Effects of thief zones on displacement efficiency: Microfluidic pore-scale and conformance control analysis

•Thief zones result in intense fingering and by-passing effects during water flooding.•Tertiary polymer flood exhibits a better conformance control to alleviate the thief zone challenges.•Viscous cross-flow between permeability zones occurs in the polymer flood, promoting the overall displacement. Thief zones are regions with high permeability, usually formed in mature or depleted reservoirs after water flooding, which can further reduce the overall recovery during subsequent EOR stages. While existing conformance control solutions alleviate the impact of thief zones, selecting an optimal solution based on current laboratory methods is challenging, since current methods fail to isolate the impact of thief zones, lack tight control of initial conditions, and use long evaluation times as well as large sample volumes. In this study, a microfluidic analogue is designed and fabricated with multiple permeability regions, representing thief zones in a reservoir, to evaluate the performance of polymer flood in a porous medium. The analogue contains four connected and distinct heterogenous zones of different permeability, in which the fluid dynamics and relevant conformance control are resolved during secondary and tertiary polymer floodings, once thief zones are apparent. The secondary water flooding did not exhibit an efficient displacement, with fingering and by-passing effects predominantly observed in higher permeability zones and minor oil displacement in the lowest permeability zone. Despite the fingers created by water flooding, polymer flooding showed much higher displacement efficiency throughout all four zones, resulting in significant incremental oil recovery. The secondary polymer flooding also produced more oil than the secondary water flood due to improvement in the mobility control. Lastly, pore-scale mechanisms such as viscous cross-flow between zones was observed during the polymer flood.