and pore structure remains unclear.\nThis research conducts a case study on surfactant–polymer (SP)\nsolutions, which demonstrated success in a pilot test at Karamay conglomerate\nreservoirs. Microfluidics is employed to model the pore structure\nof sandstone and conglomeratic sandstone. Furthermore, an image-based\nrecovery characterization algorithm is implemented to evaluate the\nmobilization efficiency within multisize pores. The experiments indicate\na wider optimal viscosity range for conglomeratic-sandstone-structure\nchips than for sandstone-structure chips. The observation supports\nthe hypothesis that the best-matched pore proportion determines the\nrecovery factor, even in various pore structure. The optimal viscosity\nSP solution can maximize the EOR due to matching the highest pore\nproportion. Furthermore, the viscosity relational diagrams, which\nenable matching the most pore, are established for multipermeability\nreservoirs through microfluidic experiments and the developed upscaling\nmodel. The findings are significant for viscosity optimization in\noil and gas recovery.